Somatosensory-visual effects in visual biological motion perception.

Social cognition is dependent on the ability to extract information from human stimuli. Of those, patterns of biological motion (BM) and in particular walking patterns of other humans, are prime examples. Although most often tested in isolation, BM outside the laboratory is often associated with multisensory cues (i.e. we often hear and see someone walking) and there is evidence that vision-based judgments of BM stimuli are systematically influenced by motor signals. Furthermore, cross-modal visuo-tactile mechanisms have been shown to influence perception of bodily stimuli. Based on these observations, we here investigated if somatosensory inputs would affect visual BM perception. In two experiments, we asked healthy participants to perform a speed discrimination task on two point light walkers (PLW) presented one after the other. In the first experiment, we quantified somatosensory-visual interactions by presenting PLW together with tactile stimuli either on the participants' forearms or feet soles. In the second experiment, we assessed the specificity of these interactions by presenting tactile stimuli either synchronously or asynchronously with upright or inverted PLW. Our results confirm that somatosensory input in the form of tactile foot stimulation influences visual BM perception. When presented with a seen walker's footsteps, additional tactile cues enhanced sensitivity on a speed discrimination task, but only if the tactile stimuli were presented on the relevant body-part (under the feet) and when the tactile stimuli were presented synchronously with the seen footsteps of the PLW, whether upright or inverted. Based on these findings we discuss potential mechanisms of somatosensory-visual interactions in BM perception.

Introduction

Actions of other individuals are a fundamental source of information for social agents and as such, the ability to exploit the sensory information they generate is important for social cognition. Research has focused on the ability of human observers to interpret biological motion (hereafter BM) of their conspecifics, and in particular patterns of BM with walking humans. However, attempts to understand how the brain extracts information from the BM of human walking have so far focused almost exclusively on processing within the visual system. A seminal study by Gunnar Johansson (1973) reported that actors defined solely by point lights attached to the major joints of an otherwise invisible body are, once set in motion, highly perceivable. Since then, these so-called point light walkers (hereafter PLWs) have been considered a classic ‘form-from-motion’ stimulus, and have been deployed as the gold standard tool used to further understand the perceptual and neural mechanisms supporting BM perception. Perception of such PLW patterns survives information degradation, as when PLWs are embedded in ‘noise’ that mimics the motion of ‘signal’ point lights [1,2], and when the number of point lights is reduced to represent the major bodily joints only [3,4].

Numerous other PLW studies confirm the remarkable proficiency of human visual BM perception, by showing that observers can quickly discern characteristics including the sex [3,5-10], age [3,8], vulnerability [11], and even emotions of the actor being depicted [12-14]. Further, Loula and colleagues showed that human observers are more sensitive to their own action compared to friends’ and strangers’ actions during identification and discrimination tasks, suggesting that motor signals play an important role for BM analysis [15]. Another study involving hemiplegic patients with motor system lesions revealed degraded visual sensitivity to point-light actions that correspond to their compromised limbs, but not to point-light actions that correspond to their functional limbs [16]. Furthermore, when asked to estimate the terminal location of a moving point-like arm that vanished after 60% of its movement, observers improved their performances when self-generated movements were presented [17]. All these results highlight the motor contributions to BM perception and support the common coding theory according to which actions are represented through perceptual and motor coding systems [18]. Other results support this theory, showing for instance that producing a running activity briefly prior to the task improved participants’ perceptual judgements regarding the direction of a point-light runner [19], that observers’ own activities influence the perception of activities of other people [20] or that observers demonstrate maximum sensitivity to actions most familiar to them and reduced sensitivity to actions unfamiliar to them [21] (see [22] and [23] for a review). These findings are consistent with the idea that observers spontaneously simulate, in their own sensorimotor planning system, the actions they observe [24,25], simulation that partially mediates action perception by embodying observed actions [23,26,27].

Furthermore, it has been shown that tactile sensations contribute not only to coding the properties of the external object but also to the formation of a mental body representation, defined as “an abstract representation of one’s own body, derived from sensory input but capable of being dissociated from it, and playing a functional role in perception and/or action” [28]. These representations, containing representations of the body itself, are thought to reciprocally influence primary tactile processing and to modulate not only the perception of one’s own body, but also the perception of other external object. Specifically, it has been shown that visual information related to the body could affect tactile sensation, a cross-modal effect termed visual enhancement of touch (VET) (see [28] for a review).

Thus, based on the observation of the contribution of motor mechanisms in BM processing, the cross-modal visuo-tactile mechanisms in the perception of bodily stimuli [28], and the finding of highest BM sensitivity to one’s own BM stimuli [15], we hypothesized that perceptions of visually-defined BM would be subject to the influence of somatosensory input. For this, we used a two-interval forced choice paradigm and tested discrimination of PLW speed in the presence of tactile cues. We carried out two experiments aiming at evaluating the role of tactile signals for BM perception along three main lines. First, the discriminability of visual BM was compared when tactile stimulation was applied to the feet (experimental tactile condition—ET) or to the forearms (body-site control tactile condition—CT), compared to a baseline condition without tactile stimulation (only vision condition–V). Assuming that tactile-visual interactions are specific to the foot stimulation, we predicted that task-relevant tactile stimulation (ET) would enhance performance discriminating information from PLWs. Specifically, it was predicted that such stimulation would result in subjects detecting smaller differences in speed between walkers across presentation pairs. By contrast, it was predicted that the relatively task-irrelevant tactile forearm stimulation (CT) condition would result in performance outcomes that were comparable to those in the baseline condition (V). Second, we assessed the temporal specificity of somato-visual interactions. Within the audio-visual motion processing literature [29,30], cross-modal integration is optimized under conditions of temporal co-incidence. If the somatosensory-visual BM processing effect is of a similar nature, we expected that temporally coincident (synchronous) foot stimulation (i.e. tactile cue is applied when the seen PLW touches the ground) would result in better performance than asynchronous stimulation (i.e. tactile cue is randomly applied during the PLW cycle). Inclusion of such a manipulation also allowed us to test an alternative account of the effects observed in the first experiment, within which the application of tactile stimulation may have had its effect solely by focusing attention on the body part most relevant to performance on the visual task (see [31] and [4] for a discussion of the significance of the lower limbs for PLW interpretation). In other words, it allowed us to test whether the effects we observed may have arisen via attention modulation mechanisms rather than BM-specific cross-modal effects per se. Were that the case then asynchronous and synchronous stimulation of the relevant body part (the foot) should similarly facilitate visual performance. Finally, we tested whether the aforementioned tactile-visual interactions were related to global BM perception or related to other lower-level motion-processing mechanisms. To that end, we relied on the classic method of presenting PLW upside-down, as it is known to specifically impair BM processing while conserving all local motion features as their upright counterparts [4,32,33]. Changing orientation from upright to inverted impede spontaneous recognition of PLWs, making for instance more difficult to detect a camouflaged PLW within a mask [31]. Furthermore a recent study using 9.4T functional MRI has shown that different neural circuits are activated for inverted or upright PLWs processing, with activation of left hemispheric anterior networks engaged in decision making and cognitive control for inverted BM, and activation of right hemispheric multiple networks in response to upright BM [34]. We here quantified the impact of tactile cues on the discriminability of inverted PLW and predicted that performance for upright PLWs would be better than for their inverted counterparts.

Materials and methods

Subjects

A total of 43 participants were recruited (22 in Experiment 1, mean age: 27.19 years +/- 3.43 years SD; 8 women, binomial test: p = 0.38; 21 in Experiment 2: mean age: 24.05 years +/- 2.59 years SD; 7 women, binomial test: p = 0.19). Participants in experiments 1 and 2 were not the same. Participants were recruited through printed and electronic advertisements on notice boards at various sites in the Ecole Polytechnique Fédérale de Lausanne (EPFL). After contacting the experimenter, participants received the participant information sheet explaining the procedure and the goal of the study as well as the exclusion criteria (uncorrected vision deficit, somatosensory deficit). Individuals had normal or corrected-to-normal vision and had no known somatosensory processing deficits. All participants were naive to the purpose of the study.

The local Ethical Committee of the Ecole Polytechnique Fédérale de Lausanne (IRB of EPFL; Switzerland) approved the experiment, which was in accordance with the Declaration of Helsinki (1964). Participants gave written informed consent prior to inclusion in the study and were compensated for their participation (20 Swiss francs).

In Experiment 1, data from one male subject were excluded after that individual interrupted the experiment multiple times. Furthermore, 4 subjects (1 male and 3 female) in Experiment 1 and 3 subjects (2 male and 1 female) in Experiment 2 were excluded from analyses due to attention deficit during the task, i.e. more than half of the catch trials were performed as false alarm (see below procedure part for more details). Thus, it remained 17 subjects (5 women) in Experiment 1 and 18 subjects (6 women) in Experiment 2 for the analysis.

Apparatus & stimuli

Visual stimuli

PLWs were generated using Matlab (Mathworks, Natick, MA) with extensions from the Psychophysics Toolbox [35] and comprised 15 point lights representing the ankles, knees, hips, wrists, elbows, shoulders, center of the pelvis, sternum and head of a walking individual facing the observer. The PLWs were the same for all participants. Individual point lights had a radius of 10 pixels. Presented on a ViewSonic Graphic Series G90 f+ 19 inch monitor (resolution = 1280 x 1024) positioned in front of subjects at a viewing distance of 85 cm, individual PLWs had the size of 50% of the screen’s height. Individual point lights were set at 50% contrast with the grey background. The reference frequency of the gait cycle was 0.77 cycle per second. PLW movies consisted of 129 frames representing a full gait cycle of two footsteps, lasting 1298 ms. An interval of 1500 ms separated the presentation of this reference PLW to a second PLW with a slightly faster gait frequency (0.77 + Δf cycle/sec) according to the participants performance (staircase adaptative procedure, described below). Pilot testing confirmed that all variations gave rise to strong perceptions of fluid BM.

Tactile stimuli

Somatosensory stimulation was delivered via two matching custom-built devices each comprising twelve 18mm Shaftless Vibration Motors (Precision Microdrives) fitted into small rubber mats (Fig 1). Depending on the condition, each mat was attached either to subjects’ feet, or to their forearms. Mats were carefully positioned to ensure that all motors were in contact as confirmed by the subjects. Tactile stimulation to each site was confirmed to be suprathreshold in pilot studies. Individual motors were driven via an LPT-1 port using MatLab such that onset and offset was simultaneous across motors. The duration of the tactile stimuli was adapted depending on the the duration of the visual stimulus (i.e. when PLWs walked faster, tactile stimulations were shorter) and corresponded to 20% of the duration of the total PLW gait.

Fig 1

Experiment 1 procedure.

The subject is sitting in front of a computer screen, where two successive point light walkers (PLW) are shown. Subject has to focus on the visual stimuli and to determine if the PLWs gait speed is the same or different. Depending on the conditions, a vibrating sole (shown in the bottom-right corner) is banding to his feet (ET experimental tactile), his forearms (CT control tactile) or not (V vision only). The vibrations are delivered synchronously with the steps of the PLWs.

In the ‘synchronous’ conditions of Experiment 1 and 2, stimulation onset of the laterally matched body part was timed to coincide with the point at which the PLW ‘ankle’ dot reached its lowest value on the y-axis. Indeed, for a full phase of PLW cycle, Foot 1 was stimulated between 10–30% of the visual stimuli cycle and Foot 2 between 60–80% of this cycle.

In the ‘asynchronous’ condition of Experiment 2, tactile stimulation onset was randomized during the visual stimulus. Both frequency and phase were different for the visual stimuli and the tactile stimuli. The duration of the tactile stimulation was the same as in the synchronous condition (corresponding of 20% of the total PLW gait cycle), but for each foot the phase was randomly selected. For example, this could result in Foot 1 being stimulated from 0–20% of the cycle and Foot 2 from 15–35%. The phase was randomized for each foot separately, i. e. it could also be that Foot 1 and Foot 2 were stimulated at the same time.

During the entire experiment, white noise was delivered to the participants through headphone to avoid any auditory cue related to motors sounds.

Procedure

Experiments were conducted in an experimental sound insulated and dimmed booth. Subjects were seated directly in front of the monitor with their legs resting comfortably on a box, their feet suspended above the ground, and their wrists (not their forearms) rested upon the table (Fig 1). Such body positioning allowed the tactile stimulation motors to be easily affixed to the targeted body part in each of the relevant conditions.

PLW presentation pairs were separated by 1500 ms intervals. A classical staircase adaptive procedure was used to determine the just noticeable difference between a reference PLW gait speed and a slightly different PLW gait speed. The frequency of the reference stimulus was 0.77 cycle/sec and the adaptive procedure determined an always positive small value Δf cycle/sec to add to the second stimulus, which then had a gait frequency of 0.77+Δf cycle/sec. In each trial, however, we randomly selected with equal probability if the reference stimulus was shown as stimulus 1 or as stimulus 2, so the second PLW in each trial could walk either faster or slower than the first PLW in that trial.

For each pair, subjects were required to indicate via keypress whether the two seen PLWs had the same or a different gait speed. On that basis, performance for individual participants represents the average difference at which speed discrimination was possible, with lower values indicating higher sensitivity. The experiment consisted of 50 trials per condition. In order to ensure subjects were comfortable with the task, all completed 10 practice trials prior to the start of the actual experiments.

In addition, there was a ¼ chance in each trial that both the first and the second PLW were the reference stimulus with gait frequency of 0.77 cycle/sec (catch trial). The responses to those trials were excluded from the adaptive procedure. These catch trials were introduced to keep the task challenging and to maintain the attention of the subjects. If the correct response ratio for these catch trials was below 50%, meaning that more than half of the responses were false alarm, the participant was excluded from the analyses (4 subjects in Experiment1 and 3 subjects in Experiment 2). This attention deficit is likely due to the length of the experiment (180 repetitions of PLWs pairs in Experiment 1 and 240 repetitions of PLWs pairs in Experiment 2).

Experiment 1

The first experiment was designed to test whether tactile stimulation would enhance vision-based performance discriminating information from PLWs. Three conditions were ran in a randomized order: the experimental tactile condition (ET) with tactile stimulation to the feet, the control tactile condition (CT) with tactile cues presented to the subjects’ forearms and a baseline condition, vision only (V) without tactile stimulation (Fig 1).

Results

Data were analyzed using a mixed effects probit regression with response as the dependent variable, stimulus intensity (defined as the speed difference between the two PLWs) and condition as fixed effects, with a random intercept by subject and random slopes for each fixed effect. The model revealed a clear effect of stimulus intensity (X2 = 58.40, p < 0.001) indicating that participants performed the task accurately, with a tendency to respond “different” more frequently as speed difference increased, and a main effect of condition (X2 = 7.19, p = 0.03) indicating that our manipulation slightly changed response bias (Fig 2). Importantly, the model revealed an interaction between intensity and condition (X2 = 10.08, p = 0.007), reflecting a steeper slope between response and intensity in the experimental (ET) vs. control tactile condition (CT) with tactile cues applied to the forearms (Fig 2; probit estimate = 4.44, z = 3.08, p = 0.002), but was not found in the visual only (V) vs. control tactile condition (CT) (probit estimate = 0.94, z = 0.67, p = 0.50). This indicates that visuotactile cues delivered to the feet, but not the upper limbs, improves the discrimination of BM perception. In addition, there was a significant difference between the experimental visuotactile and visual condition (probit estimate = 3.07, z = 2.10, p = 0.04), indicating a steeper slope between response and intensity in the ET condition.

Fig 2

Experiment 1 results.

Mixed effects probit regression between response and stimulus intensity for the experimental tactile (ET in blue), control tactile (CT in red), and visual condition (V in green). The histograms indicate at the top the distribution of yes (different speed of the PLWs) responses and at the bottom the distribution of no (same speed of the PLWs) responses. Each histogram represents the density of a given response distribution. These results show that additional tactile stimulus had an effect on BM discriminability only if the tactile stimuli a delivered under the feet of the observers.

Experiment 2

In order to further explore the parameters of the effect of Experiment 1, we ran in the second experiment a two by two factorial design with one factor as the congruency of the tactile stimulation (Sync and Async conditions) and a second factor as the orientation of the visual stimuli (Upright and Inverted conditions) (Fig 3). Each of the four conditions were presented in a randomized order.

Fig 3

Experiment 2 procedure.

The subject is sitting in front of a computer screen, where two successive point light walkers (PLW) are shown. Subject has to focus on the visual stimulus in order to determine if the PLWs gait speed is the same or different. A vibrating sole is banding to his feet. The specificity of the effect on BM perception is tested by presenting an upright PLW (Upright condition) or an inverse PLW (Inverted condition). In order to investigate the cross-modal integration of the tactile and visual stimuli, their temporal coincidence is modulated in a way that the vibrations are either synchronous (Sync condition) or asynchronous (Async condition) with the steps of the PLWs.

Data were analyzed using a mixed probit regression with response as the dependent variable, stimulus intensity (defined as the speed difference between the two PLWs), temporal condition and orientation as fixed effects, with a random intercept by subject and random slopes for intensity and orientation. The model revealed an interaction between orientation and stimulus intensity (X2 = 13.48, p < 0.001), revealing better performance in the upright vs. inverted condition. Importantly, a significant interaction between intensity and synchrony (X2 = 4.77, p = 0.029) revealed a steeper slope for synchronous vs. asynchronous stimulation, suggesting that BM perception was better in conditions when the tactile foot cue coincided and the PLW ‘ankle’ dot reached its lowest value at the same time. However, unlike our prediction, no interaction between orientation and synchrony was found (p = 0.99), indicating that the effect of synchrony was not specific to upright PLW (Fig 4).

Fig 4

Experiment 2 results.

Mixed effects probit regression between response and stimulus intensity as a function of temporal condition (synchronous (blue) or asynchronous (red) tactile stimulation) for inverted orientation (left panel) and upright orientation (right panel). Each histogram represents the density of a given response distribution. These results show that tactile stimuli synchronously timed with the visual stimuli systematically improved BM discriminability. Both BM and non-BM (inverted visual stimulus) were better discriminated in the presence of synchronous tactile cues.

Discussion

The primary aim of these experiments was to investigate possible effects of somatosensory cues on the perception of visually defined biological motion. We designed Experiment 1 to compare the discriminability of visual BM when tactile stimulation was applied to the participants’ feet (ET) or forearms (CT), or when no tactile stimulation was applied (V). Experiment 2 was aimed to further assess the temporal specificity of somatosensory-visual interactions uncovered in Experiment 1, and to test whether they were specific to BM processing.

Considered collectively data from the conditions tested in Experiment 1 suggest two key findings. Firstly, as we found differences in BM discriminability between the experimental condition (ET) and the visual only (V) condition, they indicate that the additional tactile stimulus per se had an effect on BM perception. Secondly, the tactile effect on vision-based discrimination was body-part specific, with facilitation being observed when tactile stimulation was delivered to subjects’ feet (ET), but not when the same stimulus was applied to a non-relevant (forearm) body part (CT). As the frequency and the phase of both visual and tactile stimuli were the same in this experiment, the tactile input itself (through its frequency and its duration) could have conveyed additional information about the speed of the PLW. It is then possible that the improvement in BM discriminability was only due to this additional information. However, the finding that the same tactile stimuli improved BM discrimination only when they were delivered under the feet makes this explanation unlikely. There was indeed no difference in BM discrimination when no tactile stimulation occurred (V) and when tactile stimulation was applied on the forearms (CT). Furthermore, given that tactile stimulation to each site was qualitatively matched in pilot studies (and informally by each subject during the experimental set-up), it is unlikely these effects arise through any dissimilarity in tactile perception across stimulation sites, although we cannot exclude it.

The manipulation of congruency in Experiment 2 shows that improvement in BM discriminability with feet tactile stimulation only happened when the tactile stimulation was delivered synchronously with the visual stimulus. This indicates that the increase in discriminability was driven by the coherent interaction between the visual and the tactile stimuli (cross-modal interaction). This finding fits well with the temporal rule of the multisensory integration theory [36], stating that cross-modal integration occurs when the constituent unisensory stimuli arise at approximately the same time. Furthermore, as predicted, results of Experiment 2 argue against a purely attention-modulated account: if tactile foot stimulation simply directed visual attention towards the body part most relevant for making the visual discrimination, then performance in the asynchronous condition should have yielded comparable performance levels to those in the synchronous condition. This was not the case. However, as the frequency and the phase are different for the visual and tactile stimuli in the asynchronous condition, we cannot exclude, based only on Experiment 2, that the improvement in BM discriminability in the synchronous condition comes from additional information about the speed of the PLW conveyed by the tactile stimulation itself.

The manipulation of the orientation of the PLWs showed that this effect of synchronous visuotactile stimulation occurred equally for upright and inverted PLW. Orientation manipulation of PLW is known to have a catastrophic effect on ability to extract information from PLWs: as in the case of static face representations, stimulus processing is adversely impacted when figures are presented upside down [4,12,32], even if observers are informed of seeing an inverted visual stimulus [32], a finding that has been taken as evidence that the neural mechanisms subserving BM processing are orientation-tuned. Although we found that discriminability was overall better with upright visual stimuli, our results indicate that both BM and non-BM were better discriminated in the presence of synchronous tactile cues. This finding suggests that synchronous tactile cues does not interact in the extraction of BM features.

Taken together, the present findings extend the demonstration of cross-modal effects on motion processing to visuotactile cues, in line with what has been shown in the audiovisual domain [37,38].

However, this study presents several limitations. First, our sample of participants was not balanced for gender. Only 8 women of 22 subjects participated in Experiment 1 and 7 women of 21 subjects in Experiment 2. Previous studies have shown differences in BM perception depending on gender, showing for instance that female and male observers were found to judge differently whether a frontal PLW is facing toward or backward them [39], as well as emotional expressions [40]. Of note, post-hoc analysis revealed better performance in PLW discrimination in male observers in Experiment 2 (77.1% vs. 74.4% t(10.05) = 2.48, p = 0.03), but not in Experiment 1 (t(12.19) = 1.41, p = 0.18). A supplementary binomial mixed-effects regression including gender as a covariate in Experiment 2 revealed that the interaction between intensity and synchrony remained significant (X2 = 11.41, p < 0.001), and was not modulated by gender (X2 = 0.66, p = 0.41). Future studies will be required to assess whether this effect is anecdotal or whether cross-modal effects on motion processing depends on gender. Secondly, we cannot exclude the possibility that differences in tactile perception between the forearms and the feet may have resulted from difference in sensitiveness between these two body parts. We hypothesized during the study conception that the feet would be more relevant than forearms regarding walking actions, and therefore predicted that performance would be better with tactile stimulation on this site than elsewhere on the body. Finally, we also note that our stimulus combination may not be an ecologically valid representation of external walking cues (it is unlikely that a seen walker also generates reliable externally generated somatosensory cues, even at a subthreshold level). As we observed an effect only when the tactile stimulation was applied under the feet in Experiment 1, we speculate this effect could arise by tapping into the very system within which external/internal neural action representations merge. In other words, it is possible that externally sourced somatosensory stimulation may up-regulate the usual vision-based embodiment of the observed walking action (for established cases of somatosensory-visually driven stimulus embodiment see for e.g. [41]; [42]). This hypothesis goes in line with the idea that tactile sensation contribute to a body representation, which in turn influences the perception of external object [28]. Loula and colleagues’ results, having highlighted the motor experience contribution to BM perception, suggest also that cues on the observer could influence perception of others, as a self-influence on BM perception [15]. In line with this role of motor simulation in BM perception, it is possible that tactile stimulations under the feet (and not on the forearms) improve the observers’ motor simulation of the walking motion in their sensorimotor planning. This simulation, so reinforced by tactile congruent cues, could then improves the discriminability of PLW. Another study report such possible reinforcement of body representation by multimodal perception: investigating agency for walking avatars, Menzer and colleagues (2010) has shown that agency for one’s own footsteps decreased rapidly when a temporal delay was introduced between the participants’ footsteps and the auditory consequences of those footsteps [43]. Interestingly the results were comparable with those of studies manipulating visual cues [44,45], reflecting thus common, supramodal, mechanisms in the conscious action monitoring of auditory and visual action consequences [43], mechanisms that are compatible with the aforementioned forward sensory system, that is independent of the sensory modality tested. How precisely such embodiment influences perceptual sensitivity to the motion of ‘another’ is the subject for future research.

3 Sep 2019

PONE-D-19-17841

Somatosensory-visual effects in visual biological motion perception

PLOS ONE

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Reviewer #1: Authors depeen somatosensory effect, in particular tactile foot stimulation, on BM visual-perception, basing on evidence of brain areas integrating auditory and visual signals. I found the study interesting and the purpose of the research relevant. I would suggest some clarifications in order to make the manuscript clearer:

ABSTRACT: Even though it explains and justifies the aim of the research, it does not give space to the method. I suggest adding method of the research in term of the design, number of experiments, tasks, participants....

INTRODUCTION: I found the introduction section well written, reporting background related to research questions. Nevertheless, I did not find a section showing literature that lead authors to investigate temporal specificity of somato-visual interactions. I would suggest integrating in introduction this part.

When authors present experiment 1 conditions (ET, CT,V), while they well describe ET and CT, they do not specify baseline condition (V).

METHODS: I would suggest including chi-square analysis to report differences in gender distribution (females:males) in both experiments. In experiment 2 7/21 females are included, that means they are 1/3 of the sample. Is there a significant difference between males and females? A growing literature is documenting a gender effect in BM perception (Sokolov et al.,Frontiers in Psychology, 2011; Krüger et al., 2013, PloS One; Pavlova et al., 2014, Cerebral Cortex...).

I also suggest adding more information regarding recruitment of participants: How and where participants were recruited? What about their education level? Which were inclusion and exclusion criteria?

Are PLW stimuli balanced in males and females stimuli?

DISCUSSION: I would suggest starting discussion with a resume of the research question and experiment design.

Reviewer #2: This paper aims to examine the somatosensory effects in visual biological motion perception. For this, the authors propose two studies where they used a two-interval forced choice paradigm and tested the discrimination of point-lights walkers (PLW) speed in the presence of tactile cues. More precisely, the first experiment compares the performances in speed discrimination of PLW when the visual presentation is associated or not or incongruent tactile cues. The second experiment tests the role of time synchrony and recognition of the global motion in the tactile effects. Globally, the analyses show that the speed discrimination of PLW is improved when the visual stimulation is associated with congruent tactile cue (stimulation on the feet) but this effect is only observed when the tactile stimulation is synchronous with the visual stimuli. Interestingly, this effect does not rely with the global recognition of PLW because it persists even the PLW are presented upside-down. The authors interpret their results as somatosensory-visual interactions in BM perception with particular reference to the human mirror neuron system and multisensory mechanisms in action perception.

Globally, I find the study timely and very interesting. Introduction is well documented, and methods and results are clear. However, I have some suggestions to improve the manuscript.

Introduction:

I find the introduction clear and well documented even the authors could cite more recent studies about PLD.

See for example

- Bidet-Ildei, Chavin & Coello, 2010 for the discriminability on PLD in scrambled masking dots

- Martel, Bidet-Ildei & Coello, 2011 for the sensitivity to the own actions.

I think that it misses in introduction something about the upside-down effect. It is surprising since this effect is specifically studied in Experiment 2.

Method:

-Are the same participants in Experiment 1 and 2?

- Why the authors choose to describe both experiments and after to present the results? I find difficult to follow. I think that the manuscript could be clarified if the authors finished first the Experiment 1 and after proposing the Experiment 2. Moreover, in this configuration, they can more specified the specific objectives of each experiment and to add a specific discussion.

Discussion

The authors explain their results with sensorial interactions but is it possible to imagine that their results could be due to more precise motor simulation of the PLW?. Indeed, it would be logical that a tactile stimulation on the feet can reinforce the motor simulation of a walking motion and so improve the discriminability of PLW? Even this framework has difficulty to explain the results in Experiment 2 (the tactile effect with upside-down PLW), I think important to envisage this possibility. If the authors cut their study in two separate experiments, this hypothesis could be envisaged in the discussion of Experiment 1.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: Yes: Christel Bidet-Ildei

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

7 Oct 2019

RESPONSE TO REVIEWERS

Dear Dr. Pavlova,

Thank you for giving us the opportunity to submit a revised draft of our manuscript titled “Somatosensory-visual effects in visual biological motion perception” to PlosOne. We appreciate the time and effort that you and the reviewers have dedicated to providing your valuable feedback on our manuscript. We are grateful to the reviewers for their insightful comments on our paper. We have been able to incorporate changes to reflect most of the suggestions provided by the reviewers. We have highlighted the changes within the manuscript.

Here is a point-by-point response to the reviewers’ comments and concerns. Please, find below the reviewers’ comments repeated in italics and our responses inserted after each comment.

We look forward to hearing from you in due time regarding our submission and to respond to any further questions and comments you may have.

Sincerely,

Pierre Progin and co-authors

Comments from Reviewer #1: Authors depeen somatosensory effect, in particular tactile foot stimulation, on BM visual-perception, basing on evidence of brain areas integrating auditory and visual signals. I found the study interesting and the purpose of the research relevant. I would suggest some clarifications in order to make the manuscript clearer:

ABSTRACT:

Even though it explains and justifies the aim of the research, it does not give space to the method. I suggest adding method of the research in term of the design, number of experiments, tasks, participants....

Response: Thank you for pointing this out. We agree with this comment. Therefore, we have added information about the methodological part of the study in the abstract:

In two experiments, we asked healthy participants to perform a speed discrimination task on two point light walkers (PLW) presented one after the other. In the first experiment, we quantified somatosensory-visual interactions by presenting PLW together with tactile stimuli either on the participants’ forearms or feet soles. In the second experiment, we assessed the specificity of these interactions by presenting tactile stimuli either synchronously or asynchronously with upright or inverted PLW.

INTRODUCTION:

I found the introduction section well written, reporting background related to research questions. Nevertheless, I did not find a section showing literature that lead authors to investigate temporal specificity of somato-visual interactions. I would suggest integrating in introduction this part.

Response: We agree with this point and have revised the introduction accordingly. A first reference to a seminal neuroanatomical study (Meredith 1987) was added in the contextual part of the introduction:

Several studies have now shown that auditory information influences visual BM processing across a range of tasks (10,46–48), given that the two signals are temporally linked (49). As Meredith and Stein found in cat superior colliculus (50), multimodal neurons in STS are more responsive to coincident and colocalized multimodal stimuli compared to spatially and temporally incoherent stimuli (51).

A reference to audio-visual integration was also added to the description of the second experiment at the end of the introduction:

Within the audio-visual motion processing literature (49,51), cross-modal integration is optimized under conditions of temporal co-incidence. If the somato-visual BM processing effect is of a similar nature, we expected that temporally coincident (synchronous) foot stimulation (i.e. tactile cue is applied when the seen PLW touches the ground) would result in better performance than asynchronous stimulation (i.e. tactile cue is applied at other times).

When authors present experiment 1 conditions (ET, CT,V), while they well describe ET and CT, they do not specify baseline condition (V).

Response: We would like to thank the reviewer for noticing this omission. We now describe the baseline condition in the introduction:

First, the discriminability of visual BM was compared when tactile stimulation was applied to the feet (experimental tactile condition - ET) or to the forearms (body-site control tactile condition - CT), compared to a baseline condition in which no tactile stimulation was applied (only vision condition – V).

METHODS:

I would suggest including chi-square analysis to report differences in gender distribution (females:males) in both experiments. In experiment 2 7/21 females are included, that means they are 1/3 of the sample. Is there a significant difference between males and females? A growing literature is documenting a gender effect in BM perception (Sokolov et al.,Frontiers in Psychology, 2011; Krüger et al., 2013, PloS One; Pavlova et al., 2014, Cerebral Cortex...).

Response: We agree with this and have run a binomial test to know if our groups were significantly unbalanced for gender. In Experiment 1, we had 8 females on 21 participants, which corresponded to a p-value = 0.38. In Experiment 2, we had 7 females on 21 participants, with a corresponding p-value = 0.19. Given that groups were not significantly unbalanced, and considering that our sample size is rather small, we decided not to add this factor as a covariate of interest. However, following the reviewer’s suggestion, we now discuss the relevance of gender for BM perception:

Although our samples were not significantly unbalanced (see methods), the influence of gender on BM perception is worth considering. Indeed, female and male observers were found to judge differently whether a frontal PLW is facing toward or backward them (57), as well as emotional expressions (58, 59). Future studies will be required to assess whether cross-modal effects on motion processing depend on gender.

I also suggest adding more information regarding recruitment of participants:

Response: We now mention more details about recruitment:

Participants were recruited through printed and electronic advertisements on notice boards at various sites in the Ecole Polytechnique Fédérale de Lausanne (EPFL). They were all student at the EPFL. After contacting the experimenter, participants received the participant information sheet explaining the procedure and the goal of the study as well as the exclusion criteria (uncorrected vision deficit, somatosensory deficit).

How and where participants were recruited?

Response: Participants were recruited through advertisement on notice board in the campus EPFL at Lausanne.

What about their education level?

Response: All participants were student at EPFL, in bachelor or master degrees.

Which were inclusion and exclusion criteria?

Response: Inclusion criteria were:

1. Age between 18 and 60 years.

2. Willing to refrain from drinking alcohol at the testing days and from consuming

psychoactive substances 2 weeks before testing days and for the duration of the

study.

3. Willing and capable to give informed consent for the participation in the study

after it has been thoroughly explained.

4. Absence of neurological and major physical impairment, with normal or corrected to normal sensory abilities.

5. Informed consent form was signed.

Exclusion criteria were:

1. Prior participation in a similar study that could bias the current results

2. Presence of major internal or neurological disorders.

3. Non-compliance to the instructions of the experimenter or an inappropriate behavior hindering the normal progress of the experiment.

Are PLW stimuli balanced in males and females stimuli?

Response: PLWs were neutral regarding gender features and all stimuli were the same (PLWs shown in a frontal view) for every participant. We now specify this point in the revised manuscript in the “Apparatus and stimuli” part:

The PLWs were the same for all participants.

DISCUSSION:

I would suggest starting discussion with a resume of the research question and experiment design.

Response: We agree with this suggestion and have modified the discussion accordingly:

The primary aim of these experiments was to investigate possible effects of somatosensory cues on the perception of visually defined biological motion. We designed Experiment 1 to compare the discriminability of visual BM when tactile stimulation was applied to the participants’ feet (ET) or forearms (CT), or when no tactile stimulation was applied (V). Experiment 2 was aimed to further assess the temporal specificity of somatosensory-visual interactions uncovered in Experiment 1, and to test whether they were specific to BM processing.

Comments from Reviewer #2: This paper aims to examine the somatosensory effects in visual biological motion perception. For this, the authors propose two studies where they used a two-interval forced choice paradigm and tested the discrimination of point-lights walkers (PLW) speed in the presence of tactile cues. More precisely, the first experiment compares the performances in speed discrimination of PLW when the visual presentation is associated or not or incongruent tactile cues. The second experiment tests the role of time synchrony and recognition of the global motion in the tactile effects. Globally, the analyses show that the speed discrimination of PLW is improved when the visual stimulation is associated with congruent tactile cue (stimulation on the feet) but this effect is only observed when the tactile stimulation is synchronous with the visual stimuli. Interestingly, this effect does not rely with the global recognition of PLW because it persists even the PLW are presented upside-down. The authors interpret their results as somatosensory-visual interactions in BM perception with particular reference to the human mirror neuron system and multisensory mechanisms in action perception.

Globally, I find the study timely and very interesting. Introduction is well documented, and methods and results are clear. However, I have some suggestions to improve the manuscript.

Response: We would like to thank the reviewer for noting the quality of our work.

INTRODUCTION:

I find the introduction clear and well documented even the authors could cite more recent studies about PLD.

See for example

- Bidet-Ildei, Chavin & Coello, 2010 for the discriminability on PLD in scrambled masking dots

- Martel, Bidet-Ildei & Coello, 2011 for the sensitivity to the own actions.

Response: In line with the reviewer’s suggestion, we now cite these relevant studies in the introduction:

Furthermore, when asked to estimate the terminal location of a moving point-like arm that vanished after 60% of its movement, observers improved their performances when self-generated movements were presented (17).

Other results support this theory, showing for instance that producing a running activity briefly prior to the task improved participants’ perceptual judgements regarding the direction of a point-light runner (19),

I think that it misses in introduction something about the upside-down effect. It is surprising since this effect is specifically studied in Experiment 2.

Response: We fully agree with the reviewer, and apologize for not citing this work earlier. We added a paragraph at the end of the introduction to emphasize this point:

To that end, we relied on the classic method of presenting PLW upside-down, as it is known to specifically impair BM processing while conserving all local motion features as their upright counterparts (4,54,55). Changing orientation from upright to inverted impede spontaneous recognition of PLWs, making for instance more difficult to detect a camouflaged PLW within a mask (54).

This effect is then mention in the discussion:

Orientation manipulation of PLW is known to have a catastrophic effect on ability to extract information from PLWs: as in the case of static face representations, stimulus processing is adversely impacted when figures are presented upside down (4,12,54), even if observers are informed of seeing an inverted visual stimulus (54), a finding that has been taken as evidence that the neural mechanisms subserving BM processing are orientation-tuned.

METHODS:

-Are the same participants in Experiment 1 and 2?

Response: No, participants from Experiment 1 and 2 were not the same. We now mention this important point in the method part:

Participants in experiments 1 and 2 were not the same.

- Why the authors choose to describe both experiments and after to present the results? I find difficult to follow. I think that the manuscript could be clarified if the authors finished first the Experiment 1 and after proposing the Experiment 2. Moreover, in this configuration, they can more specified the specific objectives of each experiment and to add a specific discussion.

Response: We would like to thank the reviewer for this advice. The reviewer is right to point that describing both experiment before the results made the reading of the manuscript more difficult. We have now moved the result section of Experiment 1 right after the description of the corresponding methods, and only then describe Experiment 2 and its results. However, we have kept the discussion of both experiments in the general discussion as we think it helps to keep in mind an overview of both experiments before hypothesizing possible mechanisms underlying the reported effects.

Please note that figure number changed with this new configuration: former Fig. 3 is now Fig. 2, and former Fig. 2 is now Fig. 3.

DISCUSSION:

The authors explain their results with sensorial interactions but is it possible to imagine that their results could be due to more precise motor simulation of the PLW? Indeed, it would be logical that a tactile stimulation on the feet can reinforce the motor simulation of a walking motion and so improve the discriminability of PLW?

Response: We fully agree with this point. We consider that tactile stimulation may up-regulate the vision-based embodiment of the observed walking action. Accordingly, it is possible that tactile stimulation improves the simulation of action in subjects’ own sensorimotor planning system. We apologize that this idea was not highlighted more clearly in the manuscript. We now emphasize this hypothesis in the text:

In line with this role of motor simulation in BM perception, it is possible that tactile stimulations under the feet improve the observers’ motor simulation of the walking motion in their sensorimotor planning. This simulation, so reinforced by tactile congruent cues, could then improves the discriminability of PLW. Another study reports such possible reinforcement of body representation by multimodal perception:

Even this framework has difficulty to explain the results in Experiment 2 (the tactile effect with upside-down PLW), I think important to envisage this possibility. If the authors cut their study in two separate experiments, this hypothesis could be envisaged in the discussion of Experiment 1

Response: The reviewer is right to point that results of Experiment 2 show that the effect isn’t specific to biological motion. This finding of Experiment 2 suggests indeed that synchronous tactile cues does not interact in the extraction of BM features, but in other motion features. We acknowledge that important point in the discussion.

Although we understand the reviewer’s point of view regarding the discussion format, we preferred discussing both experiments together to provide a more global view on the involved mechanisms. We are willing to change format should the reviewer and editor argue in this sense.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

30 Oct 2019

PONE-D-19-17841R1

Somatosensory-visual effects in visual biological motion perception

PLOS ONE

Dear Dr Progin,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Two Reviewers who reviewed the previous version of your manuscript submitted their reports. I also had an attentive look at this version. I believe that you have address the following issues: (i) Intro: You refer to work that supports co9nnection between perception and production of body motion. However, there are also reports that are not in aggrement with this view (e.g., from my own lab Pavlova et al., 2003, Brain). There are also several papers (e.g., Pavlova et al., 2017 Cerebral Cortex) with 9.4 fMRI and biological motion that you may wish to discuss.(ii) REviewers did already draw your attention to unbalanced number of females and males in your experiments. Although you replied that binormial analysis does not show significant difference in number of female/male observers, you have to show that there were no gender difference in performance on your tasks to consider these groups homogeneous. (iii) VERY IMPORTANT: In Method section (Subjects) you are writing that several participants (4 in Exp. 1 and 3 in Exp. 2) had been excluded because of attentional problems. You wrote: (see below for more details'. Please explain the reasons in the text where you did mention exclusion, and indicate how many females and males had been excluded, and how many of them  entered your data analysis. (iv) You are writing that your participants were students. However, you did mention that one of your inclusion criteria  was age till 60 yrs. Is it not confusing? Please carefully address these issues in your revision.

We would appreciate receiving your revised manuscript by Dec 14 2019 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Marina A. Pavlova, PhD

Academic Editor

PLOS ONE

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: (No Response)

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: (No Response)

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: I thank authors that fully addressed all my concerns. I find the manuscript suitable for publication.

Reviewer #2: The authors did a very good job of editing. The paper is improved and I highly recommend the publication.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: Yes: Christel Bidet-Ildei

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

26 Nov 2019

RESPONSE TO EDITOR

Dear Dr. Pavlova,

Thank you for providing us your valuable feedback on our manuscript titled “Somatosensory-visual effects in visual biological motion perception”. We are grateful for your insightful comments on our paper. We have been able to incorporate changes to reflect most of the suggestions you provided. We have highlighted these changes within the manuscript.

Here is a point-by-point response to your comments and concerns. Please, find below your comments repeated in italics and our responses inserted after each comment.

We look forward to hearing from you in due time regarding our submission and to respond to any further questions and comments you may have.

Sincerely,

Pierre Progin and co-authors

EDITOR’S COMMENTS TO AUTHOR:

Comments to the Author: Two Reviewers who reviewed the previous version of your manuscript submitted their reports. I also had an attentive look at this version. I believe that you have address the following issues:

(i) Intro: You refer to work that supports co9nnection between perception and production of body motion. However, there are also reports that are not in aggrement with this view (e.g., from my own lab Pavlova et al., 2003, Brain). There are also several papers (e.g., Pavlova et al., 2017 Cerebral Cortex) with 9.4 fMRI and biological motion that you may wish to discuss.

Response: We thank the Editor for pointing us to these relevant studies, which we now cite in the introduction:

“The planning system might be constitutional for the brain, as motor experience per se doesn’t seem to be necessary for BM perception. A study with young patients with periventricular leukomalacia revealed indeed that patients with early-life impaired motor ability showed the same sensitivity to BM that patients without motor disorder, leading to the assumption that a hard-wired network for both perception and production of BM might be inherent for the brain (28).”

“Furthermore a recent study using 9.4T functional MRI has shown that different neural circuits are activated for inverted or upright PLWs processing, with activation of left hemispheric anterior networks engaged in decision making and cognitive control for inverted BM, and activation of right hemispheric multiple networks in response to upright BM as compared with scarce activation to inverted displays (57).”

(ii) REviewers did already draw your attention to unbalanced number of females and males in your experiments. Although you replied that binormial analysis does not show significant difference in number of female/male observers, you have to show that there were no gender difference in performance on your tasks to consider these groups homogeneous.

Response: As requested, we computed the average performance across female and male observers. We found no difference for Experiment 1 (t(12.19) = 1.41, p = 0.18) and slightly better performance for males in Experiment 2 (77.1% vs. 74.4% t(10.05) = 2.48, p = 0.03). We now mention this possible issue in the discussion:

“Although our samples were not significantly unbalanced (see methods), the influence of gender on BM perception is worth considering. Indeed, we found no difference for Experiment 1 (t(12.19) = 1.41, p = 0.18) but slightly better performance for males in Experiment 2 (77.1% vs. 74.4% t(10.05) = 2.48, p = 0.03). This is in line with previous findings showing that female and male observers were found to judge differently whether a frontal PLW is facing toward or backward them (59), as well as emotional expressions (60). Future studies will be required to assess whether cross-modal effects on motion processing depend on gender. “

(iii) VERY IMPORTANT: In Method section (Subjects) you are writing that several participants (4 in Exp. 1 and 3 in Exp. 2) had been excluded because of attentional problems. You wrote: (see below for more details'. Please explain the reasons in the text where you did mention exclusion, and indicate how many females and males had been excluded, and how many of them entered your data analysis.

Response: We apologize if exclusion criteria were not explained in sufficient details. They are specified under the procedure section:

“In addition, there was a ¼ chance in each trial that both the first and the second PLW were the reference stimulus with gait frequency of 0.77 cycle/sec (catch trial). The responses to those trials were excluded from the adaptive procedure. These catch trials were introduced to keep the task challenging and to maintain the attention of the subjects. If the correct response ratio for these catch trials was below 50%, meaning that more than half of the responses were false alarm, the participant was excluded from the analyses (4 subjects in Experiment1 and 3 subjects in Experiment 2).”

We now mention in the subject section that explanation are in the procedure part of the manuscript, and indicate the gender of the 7 excluded subjects:

“In Experiment 1, data from one male subject were excluded after that individual interrupted the experiment multiple times. Furthermore, 4 subjects (1 male and 3 female) in Experiment 1 and 3 subjects (2 male and 1 female) in Experiment 2 were excluded from analyses due to attention deficit during the task, i.e. more than half of the catch trials were performed as false alarm (see below procedure part for more details). Thus it remained 17 subjects (5 women) in Experiment 1 and 18 subjects (6 women) in Experiment 2 for the analysis.”

(iv) You are writing that your participants were students. However, you did mention that one of your inclusion criteria was age till 60 yrs. Is it not confusing? Please carefully address these issues in your revision.

Response: We are sorry if this point is confusing. One of the inclusion criteria for our behavioral experiments is an age between 18 and 60 years. As the participants were recruited through printed advertisements on notice boards at various sites in the EPFL campus, they were actually all student at the EPFL, but it wasn’t formally a inclusion criteria. The mean age was 27.19 years (with a 34 yo oldest subject) in Experiment 1 and 24.04 years (with a 29 yo oldest subject) in Experiment 2.

We’ve now removed the sentence stating that they were students.

Submitted filename: Response to Editor_26.11.19.docx

Click here for additional data file.

6 Dec 2019

PONE-D-19-17841R2

Somatosensory-visual effects in visual biological motion perception

PLOS ONE

Dear Dr Progin:

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

I now had a possibility to look at your reply. Thank you for your efforts. I must admit that there is a problem, which requires youir attention: you wrote that whereas there were no gender differences in Experiment 1 (please indicate whether you did check the data sets for normality of distribution, if the data is not normally distributed, yoiu are unable to use parametric statistics such as t-Student), in Experiment 2 the gender differences are significant (p < 0.03).[Please indicate whether one-tailed or two-tailed statistics is used]. In this latter case, you are unable to consider the whole group consisting of femakes and makles as homogenious. Instead, you have to proceed with the data of females and males separately. I am very sorry, but you really have to address these important issues.  The other issue is exclusion of subjects: you have to explain why so many participants made false alarms. Were they anxious? Was the task extremely demanding?

We would appreciate receiving your revised manuscript by Jan 20 2020 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Marina A. Pavlova, PhD

Academic Editor

PLOS ONE

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

9 Jan 2020

RESPONSE TO EDITOR

Dear Dr. Pavlova,

We would like to thank you for further assessing our manuscript, and apologize for the delayed response due to the holiday break.

Here is a response to your comments and concerns.

Comments to the Authors: I now had a possibility to look at your reply. Thank you for your efforts. I must admit that there is a problem, which requires youir attention: you wrote that whereas there were no gender differences in Experiment 1 (please indicate whether you did check the data sets for normality of distribution, if the data is not normally distributed, yoiu are unable to use parametric statistics such as t-Student), in Experiment 2 the gender differences are significant (p < 0.03).[Please indicate whether one-tailed or two-tailed statistics is used]. In this latter case, you are unable to consider the whole group consisting of femakes and makles as homogenious. Instead, you have to proceed with the data of females and males separately. I am very sorry, but you really have to address these important issues. The other issue is exclusion of subjects: you have to explain why so many participants made false alarms. Were they anxious? Was the task extremely demanding?

Response: We indeed had verified that our data was normally distributed with a Shapiro-Wilk normality test. This was the case both in experiment 1 (W = 0.96, p = 0.67) and in experiment 2 (W = 0.96, p = 0.56). Following standard guidelines, and because we had no a priori hypothesis concerning the influence of gender in our study, we used a two-tailed test to compare a posteriori performance between males and females.

Instead of proceeding with the data of females and males separately in experiment 2, we added this as a covariate in our analysis. We consider this to be a better approach, as it avoids losing statistical power as would be the case with data splitting. Doing so, the mixed-effects logistic regression yielded essentially similar results as the ones we reported originally. Most importantly, the interaction between stimulus intensity and synchrony remained significant (X² = 11.05, p < 0.001). We added this important point to the revised manuscript.

Finally, regarding subjects exclusion, we now report in the revised manuscript that some participants had to be excluded because of attention deficit during the task, likely due to the length of the experiment (180 repetitions of PLWs pairs in experiment 1 and 240 repetitions of PLWs pairs in experiment 2). To our knowledge, the task did not trigger anxiety, but was indeed quite demanding.

We thank you once again for your thorough editorial work, and hope that our study is now suitable for publication in PLOS ONE.

Best regards,

Pierre Progin, on behalf of all coauthors.

Submitted filename: Response to Editor_06.01.20.docx

Click here for additional data file.

13 Mar 2020

PONE-D-19-17841R3

Somatosensory-visual effects in visual biological motion perception

PLOS ONE

Dear Dr. Progin:

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

We received feedback from Reviewer 3 (you can find her/his comments below). I believe that two issues are of importance: 1. Stricktly speaking your groups are imbalannced in regard to gender. Experiment 1 contained 8 female and 22 male subjects, Experiment 2 contains 21 male and 7 femnale participants. Binomial test is of no help here. If you have such an imbalanced design, your statisticakl outcome can be affected heavily.  2. The difference between female and male participanrts WAS significant. Significant difference can't be negligible. Please address these issues in your work.

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PLOS ONE

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Reviewers' comments:

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Comments to the Author

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Reviewer #3: All comments have been addressed

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Reviewer #3: Yes

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Reviewer #3: Yes

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Reviewer #3: Your paper is clean and easy to read but I think it needs another revision step.

I have a few concerns that follow. To sum them up [1] we need to pay attention to variables operationalization and correspondent stimuli effectiveness for cross modality stimulation (mirror neuron like integration vs higher order cognitive-attentional convergent information about a variable); [2] it is not clear the gender effect; we should [3] pay attention to minor language problems.

I won't use locutions as "in my opinion" etc. because tautological. I apologize for any unpoliteness that should be perceived.

[1] The effectiveness of the asynchronous condition depends on the following fact.

The stimulus in its simplest representation can be described in terms of frequency and phase. The asynchronous condition has to be both different in "frequency" and "phase", otherwise we cannot discard the simplest interpretation that subjects use a low level property of perception (frequency) to give their responses.

I try to be clearer.

The stimulus PLW can be described in terms of frequency of an event ("the [perceived] foot touches the ground"): the higher the frequency, the higher the psychophysical variable we want to measure (the [perceived] velocity of the [perceived] walker). In the synchronous condition there is another stimulus (processed in another perceptual modality) which matches with plw both in phase and in frequency (or at least - please clarify this point - with a frequency which is an integer multiple or divisor of the frequency of the plw event). For example:

SAME PHASE - SAME FREQUENCY

time plw foot stimulus

1 1 1

2 0 0

3 0 0

4 1 1

5 0 0

6 0 0

7 1 1

8 0 0

9 0 0

...

DIFFERENT PHASE - SAME FREQUENCY

time plw foot stimulus

1 1 0

2 0 1

3 0 0

4 0 0

5 1 0

6 0 1

7 0 0

8 0 0

9 1 0

10 0 1

11 0 0

12 0 0

...

SAME PHASE - DIFFERENT FREQUENCY (but integer multiple: frequency foot touch = 2 * frequency plw)

time plw foot stimulus

1 1 1

2 0 0

3 0 1

4 0 0

5 1 1

6 0 0

7 0 1

8 0 0

9 1 1

10 0 0

11 0 1

12 0 0

...

ecc.

The point is: the asynchronous condition should be different both in phase and in frequency with no frequency at all preferably (this can be set giving a random order of the "1's" in the event representation, but maintaining that the total number of "ones" is the same for both stimuli. In analytic terms the two [continuous] event (y) - time (x) functions should have the same integral but one should be a sine(x+shift) and the other some random thing with the same area.

Please add some further consideration about the effectiveness of your "somatosensory drum effect" (some tips about implicit questions you could ask again yourself):

- how can I distinguish the pure "rythm" attentive effect from the "cross modality - mirror neuron like one"? Is my Asynchronous condition enough as it stands? etc.

- How long does each PLW stimulus last?

- is it necessary to refer to the [complex and not universally accepted] theory of mirror neurons or the results can be [easily] explained by other (simpler) mechanisms? In other words if we cannot exclude a simpler process of converging information (different stimuli of which frequencies correlates and help the performance which has a "computation time" compatible with higher order processes), the discussion, say, of rows 461 - 499 has to be "mitigated". Could some further considerations reagarding the comparison of results for forearm vs foot stimulation help? Should this considerations refer also to possible different sensitiveness of those areas (example rows 311-312)?

[2] balanced design are always preferable; your design is not regarding gender and it's unbalanced from the very beginning (it is not only a matter of subjects exclusion for data or inclusion problem). I would not mention at all gender variable effect. If you are somehow obliged to, I'd rather be more explicit about the reasons that make compatible the following statements:

rows 415-417: gender variable has a main effect in experiment 2 and does not have it in experiment 1.

rows: 420-422: the effect of intensity x synchrony remain significant on the dependent variable (the fuzzy-step|sygmoid|error function like decision function) if in the model you add gender variable explicitly.

For what I can understand: on one hand, on its own, if you use t-test, gender has no effect in experiment 1 but has an effect in experiment 2; on the other hand, it does not "significantly modulate" the combined effect of stimulus (plw) intensity and synchrony (with somatosensory stimulation). Those results are compatible without the need of giving any further information on experiment 1; on experiment 2 we are lead to think that (a) the "effect size" of intensity x synchrony is high enough to "cover" the one of gender on its own or [inclusive] (b) that the interaction gender x intensity x synchrony is "small" if significant. If this representation of what you mean in rows 414-424 is correct, please give some statistical effect size measure and be more analytic in its explanation, above all whether if the above a, b or both are true.

In layman terms you should represent you results in order to explain both (1) why gender has effect in experiment 2 and not in experiment 1 and (2) why gender does not affect the intensity x synchrony effect.

Maybe it could be helpful to "let us see" the model. I guess it is something like this:

to each discrete independent variable V_k with number of levels N_k we associate N_k variables X_h with values 0 or 1.

to each numerical/continuous independent variable W_i we associate a numerical variable X_i with the same domain.

So from

[subject, intensity, synchrony, gender]

we obtain

x_1, ..., x_n, with n = number_of_subjects

x_n+1 =intensity

x_n+2 = asynchronous_stimulation

x_n+3 = synchronous_stimulation

x_n+4 = female

x_n+5 = male

and the model WITHOUT interaction is

y = sum(a_j * x_j) + error

For the one with interaction you add more variable such as

x_n+6 = (x_n+1)*(x_n+2 == 1) that is intensity for asynchronous stimulation and so on.

If my rough resume is somehow correct, it should be the case that

y = ... + a*(intensity_x_synchrony) + b*(intensity_x_synchrony_x_gender) + c*(gender) + ...

gives

a significant and strong

b significant and small (or not significant)

c significant (for experiment 2) and small (comparing to a)

am I correct? Nevertheless I suggest you try to find a way to make it clearer.

[3] Miscellanea and minor concerns

Row 244: "...was randomized between each body part ...": check for "between" as it's used for a set of two elements, so it should be "among", because I guess that the body parts are more than 2. Moreover I think you may want something like "... randomized across ... " or "... randomly sampled among all body parts ...". However, if by "body parts" you do not mean those related to moving dots but those of the experimental subject, please be more clear about why you should randomize "left and right" stimulation (I guess because you have only one tool). Nevertheless please check this part with a native speaker.

Row 414: "Although ... NOT ..., the influence is worth considering". I cannot understand this sentence. "Although B, A" means that "A" usually implies "not-B"; in other words "A although B" should be something like "A even if not-B".

A="the influence of gender on BM perception is worth considering"

B="our samples were not significantly unbalanced for gender"="our samples were balanced enough for gender"

so to maintain an intuitive semantics it should be "A because B". Please clarify this point.

Row 450: "STS, with its central [...] actions (see [...]) has been implicated [...]" should be

"STS, with its central [...] actions (see [...]), has been implicated [...]" (the comma is missing). Please double check the paper because there are other punctuation errors.

**********

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13 May 2020

Pierre Progin Mai 12th, 2020

Laboratory of Cognitive Neuroscience

Swiss Federal Institute of Technology

Campus Biotech H4

Chemin des Mines 9

CH-1202 Genève

Switzerland

RESPONSE TO REVIEWERS

Dear Dr. Pavlova,

Thank you for providing us your valuable feedback on our manuscript titled “Somatosensory-visual effects in visual biological motion perception”. We appreciate the time and effort that you and the reviewer have dedicated to our manuscript and are grateful to the reviewer for her/his insightful comments on our paper. We have been able to incorporate changes to reflect most of the suggestions provided by the reviewers. We have highlighted the changes within the manuscript.

Here is a point-by-point response to the reviewer’s comments and concerns. Please, find below the reviewer’s comments and our responses inserted after each comment.

We thank you once again for your thorough editorial work, and hope that our study is now suitable for publication in PLOS ONE.

Best regards,

Pierre Progin, on behalf of all coauthors.

EDITOR’S COMMENTS TO AUTHOR:

Comments to the Author: We received feedback from Reviewer 3 (you can find her/his comments below). I believe that two issues are of importance:

1. Stricktly speaking your groups are imbalannced in regard to gender. Experiment 1 contained 8 female and 22 male subjects, Experiment 2 contains 21 male and 7 femnale participants. Binomial test is of no help here. If you have such an imbalanced design, your statisticakl outcome can be affected heavily.

2. The difference between female and male participanrts WAS significant. Significant difference can't be negligible. Please address these issues in your work.

Response: We now make the following statement as an important limitation in the discussion:

“[...] our sample of participants was not balanced for gender. Only 8 women of 22 subjects participated in Experiment 1 and 7 women of 21 subjects in Experiment 2. Previous studies have shown differences in BM perception depending on gender, showing for instance that female and male observers were found to judge differently whether a frontal PLW is facing toward or backward them (39), as well as emotional expressions (40). Of note, post-hoc analysis revealed better performance in PLW discrimination in male observers in Experiment 2 (77.1% vs. 74.4% t(10.05) = 2.48, p = 0.03), but not in Experiment 1 (t(12.19) = 1.41, p = 0.18). A supplementary binomial mixed-effects regression including gender as a covariate in Experiment 2 revealed that the interaction between intensity and synchrony remained significant (X² = 11.41, p < 0.001), and was not modulated by gender (X² = 0.66, p = 0.41). Future studies will be required to assess whether this effect is anecdotal or whether cross-modal effects on motion processing depends on gender”.

REVIEWER’S COMMENTS TO AUTHOR:

Comments from Reviewer #3: I have a few concerns that follow. To sum them up [1] we need to pay attention to variables operationalization and correspondent stimuli effectiveness for cross modality stimulation (mirror neuron like integration vs higher order cognitive-attentional convergent information about a variable); [2] it is not clear the gender effect; we should [3] pay attention to minor language problems. I won't use locutions as "in my opinion" etc. because tautological. I apologize for any unpoliteness that should be perceived.

[1] The effectiveness of the asynchronous condition depends on the following fact. The stimulus in its simplest representation can be described in terms of frequency and phase. The asynchronous condition has to be both different in "frequency" and "phase", otherwise we cannot discard the simplest interpretation that subjects use a low level property of perception (frequency) to give their responses. I try to be clearer.

The stimulus PLW can be described in terms of frequency of an event ("the [perceived] foot touches the ground"): the higher the frequency, the higher the psychophysical variable we want to measure (the [perceived] velocity of the [perceived] walker). In the synchronous condition there is another stimulus (processed in another perceptual modality) which matches with plw both in phase and in frequency (or at least - please clarify this point - with a frequency which is an integer multiple or divisor of the frequency of the plw event).

For example:

SAME PHASE - SAME FREQUENCY

time plw foot stimulus

1 1 1

2 0 0

3 0 0

4 1 1

5 0 0

6 0 0

7 1 1

8 0 0

9 0 0

...

DIFFERENT PHASE - SAME FREQUENCY

time plw foot stimulus

1 1 0

2 0 1

3 0 0

4 0 0

5 1 0

6 0 1

7 0 0

8 0 0

9 1 0

10 0 1

11 0 0

12 0 0

...

SAME PHASE - DIFFERENT FREQUENCY (but integer multiple: frequency foot touch = 2 * frequency plw)

time plw foot stimulus

1 1 1

2 0 0

3 0 1

4 0 0

5 1 1

6 0 0

7 0 1

8 0 0

9 1 1

10 0 0

11 0 1

12 0 0

...

ecc.

The point is: the asynchronous condition should be different both in phase and in frequency with no frequency at all preferably (this can be set giving a random order of the "1's" in the event representation, but maintaining that the total number of "ones" is the same for both stimuli. In analytic terms the two [continuous] event (y) - time (x) functions should have the same integral but one should be a sine(x+shift) and the other some random thing with the same area

Response: We thank the reviewer for noting this important aspect and we apologize if our description of the tactile stimuli were not clear enough in the original manuscript.

Actually, in the synchronous stimulation, the visual stimuli and the tactile stimuli had the same frequency and phase. The visual stimuli consisted of two stimuli, PLW 1 and PLW 2, separated by 1500 ms intervals. The gait frequency was 0.77 cycles per second for PLW 1 and (0.77 + �f cycle/sec) for PLW 2 according to the participants performance (staircase adaptative procedure). PLW 1 or PLW 2 could be presented with equal probability as the first visual stimulus for each trial. The duration of the tactile stimuli was adapted depending on the duration of the visual stimulus (i. e. if the PLW walked faster, the tactile stimulation was shorter) and corresponded to 20% of the duration of the total PLW gait. Indeed, for a full phase of a walking cycle, Foot 1 was stimulated between 10-30% of the visual stimuli cycle and Foot 2 between 60-80% of this cycle. If the cycle was longer/shorter, these stimulation windows were adjusted accordingly.

To complete your example, we can schematize this as (in row “PLW” 1 indicated that the feet of the PLW touch the ground, in rows “Foot” 1 indicated that tactile stimulations are activated):

SAME PHASE - SAME FREQUENCY

Time PLW1 Foot1 Foot2

1 0 0 0

2 1 1 0

3 1 1 0

4 0 0 0

5 0 0 0

6 0 0 0

7 1 0 1

8 1 0 1

9 0 0 0

10 0 0 0

Interval 1500 ms

Time PLW2 Foot1 Foot2

11 0 0 0

12 1 1 0

13 1 1 0

14 0 0 0

15 0 0 0

16 0 0 0

17 1 0 1

18 1 0 1

19 0 0 0

20 0 0 0

In the asynchronous condition, both frequency and phases were different for the tactile and visual stimuli. The duration of the tactile stimulation was the same as in the synchronous condition (corresponding of 20% of the total PLW gait cycle), but for each foot the phase was randomly selected. For example, this could result in Foot 1 being stimulated from 0-20% of the cycle and Foot 2 from 15-35%. The phase was randomized for each foot separately, i. e. it could also be that Foot 1 and Foot 2 were stimulated at the same time.

For example, it could be like this:

DIFFERENT PHASE – DIFFERENT FREQUENCY

Time PLW1 Foot1 Foot2

1 0 1 0

2 1 1 1

3 1 0 1

4 0 0 0

5 0 0 0

6 0 0 0

7 1 0 0

8 1 0 0

9 0 0 0

10 0 0 0

Interval 1500 ms

Time PLW2 Foot1 Foot2

11 0 0 0

12 1 0 0

13 1 0 0

14 0 1 0

15 0 1 0

16 0 0 0

17 1 0 0

18 1 0 1

19 0 0 1

20 0 0 0

We added these important methodological details in the Method section:

“The duration of the tactile stimuli was adapted depending on the duration of the visual stimulus (i.e. when PLWs walked faster, tactile stimulations were shorter) and corresponded to 20% of the duration of the total PLW gait.

In the ‘synchronous’ conditions of Experiment 1 and 2, stimulation onset of the laterally matched body part was timed to coincide with the point at which the PLW ‘ankle’ dot reached its lowest value on the y-axis. Indeed, for a full phase of PLW cycle, Foot 1 was stimulated between 10-30% of the visual stimuli cycle and Foot 2 between 60-80% of this cycle.”

“In the ‘asynchronous’ condition of Experiment 2, tactile stimulation onset was randomized during the visual stimulus. Both frequency and phase were different for the visual stimuli and the tactile stimuli. The duration of the tactile stimulation was the same as in the synchronous condition (corresponding of 20% of the total PLW gait cycle), but for each foot the phase was randomly selected. For example, this could result in Foot 1 being stimulated from 0-20% of the cycle and Foot 2 from 15-35%. The phase was randomized for each foot separately, i. e. it could also be that Foot 1 and Foot 2 were stimulated at the same time.

Please add some further consideration about the effectiveness of your "somatosensory drum effect" (some tips about implicit questions you could ask again yourself):

- how can I distinguish the pure "rythm" attentive effect from the "cross modality - mirror neuron like one"? Is my Asynchronous condition enough as it stands? etc.

Response: For Experiment 1, we consider unlikely that the effect comes from a “drum” or “rhythm” effect, as it was not present when the same tactile stimulation was delivered on the forearms. The experimental tactile condition (ET) and control tactile condition (CT) contained indeed the same visual and tactile information about the speed of the PLW.

We now add this to the Discussion:

“As the frequency and the phase of both visual and tactile stimuli were the same in this experiment, the tactile input itself (through its frequency and its duration) could have conveyed additional information about the speed of the PLW. It is then possible that the improvement in BM discriminability was only due to this additional information. But the finding that the same tactile stimuli improved BM discrimination only when they were delivered under the feet makes this explanation unlikely. There was indeed no difference in BM discrimination when no tactile stimulation occured (V) and when tactile stimulation was applied on the forearms (CT).”

For Experiment 2, as the frequency and the phase are different for the visual and tactile stimuli in the asynchronous condition, it is possible that the better speed discrimination in the synchronous condition comes from such “rhythm” matching between the two sensory inputs.

We now discuss this possibility in the Discussion:

“However, as the frequency and the phase are different for the visual and tactile stimuli in the asynchronous condition, we cannot exclude, based only on Experiment 2, that the improvement in BM discriminability in the synchronous condition comes from additional information about the speed of the PLW conveyed by the tactile stimulation itself.”

- How long does each PLW stimulus last?

Response: We used the monitor refresh rate as the reference (100Hz) and each frame showed a different dot pattern. The gait frequency of 0.77 cycles per second was used as the base gate frequency, meaning that 129 frames were shown for a full PLW cycle, lasting 1298 ms. An interval of 1500 ms separated the presentation of this reference PLW to a second PLW with a slightly faster gait frequency (0.77 + �f cycle/sec) according to the participants performance (staircase adaptative procedure). For instance, if during the previous trials the discrimination was incorrect with a �f of 0.2 cycle/sec, then the second walker would have 100/(0.77+0.2) = 103 frames for a full cycle, lasting 1030 ms. Then the total duration varied for each trial, with a maximum of 4096 ms (1298 + 1500 + 1298 ms).

- is it necessary to refer to the [complex and not universally accepted] theory of mirror neurons or the results can be [easily] explained by other (simpler) mechanisms?

In other words if we cannot exclude a simpler process of converging information (different stimuli of which frequencies correlates and help the performance which has a "computation time" compatible with higher order processes), the discussion, say, of rows 461 - 499 has to be "mitigated". Could some further considerations regarding the comparison of results for forearm vs foot stimulation help?

Response: Following the Reviewer’s suggestion, we have left the reference to mirror neurons, which was not the aim of this study, and now also propose alternative hypotheses regarding the difference of results for forearm and feet stimulation in the Discussion section (i.e., difference in sensitiveness between the two body parts, higher relevance of the feet for an embodied representation of walking actions).

Should this considerations refer also to possible different sensitiveness of those areas (example rows 311-312)?

Response: The sensitivity of the tactile simulation on the arm was tested in a pilot study. Moreover, participants reported strong tactile sensations during the task. Accordingly, we argue that it is unlikely that performance difference in Experiment 1 are due to a difference in sensitiveness between feet and forearms, although we cannot exclude this. We now have mentioned this limitation in the revised Discussion:

“Secondly, we cannot exclude the possibility that differences in tactile perception between the forearms and the feet may have resulted from difference in sensitiveness between these two body parts. We hypothesized during the study conception that the feet would be more relevant than forearms regarding walking actions, and therefore predicted that performance would be better with tactile stimulation on this site than elsewhere on the body.”

[2] balanced design are always preferable; your design is not regarding gender and it's unbalanced from the very beginning (it is not only a matter of subjects exclusion for data or inclusion problem). I would not mention at all gender variable effect.

Response: We share the reviewer’s opinion, but decided to follow the editor’s suggestion and report post-hoc effects on gender (see above).

If you are somehow obliged to, I'd rather be more explicit about the reasons that make compatible the following statements:

rows 415-417: gender variable has a main effect in experiment 2 and does not have it in experiment 1.

rows: 420-422: the effect of intensity x synchrony remain significant on the dependent variable (the fuzzy-step|sygmoid|error function like decision function) if in the model you add gender variable explicitly.

For what I can understand: on one hand, on its own, if you use t-test, gender has no effect in experiment 1 but has an effect in experiment 2; on the other hand, it does not "significantly modulate" the combined effect of stimulus (plw) intensity and synchrony (with somatosensory stimulation). Those results are compatible without the need of giving any further information on experiment 1; on experiment 2 we are lead to think that (a) the "effect size" of intensity x synchrony is high enough to "cover" the one of gender on its own or [inclusive] (b) that the interaction gender x intensity x synchrony is "small" if significant. If this representation of what you mean in rows 414-424 is correct, please give some statistical effect size measure and be more analytic in its explanation, above all whether if the above a, b or both are true.

Response: Following the reviewer’s suggestion, we conducted a supplementary binomial mixed-effects regression including gender as a covariate of (no) interest in Experiment 2. We are pleased to report that the interaction between intensity and synchrony remained significant (X² = 11.41, p < 0.001), and was not modulated by gender (X² = 0.66, p = 0.41).

In layman terms you should represent you results in order to explain both (1) why gender has effect in experiment 2 and not in experiment 1 and (2) why gender does not affect the intensity x synchrony effect.

Maybe it could be helpful to "let us see" the model. I guess it is something like this:

to each discrete independent variable V_k with number of levels N_k we associate N_k variables X_h with values 0 or 1.

to each numerical/continuous independent variable W_i we associate a numerical variable X_i with the same domain.

So from

[subject, intensity, synchrony, gender]

we obtain

x_1, ..., x_n, with n = number_of_subjects

x_n+1 =intensity

x_n+2 = asynchronous_stimulation

x_n+3 = synchronous_stimulation

x_n+4 = female

x_n+5 = male

and the model WITHOUT interaction is

y = sum(a_j * x_j) + error

For the one with interaction you add more variable such as

x_n+6 = (x_n+1)*(x_n+2 == 1) that is intensity for asynchronous stimulation and so on.

If my rough resume is somehow correct, it should be the case that

y = ... + a*(intensity_x_synchrony) + b*(intensity_x_synchrony_x_gender) + c*(gender) + ...

gives

a significant and strong

b significant and small (or not significant)

c significant (for experiment 2) and small (comparing to a)

am I correct? Nevertheless I suggest you try to find a way to make it clearer

Response: The reviewer is correct. We provide below the table corresponding to the analysis of Deviance (Type II Wald chi square tests) of the model: resp ~ intensity* synchrony * orientation * sex

--Please see the "response to reviewers_12.05.20.docx" document to access the table--

[3] Miscellanea and minor concerns

Row 244: "...was randomized between each body part ...": check for "between" as it's used for a set of two elements, so it should be "among", because I guess that the body parts are more than 2. Moreover I think you may want something like "... randomized across ... " or "... randomly sampled among all body parts ...". However, if by "body parts" you do not mean those related to moving dots but those of the experimental subject, please be more clear about why you should randomize "left and right" stimulation (I guess because you have only one tool). Nevertheless please check this part with a native speaker.

Response: We would like to thank the reviewer for this important comment. We meant among each body parts. We now give more details about the tactile stimuli in the Apparatus and Stimuli section:

“In the ‘asynchronous’ condition of Experiment 2, tactile stimulation onset was randomized during the visual stimulus. Both frequency and phase were different for the visual stimuli and the tactile stimuli. The duration of the tactile stimulation was the same as in the synchronous condition (corresponding of 20% of the total PLW gait cycle), but for each foot the phase was randomly selected. For example, this could result in Foot 1 being stimulated from 0-20% of the cycle and Foot 2 from 15-35%. The phase was randomized for each foot separately, i. e. it could also be that Foot 1 and Foot 2 were stimulated at the same time.”

Row 414: "Although ... NOT ..., the influence is worth considering". I cannot understand this sentence. "Although B, A" means that "A" usually implies "not-B"; in other words "A although B" should be something like "A even if not-B".

A="the influence of gender on BM perception is worth considering"

B="our samples were not significantly unbalanced for gender"="our samples were balanced enough for gender"

so to maintain an intuitive semantics it should be "A because B". Please clarify this point.

Response: The reviewer is right, this sentence was incoherent. Actually there was a mistake in the first part of the sentence, it should have been: “although our samples were not significantly balanced for gender �…�” and not “�…� unbalanced �…�”. We don’t use this sentence anymore in the manuscript.

Row 450: "STS, with its central [...] actions (see [...]) has been implicated [...]" should be

"STS, with its central [...] actions (see [...]), has been implicated [...]" (the comma is missing). Please double check the paper because there are other punctuation errors.

Response: We thank the reviewer for these observations. We have double checked the revised manuscript for punctuation errors.

Submitted filename: response to reviewers_12.05.20.docx

Click here for additional data file.

19 May 2020

Somatosensory-visual effects in visual biological motion perception

PONE-D-19-17841R4

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29 May 2020

PONE-D-19-17841R4

Somatosensory-visual effects in visual biological motion perception

Dear Dr. Progin:

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