Visual presentation of hand image modulates visuo-tactile temporal order judgment.

Perceptual systems can distinguish among a variety of inputs in the temporal domain, including even different sensory inputs. This process has been investigated mainly by using a temporal task (temporal order judgment: TOJ). For example, studies have reported estimated critical limits (just noticeable difference: JND) of the TOJ between a visual stimulus and a tactile stimulus (visuo-tactile TOJ, e.g., flashes and vibrations) fell within a certain temporal range. Recent studies have also suggested that the visual presentation of a hand image could modulate visuo-tactile integrations in the temporal domain, but these studies did not thoroughly examine such effects by using temporal tasks. Here, we investigated the effect of visual presentation of a hand image on visuo-tactile TOJ. In our experiments, a visual stimulus was presented on the index finger of a hand image and a tactile stimulus was presented on the index finger of a participant's hand. We found that the JND of visuo-tactile TOJ became larger when a forward hand image was presented than when inverted hand or arrow images were presented. However, this effect was not observed for the TOJ between an auditory stimulus and a visual stimulus. Thus, the visual presentation of a hand image whose angle corresponds to that of one's own hand could selectively degrade visuo-tactile TOJ. This finding indicates that visual hand images implicitly enhance the internal proximity between the visual and tactile stimuli and make them difficult to distinguish from each other in the temporal domain.