Details of the construction of perception: a closer look at illusory contours.

Are we aware of neural activity in primary visual cortex? This question was asked in a classic paper written by Nobel laureate Francis Crick and Christof Koch more than a decade ago (Crick and Koch, 1995). By now, several studies have addressed the question, but so far consensus has not been reached (Rees, 2007; Tong, 2003). A recent study by Maertens et al. (2008) presents new relevant data on this issue. They demonstrated that activity in primary visual cortex (V1) is correlated with the perception of subjective contours. Maertens et al. used Kanizsa figures to induce a percept of illusory contours (IC, Figure 1), which are subjective in the sense that there is no actual change in luminance at the location of the perceived contour.

Figure 1

Kanizsa stimuli used by Maertens et al. (. Illusory contours were generated by inducers (black “pack-men”) shown at different angles. The task was to judge whether the illusory contour was convex or concave. As control condition, the pack-men were misaligned which does not induce illusory contours.

The (illusory) perception of the contours is dependent on coherent orientation of the inducers. Hence, by either aligning or misaligning the inducers, very similar (local) physical parameters can create two subjectively different visual experiences. IC can therefore be used to provide information on neural correlates of conscious perception.

An important feature of Maertens et al.’s experiment was the localization of neural activity specific for contour perception. This was accomplished by using spatially specific localizer stimuli for the contours and inducers separately, as well as by using a spatially more precise imaging sequence than traditionally used ones. This enabled the conclusion that the IC were related to retinotopically specific regions within V1, thereby extending previous research on IC where IC-related activity at such a level of detail was not observed. The results also add to existing evidence regarding V1 involvement in conscious perception. In previous research on conscious perception, some but not other studies have observed correlated activity in V1. A factor that could account for the inconsistent results is the specific task requirements. Specifically, it has been suggested that there is a reverse hierarchical sequence for conscious perception such that global, holistic features are perceived first and the details are added if required (Hochstein and Ahissar, 2002). In the Maertens et al. paper, the angle of the induced contrasts was varied between trials and the participant was required to judge the angle on each trial (Figure 1). Hence, the task mandated a fine-scaled decision which required detail to be extracted – detail at such a fine scale that it may only be present in V1 due to its small receptive fields – and might therefore have induced correlated V1 activity.

A hypothesis put forth by Crick and Koch in their 1995 paper was that, while activity in V1 may be necessary for conscious perception, it may not be sufficient. According to Crick and Koch, sensory regions need to be connected to prefrontal cortex to enable conscious perception. Whether involvement of frontal regions is necessary remains controversial (Eriksson et al., 2008), but a role of prefrontal cortex is indicated by a large number of neuroimaging studies showing a correlation between activity in frontal regions and conscious perception (Naghavi and Nyberg, 2005; Rees et al., 2002). In the Maertens et al. study, a limited part of the brain was sampled to improve spatial resolution. While this enabled the retinotopic mapping of the IC and thus constituted a great strength of study, this level of detail comes with a cost as nothing could be concluded regarding frontal involvement in the creation of IC. Future work, building on the methods used in the Maertens et al. study along with whole-brain sampling could shed light on the involvement of higher-order cortical regions and necessary vs. sufficient conditions for conscious perception.

In sum, the Maertens et al. study is an excellent demonstration of the constructive nature of perception (von Helmholtz, 1910). Moreover, it shows that this constructive process is not confined to higher levels of the visual hierarchy, but can take place even at the lowest level of cortical processing.