Surround suppression effect in human early visual cortex contributes to illusory contour processing: MEG evidence

European Journal of Neuroscience. 2014.

Chernyshev B. V., Pronko P., Kravchenko A., Stroganova T.

Presently there is a significant controversy concerning the role of the early visual cortices in illusory contour processing, and contradictory data were reported concerning even the mere existence and/or the sign of the effect. The evidence of iso-orientation surround suppression by collinear borders in V1 hints that the initial response to the Kanizsa figure may be smaller compared with controls. We attempted to find the direct evidence, as well as localization and timing of early suppression effect produced by Kanizsa square in adult non-clinical population. We used two sizes of stimuli in order to test the validity of the effect at two different levels of eccentricity; the stimuli sized 4.5° and 9.0° were presented to the participants centrally in passive viewing conditions. By applying strict spatial and temporal restrictions as well as using threshold-free cluster enhancement technique followed by permutation statistics, we were able to detect the inverted illusory contour effect – relative suppression of the response to the Kanizsa figure compared with the control stimulus within the 40-120 ms time window after the stimulus onset. In view of the current literature, this effect is most likely explained by iso-orientation suppression – a salient feature of V1 neuronal responses to collateral luminance borders, which are present in Kanizsa figures and absent in control stimuli. The effect may relate to the principle of "sparse" coding, according to which V1 suppresses representations of inner parts of collinear assemblies as being informationally redundant.

Keywords: visual perception, Kanizsa illusion, primary visual cortex, surround suppression, sparse coding