Early suppression effect in human primary visual cortex during Kanizsa illusion processing: a magnetoencephalographic evidence
Detection of illusory contours such as Kanizsa figures is known to depend primarily upon the lateral occipital complex. Yet there is no universal agreement on the role of the primary visual cortex in this process; some existing evidence hints that an early stage of the visual response in V1 may involve relative suppression to Kanizsa figures compared with controls. Iso-oriented luminance borders, which are responsible for Kanizsa illusion, may evoke surround suppression in V1 and adjacent areas leading to the reduction in the initial response to Kanizsa figures. We attempted to test the existence, as well as to find localization and timing of the early suppression effect produced by Kanizsa figures in adult non-clinical human participants. 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 in combination with 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. The effect was located mostly in V1. The current finding is highly compatible with the explanation involving surround suppression evoked by iso-oriented collinear borders. The effect may be related to the principle of sparse coding, according to which V1 suppresses representations of inner parts of collinear assemblies as being informationally redundant. Such a mechanism is likely to be an important preliminary step preceding object contour detection.