Distributed illusory figure-ground segmentation signatures across the dorsal and ventral streams

Abstract

Visual illusions give rise to percepts that are more complex than the incoming sensory information, thereby exposing the inferential nature of perception. Early visual cortex (EVC) can represent not only physical input, but also illusory content of perception. Specifically, much like in real shape perception, during perception of an illusory shape the topographic representation of the shapes surface is enhanced, while the regions surrounding it are suppressed, reflecting the figure-ground segmentation mechanisms on the neural level. It remains unclear whether such topographically specific figure-ground segmentation signatures are present in higher topographic maps along the visual hierarchy. To test this, we measured brain activity of 30 healthy human participants using functional magnetic resonance imaging (fMRI). The participants performed a central fixation task while passively observing a surrounding configuration of four inducers, which either formed or did not form an illusory shape. Using an additional functional localizer scan we could identify voxels uniquely representing the illusory surface and the background in the EVC, posterior parietal topographic region IPS0/V7 and several other maps along the dorsal and ventral streams. Apart from the EVC, we found signatures of figure-ground segmentation in several topographic maps. Illusory surface enhancement was observed in IPS0/V7, but without background suppression. Ventral topographic maps VO1 and VO2 showed background suppression only. Finally, other extrastriate areas, V3a, V3b and hV4, displayed both surface enhancement and background suppression. Our results demonstrate that topographically specific illusory shape responses are distributed along multiple topographic maps beyond the EVC. Furthermore, they point to the difference in neural signatures of figure-ground segmentation at various hierarchical levels and along different processing streams, stressing the independence of the surface enhancement and background suppression mechanisms.