The Representation of Illusory and Real Contours in Human Cortical Visual Areas Revealed by Functional Magnetic Resonance Imaging

TLDR

Illusory contours, perceived edges lacking local stimulus borders, illustrate that perception actively generates features absent from retinal input and are thought to engage mechanisms overlapping with real contours, though their neural basis remains uncertain. Using fMRI, the study recorded activity from nine retinotopically defined visual areas while participants viewed real and illusory contours, sampling signals independently from each region. Results revealed overlapping processing sites, with most areas responding to both contour types, but differing activation strengths across areas and contour types, including distinct responses of two illusory contour types in V3A–V8, invariant activation across stimulus sizes yet tied to spatial frequency, and a striking similarity between illusory contour‑defined shapes and stereoscopic depth shapes, indicating a role for the lateral occipital region in surface perception.

Abstract

Illusory contours (perceived edges that exist in the absence of local stimulus borders) demonstrate that perception is an active process, creating features not present in the light patterns striking the retina. Illusory contours are thought to be processed using mechanisms that partially overlap with those of “real” contours, but questions about the neural substrate of these percepts remain. Here, we employed functional magnetic resonance imaging to obtain physiological signals from human visual cortex while subjects viewed different types of contours, both real and illusory. We sampled these signals independently from nine visual areas, each defined by retinotopic or other independent criteria. Using both within- and across-subject analysis, we found evidence for overlapping sites of processing; most areas responded to most types of contours. However, there were distinctive differences in the strength of activity across areas and contour types. Two types of illusory contours differed in the strength of activation of the retinotopic areas, but both types activated crudely retinotopic visual areas, including V3A, V4v, V7, and V8, bilaterally. The extent of activation was largely invariant across a range of stimulus sizes that produce illusory contours perceptually, but it was related to the spatial frequency of displaced-grating stimuli. Finally, there was a striking similarity in the pattern of results for the illusory contour-defined shape and a similar shape defined by stereoscopic depth. These and other results suggest a role in surface perception for this lateral occipital region that includes V3A, V4v, V7, and V8.