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

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 contourdefined 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.

Spatial attention facilitates selection of illusory objects: Evidence from event-related brain potentials

“…In the present experiment and in our previous study (Martinez et al, 2006), source analyses of the N1 associated with object-selective attention also indicated a principal neural generator in lateral occipital cortex (area LOC). Activation in this same LOC region has been observed in numerous object recognition studies using a variety of task stimuli, including objects defined by illusory contours (Mendola et al 1999;Murray et al 2004), partially occluded objects (Lerner et al 2001) and scrambled objects (Malach et al 1995). On the basis of these studies and others it has been suggested that the LOC region plays an important role in the encoding and recognition of objects.…”

Illusory contours: a window onto the neurophysiology of constructing perception

“…In addition, the IC effect is insensitive to variations in the contrast polarity (i.e. whether inducers are black on a gray background or vice versa) [8] (see also [39]), ruling out an explanation in terms of perceived brightness enhancement (see also [40]). The IC effect is mediated by the presence of the illusory figure but not collinear lines [41].…”

“…In agreement are data concerning the relative insensitivity of IC processes to the spatial frequency of the misaligned gratings used as inducers (cf. Figure 6 in [40] as well as [55] for corresponding single-unit data).…”

“…Mendola et al [40] conducted arguably the most comprehensive fMRI study to date in terms of range of stimulus parameters combined with retinotopic mapping and cortical surface reconstructions. Activations were stronger within LOC for Kanizsa-type ICs vs. control stimuli and luminance-defined contours, with no reliable modulations in lower-tier regions (cf.…”

Automatic volumetric segmentation of human visual retinotopic cortex

“…These results were replicated by DeYoe et al (1996) and Engel et al (1997), also utilizing a cortical surface-based analysis but manually distinguishing the borders based on polar-angle information. Further phase-encoded mapping studies with modifications of the stimulus revealed visual areas V3A (DeYoe et al, 1996;Tootell et al, 1997Tootell et al, , 1998c, V3B (Press et al, 2001;Smith et al, 1998), V7 (Mendola et al, 1999;Press et al, 2001;Tootell et al, 1998a,c;Tootell and Hadjikhani, 2001), V8 , a putative homologue of the lateral intraparietal area (LIP) (Sereno et al, 2001), and retinotopic organization in human MT/V5 (Huk et al, 2002).…”