Illusory Contour And Surface Completion Mechanisms In Human Visual Cortex PDF Download

Abstract: The human visual system processes images projected to the retina and transforms this information into representations of objects in the real world. A fundamental stage in the transformation is extraction of visual contours and surfaces. In part, contour identification is performed by neurons sensitive to luminance edges (Hubel & Wiesel, 1962); however, contours and surfaces can also be perceived in instances without explicit luminance edges (Kanizsa, 1955; Schumann, 1900). These phenomena, known as illusory contours, drive visual cortical responses (Mendola et al., 1999; von der Heydt et al., 1984) and offer an important window into the mechanisms of contour and surface perception. First, a novel display, the Chomping Pacman (CP) illusion, is described. Illusory contour clarity is enhanced relative to traditional illusory contour figures. The CP illusion is combined with a method of adjustment technique to dissect contributions of three mechanisms of contour completion: extrapolation, interpolation, and figural processes. When occlusion cues are present, one or more mechanisms are blocked, yet modal (or visible) contours are still observed to persist. These studies question contour theories based solely on local cues without figural mechanisms. Second, functional MRI (fMRI) was employed to image human brain activity when subjects viewed the CP illusion. Activity in lateral occipital (LO) cortex was observed when contours were formed with or without the percept of a complete figure, indicating LO is a site of both contour and figure completion, contrary to previous reports of figure completion only in LO (Stanley & Rubin, 2003). Figure activity was found in all midlevel visual regions, intraparietal sulcus (IPS), LO, MT and V4, suggesting figure-based mechanisms of completion involve areas in addition to LO. Finally, fMRI was utilized to investigate changes in brain activity that occur with increases in the perceptual vividness of an illusory contour figure. Perceptual vividness was enhanced via coherent motion cues and via neon color spreading (NCS) in the Chomping Pacman display. A brain area encoding perceptual vividness should give greater activation for illusory figures than for non-figures, for motion illusory figures than for static illusory figures, and for NCS illusory figures than for achromatic illusory figures. Five visual cortical areas met these criteria, areas: V3B, hV4, LOCvt, IPS2, and IPS3. These results demonstrate that perceptual vividness of illusory contour figures is reflected in the activity in both the dorsal and ventral visual streams for both color and motion percepts.