Illusory Contour Processing

Experiments conducted in macaque monkeys indicate that area V2 contains neurons that are selective for illusory contour stimuli, whereas such responses are weak or not present in area V1. In order to determine which areas in human visual cortex are responsive to illusory contours, fMRI techniques were applied to map the areal distribution of responses to illusory contours of the Kanizsa and displaced grating types. Kanizsa stimuli were compared with aligned and rotated inducers, and the resultant activations were mapped onto retinotopically defined visual areas. For individual subjects, Kanizsa stimuli produced robust activations in intermediate level visual areas V3A, V4, V7, and V8 but not in areas V1, V2, V3, or VP. When an across-subject analysis with restricted regions of analysis (ROIs) was applied to these data, a small, but statistically significant response was also observed in these lower visual areas. When displaced grating stimuli were used to produce illusory contours, strong activations were observed in the intermediate cortical areas and weaker activations were seen in areas V1, V2, V4, and VP. Thus, in agreement with previous single-cell and optical recording in macaque monkeys, area V2 contains neurons responsive to illusory contours based on displaced grating stimuli. Unlike the work in macaques, area V1 contained illusory contour responses that were indistinguishable from those observed in area V2. In addition, these results indicate that human area V2 contains a weak, yet significant signal representing stimuli of the Kanizsa type. In contrast, higher cortical areas contained much stronger responses to both types of illusory contours.



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