Functional Imaging of Coherent Visual Perception in the Monkey and the Human
The question of how local image features on the retina are integrated into perceived global shapes is central to our understanding of human visual perception. However, the neural mechanisms that mediate unified shape perception in the primate brain remain largely unknown. Our recent fMRI studies on both monkeys and humans addressed this question by using an adaptation paradigm, in which stimulus selectivity was deduced by changes in the course of adaptation of a pattern of randomly oriented elements. Accordingly, we observed stronger activity when orientation changes in the adapting stimulus resulted in a collinear contour than a different random pattern. This selectivity to collinear contours was observed not only in higher (occipitotemporal) visual areas that are implicated in shape processing, but also in early (retinotopic) visual areas where selectivity depended on the receptive field size. These findings suggest that unified shape perception in both monkeys and humans involves multiple visual areas that may integrate local elements to global shapes at different spatial scales. Further human fMRI studies showed decreased detection performance and fMRI activations when misalignment of the contour elements disturbed the perceptual coherence of the contours. However, grouping of the misaligned contour elements by disparity resulted in increased performance and fMRI activations. These studies provide evidence for the role of early perceptual organization processes and their interactions with higher stages of visual analysis in unified visual perception in the primate brain.