Integration of local features into global shapes: monkey and human fMRI studies

The perception of global visual shapes entails the integration of local image features into global configurations. Traditionally, the visual system is thought to be hierarchically organized in early visual areas (V1, V2, V3, V4) that are involved in the analysis of simple local features and higher visual areas (regions in the inferotemporal cortex) that are implicated in the processing of complex global shapes. We investigated the integration of local image features into global shapes across visual areas in the monkey and the human brain using fMRI. An adaptation paradigm was used, 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 after adaptation when orientation changes in the adapting stimulus resulted in a collinear shape than a different random pattern. This selectivity to collinear shapes was observed not only in higher visual areas, but also in early 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.