Processing of global vs. Local shape information in the human visual cortex

Abstract

Coherent visual perception requires the integration of local elements into global shapes. However, the involvement of the various visual areas in the integration of local features into global shapes remains largely unknown. We used event-related fMRI to test for local and global shape processing in visual areas known to be involved in the processing of shapes. The stimuli consisted of images of houses or faces (global shapes) rendered by smaller images of stimuli from these categories (local shapes). We tested four conditions: a) global faces rendered by local faces; b) global faces rendered by local houses; c) global houses rendered by local faces and d) global houses rendered by local houses. Subjects were instructed to judge whether global and local shapes where from the same or different categories. We tested for responses in the Fusiform Face Area (FFA) known to respond selectively to faces (Kanwisher et al., 1997) and the Parahippocampal Place Area (PPA) known to be involved in the analysis of spatial layout (Epstein et al., 1998). Our results showed strong fMRI responses for global faces in the FFA and global houses in the PPA independent of the stimulus category at the local level. Lower category specific responses to the local shapes were observed when the global shapes were from a different category than the local shapes. These results suggest differential processing of global and local shape information in category selective areas. Further studies will test for the processing of global and local shape information at different stimulus scales