Global and Local Mechanisms of Shape Processing in the Human Visual Cortex

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.Event-related fMRI was used to test for local and global mechanisms of shape processing in higher visual object related areas. We tested for responses in the Fusiform Face Area (FFA) known to respond selectively to faces [1] and the Parahippocampal Place Area (PPA) known to be involved in the analysis of spatial layout [2]. 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. 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 specic responses to the local shapes were observed when the global shapes were from a different category than the local shapes. Further studies tested for fMRI responses at different stimulus scales and attentional shifts. Stronger responses to the local faces in the FFA and local houses in the PPA were observed compared to global faces and global houses. Our results suggest differential processing of global and local shape information in category selective areas. Furthermore, attention and spatial scale inuence the processing of local and global shape information