Functionally distinct sub-regions in the lateral occipital complex revealed by fMRI responses to abstract 2-dimensional shapes and familiar objects

Abstract

The Lateral Occipital Complex (LOC) has been consistently implicated in the perception of objects, object fragments, and shapes. The purpose of this study was to functionally characterize sub-regions within the LOC by mapping fMRI responses to images of abstract 2-Dimensional shapes and everyday objects. FMRI data were collected for 5 observers while they viewed images of objects or abstract 2-D shapes, contrasted with phase-scrambled controls of the same images in a block design. Based on these data, the LOC was divided into 3 sub-regions. A posterior region in the lateral occipital sulcus (pLOC) was more active for both objects and 2-D shapes than controls. An anterior region including the fusiform gyrus and collateral sulcus (aLOC) was more active for objects versus controls but not for 2-D shapes versus controls. A third region, posterior to aLOC and ventral to pLOC (potentially V4v), was more active for objects than controls but more active for controls than 2-D shapes (Obj+Shp-). This is consistent with reports that V4v is sensitive to local stimulus features (Grill-Spector et al, 1998) - objects and phase-scrambled controls contained more local heterogeneity than the 2-D shapes which were internally homogeneous. In addition to the localizer experiments we also performed an event-related adaptation experiment. On each trial two images (objects or shapes) were presented in rapid succession and were either identical or different. We found more adaptation in posterior than anterior regions. Interestingly, the observed adaptation was stimulus selective: in aLOC there was significant adaptation to 2-D shapes but not objects, while in Obj+Shp- this pattern was reversed. When adaptation wasn't observed it was likely due to saturation from across-trial adaptation (the same object or shape images were used on each trial). This pattern of adaptation is consistent with the findings from the localizer scans further supporting the functional distinction of these regions.