Extrastriate cortex and medial temporal lobe regions respond differentially to visual feature overlap within preferred stimulus category.

It has been proposed that domain-specific regions in extrastriate cortex, parahippocampal cortex and the medial temporal lobe (MTL, particularly the hippocampus, HC, and perirhinal cortex, PrC) may respond differently to the degree of feature complexity present in sets of visual stimuli, with the latter two regions tuned to represent the differences among stimuli with a high degree of visual overlap or featural ambiguity (Graham, Barense, & Lee, 2010; Cowell, Bussey, & Saksida, 2010a). To test this prediction, healthy participants viewed blocks containing visually similar or visually different exemplars from four stimulus categories (scenes, faces, inanimate objects and animate objects). Independent functional regions of interest were identified in extrastriate and MTL regions that were preferentially responsive to one or more of these visual categories, and the main experimental data interrogated for any evidence of an interaction between visual category and degree of feature overlap. In PrC and posterior HC (PostHC) viewing sets of stimuli with a large number of overlapping features resulted in greater activity than blocks containing items that were more visually distinct. The opposite pattern was found in fusiform face area (FFA), parahippocampal place area (PPA) and lateral occipital complex (LOC). The increased response in the HC and PrC to high visual similarity was seen only for visual categories that effectively activate these regions (PrC-faces and objects; PostHC-scenes). This study confirms that regions throughout the visual ventral stream, parahippocampal cortex and MTL are engaged differentially by visual complexity, consistent with recent lesion experiments in which MTL damage affects discrimination and learning of, as well as recognition memory for, exemplars with a high degree of visual feature overlap.