Abstract
Several modules along the ventral visual pathway selectively respond to specific image categories: fusiform face area (FFA) to faces, parahippocampal place area (PPA) to scenes, and extrastriate body area (EBA) to bodies. The existence of these category-specific regions suggests that there are distinct branches in the visual-processing hierarchy. The relative levels of processing or abstraction among these category-specific regions are not known. Here we sought to determine the relative levels of processing for FFA and PPA using fMRI. We used noise-masked stimuli consisting of both a face and a scene, transparently superimposed. Subjects performed a face task and a scene task in separate scans using the same set of stimuli. For the scene task, subjects decided whether the scenes in two successively presented images were from the same scenery; for the face task, they determined if the two faces were of the same individual. The transparency of the faces and scenes were adjusted such that the accuracies for both tasks were similar. For each ROI (V1-hV4, LO, pFs, FFA, PPA) and task, we measured BOLD amplitude as a function of image SNR. We found that, for both tasks, the log-log slope of the BOLD response function increased monotonically from low- to high-level visual areas. The log-log slopes of the BOLD response functions of FFA during the face task and PPA during the scene task placed both areas at a similar level of processing as pFs (Tjan, Lestou and Kourtzi, 2006). However, FFA and PPA differed in that FFA was modulated equally by image SNR during both tasks, while PPA was not modulated at all during the face task, but was strongly modulated during the scene task. This suggests that face processing in FFA is involuntary while scene processing in PPA is task-dependent.
NIH R03-EY016391, R01-EY017707.