The encoding of category-specific versus nonspecific information in human inferior temporal cortex

Several brain areas in the inferior temporal (IT) cortex, such as the fusiform face area (FFA) and parahippocampal place area (PPA), are hypothesized to be selectively responsive to a particular category of visual objects. However, how category-specific and nonspecific information may be encoded at this level of visual processing is still unclear. Using fMRI, we compared averaged BOLD activity as well as multi-voxel activation patterns in the FFA and PPA corresponding to high-contrast and low-contrast face and house images. The averaged BOLD activity in the FFA and PPA was modulated by the image contrast regardless of the stimulus category. Interestingly, unlike the univariate averaged BOLD activity, multi-voxel activation patterns in the FFA and PPA were barely affected by variations in stimulus contrast. In both the FFA and PPA, decoding the categorical information about whether participants saw faces or houses was independent of stimulus contrast. Moreover, the multivariate pattern analysis (MVPA) results were highly stable when either the voxels that were more sensitive to stimulus contrast or the voxels that were less sensitive were used. Taken together, these findings demonstrate that both category-specific (face versus house) information and nonspecific (image contrast) information are available to be decoded orthogonally in the same brain areas (FFA and PPA), suggesting that complementary neural mechanisms for processing visual features and categorical information may occur in the same brain areas but respectively be revealed by averaged activity and multi-voxel activation patterns. Whereas stimulus strength, such as contrast, modulates overall activity amplitudes in these brain areas, activity patterns across populations of neurons appear to underlie the representation of object category.