Intrinsic Structure of Visual Exemplars and Category Representations in Macaque Brain

Abstract

The neural mechanisms underlying the ability to identify and categorize a wide variety of objects effortlessly have been the focus of many studies. Previous studies, especially neuroimaging ones, have typically investigated brain responses to different predefined categories without distinguishing responses to individual object exemplars. Thus, it is unclear whether the commonly defined categorical structure actually reflects the intrinsic organization of individual object representations in the brain. To answer this question, three monkeys were presented with 96 images of isolated real-world objects while they performed a fixation task in a rapid event-related fMRI experiment. Object representations were measured using fMRI and analyzed at the single-image level. We found that the multi-voxel response patterns to individual objects in the inferior temporal (IT) cortex encode the animate-inanimate categorical division, with a subordinate cluster of faces within the animate category. These results are consistent with previous results from both monkey electrophysiology and human fMRI studies, providing the missing link between those previous studies. Furthermore, we found differences in object representations between TEO and TE: individual object representations formed only an animate cluster (without any sub-clusters) in TEO, but formed both animate (with a face sub-cluster) and inanimate clusters in TE. Our findings thus support a hierarchical organization from TEO to TE in object processing. We did the same analyses in two output regions of IT cortex, namely, the amygdala and prefrontal cortex. We found the individual object representations in these two regions either did not exhibit any categorical structure or exhibited a categorical structure different from that in IT cortex. Despite the different object representations in the IT cortex, amygdala and prefrontal cortex, there were significant representational connectivities among these regions. Our findings reveal a hierarchical organization in the intrinsic structure of individual object representations and categorization in the non-human primate brain.

Meeting abstract presented at VSS 2013