Selectivity of audiovisual ECoG responses revealed under naturalistic stimuli in the human cortex

A fundamental debate in the study of cortical sensory systems concerns the scale of functional selectivity in cortical networks. Brain imaging studies have repeatedly demonstrated functional selectivity in entire cortical areas and networks using predetermined stimuli. However, it is not clear to what extent these networks are heterogeneous, i.e., whether the selectivity profiles in subregions within each sensory network show significant dissimilarity. Here, we studied local functional selectivity in the human cortex using naturalistic movie clips shown to 12 patients implanted with intracranial electrocorticography electrodes (590 in total), providing extensive cortical coverage. We examined the similarity of response profiles (40- to 80-Hz gamma-power modulations) across electrodes using a novel data driven approach without assuming any predefined category. Our results show that the functional selectivity of each highly responsive electrode was different from that of all other electrodes across the sensory cortex. Thus most responsive electrodes showed an activation profile that was unique in each patient and was similar to that of only 0.3% (1-2) of all other electrodes across all patients. Functional similarity between electrodes was linked to anatomical proximity. While in most electrodes the source of selectivity was complex, a small subset showed the well-documented selectivity to faces and actions. Our results indicate that the human sensory cortex is organized as a mosaic of functionally unique subregions in which each site manifests its own special response profile.