Abstract
According to a non-hierarchical view of human cortical face processing, selective responses to faces may emerge in a higher-order area of the hierarchy, in the fusiform face area (FFA) independently from face-selective responses in the occipital face area (OFA), a lower order area. Here we provide a stringent test of this hypothesis by using fMRI with a very short sampling rate (TR=500 ms) and by gradually revealing segmented face stimuli throughout strict linear descrambling of phase information. We presented 15 seconds stimulation sequence of upright, inverted and scrambled faces changing in local contrast occurring every TR while preserving low-level visual information constant throughout the sequence (Supplemental Material). A single subject statistical analysis revealed that the appearance of a face shape in the sequence triggered highly significant fMRI activation in the predefined face-selective regions FFA and OFA of all six individual brains tested, both for upright and inverted faces. Three main effects were observed (Supplemental Material). First, the BOLD signal rose above baseline in the OFA before any differentiation between upright and scrambled faces, but not in the FFA for the majority of subjects. Second and in contrast with this result, the difference between upright and scrambled faces emerged later in the OFA than the FFA, showing face detection based on structural information first in the higher-order region of the cortical face network. Third and in line with behavioral responses, the face versus scrambled face difference appeared earlier in the sequence for upright than inverted stimuli, both in the FFA and OFA, thus revealing a face inversion effect in the time-domain for the first time in fMRI. Overall, these results support the non-hierarchical view of human cortical face processing and open new perspectives for time-resolved analysis at the single subject level of fMRI data obtained during continuously evolving visual stimulation.
Meeting abstract presented at VSS 2017