A gradual increase of face-selectivity along the human ventral visual pathway: evidence from intracerebral recordings with fast periodic visual stimulation

Abstract

Face perception involves a large set of regions distributed along the ventral temporal cortex and thus represents an interesting model to help understanding how visual information is processed along the ventral visual pathway. Here we shed light on the functional organization of this cortical network by recording focal intracerebral electroencephalogram in 20 epileptic patients implanted with depth electrodes. We objectively defined and quantified face-selective responses in the frequency domain by means of fast periodic visual stimulation with natural images (Rossion et al., in press, JOV). Patients were presented with various object categories (houses, animals, plants, etc.) at a base frequency of 6 Hz (6 images/second) with variable face stimuli interleaved in this sequence at regular intervals of 5 stimuli (face frequency = 6 Hz/5 = 1.2 Hz) (Figure 1A). They were instructed to detect colour changes of the fixation cross. Robust face-selective responses occurring exactly at the 1.2 Hz oddball frequency and its harmonics (2.4, 3.6, etc.) were found in the lateral occipital cortex and in the fusiform gyrus (FG), but also in the antero-inferior temporal cortex (AIT: anterior collateral and occipito-temporal sulci) and in the temporal pole (TP). Interestingly, there was a posterior-anterior gradient of face-specificity: face/base frequency ratio increased along the ventral temporal regions (see Figure 1B for typical responses in 3 patients). Although the magnitude of the face-selective responses were the largest in regions that also responded to all visual stimuli (FG), pure response to faces (oddball responses in the absence of any 6 Hz general visual responses) were observed only in the AIT and in the TP (e.g., patients 2 and 3, Figure 1B). These findings point to a sharpening of face-selectivity along the ventral visual pathway and reveal that exclusive response to faces can be found at a macroscopic (cell populations) level of cortical organization. Meeting abstract presented at VSS 2015.