Relative sensitivity to low- vs. high-level visual properties in face-sensitive regions of the human ventral occipito-temporal cortex: evidence from intra-cerebral recordings

Abstract published in Journal of Vision, 15(12):751, 2015International audienceNeuroimaging studies have identified a network of regions in the human ventral occipito-temporal cortex (VOTC) sensitive to faces. Using intra-cerebral electrophysiological recordings, we compared the relative responsivity of the different regions to face-structure using a periodic visual stimulation paradigm. Recordings were made in 17 epileptic patients implanted with linear-array depth-electrodes (>1000 contacts in the VOTC). In Experiment 1, we presented sequences of phase-scrambled and intact face stimuli (equalised for low-level image properties) alternating at a fixed frequency of 3Hz (6 images/s) while patients completed an orthogonal task. Significant periodic EEG responses (SSVEPs) reflecting high-level face structure processing were found at 3 Hz, while responses reflecting low-level visual processing (e.g. local contrast change) were recorded at 6 Hz. An index of high-level processing (3Hz/(3Hz+6Hz)) showed a posterior-to-anterior gradient, reflecting increased sensitivity to face structure going from the occipital to the temporo-polar cortex, with a right hemisphere lateralisation. In Experiment 2, we further investigated face structure processing with a sweep VEP paradigm (Ales et al., 2012; 4/17 patients). Stimulation was similar to Experiment 1 except the visibility (phase coherence) of the face image was parametrically varied so that a face gradually emerged over the course of a sequence. The patients performed a face detection task during the trials. Responses at 3Hz, objective markers of face detection, were found in all patients on contacts in the lateral fusiform gyrus and inferior occipital gyrus that overlapped with those defined in Experiment 1. All regions showed non-linear voltage vs coherence functions at 3Hz, with thresholds lying around ≈40% coherence. Sites in the lateral fusiform gyrus had a distinct step-like response profile suggestive of categorical face detection. These findings demonstrate that face-sensitive regions are organised with a posterior-to-anterior gradient of increasingly selective high-level visual processing. These regions appear to detect face structure in a non-linear/categorical manner. Meeting abstract presented at VSS 2015