How the brain differentiates human and monkey faces: Neuromagnetic evidence from spatiotemporal dynamics.

Face recognition is an important aspect of human non-verbal communication. Event-related potentials or magnetic fields, such as the N170/M170 component, are considered essential neural markers of face processing. Compared to upright human faces, inverted human faces and upright but not inverted animal faces cause longer latencies and larger amplitudes of these components. However, the mechanisms underlying this factor remain unclear. To elucidate the spatiotemporal dynamics of the processing of inverted human and animal faces, we recorded face-selective responses (M170) to upright and inverted human and monkey faces using a 306-channel whole-head magnetoencephalography. Sensor analysis showed an increased M170 latency and amplitude for inverted human and upright animal faces. However, in the source analysis, the observed modulations of the estimated spatiotemporal dynamics were different from the sensor results: irrespective of species, upright faces activated wider areas in the ventral and dorsal visual regions compared with inverted faces. Additionally, face orientation differentially modulated the anterior region of the fusiform gyrus (FG) in both face categories. These results suggest that spatiotemporal dynamics differ in face orientation regardless of category and that the FG contributes little or nothing to the M170 modulation recorded in the scalp sensor. Furthermore, we demonstrated that inverted human and animal faces are processed via different mechanisms.