An electrophysiological study of the initial step of face perception

We studied the initial step of face perception examining the visual evoked potentials (VEPs) elicited by mosaic pictures. Ten healthy right-handed adult volunteers served as subjects. The threshold level of perception was determined psychophysically, and three different mosaic levels (subthreshold (SUB), threshold (THR) and suprathreshold (SUP)) for a familiar face (FF), an unfamiliar face (UF) and an object (OB) were used as the visual stimuli, which were presented for 250 ms in a random order. VEPs were recorded from 15 electrodes according to the international 10-20 system, referring to an electrode at nose tip. An epoch of 700 ms including a 200-ms prestimulus baseline was collected. The peak amplitudes and latencies of major components (N100 at O1 and O2 and N170 at T5 and T6) were analyzed. The peak latencies of N100 and N170 were shortened by decreasing the mosaic levels for FF and UF, but there were no significant changes for OB. The latency difference between N100 and N170 for FF and UF was shortened by decreasing mosaic levels especially that of FF was significantly shorter at THR. The N100 amplitude was unchanged for all pictures, while the N170 amplitude was increased at THR and SUP for FF and UF. Therefore, our findings suggest that the first step of face perception is already set in the primary visual cortex and familiarity can facilitate the cortico-cortical processing of facial information.