The time-frequency representation of the ERPs of face processing

In ERP studies of face processing, N170 and VPP were considered to reflect the structural encoding of faces in many researches. However, whether the VPP is the positive counterpart of the N170 from the same brain source is still a matter of debate. In present study, in order to address the debate we studied the oscillatory activities of event-related potentials (ERPs) during face processing. We recorded the ERPs in response to upright and inverted faces, and analyzed the time-frequency representation (TFR) by using the Smoothed Pseudo Wigner-Ville Distribution (SPWVD). The TFR of ERPs suggests that the oscillations at occipito-temporal sites (T5/T6) where the N170 was observed, were centralized in a higher frequency band (4-10 Hz) while the oscillations at the fronto-central site (Cz) with salient VPP, were more centralized in a lower frequency range(0-5 Hz). Therefore, we concluded that the face-sensitive N170 and VPP, associated with 'the structural encoding' stage of the face processing in Bruce & Young's model, reflect the activities from two different sources. Furthermore, different oscillatory activities were obtained at different face processing stages: 0-10 Hz oscillations around 100-200 ms post-stimulus are involved in face encoding processing, 0-4 Hz oscillations around 300-400 ms post-stimulus are involved in the later face identification and the enhanced activities in response to the inverted faces indicate a higher number of synchronous neuronal populations.