Face processing stages: impact of difficulty and the separation of effects.

Cognitive models of face perception suggest parallel levels of processing yet there is little evidence of these levels in studies of brain function. Series of faces that engage different processes ((photographs, schematic and Mooney faces (incomplete two-tone faces)) were presented upright, inverted and scrambled; subjects performed a face/non-face discrimination while event-related potentials (ERPs) were recorded. Different patterns in N170 latency and amplitude provided evidence of multiple steps in face processing, which can be seen at the ERP level. We showed that first-order configural and holistic processing were evident at the N170. N170 latency indexed task difficulty for the upright faces, yet the face inversion effect was independent of difficulty. N170 amplitude inversion effect was unique to photographic faces. Separable ERP effects were found for the processing engaged by the three face types, although the P1 and N170 sources did not differ. Thus, it appears that common brain sources underlie the early processing stages for faces (reflected in the P1 and N170), whereas the P2 showed activation of primary visual areas for the non-photographic faces and reactivation of the same regions as the N170 for the photographic faces.