Previous neural research on face perception has mainly focused on the distinction between faces and non-face stimuli. However, the brain mechanisms for differentiating one face from another are not well understood. In the present study, using scalp-recorded event-related potentials (ERPs), we investigated the brain responses to faces that varied in identity strength as a result of morphing individual faces to an "average" face in steps of 10%. Participants performed a face identification task. Behavioral results showed categorical boundaries of face identification at 30% and 70%. Face identity strength related to initial brain responses occurring shortly after 200 ms in the ventral P2 and the N250 components: stronger identity strength was associated with a smaller P2 and a larger N250. In contrast, the brain responses within 200 ms, as reflected by the P1, the N170, and the dorsal P2 component, were not affected by face identity strength. Consistent with recent imaging studies and animal research, our results provide the ERP evidence for brain responses to variations in face identity strength relative to an "average" face. Furthermore, with the high temporal resolution of ERPs, our results help to clarify the timing of neural events that are associated with the different stages involved in recognizing individual faces, thus providing a timeline for the classical face recognition model in the brain.
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