Abstract
Recognizing a familiar person from his/her face is a fundamental brain function. Surprisingly, to date the actual speed of categorizing a face as familiar remains largely unknown. Here we seek to clarify this question by using a Go/No-go familiarity judgment task with photographs of personally familiar (same classroom as the participant) and well-matched pictures of unfamiliar faces, which required speeded responses to individually presented face stimuli. During the recording of high-density event-related potentials (ERP, 128 channels), two groups of young adult participants were instructed either to respond when a photograph of a personally familiar face was presented (n = 11, 6 females), or when the face was unfamiliar (n = 12, 7 female). Face stimuli contained external features (hair), but external indicators of identity were carefully removed (clothes, ….). Each face stimulus appeared for 100ms, followed by a blank screen (1500-1700ms). Behaviorally, faces could be classified as familiar as early as 310-320ms (average RT, 450 ms), about 80ms faster than when unfamiliar face categorization was required. ERP differential waveforms between Go and No-go responses when detecting familiarity showed the earliest difference at occipito-temporal cortex shortly after 200 ms, starting in the right hemisphere, and 10 ms later in the left hemisphere. Differences appeared about 50 ms later for the Go-unfamiliar decision task, with no differences in lateralization of onset times. There were no clear effects of face familiarity on earlier visual event-related potentials (P1, N170). These earliest effects observed in electrophysiological recordings are compatible with the behavioral output taking place at about 100 ms later. They indicate that the human brain needs no more than 200ms following stimulus onset to recognize a familiar person based on his/her face only, a time frame that puts strong constraints on the time-course of face processing operations in the human brain.