How the Percept of Face Familiarity Unfolds Over Time

A single glance at a face can evoke a sense of familiarity, yet how this percept unfolds over time remains unclear. We adopted a frequency-tagging EEG paradigm to investigate neural responses to face familiarity across different SOAs. Streams of unfamiliar faces were presented at base frequencies of 8, 12, 16, and 20 Hz (SOAs: 125, 83, 62.5, and 50 msec), with personally familiar faces inserted periodically at 2 Hz. Significant oddball responses were observed at all four SOAs over occipito-temporal regions, indicating reliable discrimination between familiar and unfamiliar faces even at the shortest SOA of 50 msec. Time domain analysis revealed a significant N170 component in all four conditions, with amplitudes increasing progressively as SOA lengthened. In contrast, a reliable P300 component emerged only at SOAs of 83 msec and longer. These findings suggest two temporally distinct stages of familiar face processing: an early stage indexed by the N170 that scales with available processing time and likely reflects familiarity-sensitive perceptual processing and a later stage indexed by P300 that requires a longer processing window and likely reflects higher-order evaluative processing.