A proportionally suppressed and prolonged LPP acts as a neurophysiological correlate of face identity aftereffect

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The recognition of facial identity is fundamentally important in daily life and social interactions. Here, we asked how the perception of facial identity is influenced by the set of preceding faces that observers are exposed to. We morphed the faces of Adam and Bob to generate a continuum of face images, and asked subjects to judge the identity of those face images to be Adam or Bob after they were exposed to a face image within the continuum. We found that after adaptation to Bob, the subjects were apt to identify a subsequently presented ambiguous face to be Adam, and vice versa, i.e., the face identity aftereffect (FIAE). We simultaneously recorded EEG during the task and analyzed the event-related potentials in response to the test faces, and found that adapting to a face identity (e.g., Bob) suppressed the amplitude, and prolonged the latency, of the late positive potential (LPP) in response to subsequent faces of the same identity (e.g., Bob). Further analyses of the LPP in response to each test face within the continuum revealed a proportional changing trend, i.e., the strongest LPP suppression happened when the adaptor and the test face were the most similar, while the similarity declined, the LPP gradually recovered from suppression. The modulations of LPP amplitude and latency by the identity similarity between the adaptor and test face strongly indicate that the proportionally suppressed and prolonged LPP activity acts as a neurophysiological correlate of FIAE.