Non-phase-locked alpha oscillations are involved in spontaneous racial categorization of faces.

Racial categorization of faces has a notable impact on human behavior, but its neural underpinnings remain unresolved. Previous electroencephalography (EEG) research focused on contributions of phase-locked neural activities to racial categorization of faces. We investigated functional roles of non-phase-locked neural oscillations in spontaneous racial categorization of faces by recording EEG from Chinese adults who performed an individuation task on Asian/White faces in Experiment 1 and on Asian/Black faces in Experiment 2. We quantified neural processes involved in spontaneous racial categorization of faces by examining repetition suppression of non-phase-locked neural oscillations when participants viewed faces of one race presented repeatedly in the same block of trials (repetition condition), or faces of two races presented alternately in the same block of trials (alternating condition). We found decreased power of alpha (9-13 Hz) oscillations in the repetition than alternating conditions at 80-240 ms over frontal-central electrodes induced by White/Black (but not Asian) faces. Moreover, larger repetition suppression of alpha oscillations in response to White/Black (vs. Asian) faces predicted greater implicit negative attitudes toward White/Black faces across individuals. Our findings suggest that non-phase-locked alpha oscillations are engaged in spontaneous racial categorization of faces and are associated with implicit negative attitudes toward other-race faces.