How Faces are Special: an ERP Study for Human Subliminal Face Processing

We demonstrate the perceptual specialization to faces that occurs at early visual stages, before conscious face recognition. Faces (neutral and fearful) or objects were briefly presented, followed by a 1000-ms mask stimulus. Event-related potentials (ERPs) in a subliminal presentation task were recorded at Oz, Cz, Pz, T5 and T6 (international 10-20 system). To determine the effect of subliminal face stimulation on ERPs, we firstly used subthreshold, threshold, and suprathreshold presentations with exposure durations of approximately 20, 30 and 300 ms respectively. Secondly, we adopted upright and inverted face presentations to examine whether ERPs elicited by subliminally presented faces resulted from face-specific brain responses rather than from the similarity of the physical features. The occipital P100 and N150 amplitudes for faces were significantly different from those for non-face stimuli in the subthreshold condition. Moreover, the occipital N150 amplitudes for the faces were significantly smaller than those for objects in the subthreshold condition. Conversely, there was no significant effect of subliminal stimulation with faces on the temporal N170 amplitude. However, the temporal N170 amplitudes for the faces, but not for the objects, increased in the threshold and suprathreshold conditions. Finally, the occipital P100 amplitude for inverted faces was significantly smaller than that for upright faces in the subthreshold condition. Furthermore, the face-specific response of the occipital N150 disappeared when inverted faces were presented. Our results suggest that output from the unconscious face processing route is integrated with conscious face processing activity, before the temporal face specific brain area perceives the necessary information.