Reversal of the face-inversion effect in N170 under unconscious visual processing.

Many studies using electroencephalography consistently reported a larger N170 (N1) response in the visual cortices to inverted than upright face images (the face inversion effect in N1, FIE-N1). Here we report this robust effect is diminished and even reversed when face stimuli are processed unconsciously. We measured visual-evoked potentials to neutral faces either visible or rendered invisible by an inter-ocular suppression. In visible condition, we observed a larger N1 to inverted than upright faces, which replicated the traditional FIE-N1. When those faces became invisible, however, neural responses to the inverted faces were greatly reduced compared to visible condition, whereas those to the invisible upright faces were relatively preserved. Consequently, N1 amplitudes were found to be larger in upright, rather than inverted, faces in invisible condition, which was opposite to the traditional FIE-N1 (upright<inverted) in visible condition. Those results highlighted a special mechanism in the brain for the processing of the upright, but not inverted, face (e.g. fusiform face area) that retains vigorous responses even when the face becomes invisible.