Electromagnetic responses to invisible face stimuli during binocular suppression.

When two incompatible monocular images compete for perceptual dominance, one can dominate awareness while the other is suppressed and invisible. It has been suggested that binocular suppression of this kind involves competitive neuronal interactions at multiple levels of the visual processing hierarchy. Here we asked whether in such situations, complex stimulus information is still preserved at advanced visual processing stages despite competitive interactions at early levels of the visual system. We tested whether face stimuli made invisible through binocular suppression would evoke category-specific responses in the human brain, using magnetoencephalography. During scanning participants were presented with face or house stimuli that were either visible or rendered invisible using continuous flash suppression (CFS). Online behavioural assessment with a two-alternative forced choice task confirmed that participants were neither able to detect target stimuli on CFS trials nor to correctly classify the stimuli as faces or houses. Even though the amplitude of the M170, a well-established face-specific signal, was strongly reduced by CFS, there was still a significantly greater signal in response to invisible face compared to invisible house stimuli at the latency and the location of the M170 to visible stimuli. Thus, visual object stimuli undergo category-specific processing in the ventral visual pathway even when profoundly suppressed from awareness by binocular competition.