Semantic processing in subliminal face stimuli: an EEG and tDCS study.

Whether visual subliminal processing involves semantic processing is still being debated. To examine this, we combined a passive electroencephalogram (EEG) study with an application of transcranial direct current stimulation (tDCS). In the masked-face priming paradigm, we presented a subliminal prime preceding the target stimulus. Participants were asked to determine whether the target face was a famous face, indicated by a button press. The prime and target pair were either the same person's face (congruent) or different person's faces (incongruent), and were always both famous or both non-famous faces. Experiments were performed over 2 days: 1 day for a real tDCS session and another for a sham session as a control condition. In the sham session, a priming effect, reflected in the difference in amplitude of the late positive component (250-500 ms to target onset), was observed only in the famous prime condition. According to a previous study, this effect might indicate a subliminal semantic process [10]. Alternatively, a priming effect toward famous primes disappeared after tDCS stimulation. Our results suggested that a subliminal process might not be limited to processes in the occipital and temporal areas, but may proceed to the semantic level processed in prefrontal cortex.