A NEUROPHYSIOLOGICAL APPROACH TO THE MECHANISMS OF CONSCIOUS AND UNCONSCIOUS PROCESSING OF BRIEFLY PRESENTED WORDS
 
Matsumoto, A., Iidaka, T., Nomura, M., Ohira, H., Nagoya University and Kashikura, K., Fukui Medical University Biological Imaging Research Center, Japan
 
Although many psychological studies have demonstrated that a word that is presented subliminally can be processed unconsciously, little known about the neurophysiological mechanisms underlying the processing. We measured event-related potential (ERP) to examine the effect of subliminal presentation of a word on a subsequent word-stem completion task. Furthermore, we tried to specify the components of ERP related to conscious and unconscious processing of words using the process dissociation procedure (Jacoby, 1991). A word-stem was presented following the masked prime word, and then subjects performed the word-stem completion task. In inclusion and exclusion conditions, the likelihood that a word-stem was completed by the prime word was significantly higher than the baseline likelihood that the word-stem was incidentally completed by the prime word. This result indicates that subliminally presented words were perceived unconsciously. ERP waveforms were compared between "old" responses in which the word-stems were completed by the prime words and "control" responses in which the word-stems did not correspond with the prime words. In the exclusion condition, Significantly larger P220 amplitude was found in old responses than in control responses at left precentral region (C3). In the inclusion condition, the amplitude of the negative component in control responses at 400-500ms time window was significantly larger in the control responses than in the old responses on bilateral parietal regions. We speculated that the difference of amplitude in P220 reflects unconscious activation of the subliminally presented word, and N400 reflects conscious processing.