MAGNETOENCEPHALOGRAPHIC STUDY ON THE CEREBRAL NEURAL ACTIVITIES RELATED TO THE PROCESSING OF VISUALLY PRESENTED CHARACTERS

Neuromagnetic fields were recorded from normal subjects to study the t ime course of cerebral neural activation while they performed a matchi ng task of visual stimuli in which sequentially presented Japanese cha racters or unreadable pseudo-characters were compared according to pho nological (reading of the characters) or graphical (geometry of the ps eudo-characters) identity. In response to the single real-character or pseudo-character which was presented the latest distinct magnetic fie ld components were observed, from which current dipole sources of the fields were localized in the individual magnetic resonance images of t he brain. In the phonological identification, the sources were found i n the parieto-occipital extrastriate cortex at 155-210 ms following th e character presentation, and in the posterior temporal region (part o f the Wernicke's area) and the posterior superior temporal region of t he visual/auditory association cortex at 210-410 ms. The activity in t hese temporal regions was left hemisphere dominant, and may be the neu ral basis of phonological processing of the visual characters. In the graphical identification, sources occurring at 125-250 ms were noted i n the inferior temporo-occipital region, and those at 180-460 ms in th e posterior temporal and posterior superior temporal regions of the ri ght hemisphere. These results indicate that the activities in the temp oral area are lateralized to the left for the phonological processing and to the right for the graphical processing.