[Cerebral lateralization in spoken language processing measured by multi-channel near-infrared spectroscopy (NIRS)]

Language processing is lateralized to the left hemisphere in 95% of people as assessed by invasive techniques like Wada test(intracarotid amobarbital test) and cortical electric stimulation. Here we report a new non-invasive method of assessing language dominance by measuring speech-evoked local cerebral responses in hemoglobin(Hb) concentration with near infrared spectroscopy(NIRS). The activation of the bilateral auditory areas to phoneme and pitch contrasts was investigated in twenty right-handed(RH) and ten non-right-handed (NRH) normal-hearing native speakers of Japanese with a 24-channel NIRS system using 830 and 780 nm lasers. Three analysis-synthesized, inflected forms of a Japanese verb/iku/; (a)/itta/, (b)/itte/, and (c)/itta?/, were used as stimuli, delivered diotically at a rate of one word per second on average. In phoneme and pitch contrast conditions, a baseline block containing (a) and a test block including (a) and (b) (phoneme-contrast) or (a) and (c) (pitch contrast) were alternated and repeated several times. Laterality indices, LI = (L - R)/(L + R), calculated from the peaks of the selectively averaged Hb responses in the left(L) and right(R) auditory areas, were compared between the two contrast conditions. A significant leftward shift of LI in the phoneme condition relative to the pitch condition was observed in 85% of the RH subjects, with a significant group difference in the same direction. In contrast, 50% of the NRH subjects showed an inverse shift of LI between the two conditions, but no significant overall shift of LI was observed in this group. These results suggest that multi-channel NIRS could help non-invasively determine the dominant temporal cortex in speech perception, independent of speech production.