Functional lateralization in stutterers during spoken word processing, measured by near-infrared spectroscopy

Near-infrared spectroscopic measurement of cerebral hemodynamics was performed to investigate cerebral dominance during auditory language processing in adults who stutter. Analysis-synthesized Japanese words including phoneme (/itta/ and /itte/) and intonation (/itta/ and /itta?/) contrasts were used for stimuli. The baseline block contained only /itta/, whereas the contrast block consisted of either the phoneme or intonation pair, with the contrast words presented in random order at equal probabilities. To analyze cerebral lateralization, we calculated a laterality index (LI) from the peaks of the left and right total Hb responses for each contrast block, compared to the preceding baseline block in the auditory area. The results showed that there were no significant differences in LI between the phoneme and intonation contrast responses in those who stutter. Within-subject analysis showed no subject with a significant leftward shift of LI in the phoneme condition relative to the intonation condition, although it has been previously shown that 85% of nonstutterers show significant left dominance for phoneme processing. These results confirm that stuttering is correlated to abnormal cerebral dominance for processing speech.