The Japanese journal of neuropsychology

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ArticleTitle	Speech acquisition in infants and second-language learners
Language	J
AuthorList	Koichi Mori, Izumi Furuya, Yutaka Sato, Yasuyo Minagawa-Kawai, Ryoko Hayashi
Affiliation	Department of Rehabilitation for Sensory Functions, Research Institute, National Rehabilitation Center for Persons with Disabilities
Publication	Japanese Journal of Neuropsychology: 21 (1), 26-34, 2005
Received
Accepted
Abstract	Does the ability to discriminate speech sounds predict the ability to perceive the same as speech? We addressed this issue by assessing the lateralization of regional cerebral activation evoked by speech sound contrasts, with functional multi-channel, near-infrared spectroscopy (fNIRS). FNIRS can be applied as easily and as non-invasively as EEG to subjects of all ages, yet with superior sensitivity to detect regional cerebral responses. Sound stimuli comprising analysis-synthesized words with phonemic or intonational contrasts were played back at the rate of one word per 1 or 1.25 sec in a pseudo-randomized order under a block design paradigm. Bilateral responses in the temporal cortex were measured, and tested for left/right (L/R) dominance with their laterality indices (LI)=(L-R)/(L+R). While 85% of normal right-handed adults showed significant leftward dominance in the LI for the phonemic contrast to that for the intonational contrast, no significant dominance was found in the group of right-handed adults who stutter. There was an inverse correlation between severity of stuttering and the LI for the phonemic contrast, suggesting that the abnormal lateralization of the speech responses in the brain is closely related to the pathophysiology of stuttering. In normally developping infants, a significant difference begins to appear between the LI's to the phonemic and intonational contrasts only around 12 months of age, which suggests that the speech-specific cortical responses become predominant in the responses to the stimuli at this age. In an infant with Down's syndrome, the development of the lateralization was delayed by several months. With fNIRS, the linguistic developmental stage could be assessed before any behavioral speech activity or responses could be observed. Although the long and short phonemic contrasts based solely on durational differences are commonly seen in Japanese, they are rare among the world languages, which poses a problem to second-language learners of Japanese. Korean subjects who had come to Japan as adults and had learned to speak Japanese fluently, showed that they could discriminate long/short Japanese vowels categorically in an identification test just like native Japanese speakers. However, their brain responses measured with fNIRS to the durational contrast across the "phonemic" boundary did not show lateralization to the left, unlike Japanese natives, which suggests that their categorical perception may be acoustic or at least language-general rather than phonemic or language-specific, and may not be available in time or in the same stream for linguistic comprehension of running speech.
Keywords	infants, second language acquisition, speech development, functional near infrared spectroscopy; fNIRS, functional cerebral lateralization