Cortical representation of phonemic and phonetic contrasts in Japanese vowel

Phonemic changes in spoken words alter linguistic meanings, whereas allophonic phonetic variations do not. The Japanese vowel /u/ is produced as devoiced [u˚] when surrounded by voiceless consonants, otherwise as voiced [u]. Due to this allophonic variation, Japanese single vowel /u/ has a phonetic contrast between [u˚] and [u] without any phonemic change. The present study investigated how Japanese speakers process phonemic and phonetic contrasts using voiced and devoiced vowel [u] and [u˚]. Two oddball experiments were carried out. Under phonemic condition, a frequent stimulus [t∫i˚ta] was contrasted with a deviant [tsu˚ta], and a frequent [tsu˚ta] with a deviant [t∫i˚ta]. Under phonetic condition, a frequent [tsu˚ta] was contrasted with a deviant [tsuta], and a frequent [tsuta] with a deviant [tsu˚ta]. The subjects were 14 native Japanese. The equivalent current dipole moment (ECDM) was estimated from the mismatch magnetic field (MMF). The ECDM in the left hemisphere was significantly larger than that in the right hemisphere under phonemic condition (p < 0.01), whereas the ECDM was not significantly different between the hemispheres under phonetic condition (p = 0.4870). Moreover, under phonetic condition, the ECDM elicited by the voiced deviant was significantly larger than that elicited by the devoiced deviant in both hemispheres (p < 0.01), while there were no significant deviant-related differences in ECDM under phonemic condition in both hemispheres. Although significant MMF was elicited for phonetic contrasts between the voiced and the devoiced vowels, these mismatch responses were significantly different from those observed under phonemic contrasts. The phonemic contrasts elicited significantly larger responses in left than right hemisphere, whereas the phonetic contrasts did not.