Perception of a Japanese vowel length contrast by Japanese and American English listeners: behavioral and electrophysiological measures.

This study examined the role of automatic selective perceptual processes in native and non-native listeners' perception of a Japanese vowel length contrast (tado vs. taado), using multiple, natural-speech tokens of each category as stimuli in a "categorial oddball" design. Mismatch negativity (MMN) was used to index discrimination of the temporally-cued vowel contrast by naïve adult American listeners and by a native Japanese-speaking control group in two experiments in which attention to the auditory input was manipulated: in Exp 1 (Visual-Attend), listeners silently counted deviants in a simultaneously-presented visual categorial oddball shape discrimination task; in Exp 2 (Auditory-Attend), listeners attended to the auditory input and implicitly counted target deviants. MMN results showed effects of language experience and attentional focus: MMN amplitudes were smaller for American compared to Japanese listeners in the Visual-Attend Condition and for the American listeners in the Visual compared to Auditory-Attend Condition. Subtle differences in topography were also seen, specifically in that the Japanese group showed more robust responses than the American listeners at left hemisphere scalp sites that probably index activity from the superior temporal gyrus. Follow-up behavioral discrimination tests showed that Americans discriminated the contrast well above chance, but more poorly than did Japanese listeners. This pattern of electrophysiological and behavioral results supports the conclusion that early experience with phonetic contrasts of a language results in changes in neural representations in the auditory cortex that allow for more robust automatic, phonetic processing of native-language speech input.