English Vowel Discrimination and Perceptual Assimilation by Japanese Listeners

This study examined whether the discrimination accuracy of nonnative vowels could be predicted by how listeners assimilate nonnative phones into their L1. The results demonstrated that Japanese listeners discriminated between English /æ/ and /ʌ/ better than they did between /ɑ/ and /ʌ/, although they categorized all those stimuli as the Japanese /a/. Given that the acoustic distance between stimuli was controlled to be identical, this result was attributed not to the acoustic difference but to the category-goodness difference. The goodness-of-fit to the Japanese /a/ phoneme differed between the English /æ/ and /ʌ/ but not between the English /ɑ/ and /ʌ/, suggesting that it is more difficult to discriminate between vowels when the category-goodness difference between two nonnative stimuli is smaller. In addition, this study examined the relationship between perceptual assimilation and the focalization effect. Focalization affects directional asymmetry in a manner that renders detecting a sound change from a more-focal to a less-focal vowel more difficult than detecting a change in the opposite direction. The results demonstrated that this directional asymmetry is only observed when listeners assimilate two nonnative phones into a single L1 phonemic category, with no category-goodness difference between the two nonnative phones.