English Vowel Discrimination and Perceptual Assimilation by Japanese Listeners

Area of Science:

• Phonetics
• Second Language Acquisition
• Psychoacoustics

Background:

• Listener's native language (L1) influences perception of nonnative speech sounds.
• Perceptual Assimilation Model (PAM) explains how nonnative phonemes are mapped to L1 categories.

Purpose of the Study:

• To investigate if vowel discrimination accuracy is predicted by perceptual assimilation of nonnative vowels into the listener's L1.
• To examine the relationship between perceptual assimilation and the focalization effect in vowel discrimination.

Main Methods:

• Acoustically controlled English vowels /æ/, /ʌ/, and /ɑ/ were presented to Japanese listeners.
• Listeners performed discrimination tasks and perceptual assimilation tasks.
• The focalization effect was assessed by examining directional asymmetry in sound change detection.

Main Results:

• Japanese listeners showed better discrimination between English /æ/ and /ʌ/ than between /ɑ/ and /ʌ/, despite categorizing all as Japanese /a/.
• Discrimination difficulty correlated with the goodness-of-fit to the Japanese /a/ category; smaller differences led to greater difficulty.
• Directional asymmetry (focalization effect) was observed only when two nonnative vowels assimilated to a single L1 category without significant goodness-of-fit differences.

Conclusions:

• Category goodness, not just acoustic distance, significantly impacts nonnative vowel discrimination.
• The focalization effect is linked to the assimilation of nonnative sounds into a single, undifferentiated L1 category.