Vowel Identification by Monolingual and Bilingual Listeners: Use of Spectral Change and Duration Cues

Recent studies have shown that even highly-proficient Spanish-English bilinguals, who acquired their second language (L2) in childhood and have little or no foreign accent in English, may require more acoustic information than monolinguals in order to identify English vowels and may have more difficulty than monolinguals in understanding speech in noise or reverberation (Mayo, Florentine, & Buus, 1997). One explanation that may account for this difference is that bilingual listeners use acoustic cues for vowel identification differently from monolinguals (Flege, 1995). In this study, we investigated this hypothesis by comparing bilingual listeners’ use of acoustic cues to vowel identification to that of monolinguals for six American English vowels presented under listening conditions created to manipulate the acoustic cues of vowel formant dynamics and duration. Three listener groups were tested: monolinguals, highly proficient bilinguals, and less proficient bilinguals. Stimulus creation included recording of six target vowels (/i, I, eI , E, Q, A/) in /bVd/ context, spoken in a carrier phrase by four American monolinguals (two females, two males). Six listening conditions were created: 1) whole word, 2) isolated vowel, 3) resynthesized with no change, 4) resynthesized with neutralized duration, 5) resynthesized with flattened formants, and 6) resynthesized with flattened formats and neutralized duration. The resynthesized stimuli were created using high-fidelity synthesis procedures (Straight; Kawahara, Masuda-Katsuse, & Cheveigné, 1998) and digital manipulation. A six-alternative forced choice listening task was used. The main experiment was composed of 240 isolated vowel trials and 48 whole word trials. Data from 17 monolinguals, 25 highly proficient bilinguals, and 18 less proficient bilingual listeners indicate a consistent but relatively small decrease in performance for the proficient bilinguals compared to the monolinguals, a substantially greater decrease in performance for the less proficient bilinguals compared to the proficient bilinguals, and a greater decrease in performance due to formant flattening than to duration neutralization for all groups. In support of the hypothesis of differing cue use by bilinguals, the data showed significantly different patterns of performance across vowels and listening conditions for the three listener groups