The Role of Vowels in the Discrimination Abilities of Bilingual Adults and Children

Abstract

The discrimination of English vowel contrasts in real and novel words by 101 bilingual Spanish/English- and native English-speaking adults and children was examined. Bilingual-speaking adults and children had significant difficulty with certain English vowel contrasts, while native English participants had none. The vowels that affected discrimination accuracy were those absent from Spanish and those more similar to one another in the F1-F2 acoustic range.

The main non-linguistic factors that affected bilingual-speaking adult and children's discrimination accuracy were the age of acquisition of English and the overall percentage of time devoted to communication in English. While production tends to be the focus of most practitioners working with second language learners, these results suggest practitioners should consider discrimination and vowel quality when working with second language learners with speech or language disorders.

Introduction

The ability to discriminate sounds of a second language (L2) is essential to the acquisition of a new language. Some sounds in a learner's first language (L1) may be absent from the new language, and sounds of the new language may be absent from the learner's first language. Another factor that may affect discrimination is the variation among native L2 speakers in their production of sounds. Vowels are affected by dialect variation more often than consonants (MacKay, 1997), evidenced in the variability in the production of these sounds by native L2 speakers (Hillenbrand, Getty, Clark, & Wheeler, 1995). Vowels more similar to one another also may present difficulty, such as the vowels found in the words pod and pawed. These vowel contrasts present discrimination difficulty for children (Levey & Schwartz, 2002) and adults (Flege, 1991; Hillenbrand & Gayvert, 1993; Hillenbrand et al., 1995; Levey, 2004; Levey & Cruz, 2004; Lieberman & Blumstein, 1988). In contrast, the more dissimilar vowels /i/ and /u/, found in the words beat and boot, are discriminated with greater accuracy (Bond & Robey, 1983; Levey, 2004; Levey & Schwartz; Polka, Jusczyk, & Rvachew, 1995).

The ability to discriminate vowels derives from their acoustic cues. These cues allow listeners to identify and/or discriminate vowels based on their formant frequencies (Borden, Harris, & Raphael, 2003). A formant is defined as a peak of resonance in the vocal tract. The first (F1) and second (F2) formants of the vowel produced by a speaker, and the relationship between these formants, play an important role in vowel identification. While there is variabiality in the production of vowels, the vowel /i/ is always produced with a low frequency first formant and a high frequency second formant. In contrast, the vowel /u/ is always produced with a lower first and second formant than the vowel /i/, with a smaller frequency gap between these formants. These differences result in a listener's ability to discriminate these vowel contrasts. The difficulty in discriminating certain vowel contrasts, such as /-/ and //, is based on their closer proximity or adjacency to one another in the F1-F2 acoustic range (Bond & Robey, 1983; Hillenbrand & Gayvert, 1993). The greater distance between vowels in this acoustic range, such as /i/ and /u/, allows a listener to discriminate these vowel contrasts with greater accuracy (Bond & Robey, 1983).

Non-linguistic factors may also affect discrimination, such as more experience with, and earlier age of acquisition of, L2. Experience with L2 may provide more opportunities to interact with native L2 speakers, thus leading to more accurate discrimination and production of sounds in L2 (Best & Strange, 1992; Flege, Bohn, & Jang, 1997; Flege, 1993; Flege & Liu, 2001; Kuhl & Iverson, 1995). There is a consensus among investigators that earlier age of acquisition provides an advantage, given early exposure to L2 (Flege, 1992; Singleton, 1995). Other factors may correlate with skills in discrimination or speech sound production, such as the motivation to learn a new language. However, discrimination has been found to be a significant factor in learning a new language L2 (Flege, 1987, 1992, 1995; Flege & Liu, 2001). In spite of this finding, production continues to be the focus of most practitioners who provide services to L2 learners (Rochet, 1995), often ignoring the possibility of difficulty in the perception of the differences of sounds.