Patterns of Acoustic Errors in the Imitation of Speech

Abstract

Children with disordered vowel phonologies exhibit systematic substitution patterns, but the reason for these substitutions is a matter of debate. We describe two experiments that potentially shed light on this issue. In the two experiments, adult male speakers with normal phonologies attempted to imitate vowel-like target sounds they had produced earlier. In the first experiment, "serial imitation," speakers were presented with a self-produced target and their imitations of the target were played back as the target for the next imitation. This continued for ten steps, resulting in a chain of imitations. In the second experiment, "multiple imitation," speakers imitated each self-produced target several times. Formant analysis of the targets and the imitations showed that (a) the speakers were unable to imitate themselves accurately, (b) the imitations deviated from the targets in systematic ways, and (c) the deviations did not appear to be structured by the linguistic background of the speaker. The patterns of deviation were reminiscent of the substitution patterns exhibited in vowel disorders. Therefore, we propose a hypothesis for the cause of the deviations and discuss its implications for the etiology and treatment of vowel disorders.

Introduction

In speech research, consonants have historically been seen as more indicative of speech-specific processing than vowels, and therefore more theoretically interesting. Likewise, in clinical speech pathology, more attention has been paid to consonantal disorders than vowel disorders. There is a pragmatic aspect to this: the diagnosis of vowel phonological disorders is more difficult because of the confound between normal developmental change (e.g. producing a diphthong as a monophthong) and dialect variation.

However, in recent years there has been an increased interest in describing, diagnosing and treating vowel disorders. Pollock (2002) gives a good overview of the issues, and also a preliminary estimate of the incidence of vowel disorders. She reports that at 36 months of age, 4% of children with normal consonant phonologies had mild vowel problems, although none had severe problems. Of the children with disordered consonant phonologies, 35% had mild vowel problems and 9% had severe problems (Pollock, 2002; Table 3-4).

Despite the difficulty of diagnosis, researchers have been able to identify some systematic patterns in disordered vowel production, such as lowering (/e/ ? /a/), fronting (/?/ ? /a/), diphthong reduction (/e?/ ? /e/), and a "preference for peripheral vowel quality" (Reynolds, 2002). It is not clear why these patterns are preferred over others. One approach is to look for similar patterns in diachronic and synchronic change. For example, lowering/raising, coloring (fronting and/or rounding), bleaching (lowering and/or unrounding), and tensing/laxing are all diachronic processes (Donegan, 2002). Likewise, certain kinds of vowel errors, such as the preference for /i,a,u/, may be described by formalisms for synchronic change (Ball, 2002).

Another approach to understanding why certain patterns are preferred is to think about the perceptual representations of vowels: Are they coded discretely or continuously? Is perception more variable (or unstable) in certain parts of the vowel space? What kinds of perception and production errors can result from such instability? In this paper, we discuss two experiments that address these issues and concerns.