Academic journal article Journal of Medical Speech - Language Pathology

Stress Identification in Speakers with Dysarthria Due to Cerebral Palsy: An Initial Report

Academic journal article Journal of Medical Speech - Language Pathology

Stress Identification in Speakers with Dysarthria Due to Cerebral Palsy: An Initial Report

Article excerpt

Contrastive stress drills are often used in intervention for dysarthria. These skills are usually not addressed, however, until the speaker has achieved a base level of segmental clarity. In addition to reducing monotony and improving naturalness, contrastive stress has been observed to increase articulatory displacement, which may impact segmental clarity. Thus, despite poor speech sound intelligibility, speakers with severe dysarthria may benefit from exploiting contrastive prosody to convey their intentions. This article reports on everyday listeners' ability to identify stress placement in the utterances of three speakers with severe dysarthria due to cerebral palsy. Each speaker produced five, four-word phrases with emphasis on one of the four words or neutrally. Twelve listeners, unfamiliar with dysarthric speech, were asked to identify the phrase produced and the locus of stress. While phrase identification ranged from 94-100%, stress identification ranged from 78-97% across speakers. Acoustic analyses of perceptual errors revealed that listeners relied on fundamental frequency cues more than intensity and duration for stress identification. Implications of these findings for improving speech intelligibility and naturalness through stress patterning drills are discussed.

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Contrastive stress drills are often used in the intervention of prosodic disturbances accompanying dysarthria. Placing contrastive stress on a given word within a phrase provides listeners with semantic and syntactic cues about the speaker's intent and thus may be particularly useful when the speech signal is degraded in terms of intelligibility. In practice, these skills are usually not addressed until the speaker has achieved a base level of segmental clarity. There is evidence, however, that in addition to reducing monotony and improving naturalness (Hartelius, Wising, & Nord, 1997; Kearns & Simmons, 1988), contrastive stress tasks increase articulatory displacement (Engstrand, 1988; Kent & Netsell, 1971), which may in turn impact segmental clarity. Thus, speakers with severe dysarthria may benefit from exploiting contrastive prosody to convey their intentions and may help in resolving communication breakdowns.

Healthy speakers convey contrastive emphasis on a specific word within an utterance by increasing its vocal fundamental frequency (F0), intensity, and syllable duration (Bolinger, 1961; Fry, 1955; Lehiste, 1970; Morton & Jassem, 1965). While F0 is thought to be the prominent prosodic signal for conveying emphatic stress, changes in intensity and duration are also acknowledged as salient cues (Morton & Jassem, 1965). The acoustic correlates of perceived stress are further complicated by the fact that these cues also serve many other functions, including marking phrase boundaries (Campbell & Isard, 1991; Klatt, 1976; Scott, 1982) and signaling questions (Eady & Cooper, 1986). As a result, the location of stress within the utterance may influence the extent to which each cue is used to signal stress (Cheung, Holden, & Minifie, 1973). Furthermore, duration effects may not be restricted solely to the stressed syllable (Folkins, Miller, & Minifie, 1975; Kimelman, 1991; Weismer & Ingrisano, 1979). For example, final syllables may be longer in duration even when unstressed (Campbell & Isard, 1991; Klatt, 1976; Scott, 1982). While the proportional use of these cues has been shown to vary from speaker to speaker (Peppe, Maxim, & Wells, 2000), listeners are able to identify stress even when the cue combination used differs from their own (Howell, 1993).

In contrast to our understanding of contrastive stress in healthy speech, little is known about how speakers with dysarthria manipulate their vocal apparatus to achieve stress and whether everyday listeners can perceive their contrasts. Previous studies have shown that the increased physiological effort required for signaling contrastive stress may lead to inaccurate, inconsistent, exaggerated, and bizarre stress patterning in dysarthria (Liss & Weismer, 1992, 1994; Netsell, 1973; Yorkston, Beukelman, Minifie, & Sapir, 1984). …

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