Читать книгу The Handbook of Language and Speech Disorders - Группа авторов - Страница 64

Adult speakers without communication disorders are generally assumed to be fully intelligible. However, for children, acquisition of adult‐like intelligible speech is a protracted developmental process, beginning early in the first year of life with vocal play, babbling, and word approximations, and continuing through childhood. Segmental development (acquisition of speech sounds) has been well documented in the literature. Expected age of acquisition for consonants and vowels in single words has been characterized using expert perceptual techniques (McLeod & Crowe, 2018; Smit, Hand, Freilinger, Bernthal, & Bird, 1990b). These data have been very useful to clinicians for assessing children’s speech sound development and identifying children with speech sound disorders. Developmental data indicate that English‐speaking children produce most speech sounds accurately by about 5–6 years, with adult‐like mastery expected at about 8 years (Sander, 1972; Smit, Hand, Freilinger, Bernthal, & Bird, 1990a). However, studies suggest that intelligibility is not readily predictable from phoneme data (Ertmer, 2010; Weismer, 2008; Whitehill, 2002). For example, studies have indicated that measures such as percentage of consonants correct (PCC) have a weak relationship with intelligibility (Ertmer, 2010). Generally, the number of articulation errors is negatively correlated with intelligibility; however, individuals can have significant articulation errors and still be highly intelligible (Whitehill, 2002).

Although it is clear that children acquire intelligible speech gradually, the precise course of development of intelligibility in typical children and the range of expected variability over the full course of development is not well understood. Problems that have plagued the historical literature include methodological differences among studies, such as whether intelligibility was measured objectively or subjectively, whether listeners were “experts” (e.g., speech‐language pathologists or phoneticians or naïve listeners), and the nature of speech material (elicited vs. spontaneous; single words vs. sentences vs. discourse or conversation). These differences among studies have led to conflicting reports on intelligibility development. Across studies, findings are discrepant and difficult to reconcile, for example, intelligibility of 3‐year‐old children has varied for different studies between about 53% and 96% (Chin, Tsai, & Gao, 2003; Flipsen, 2006; Morris, Wilcox, & Schooling, 1995; Weiss, 1982). From the existing literature it is impossible to know whether these values reflect the range of variability among typical children or whether they are a result of methodological differences between studies. Data for children at 4 years of age and older are similarly discrepant. However, one consistent and important finding is that intelligibility increases with age.

Growth curves for intelligibility development based on a large sample of typical children producing elicited utterances (single word and multiword) transcribed by unfamiliar listeners have recently been published for children up to 47 months of age (Hustad, Mahr, & Rathouz, 2020). Results indicate that there is a very wide range of variability among children at 2 years of age, with 5th and 95th percentile single word intelligibility scores of 18% and 74%, respectively. However, variability reduces somewhat with age, with 5th and 95th percentile single word intelligibility scores of 55% and 86% respectively at 4 years of age. This variability sheds some light on previous studies showing very discrepant results, suggesting that the range of typical intelligibility development is very wide, particularly for younger children. Results from Hustad and colleagues (Hustad, Mahr, & Rathouz, 2020) indicate that there is an intelligibility advantage for single word production prior to 41 months of age; after 41 months of age there is an intelligibility advantage for multiword production. Otherwise, the range of variability for typical children is similar for single word utterances and multiword utterances. Notably, typically developing children are not 100% intelligible as indicated by objective measures at 4 years of age. In addition, ongoing preliminary work comparing intelligibility development in children who speak different native languages is revealing important convergences. These results suggest that there may be some intelligibility development universals across languages. Such a finding would have critical implications for early identification of functional speech deficits in children.

A key issue is the ability to differentiate between children whose intelligibility falls within the range of age‐level expectations from those whose performance is delayed or disordered with regard to age‐level milestones. For many children, intelligibility reductions beyond age‐expectations may have a significant detrimental impact on functional communication and on social participation, leading to important negative educational consequences, since speech is a primary modality through which children in the early grades demonstrate their learning. Accurate differential diagnosis of intelligibility deficits, early identification, and treatment to improve intelligibility is critical for these children.

Studies are currently underway that seek to identify cut points for typical intelligibility development and to validate the diagnostic accuracy of intelligibility cut points for separating children who have mild or subtle speech motor disorders from those who are in the lower percentiles of typical development. Recent work employing receiver operating characteristic (ROC) curves (DeLong, DeLong, & Clarke‐Pearson, 1988) suggests that intelligibility scores differentiate between children with cerebral palsy (CP) who have speech motor impairment and typically developing peers with nearly perfect certainty (area under curve = .99). These data further suggest that at 5 years of age nearly all typically developing children had intelligibility scores above 87%, while the vast majority of children with CP and speech motor impairment had intelligibility below 72% at the same age (Hustad, Sakash, Broman, & Rathouz, 2019). These findings are consistent with earlier work suggesting that the range of intelligibility between 75% and 85% may represent a “gray area” for determining whether a 5‐year‐old child was performing at an age‐appropriate level (Hustad, Oakes, & Allison, 2015). The determination of cut points of this nature for children across the age span is currently ongoing, with a focus on children who have CP and are thus at considerable risk for speech motor disorders.

In other work focused on intelligibility development in children, studies have shown that children with CP experience their greatest growth in single word intelligibility between the ages of 36–60 months, but that intelligibility is still developing through 8 years (Hustad, Sakash, Natzke, Broman, & Rathouz, 2019). Results indicate that growth is impacted by speech and language profile characteristics, such that children with CP who do not show evidence of speech motor involvement have their greatest growth at earlier ages and reach higher intelligibility levels at 8 years of age than those who do have speech motor involvement. Further, children with comorbid receptive language impairment and speech motor impairment lag behind their peers who do not have receptive language impairment but do have speech motor involvement (Hustad, Mahr, Broman, & Rathouz, 2020). Clearly, many attributes of the speaker impact intelligibility and its development.