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.
4.5 Perceptual Learning
Historically, there has been a strong emphasis on the study of the speech signal and on disorder characteristics of the speaker to understand and explain intelligibility deficits. This is observed in the clinic, with efforts to manage intelligibility impairments focused largely on the speaker. For example, for clients with dysarthria and an associated intelligibility deficit, the target is often to improve the speaker’s production of speech or give them strategies and/or technology to compensate for the impairment (Duffy, 2012). However, the past decade has seen a surge in studies that examine listeners and their contribution to intelligibility and potential remediation of impairment. Focusing on the role that listeners play in the construct of intelligibility may lead to new interventions that adjunct traditional speaker‐focused treatments or intervention alternatives when the speakers themselves are not viable treatment candidates.
When listeners encounter speech that is difficult to understand, they rapidly adapt their perceptual systems, mapping the non‐canonical acoustic cues onto linguistic categories stored in memory (i.e., cue‐to‐category mapping). This is known as perceptual learning. According to theoretical models, the cue‐to‐category mapping process is driven by signal predictability (Kleinschmidt & Jaeger, 2015). So long as there is some level of regularity in the degraded speech signal, perceptual learning should transpire. Listeners can and do adapt to the speech of speakers with dysarthria, even when significant intelligibility impairment exists. Empirical studies report statistically and clinically significant intelligibility improvements, ranging from 8 to 20 percentage points, for listeners who receive a brief familiarization experience with audio recordings of a speaker with dysarthria (e.g., Borrie & Schäfer, 2015; Borrie et al., 2012; Liss et al., 2002). This rapid perceptual adaptation to a speaker with dysarthria has been primarily studied with young, normal‐hearing adult listener populations. However, preliminary evidence indicates that older adults, including those with impaired hearing, also experience intelligibility improvements following familiarization with a speaker with dysarthria (Lansford, Luhrsen, Ingvalson, & Borrie, 2018). Further, while existing research in this area has heavily focused on how listeners learn and adapt to the speech of a specific speaker with dysarthria, recent evidence shows that familiarization with one talker with dysarthria can generalize to intelligibility improvements in a novel talker with dysarthria, with the level of intelligibility improvements regulated by the similarity of speech deficit (Borrie, Lansford, & Barrett, 2017a). Although additional studies are warranted, the theoretical and clinical implications of perceptual learning generalization across speakers who share similar patterns of impairment are many. While there is strong collective evidence that listeners perceptually adapt to the impaired speech signals of speakers with dysarthria, there may be a small subset of speakers, those with largely unpredictable speech features (i.e., the hyperkinetic speech patterns associated with the involuntary movements of Huntington’s disease) that listeners simply cannot learn (Borrie, Lansford, & Barrett, 2018; Lansford, Borrie, & Barrett, 2019).
4.6 Individual Differences
While still an emerging area of investigation in speech pathology, it would be a disservice to the discussion of intelligibility impairment if we did not touch upon individual differences. Traditional models of intelligibility and learning with normal‐hearing listeners are routinely developed on group averages. Acknowledgment of the variability in individual listener performance has, historically, been minimal. Further, differences have often been treated as a type of error or as justification to remove the data from the analysis. This, and the fact that intelligibility of adults without communication disorders generally hovers around ceiling, has contributed to a false sense of homogeneity among normal‐hearing listeners.
There is mounting evidence that listeners vary widely in their ability to decipher speech across populations and across the age span. Research findings clearly show there is considerable variability in the performance of different listeners who hear the same speaker (Hustad & Cahill, 2003; Hustad, Schueler, Schultz, & DuHadway, 2012; Lam & Tjaden, 2013; Pennington et al., 2013). In fact, studies report standard deviations within a given speaker across listeners of up to 17% for individuals with the most severe dysarthria (Hustad & Cahill, 2003). Other contexts that seem to heighten individual differences in intelligibility performance include the presence of background noise and unfamiliar accent. Findings suggest that some listeners are simply better equipped to tackle the demanding task of understanding speech in challenging conditions. Cognitive, perceptual, and linguistic skills have been shown to contribute to these individual differences. Specific to intelligibility and learning of the impairments associated with dysarthria, factors such as age, rhythm perception abilities, vocabulary knowledge, and working memory likely play a meaningful role in the individual variability observed (Bent, Baese‐Berk, Borrie, & McKee, 2016; Borrie, Lansford, & Barrett, 2017b; Lee, Sung, & Sim, 2014; McAuliffe, Gibson, Kerr, Anderson, & LaShell, 2013). A systematic approach to documenting how these and other listener parameters influence and interact with one another is needed. However, the current data suggest that individual differences should be given due consideration in the measurement and understanding of intelligibility impairment.
4.7 Summary
Speech intelligibility is a fundamental feature of spoken communication, involving the production of an acoustic signal by a speaker and the decoding of that signal by a listener. Intelligibility impairment is an issue across the lifespan, for a range of clinical populations that frequent the speech‐language pathologist’s caseload. For children, an understanding of developmental trajectories and the range of typical variability by age is critical for identifying those who have delays and disorders. As such, measurement of intelligibility is of essential import. However, as summarized in this chapter, such measurement is by no means a straightforward task. This is largely because intelligibility is not a unitary construct. It involves contributions from both the speaker and the listener, as well as the surrounding communicative environment. Additionally, while surprisingly understudied in the field of communication disorders, meaningful contributions to the construct of intelligibility likely arise from the dynamic, coordinative nature of spoken dialog, where production and perception are intricately and interdependently connected. Finally, to further complicate the measurement dilemma, these speaker, listener, and contextual contributions likely interact with one another in unique and variable ways. Thus, any measure of intelligibility must be viewed as an estimate, accounting for the speaker, listener, and context within which it was produced. While the task of an ecologically valid approach to capturing intelligibility is challenging, considerable progress has been made in the systematic study of large‐scale samples of both speakers and listeners for characterizing the many variables that contribute to intelligibility.
REFERENCES
1 Ansel, B. M.,