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4 Intelligibility Impairment
KATHERINE C. HUSTAD1 AND STEPHANIE A. BORRIE2
1 University of Wisconsin–Madison, WI, USA
2 Utah State University, Logan, UT, USA
4.1 Introduction
Intelligibility is a critical concern in speech‐language pathology, impacting a wide range of individuals across populations, with ages ranging across the lifespan. For example, intelligibility is an important developmental concern for children who are acquiring speech, including those with and without risk factors for speech impairment (Hustad, Mahr, & Rathouz, 2020). Speech intelligibility impairments in children can stem from speech sound disorders, childhood apraxia, conditions associated with neuromotor involvement or disease (cerebral palsy, childhood brain injury or stroke), genetic etiologies (Down syndrome, cleft palate), or sensory involvement (hearing impairment). In adults, intelligibility remains an important concern for individuals with many of the aforementioned childhood onset etiologies as well as for individuals with adult onset etiologies. Adult onset etiologies include those that have a degenerative course affecting speech motor control (e.g., amyotrophic lateral sclerosis, Parkinson’s disease, Huntington’s disease), those with a recovering course (e.g., stroke, traumatic brain injury during the post‐onset recovery window), and those with a persistent stable course (stroke, traumatic brain injury, cerebral palsy, and other chronic conditions). The field of motor speech disorders has had a particularly notable and longstanding interest in speech intelligibility, in part because reductions in intelligibility are very frequently associated with dysarthria (Darley, Aronson, & Brown, 1969). Improving intelligibility has long been considered a key goal of treatment for this population (Ansel & Kent, 1992).
Clinically, intelligibility measures provide an important metric that can be used for treatment decision‐making. Intelligibility measures are often used as a basis of comparison for documenting and monitoring change in speech performance (Yorkston, Beukelman, Strand, & Bell, 1999), as an index of severity of the speech disorder (Weismer & Martin, 1992), and as an indicator of functional ability (or disability) relative to “normal” performance (Yorkston et al., 1999). Critical clinical concerns such as justifying treatment for third‐party payers, and deciding whether or not to discontinue intervention, sometimes may be made on the basis of intelligibility measures.
4.2 Intelligibility Defined
Intelligibility is a complex and multifaceted construct that is dynamic in nature. It has been defined as the extent to which an acoustic signal, generated by a speaker, can be correctly recovered by a listener (Kent, Kent, et al., 1989; Kent, Weismer, Kent, & Rosenbek, 1989; Yorkston & Beukelman, 1980). Intelligibility is a dyadic construct that is not solely attributable to a speaker or to a listener. Rather, it is a product of the joint efforts of the speaker (who produces the signal) and the listener (who interprets the signal) as communication partners. To be intelligible, speech does not need to be perfect or even “normal.” In fact, productions may be characterized by a range of different errors and still be readily recoverable to listeners. The key issue in intelligibility is whether listeners are able to map acoustic cues onto the intended linguistic representations in spite of any deviant production patterns.
Intelligibility is influenced by a host of variables related to the speaker and his or her impairment(s), the listener and his or her ability to make sense of a distorted speech signal, and contextual factors such as the communicative environment, and shared knowledge between the speaker and the listener. Studies have shown that intelligibility can be markedly affected when different variables are manipulated. Examples include the length and nature of speech being produced (single words, individual sentences, narrative discourse, conversational discourse) (Hustad, Mahr, & Rathouz, 2020; Miller, Heise, & Lichten, 1951), semantic predictability of messages (Kent, Miolo, & Bloedel, 1994), availability of visual information (Borrie, 2015; Hustad & Cahill, 2003; Hustad, Dardis, & McCourt, 2007), and listener familiarity with the speaker (Borrie et al., 2012; Liss, Spitzer, Caviness, & Adler, 2002), to name but a few. Because so many variables influence intelligibility, no one measure can accurately and adequately provide a complete index of it. Kent and colleagues have suggested that “a particular talker has a range of intelligibility potentials, depending on listener familiarity, nature of the linguistic message, physical setting, motivation, effort level, and so on” (Kent et al., 1994, p. 81). Thus, any given measure of intelligibility is best considered a snapshot of performance under a specific set of circumstances. An intelligibility estimate must be interpreted cautiously and within the context it was obtained.
Within the World Health Organization’s International Classification of Functioning, Disability, and Health (ICF‐2), intelligibility has been regarded as an index of a person’s ability to engage in the activity of speaking (Dykstra, Hakel, & Adams, 2007). Intelligibility is influenced by body structures and functions, particularly the speech subsystems of articulation, resonance, phonation, and respiration, and their constituent components. However, intelligibility