Abstract
In a sample of 46 children aged 4 to 7 years with Autism Spectrum Disorder (ASD) and intelligible speech, there was no statistical support for the hypothesis of concomitant Childhood Apraxia of Speech (CAS). Perceptual and acoustic measures of participants’ speech, prosody, and voice were compared with data from 40 typically-developing children, 13 preschool children with Speech Delay, and 15 participants aged 5 to 49 years with CAS in neurogenetic disorders. Speech Delay and Speech Errors, respectively, were modestly and substantially more prevalent in participants with ASD than reported population estimates. Double dissociations in speech, prosody, and voice impairments in ASD were interpreted as consistent with a speech attunement framework, rather than with the motor speech impairments that define CAS. Key Words: apraxia, dyspraxia, motor speech disorder, speech sound disorder
A continuing question about persons with Autism Spectrum Disorder (ASD) is whether reported diminished abilities in gross, fine, and oral motor control are causally associated with reported deficits in speech acquisition and performance. The classification term for the speech deficit in question, recently adapted by the American Speech-Language-Hearing Association (ASHA; 2007a, 2007b), is Childhood Apraxia of Speech (CAS). Medical literatures and speech literatures in other countries continue to prefer several other classificatory terms for this disorder, including dyspraxia and developmental verbal dyspraxia. “Childhood” apraxia of speech differentiates congenital and early acquired forms of apraxia of speech from adult acquired forms, but creates a nosological problem because childhood apraxia of speech generally persists into adulthood. We will use the ASHA (2007a) recommended term— CAS.
The strong form of the hypothesis in the title of this paper, hereafter, the ‘CAS-ASD’ hypothesis, is that CAS is a sufficient cause of lack of speech development in at least some children classified as nonverbal ASD. The weak form of the CAS-ASD hypothesis is that CAS contributes to the inappropriate speech, prosody, and/or voice features reported in some children and adults with verbal ASD. Although the present report addresses only the weak form of the hypotheses, the conceptual framework and implications for treatment apply to both forms of the hypothesis. Forthcoming research addresses the strong form of the hypothesis. The following sections provide (a) rationales for the CAS-ASD hypothesis, (b) an overview of idiopathic speech sound disorders, and (c) a summary of speech, prosody, and voice findings in verbal ASD.
Rationales for the CAS-ASD Hypothesis
The American Speech-Language-Hearing Association Position Statement recommends the following definition of CAS:
Childhood apraxia of speech (CAS) is a neurological childhood (pediatric) speech sound disorder in which the precision and consistency of movements underlying speech are impaired in the absence of neuromuscular deficits (e.g., abnormal reflexes, abnormal tone). CAS may occur as a result of known neurological impairment, in association with complex neurobehavioral disorders of known or unknown origin, or as an idiopathic neurogenic speech sound disorder. The core impairment in planning and/or programming spatiotemporal parameters of movement sequences results in errors in speech sound production and prosody. (p. 1)
Three conceptual and empirical perspectives motivate the hypothesis that CAS may be causal to the absence of speech development in some children with ASD or in others, to perceptible differences in speech, prosody, or voice.
Motor skills
A primary rationale for the CAS-ASD hypothesis is findings indicating that persons with ASD have praxis deficits affecting imitative processes and impairing acquisition and performance of a range of motor skills. Reviews of this literature and the neural correlates of praxis findings in ASD are beyond the scope of the present report; for representative data and overviews of research during the past two decades see Dawson, Mottron, and Gernsbacher (in press); Dowell, Mahone, and Mostofsky (2009); Dziuk et al. (2007); Gernsbacher, Sauer, Geye, Schweigert, and Goldsmith (2008); Green et al. (2002); Goldman Gross and Grossman (2008); McDuffie et al. (2007); Mostofsky, Burgess, and Gidley Larson (2007); Mostofsky et al. (2006); Ozonoff et al. (2008); Page and Boucher (1998); Rogers (2009); Rogers, Bennetto, McEvoy, and Pennington (1996); Russo, Larson, and Kraus (2009); Smith and Bryson (1994); Vivanti, Nadig, Ozonoff, and Rogers (2008); and Zadikoff and Lang (2005). A parsimonious extension of the findings from studies in other motor domains is that a praxic deficit in speech may account for the failure of some children with ASD who have adequate cognitive ability and communicative intent to acquire articulate speech (the strong version of the CAS-ASD hypothesis), and for others with ASD to have atypical speech, prosody, and/or voice (the weak version of the CAS-ASD hypothesis). As reviewed in the following sections, CAS is the one subtype of speech sound disorder whose neurobehavioral substrates could account for the speech, prosody, and voice findings reported in ASD (Shriberg 2010a, 2010b). Unlike dysarthria, a class of neuromuscular speech disorders that constrains the precision of speech production, the transcoding (planning/programming) deficits that define CAS (van der Merwe, 2009) are functionally sufficient to disrupt the onset of speech and/or speech precision and stability.
A constraint on the CAS-ASD hypothesis is that many speech researchers have concluded from diverse conceptual and empirical considerations that speech is domain specific (e.g., Dewey, Roy, Square-Storer, & Hayden, 1988; Kent, 2000, 2004, 2010; McCauley, Strand, Lof, Schooling, & Frymark, 2009; Potter, Kent, & Lazarus, 2009; Smith, 2006; Weismer, 2006; Watkins, Dronkers, & Vargha-Khadem, 2002; Ziegler, 2002, 2008). The perspectives in these and other sources are that the neural substrates of apraxia of speech differ from the neural substrates posited for other motor systems and other types of apraxia (e.g., oromotor apraxia, limb apraxia, ideomotor apraxia).
A recent empirical constraint on the CAS-ASD hypothesis are results discussed in Pickett, Pullara, O’Grady, and Gordon (2009), which summarizes findings from reports of 167 individuals with nonverbal ASD who acquired speech at age 5 or older. Records indicated that these individuals learned skills including “imitating sounds, words, and phrases,” “answering simple questions,” “requesting spontaneously,” “using complete sentences,” and “speaking in spontaneous complex sentences” (p.13). Crucially for the strong version of the CAS-ASD hypothesis, however, the speech findings in Table 1 of Pickett et al. (2009) do not include information consistent with the signs of CAS described later in the present report.
Genomics
A second rationale for the CAS-ASD hypothesis is based on the possibility of common genetic origins. Whereas numerous candidate genes and regions of interest for autism spectrum disorders have been reported, the widely-studied FOXP2 transcription gene is the only gene to date associated with CAS. The origins of both disorders are viewed as strongly heritable and both involve cognitive-linguistic impairments, suggesting the possibility of genes common to both disorders (e.g., Poot et al., 2010; Vernes et al., 2008).
A constraint on the likelihood of inherited or sporadic genetic comorbidity of CAS and ASD is the wide differences in their reported prevalences, with idiopathic CAS estimated at approximately .1% (Shriberg & Kwiatkowski, 1994) and ASD reportedly at approximately 1% (Rice, 2009), a 10-fold increased risk. Unless a more highly prevalent subtype of CAS than the idiopathic form is posited for either or both nonverbal and verbal ASD, comorbid ASD and CAS would be expected to be extremely rare (i.e., 1/100,000, multiplying the individual probabilities of each disorder).
Phenotypic similarity
A third rationale is based on findings reviewed presently indicating that the speech, prosody, and voice characteristics of some children with low and high verbal ASD reportedly are similar to those found in children and adults with apraxia of speech. The validity of this claim for the CAS-ASD hypothesis, the most testable of the three rationales reviewed, requires close examination of the ASD-speech literature, in particular, findings for prosody and voice characteristics. A constraint, however, is that literature findings to date are heterogeneous and lack the conceptual organization needed for comparative analyses. Peppé, McCann, Gibbon, O’Hare, and Rutherford (2007) provide a useful perspective on the precedent speech literature in ASD:
In the research literature, numerous adjectives are used to describe atypical expressive prosody in autism, for example, dull, wooden, singsong, robotic, stilted, overprecise, and bizarre (Baltaxe & Simmons, 1985; Fay & Schuler, 1980); terms that perhaps reflect perceived characteristics of autism more than acoustic features. The fact that adjectives with opposite meanings, such as monotonous and exaggerated (Baron-Cohen & Staunton, 1994), can be used to describe this atypicality suggests a wide variation in either the perception of atypical expressive prosody or in the prosody itself. (p. 1016)
The following overview of idiopathic speech sound disorders attempts to redress this situation. The goal of this tutorial is to introduce terms and concepts needed for efficient review of the ASD-speech literature. The system described in the next section is also used later to organize findings from the present study.
A tutorial on Idiopathic Speech Sound Disorders (SSD)
The cover term Speech Sound Disorders (SSD) was adopted by the American Speech-Language-Hearing Association in 2005 to replace both the early 20th century term functional articulation disorders, and the term used for the same clinical entity from approximately 1980 to 2005, phonology disorders of unknown origin. There is no current professional consensus, however, on nomenclature for subtypes of SSD (i.e., excluding disorders of known origin, such as those due to cleft palate, Down syndrome, deafness, traumatic brain injury, or other frank cognitive, structural, sensory, motor or affective disorder). The nosology in Table 1, from a system termed the Speech Disorders Classification System (SDCS: Shriberg et al., 2010a), has evolved for genomic and other descriptive-explanatory research in SSD of currently unknown origin. As indicated, the speech classification terms and concepts in Table 1 are needed to organize both the literature review in Table 2 and findings from the present study. Technical information on perceptual and acoustic procedures used to classify participants’ speech status using the SDCS in a software environment is available elsewhere (Shriberg et al., 2010b; see also
http://www.waisman.wisc.edu/phonology/).
Table 1
Table 1
Definitions and descriptive statistics for six subtypes of speech sound disorders in the Speech Disorders Classification System (SDCS).
Table 2
Table 2
Prevalence estimates and descriptive findings for subtypes of speech sound disorders in studies of speakers with verbal Autism Spectrum Disorders (ASD). Studies are ordered chronologically within each set of findings.
Speech Delay
Speech Delay (SD) is the SDCS classification term for 3 to 9 year-old children with mildly to severely reduced intelligibility due to age-inappropriate speech sound deletions, substitutions, and distortions. As indicated in Table 1, children with SD generally do not have notable impairments in prosody or voice, an important differential diagnostic sign between SD and CAS discussed below. Relative to typically-developing children, however, children with SD have higher rates of language impairment, lowered intelligibility, and are at greater risk for reading impairment. Two American English population estimates of speech sound disorders using similar definitions and methods (Campbell et al., 2003; Shriberg, Tomblin, & McSweeny, 1999) report approximately similar point prevalence population estimates (15.6%, 15.2%, respectively) at 3 years of age (interpolated finding in Shriberg et al., 1999) and similar estimates (3.8%) at 6 years of age. A third large British English epidemiological study, also using the SDCS definition of SD, reported a population estimate at 8 years of age of 3.8% (Wren, Roulstone, Miller, Emond, & Peters, 2009).
Speech Errors
Speech Errors (SE) is the SDCS term for 6 to 9 year-old children whose speech impairment is limited to distortions of one or two English sounds or sound classes: the sibilants /s/ and /z/ and the rhotic consonant /r/ and/or the stressed and unstressed rhotic vowels (as in “bird” and “sister,” respectively). Elementary-school American English children with SE are typically not provided speech services because, as shown in Table 1, SE is generally not associated with prosody-voice impairment, language disorder, or intelligibility deficits and children with SE are not at risk for reading impairment (Shriberg, 2010b; Wren et al., 2009). Using definitions and methods for SE classification adapted from the SDCS, the Wren et al. (2009) epidemiologic study reported a point prevalence of SE at 8 years of age of 7.9%.
Persistent Speech Disorders (PSD)
Persistent Speech Disorder (PSD) is the SDCS term for speech disorders that persist past 9 years of age and for some speakers, for a lifetime. By 9 years of age, most children with histories of either SD or SE have normalized speech production, but a percentage of adolescents and adults continue to misarticulate. Children with prior SD may continue to have speech sound deletions, substitutions, and/or distortions, and children with prior SE may have persistent sibilant and/or rhotic distortions. As shown in Table 1, depending on whether such speakers have histories of SD or SE, they also may have persistent impairments in language, intelligibility, and/or reading. Flipsen’s (1999) review of survey and epidemiology studies, which also used the SD and SE classification constructs to organize the literature, yielded an estimated prevalence rate for PSD of 2.4%–3.9%.
Motor Speech Disorder (MSD)
The fourth classification entity for speakers with idiopathic SSD, Motor Speech Disorder (MSD), includes speakers of all ages whose significant intelligibility deficits are associated with motor speech impairment. As shown in Table 1, MSD subsumes three subclassifications. MSD-Apraxia of Speech (MSD-AOS) is the same clinical entity as Childhood Apraxia of Speech (CAS), a term that the American Speech-Language-Hearing Association adopted in 2007 to replace the prior terms Developmental Apraxia of Speech and Developmental Verbal Dyspraxia (the latter term continues to be used in medical contexts and in most other countries). As indicated previously, CAS will be the reference term for this classification in the present paper.
The core feature of both congenital and acquired apraxia of speech is a deficit in the planning/programming processes that transcode linguistic representations to the articulatory movements for speech. Motor Speech Disorder-Dysarthria (MSD-DYS), the second subclassification of MSD, is itself, a cover term for several subtypes of neuromuscular deficits (e.g., spastic dysarthria, ataxic dysarthria, hyperkinetic dysarthria) in the production of speech sounds (Duffy, 2005). Motor Speech Disorder-Not Otherwise Specified (MSD-NOS) is a recently proposed classification entity (Shriberg et al., 2010a) for speech signs that are not specific for apraxia or dysarthria and for speakers who have signs of motor speech involvement, but do not meet inclusionary criteria for either CAS (i.e., MSD-AOS) or MSD-DYS.
As indicated in Table 1, each of the three MSD classifications is characterized by deletions, substitutions, and distortions of sounds. Unlike SD, SE, and PSD, however, each MSD classification is also characterized by significant and persistent deficits in prosody and voice features. Speakers with MSD likely have concomitant language disorder, typically have significant intelligibility deficits, and generally are at increased risk for reading impairment. As cited previously, based on clinical referrals to one University speech clinic in a moderate-sized city, a preliminary estimate placed the population prevalence of CAS at .1% (Shriberg & Kwiatkowski, 1994). Several published and unpublished sources internationally indicate false positive rates for CAS of 80 to 90%, reflecting the lack of consensus on the inclusionary and exclusionary criteria for this disorder, especially as suspected in toddlers, preschool, and early elementary age children. There are no available prevalence estimates for MSD-DYS or MSD-NOS, although many researchers suggest that subclinical dysarthria and delays in maturation of sensorimotor systems subserving speech (i.e., MSD-NOS) may account for a substantial proportion of idiopathic speech sound disorders.
Prevalence Estimates and Speech, Prosody, and Voice Findings in Verbal ASD
The considerable body of research on the language characteristics of speakers with ASD (see Smith, 2007; Tager-Flusberg, 2009; Tager-Flusberg, Paul, & Lord, 2005 for reviews) has reported extensive heterogeneity of expressive ability among children with verbal ASD, ranging from children with only single word or simple word combinations to children with precocious levels of vocabulary and sentence structure. Tager-Flusberg and Joseph (2003) have proposed a system for classifying subtypes of language development within speakers with ASD, with other investigators raising validity issues about the system (e.g., Eigsti, Bennetto, & Dadlani, 2007; Whitehouse, Barry, & Bishop, 2007).
In contrast to the widespread intense interest in the language abilities of children with ASD, few studies have focused on the speech abilities of children, adolescents, and adults with ASD. Table 2 includes a summary of prevalence estimates for subtypes of speech sound disorders and speech, prosody, and voice impairment findings in speakers with verbal ASD. The entries in Table 2 do not include questionnaire data or single case study observations. Only information on productive speech, prosody, and voice behaviors is included, not studies of speech perception or comprehension in ASD; for reviews of the latter domains see Diehl, Bennetto, Watson, Gunlogson, and McDonough (2008); Diehl, Watson, Bennetto, McDonough, and Gunlogson (2009); McCann and Peppé (2003); and Paul, Augustyn, Klin, and Volkmar (2005). The format and content in Table 2 is the first to organize prevalence and descriptive findings using the SDCS classifications described in Table 1.
As shown in the top section of Table 2, three studies have estimated the prevalence of subtypes of speech impairment in ASD, each using definitions of one or more subtypes of speech impairment consistent with the subtypes in Table 1. Impairment consistent with Speech Delay (SD) occurred in 12% of the 3 to 9 year-old children with ASD studied by Cleland, Gibbon, Peppé, O’Hare, and Rutherford (2010). Rapin, Dunn, Allen, Stevens, and Fein (2009) reported SD and Speech Errors (SE) in 24% of participants with ASD during this age period. Cleland et al. also reported that 33% of the children with ASD studied had either SE or Persistent Speech Disorder (PSD). Shriberg, Paul, et al. (2001) reported that 33% of adolescents and adult study participants had PSD. Thus, although each of the subtypes of speech impairment in Table 1 have been reported in ASD, few between-study comparisons are possible due to differences in the age groups studied. Among the 11 studies in Table 2 in which the data could be interpreted as absence of support or support for SD in ASD (indicated by “X”), four have reported absence of support for SD and seven have reported support for SD in ASD. Velleman et al. (2010) is the only study series to date supporting speech impairment consistent with the SDCS term Motor Speech Disorder-Not Otherwise Specified. Although frank CAS was not observed in their studies, the findings Velleman and colleagues report using an array of perceptual and acoustic indices are consistent with MSD-NOS.
The remaining entries in Table 2 organize findings in the ASD literature using the Prosody (Phrasing, Rate, Stress) and Voice (Pitch, Loudness, Laryngeal Quality, Resonance) domains in the SDCS. The most well-studied prosody domain is Stress, with the 16 studies in Table 2 reporting impairments in participants’ with ASD ability to produce correct contrastive, emphatic, sentential, syntactic, and syllable stress (for reviews, see McCann & Peppé, 2003; McCann, Peppé, Gibbon, O’Hare, & Rutherford, 2008; Paul, Augustyn, et al., 2005; Paul, Bianchi, Augustyn, Klin, & Volkmar, 2008; Peppé et al., 2007). As shown in Table 2, impairments have been reported in at least one published study of ASD for each of the other 6 prosody and voice domains.
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Statement of Purpose
The primary goal of the present study was to assess the weak version of the CAS-ASD hypothesis—the hypothesis that concomitant CAS may be a sufficient causal explanation for at least some of the speech, prosody, and voice impairments reported in ASD. A secondary goal of the study was to estimate in a sample of verbal young children with ASD the prevalence of the two primary forms of speech impairment of unknown origin: Speech Delay and Speech Errors.