includes prevalence findings for three of the four types of speech impairment described previously in : Speech Delay (SD), Speech Errors (SE), and CAS. All of the ASD participants were younger than 9 years of age, and therefore ineligible for Persistent Speech Disorder (PSD). Additionally, based on the precedent literature and the sampling procedures for this study, MSD-DYS (dysarthria) was not expected in sample participants.
Table 4 Prevalence of Speech Delay (SD), Speech Errors (SE), and Motor Speech Disorder-Apraxia of Speech (MSD-AOS) in 46 children aged 4 to 7 years with Autism Spectrum Disorder (ASD). Population prevalence estimates from are included for comparison to (more ...)
Prevalence of Speech Delay (SD) in ASD
The left section of is a summary of the percentage of participants with ASD who met criteria for SD, as defined in and derived by software algorithms. Point-prevalence data from the three epidemiological studies of speech sound disorders reviewed previously are included in for ease in comparing the present prevalence findings for ASD with population estimates. The only same-age data for such comparisons is for the 6-year-old participants with ASD, at which the small number of ASD participants includes one with SD (10%) compared to the population estimates in two studies of 3.8%. A more robust comparison is the mean of 15.2% finding for SD in the 46 ASD participants aged 4 to 7 years, a finding that is essentially similar to the population prevalence of SD in 3-year-old children estimated in two large scale epidemiologic projects (Campbell et al., 2003
; Shriberg, Tomblin, & McSweeny, 1999
The prevalence findings for SD in are interpreted as indicating modest increased risk for concomitant SD in young children with ASD. This conclusion is influenced by the weight of the findings in the precedent literature in ASD summarized in . As noted, SD for 3 to 9 year-old children with ASD was estimated at 12%, generally consistent with 15.2% for the children with ASD in the present sample. Moreover, 7 of 11 studies (64%) cited in reported means or percentage data supporting increased prevalence of SD (primarily based on articulation test percentile data) in children with ASD.
Prevalence of Speech Errors (SE) in ASD
The middle section of is a summary of the percentage of participants with ASD who met criteria for SE at the minimum age of 6 years (see ) and at 7 years of age (n
= 22). Findings are subtotaled for speech errors on sibilants and rhotics, with summary percentages indicating children who had errors in one or both sound classes. The only available prevalence comparison data for SE in a large epidemiologic study is Wren and colleagues’ (2009)
prevalence estimate of 7.9% at 8 years of age. Although the prevalence finding of 31.8% SE averaged over the 6-and 7-year-old participants with ASD cannot be directly compared with the Wren et al. estimate, note that there would need to be 75% normalization rate by 9 years (i.e., from approximately 32% to approximately 8%) for the two values to be consonant. As such a one-year normalization rate is unlikely, the present findings are interpreted as support for substantially higher risk for concomitant SE in children with ASD. Although the ages of the present participants did not allow an estimate of the persistence of SD or SE past 9 years of age (i.e., PSD), it is reasonable to assume that prevalence rates past 9 years would also be higher than the 7.9% SE estimate from Wren et al. at 8 years of age. Finally, as indicated in and , the present study is the third reporting that approximately one of every three children with ASD meet criteria for SE, particularly dentalized sibilants ().
Prevalence of CAS in ASD: Percentage of Positive Marker Findings
The primary question addressed in this study is whether some children with verbal ASD have concomitant CAS. Because the speech data were obtained only from continuous speech samples, rather than from speech assessment protocols designed for comprehensive assessment of motor speech processes (e.g., lexical stress tasks, challenging word tasks, repeated measures tasks for target and error stability), only a subset of the diagnostic signs of CAS could be assessed in this study. As demonstrated in prior studies, however, sensitive and specific signs of CAS are available in continuous speech samples (Shriberg, Potter, & Strand, in press
). Specifically, core signs of CAS, including slow speaking and articulation rates, spatiotemporal vowel errors (reduced vowel space, lengthened vowels), and distorted consonants and consonant transitions, can be identified using standardized perceptual and acoustic indices of these behaviors in continuous speech. To provide the widest possible screen, the analyses to be described included signs of both CAS and MSD-NOS. As noted previously, due to a lack of consensus on the exact set of speech, prosody, and/or voice features that are sensitive to and specific for CAS, the only clinical-research approach to classification is to appeal to some minimal number of positive markers (i.e., the number of proposed indices of CAS or MSD-NOS on which a speaker suspected to have CAS is positive).
The 10 putative indices of CAS or MSD-NOS in the top panel in , titled Sensitivity, were those among a list of 38 that have consensual and construct validity to identify motor speech disorders (Shriberg et al., 2010a
). The indices sensitive to CAS or MSD-NOS are arranged by decreasing order of speakers with CAS who scored positive on the index in the continuous speech (cross-hatched bars). For example, 100% of the speakers with CAS were positive on the index termed Lengthened Vowels, as defined by having average vowel lengths (in milliseconds) longer than one standard deviation from typically developing, same-or younger-aged speakers. The dashed lines at 50% and 75% are visual aids indicating the relative sensitivity of each index. Seven of the 10 indices were obtained using acoustic methods (bolded), two of the 10 with narrow phonetic transcription, and one with prosody-voice coding. The values above each pairwise comparison are the one-tailed effect sizes, with an asterisk indicating statistical significance at the .05 level (StatXact, Cytel Software, 2001
). To minimize Type II errors of interpretation associated with cell sizes, we treat each motor speech sign family-wise (without adjustment for multiple testing), with emphasis on the magnitudes of the significant effect sizes.
Percentage of positive indices of Childhood Apraxia of Speech (CAS) and Motor Speech Disorder-Not Otherwise Specified in participants with Autism Spectrum Disorders (ASD) compared to participants with CAS.
As shown in , 100% of the participants with CAS were positive for the first 3 of the 10 indices of motor speech disorder, with 50% to over 75% of participants positive on the remaining 7 indices. In comparison, the 46 speakers with ASD (filled bars) were over 75% positive on only one index of motor speech disorder, with two other indices over 50% positive. Effect sizes for the 7 statistically-significant pairwise comparisons were large by conventional criteria (greater than .80). Space constraints prohibit discussion of the technical details of each of the indices. Essentially, each captures a different element of the precision and stability of spatiotemporal aspects of speech production. These findings are interpreted as counter-support for the hypothesis of motor speech disorder consistent with apraxia of speech or MSD-NOS in children with verbal ASD.
Findings in the bottom panel in , titled Specificity, are also interpreted as counter-support for the CAS-ASD hypothesis. To assess the possibility that there might be a subgroup of the participants with ASD who have concomitant CAS, findings for the 7 participants with SD () were compared to findings for the comparison group with CAS. For these analyses, the reference group used to standardize index raw scores was the participant group with Speech Delay (SD). Thus, whereas the data in the top panel in assess the sensitivity of the motor speech markers to identify speech impairment, the bottom panel findings in assess the specificity of the 38 indices, i.e., their ability to discriminate CAS or MSD-NOS from Speech Delay. The bottom panel includes the five indices of motor speech disorder on which the comparison group participants with CAS differed by more than one standard deviation from the comparison speakers with SD. Four of the five pairwise comparisons with the ASD participant’s performance on these indices yielded large significant effect sizes, with none of the ASD participants scoring below the SD participants.
Prevalence of CAS in ASD: Group-Averaged Prosody-Voice Findings
provides summary graphic and statistical information focusing on prosody-voice characteristics of the participants with ASD. As continuous speech samples are the primary, and for some variables, the only source for these indices, it is appropriate to compare findings for the speakers with ASD to those with each of the other comparison groups. As shown in the key to the symbols, these include in addition to the ASD target group (A-filled circles); the typically-developing speakers (T-open circles), the speakers with Speech Delay (S-open squares) and the speakers with CAS (C-open triangles). In each panel, the top section provides the numeric data on the three prosody and four voice domains for each of the four groups. Boxes around the numeric data in Panel A indicate significant one-way analyses of variance, with a key to the conventional symbols for significant p
values at the bottom of each panel. For Panels B–D, the boxes in the numeric data indicate significant effect sizes for the pairwise comparisons. As indicated below each of the three panels, the conventional effect size (ES) adjectives from Cohen (1988
; S: Small, M: Medium, L: Large), are extended for increased sensitivity to include V: Very Large and E: Extremely Large. Significant ES’s (i.e., confidence intervals not crossing 0) are underlined. These same values are plotted graphically below the numeric values. Scores of 80–90% on this measure (see dashed lines) are considered marginal impairment and scores below 80% as impairment.
Prosody-voice findings for participants with Autism Spectrum Disorders (A) compared to participants with Typical Development (T), Speech Delay (S), and Childhood Apraxia of Speech (C).
As indicated in , Panel A, significant analyses of variance results were obtained for the prosody variables of Rate and Stress and for the voice variables of Loudness, Pitch, and Laryngeal Quality. The findings in Panels B–D provide follow-up pairwise analyses to determine the source and effect sizes for all significant differences in the omnibus test. It is efficient for conceptual perspectives to review the findings by prosody-voice variable, rather than by comparison group.
As shown in , Panel A (see both the numeric and graphic sections), an average of only 70.9% of the utterances in the continuous speech of the speakers with CAS had appropriate Rate. Inspection of subcodes not shown in indicated that the remaining approximately 29% were subcoded “Too Slow” for each speaker’s age (< 2 syllables/second, including pause time). In comparison, as shown best in Panel D, the ASD participants averaged 97.7% utterances that were appropriate in Rate for their age (2–4 syllables/sec). The significant ES for this difference was >1.0 (Very Large). As shown in Panel C, ASD participants had significantly more utterances with appropriate Rate than the group average for the participants with Speech Delay, although the latter average (87.3%) was within the marginal range for the PVSP. As shown in Panel B, appropriate Rate values for ASD and same-aged, typically-developing children were within 1 percentage point of one another.
Similar to literature findings, the 67.9% average number of utterances with appropriate Stress of the CAS speakers was significantly lower than the Stress findings for the other groups (Panel A). The pairwise effect size indicated that whereas the averaged Stress value of 82.6% appropriate utterances for the participants with ASD was not significantly different from the average of the Typically-Developing speakers (Panel B) or the speakers with Speech Delay (Panel C), it was significantly higher than the speakers with CAS (ES= >.80; Large). Inspection of subcodes in the Stress analyses and of the continuous speech transcripts were completed to determine the locus of inappropriate Stress in ASD compared to CAS participants. It is efficient to defer comments on these findings to the Discussion.
Speakers with ASD had significantly fewer utterances with appropriate Loudness (81.0%), as indicated in , Panel A, with this value at the lower boundary of marginally inappropriate. The subcodes data indicated that the source of this finding was that participants with ASD were coded as having excessive loudness (Too Loud) on most of these inappropriate utterances. On this voice variable the speakers with ASD were significantly different from the Typically-Developing speakers (Panel B: Moderate ES), and the speakers with Speech Delay (Panel C: Large ES) and CAS (Panel D: Large ES).
Speakers with ASD differed significantly from speakers in the other three groups on the omnibus comparison of Pitch (Panel A). This difference was not significantly different than the average Pitch values for the Typically-Developing speakers (Panel B) or the speakers with Speech Delay (Panel C), but was significantly lower than obtained for the participants with CAS (98.5%; Moderate ES). Inspection of the subcodes indicated that speakers with ASD had utterances coded as Too High or Variably High pitch, findings also obtained in the acoustic analyses.
For the voice domain of Laryngeal Quality, speakers with Speech Delay had significantly lower percentage of utterances with appropriate laryngeal quality (61.9%) than the speakers with ASD (84.2%; Very Large ES). Inspection of the subcodes indicated that more of the speakers with Speech Delay had utterances coded as Rough.
Additional prosody-voice analyses addressed the possibility explored in the bottom panel of that the subgroup of 7 participants with ASD classified as SD might have prosody-voice findings more consistent with the values of the participants with CAS, or the less specific subtype of motor speech disorder, MSD-NOS. These subgroup analyses provided no statistical support or trends for CAS or MSD-NOS, with the prosody-voice profiles of the 7 ASD participants with SD wholly consistent with the averaged profile of the comparison speakers with SD.