Feature-driven classification reveals potential comorbid subtypes within childhood apraxia of speech.

BACKGROUND: Childhood apraxia of speech (CAS) is a neurodevelopmental disorder with heterogeneous communication and other comorbid manifestations. While previous studies have characterized speech deficits associated with CAS, few studies have examined variability in reading and language and/or other developmental comorbidities. We sought to identify comorbid subgroups within CAS that could be clinically relevant as well as genetically distinctive. METHODS: In a group of 31 children with CAS and 8 controls, we performed hierarchical cluster analysis utilizing measures of articulation, vocabulary, and reading. We also conducted a chart review of the children with CAS to examine other clinical characteristics in these children and their association with subgroup membership. RESULTS: We identified 3 comorbid subgroups within CAS of varying severity. The high severity subgroup was characterized by poor reading and vocabulary, and the moderate severity subgroup by poor reading and non-word repetition but average vocabulary, compared to the mild severity subgroup. Subgroups were indistinguishable with respect to speech sound production, the hallmark of CAS, all demonstrating poor articulation. Children in the most severe subgroup were more likely to have early problems feeding (p = 0.036). CONCLUSIONS: Children with CAS may potentially be classified into comorbidity groups based on performance on vocabulary and reading measures, providing additional insight into the heterogeneity within CAS with implications for educational interventions.

Heritability and longitudinal outcomes of spelling skills in individuals with histories of early speech and language disorders.

This study examined the spelling skills in middle childhood and adolescence in individuals with histories of early childhood speech sound disorders (SSD) with and without language impairment (LI). Youth without such histories were also included (No SSD/LI group). The heritability of spelling skills at each age level was estimated. Children with SSD were classified as SSD-only, SSD with LI but without childhood apraxia of speech (SSD + LI/ No CAS), and CAS and LI (CAS + LI). The SSD-only group did not differ in spelling from the No SSD/LI group, suggesting that SSD-only did not increase risk for poor spelling. The SSD + LI/No CAS and CAS + LI groups had poorer spelling skills than the SSD-only and No SSD/LI groups. Spelling was associated with phonological awareness in the middle childhood and adolescent samples and with rapid automatized naming in the adolescent sample. Heritability of spelling skills was stronger in adolescence than in middle childhood. Differences in the correlates of spelling and in heritability at the two ages suggest developmental changes in the factors contributing to spelling.

Importance of copy number variants in childhood apraxia of speech and other speech sound disorders.

Childhood apraxia of speech (CAS) is a severe and rare form of speech sound disorder (SSD). CAS is typically sporadic, but may segregate in families with broader speech and language deficits. We hypothesize that genetic changes may be involved in the etiology of CAS. We conduct whole-genome sequencing in 27 families with CAS, 101 individuals in all. We identify 17 genomic regions including 19 unique copy number variants (CNVs). Three variants are shared across families, but the rest are unique; three events are de novo. In four families, siblings with milder phenotypes co-inherited the same CNVs, demonstrating variable expressivity. We independently validate eight CNVs using microarray technology and find many of these CNVs are present in children with milder forms of SSD. Bioinformatic investigation reveal four CNVs with substantial functional consequences (cytobands 2q24.3, 6p12.3-6p12.2, 11q23.2-11q23.3, and 16p11.2). These discoveries show that CNVs are a heterogeneous, but prevalent, cause of CAS.

Heritability estimation for speech-sound traits with developmental trajectories.

Numerous studies have examined genetic influences on developmental problems such as speech sound disorders (SSD), language impairment (LI), and reading disability. Disorders such as SSD are often analyzed using their component endophenotypes. Most studies, however, have involved comparisons of twin pairs or siblings of similar age, or have adjusted for age ignoring effects that are peculiar to age-related trajectories for phenotypic change. Such developmental changes in these skills have limited the usefulness of data from parents or siblings who differ substantially in age from the probands. Employing parent-offspring correlation in heritability estimation permits a more precise estimate of the additive component of genetic variance, but different generations have to be measured for the same trait. We report on a smoothing procedure which fits a series of lines that approximate a curve matching the developmental trajectory. This procedure adjusts for changes in measures with age, so that the adjusted values are on a similar scale for children, adolescents, and adults. We apply this method to four measures of phonological memory and articulation in order to estimate their heritability. Repetition of multisyllabic real words (MSW) showed the best heritability estimate of 45% in this sample. We conclude that differences in measurement scales across the age span can be reconciled through non-linear modeling of the developmental process.

Psychosocial Comorbidities in Adolescents With Histories of Childhood Apraxia of Speech.

Purpose Adolescent psychosocial outcomes of individuals with histories of childhood apraxia of speech (CAS) were compared to outcomes of individuals with histories of speech sound disorders (SSD) only and SSD with language impairment (LI). It was hypothesized that individuals with more severe and persistent disorders such as CAS would report poorer psychosocial outcomes. Method Groups were compared using analyses of variance on a psychosocial assessment battery that included measures of hyperactivity and inattention, anxiety, depression, internalizing and externalizing behaviors, thought problems, and social outcomes. Results Results revealed significant group differences on self-report of social problems and parent report of hyperactivity, thought problems, and social problems at adolescence. Compared to the SSD-only group, the CAS group had significantly higher parental ratings of hyperactivity and social problems in adolescence. The CAS and SSD + LI groups did not differ on psychosocial measures, possibly due to the high rate of comorbid LI in the CAS group. The CAS group also had more individuals who scored in the borderline/clinical range on self-report of social problems than the SSD-only group. The CAS group did not differ from the SSD + LI group in the number of participants scoring in the borderline/clinical range on measures. Conclusions Individuals with histories of CAS demonstrate increased rates of social problems and hyperactivity based on parent ratings compared to adolescents with histories of SSD only; however, most do not score within the clinical range. The persistence of speech sound errors combined with self-reported and parent-reported social difficulties suggests that speech-language pathologists should be sensitive to the social and emotional impact of CAS and make appropriate referrals to mental health professionals when warranted.

Association between genes regulating neural pathways for quantitative traits of speech and language disorders.

Speech sound disorders (SSD) manifest as difficulties in phonological memory and awareness, oral motor function, language, vocabulary, reading, and spelling. Families enriched for SSD are rare, and typically display a cluster of deficits. We conducted a genome-wide association study (GWAS) in 435 children from 148 families in the Cleveland Family Speech and Reading study (CFSRS), examining 16 variables representing 6 domains. Replication was conducted using the Avon Longitudinal Study of Parents and Children (ALSPAC). We identified 18 significant loci (combined p < 10-8) that we pursued bioinformatically. We prioritized 5 novel gene regions with likely functional repercussions on neural pathways, including those which colocalized with differentially methylated regions in our sample. Polygenic risk scores for receptive language, expressive vocabulary, phonological awareness, phonological memory, spelling, and reading decoding associated with increasing clinical severity. In summary, neural-genetic influence on SSD is primarily multigenic and acts on genomic regulatory elements, similar to other neurodevelopmental disorders.

Differential Long-Term Outcomes for Individuals With Histories of Preschool Speech Sound Disorders.

Purpose The goal of this study was to determine whether adolescent outcomes for individuals with histories of early speech sound disorders (SSD) could be differentiated by speech and language skills at earlier ages (preschool, 4-6 years, and school age, 7-10 years). Method The study used a retrospective longitudinal design. Participants with and without histories of early SSD were classified in adolescence as having no SSD, resolved SSD, low multisyllabic word (MSW; difficulty with MSW repetition but no errors in conversational speech), or persistent speech disorders (errors in both conversational speech and MSW repetition). Analysis of variance was employed to determine whether early speech, language, and literacy skills distinguished these adolescent outcome groups. Results Preschool and school-age skills differed for adolescents whose SSD had resolved from those who had persistent speech errors. Adolescents with errors solely in production of MSWs (Low MSW) did not differ in early speech and language skills from adolescents who had difficulty with both MSWs and persistent errors in conversation. Conclusions Speech and language assessments earlier in childhood can help establish risks for persistent SSD and other language and literacy difficulties in adolescence. Early identification of these clinically relevant subgroups of SSD may allow for early targeted interventions. Supplemental Material https://doi.org/10.23641/asha.9932279.

Reading Outcomes for Individuals With Histories of Suspected Childhood Apraxia of Speech.

Purpose The primary aims of this study were to examine the speech-language correlates of decoding difficulties in children with histories of suspected childhood apraxia of speech (sCAS) and to identify predictors of low-proficiency reading levels. Method Participants were school-age children and adolescents, 7-18 years of age, diagnosed with sCAS (n = 40) or speech sound disorder but no sCAS (SSD-no sCAS; n = 119). The sCAS and SSD-no sCAS reading groups were compared on measures of performance IQ, oral language, phonological awareness, rapid automatic naming, diadochokinetic rates, single word articulation, and multisyllable and nonsense word repetition. Logistic regression analyses were employed to identify predictors of low-proficiency reading in the sCAS and SSD-no sCAS groups. Results Sixty-five percent of the participants with sCAS compared to 24% of those with SSD-no sCAS were classified as low-proficiency readers based on nonsense and single word decoding. Analysis failed to reveal significant differences in reading, oral language, or phonological awareness between low-proficiency readers with sCAS and low-proficiency readers with SSD-no sCAS. Oral language and phonological awareness skills were the best predictors of reading level for all participants, followed by performance on multisyllabic word repetition and diadochokinetic rate. Conclusions The language and phonological awareness deficits of children with sCAS are related to their risks for reading failure. To a lesser degree, motor speech deficits and speech sound production also increase risks for reading difficulties. The findings justify early intervention for this subset of children.

Relationship between speech-sound disorders and early literacy skills in preschool-age children: impact of comorbid language impairment.

OBJECTIVE: Disorders of articulation or speech-sound disorders (SSD) are common in early childhood. Children with these disorders may be at risk for reading difficulties because they may have poor auditory, phonologic, and verbal memory skills. METHODS: We sought to characterize the reading and writing readiness of preschool children with SSD and identify factors associated with preliteracy skills. Subjects were 125 children aged 3 to 6 years with moderate to severe SSD; 53% had comorbid language impairment (LI). Reading readiness was measured with the Test of Early Reading Ability-2 (TERA) and writing skills with the Test of Early Written Language-2 (TEWL), which assessed print concept knowledge. Linear regression was used to examine the association between SSD severity and TERA and TEWL scores and analysis of variance to examine the effect of comorbid LI. Performance on a battery of speech and language tests was reduced by way of factor analysis to composites for articulation, narrative, grammar, and word knowledge skills. RESULTS: Early reading and writing scores were significantly lower for children with comorbid LI but were not related to SSD severity once language status was taken into account. Composites for grammar and word knowledge were related to performance on the TERA and TEWL, even after adjusting for Performance IQ. Below average language skills in preschool place a child at risk for deficits in preliteracy skills, which may have implications for the later development of reading disability. CONCLUSION: Preschool children with SSD and LI may benefit from instruction in preliteracy skills in addition to language therapy.

Speech and language skills of parents of children with speech sound disorders.

PURPOSE: This study compared parents with histories of speech sound disorders (SSD) to parents without known histories on measures of speech sound production, phonological processing, language, reading, and spelling. Familial aggregation for speech and language disorders was also examined. METHOD: The participants were 147 parents of children with SSD (58 fathers and 89 mothers) who were directly tested and interviewed for family history of disorders. RESULTS: Thirty-six parents (18 mothers and 18 fathers) reported enrollment in speech therapy as children for SSD. Parents with a history of speech therapy obtained lower scores on the Multisyllabic Word Repetition, Nonword Repetition, and Tongue Twister tasks than parents without such histories. These parents also had poorer reading, spelling, and receptive language skills. Parents with histories of SSD and additional language impairments (LI) performed worse than parents with isolated SSD on all measures except Pig Latin and oral motor skills. Familial aggregation for SSD and LI was supported. In addition, the likelihood of SSD in a family member increased by a factor of 1.9 over rates of SSD found in individuals without additional family members with SSD, and the odds of LI increased by a factor of 4.1 over rates of LI found in individuals without additional family members with LI for each additional family member with SSD or LI, respectively. CONCLUSIONS: The results documented both residual effects in adulthood of childhood SSD and familial aggregation for SSD. These residual difficulties do not appear to affect educational and occupational outcomes.

The genetic bases of speech sound disorders: evidence from spoken and written language.

The purpose of this article is to review recent findings suggesting a genetic susceptibility for speech sound disorders (SSD), the most prevalent communication disorder in early childhood. The importance of genetic studies of SSD and the hypothetical underpinnings of these genetic findings are reviewed, as well as genetic associations of SSD with other language and reading disabilities. The authors propose that many genes contribute to SSD. They further hypothesize that some genes contribute to SSD disorders alone, whereas other genes influence both SSD and other written and spoken language disorders. The authors postulate that underlying common cognitive traits, or endophenotypes, are responsible for shared genetic influences of spoken and written language. They review findings from their genetic linkage study and from the literature to illustrate recent developments in this area. Finally, they discuss challenges for identifying genetic influence on SSD and propose a conceptual framework for study of the genetic basis of SSD.

Dimensions of early speech sound disorders: A factor analytic study.

UNLABELLED: The goal of this study was to classify children with speech sound disorders (SSD) empirically, using factor analytic techniques. Participants were 3-7-year olds enrolled in speech/language therapy (N = 185). Factor analysis of an extensive battery of speech and language measures provided support for two distinct factors, representing the skill dimensions of articulation/phonology and semantic/syntactic skills. To validate these factors, 38 of the children were followed to school age to re-evaluate speech and language skills and assess reading/spelling achievement. The validity of the two factors was supported by their differential associations with school-age reading and spelling achievement, persistence of SSD, and affection status in family members. A closer relationship of the family member to the proband and male gender predicted higher odds of a disorder. The findings suggest that articulation/phonology and language abilities are at least partially independent in children with SSD and that these constructs have distinct clinical and biological correlates. LEARNING OUTCOMES: The reader will develop knowledge about subtypes of speech sound disorders, understand the relationship between early speech sound disorders and later reading and spelling difficulties, and obtain information concerning familial transmission of speech sound disorders.

Psychosocial co-morbidities in adolescents and adults with histories of communication disorders.

BACKGROUND: Few studies have considered the long-term psychosocial outcomes of individuals with histories of early childhood speech sound disorders (SSD). Research on long-term psychosocial outcomes of individuals with language impairment (LI) have frequently failed to consider the effects of co-morbid SSD. The purpose of this study was to compare individuals with histories of SSD-only versus SSD with LI on these outcomes and to examine the contributions of other comorbid conditions including reading disorders (RD) and Attention Deficit Hyperactivity Disorder (ADHD). METHODS: Participants were adolescents aged 11-17 years (N=129) and young adults aged 18-33 years (N=98). Probands with SSD were originally recruited between 4 and 6 years of age and classified into SSD-only and SSD+LI groups. Siblings of these children were also assessed at this time and those without SSD or LI were followed as controls. Outcome measures at adolescence and adulthood included ratings of hyperactivity, inattention, anxiety, and depression, as well as internalizing, externalizing, social, and thought problems. Adult outcomes also included educational and employment status and quality of life ratings. Regression modeling was performed to examine the association of SSD, LI, RD, and ADHD with psychosocial outcomes using Generalized Estimating Equations. RESULTS: In the adolescent group, LI was associated with poorer ratings of psychosocial problems on all scales except depression. Histories of SSD-only, RD and ADHD did not independently predict any of the adolescent psychosocial measures. In contrast, LI in the adult sample was not significantly associated with any of the behavior ratings, though RD was related to higher ratings of hyperactivity and inattention and with higher parent ratings of internalizing and externalizing symptoms and thought problems. SSD did not predict any of the adult measures once other comorbid conditions were taken into account. CONCLUSIONS: Poor adolescent psychosocial outcomes for individuals with early childhood SSD were primarily related to comorbid LI and not to SSD per se. At adulthood, comorbid RD and ADHD may influence outcomes more significantly than LI.

Adolescent outcomes of children with early speech sound disorders with and without language impairment.

PURPOSE: In this study, the authors determined adolescent speech, language, and literacy outcomes of individuals with histories of early childhood speech sound disorders (SSD) with and without comorbid language impairment (LI) and examined factors associated with these outcomes. METHOD: This study used a prospective longitudinal design. Participants with SSD (n = 170), enrolled at early childhood (4-6 years) were followed at adolescence (11-18 years) and were compared to individuals with no histories of speech or language impairment (no SSD; n = 146) on measures of speech, language, and literacy. Comparisons were made between adolescents with early childhood histories of no SSD, SSD only, and SSD plus LI as well as between adolescents with no SSD, resolved SSD, and persistent SSD. RESULTS: Individuals with early childhood SSD with comorbid LI had poorer outcomes than those with histories of SSD only or no SSD. Poorer language and literacy outcomes in adolescence were associated with multiple factors, including persistent speech sound problems, lower nonverbal intelligence, and lower socioeconomic status. Adolescents with persistent SSD had higher rates of comorbid LI and reading disability than the no SSD and resolved SSD groups. CONCLUSION: Risk factors for language and literacy problems in adolescence include an early history of LI, persistent SSD, lower nonverbal cognitive ability, and social disadvantage.

Family pedigrees of children with suspected childhood apraxia of speech.

UNLABELLED: Forty-two children (29 boys and 13 girls), ages 3-10 years, were referred from the caseloads of clinical speech-language pathologists for suspected childhood apraxia of speech (CAS). According to results from tests of speech and oral motor skills, 22 children met criteria for CAS, including a severely limited consonant and vowel repertoire, difficulty sequencing syllables, and inconsistent and unusual errors. Family pedigrees for these children were constructed through parent interviews and direct testing of nuclear family members. Familial aggregation for speech-sound and language disorders was demonstrated with 86% reporting at least one nuclear family member affected. Based on parent report, 13 of the 22 children (59%) had at least one affected parent. However, CAS was evident in only two siblings of probands with CAS and two probands with other speech-sound disorders. Based on testing, overall affection rates of speech-sound/language disorders were higher in families of children with CAS than in families of children with other speech-sound disorders. Mothers of children with CAS demonstrated a higher affection rate than mothers of children with other speech-sound disorders. A sex-related threshold model of transmission was also supported with brothers more often affected than sisters for male probands only. If our inclusionary criteria for CAS are valid, these findings support a general verbal trait deficit hypothesis. LEARNING OUTCOMES: (1) As a result of this activity, the participant will understand potential familial risk factors for CAS; (2) will differentiate aggregation for speech-sound and language disorders in families with CAS from families of children who have other speech-sound disorders; (3) will distinguish how familial aggregation differs in families of boys and girls with CAS; (4) will determine how children with CAS differ in severity from those with other speech-sound disorders.

School-age follow-up of children with childhood apraxia of speech.

PURPOSE: The primary aim of this study was to examine differences in speech/language and written language skills between children with suspected childhood apraxia of speech (CAS) and children with other speech-sound disorders at school age. METHOD: Ten children (7 males and 3 females) who were clinically diagnosed with CAS (CAS group) were followed from the preschool years (ages 4-6) to school age (ages 8-10) and were compared with children with isolated speech-sound disorders (S group; n = 15) and combined speech-sound and language disorders (SL group; n = 14). Assessments included measures of articulation, diadochokinetic rates, language, reading, and spelling. RESULTS: At follow-up, 8 of the children with CAS demonstrated improvement in articulation scores, but all 10 continued to have difficulties in syllable sequencing, nonsense word repetition, and language abilities. The children also exhibited comorbid disorders of reading and spelling. Group comparisons revealed that the CAS group was similar to the SL group, but not the S group during the preschool years. By school age, however, the SL group made more positive changes in language skills than the CAS group. CLINICAL IMPLICATIONS: These findings suggest that the phenotype for CAS changes with age. Language disorders persist in these children despite partial resolution of articulation problems. Children with CAS are also at risk for reading and spelling problems.

Association between AVPR1A, DRD2, and ASPM and endophenotypes of communication disorders.

OBJECTIVES: Speech sound disorder (SSD) is one of the most common communication disorders, with a prevalence rate of 16% at 3 years of age, and an estimated 3.8% of children still presenting speech difficulties at 6 years of age. Several studies have identified promising associations between communication disorders and genes in brain and neuronal pathways; however, there have been few studies focusing on SSD and its associated endophenotypes. On the basis of the hypothesis that neuronal genes may influence endophenotypes common to communication disorders, we focused on three genes related to brain and central nervous system functioning: the dopamine D2 receptor (DRD2) gene, the arginine-vasopressin receptor 1a (AVPR1A) gene, and the microcephaly-associated protein gene (ASPM). METHODS: We examined the association of these genes with key endophenotypes of SSD - phonological memory measured through multisyllabic and nonword repetition, vocabulary measured using the Expressive One Word Picture Vocabulary Test and Peabody Picture Vocabulary Test, and reading decoding measured using the Woodcock Reading Mastery Tests Revised - as well as with the clinical phenotype of SSD. We genotyped tag single nucleotide polymorphisms in these genes and examined 498 individuals from 180 families. RESULTS: These data show that several single nucleotide polymorphisms in all three genes were associated with phonological memory, vocabulary, and reading decoding, with P less than 0.05. Notably, associations in AVPR1A (rs11832266) were significant after multiple testing correction. Gene-level tests showed that DRD2 was associated with vocabulary, ASPM with vocabulary and reading decoding, and AVPR1A with all three endophenotypes. CONCLUSION: Endophenotypes common to SSD, language impairment, and reading disability are all associated with these neuronal pathway genes.

Reading skills of students with speech sound disorders at three stages of literacy development.

PURPOSE: The relationship between phonological awareness, overall language, vocabulary, and nonlinguistic cognitive skills to decoding and reading comprehension was examined for students at 3 stages of literacy development (i.e., early elementary school, middle school, and high school). Students with histories of speech sound disorders (SSD) with and without language impairment (LI) were compared to students without histories of SSD or LI (typical language; TL). METHOD: In a cross-sectional design, students ages 7;0 (years;months) to 17;9 completed tests that measured reading, language, and nonlinguistic cognitive skills. RESULTS: For the TL group, phonological awareness predicted decoding at early elementary school, and overall language predicted reading comprehension at early elementary school and both decoding and reading comprehension at middle school and high school. For the SSD-only group, vocabulary predicted both decoding and reading comprehension at early elementary school, and overall language predicted both decoding and reading comprehension at middle school and decoding at high school. For the SSD and LI group, overall language predicted decoding at all 3 literacy stages and reading comprehension at early elementary school and middle school, and vocabulary predicted reading comprehension at high school. CONCLUSION: Although similar skills contribute to reading across the age span, the relative importance of these skills changes with children's literacy stages.

Speech-Sound Disorders and Attention-Deficit/Hyperactivity Disorder Symptoms.

PURPOSE: The purpose of this study was to examine the association of speech-sound disorders (SSD) with symptoms of attention-deficit/hyperactivity disorder (ADHD) by the severity of the SSD and the mode of transmission of SSD within the pedigrees of children with SSD. PARTICIPANTS AND METHODS: The participants were 412 children who were enrolled in a longitudinal family study of SSD. Children were grouped on the basis of the severity of their SSD as determined by their scores on the Goldman-Fristoe Test of Articulation and history of an SSD. Five severity groups were compared: no SSD, resolved SSD, mild SSD, mild-moderate SSD, and moderate-severe SSD. Participants were also coded for comorbid language impairment (LI), based on scores on a standardized language test. Pedigrees of children were considered to represent bilineal inheritance of disorders if there was a history for SSD on both the maternal and paternal sides of the family. Parents completed the ADHD rating scale and a developmental questionnaire for each of their children. RESULTS AND CONCLUSIONS: Children with moderate-severe SSD had higher ratings on the inattention and hyperactive/impulsivity scales than children with no SSD. Children whose family pedigrees demonstrated bilineal inheritance had higher ratings of inattention than children without bilineal inheritance. To determine the best predictors of ADHD ratings, multiple linear regression analyses were conducted. LI was more predictive of ADHD symptoms than SSD severity, bilineal inheritance of SSD, age, or gender. Findings support that LI rather than SSD is associated with ADHD.

Neural correlates of phonological processing in speech sound disorder: a functional magnetic resonance imaging study.

Speech sound disorders (SSD) are the largest group of communication disorders observed in children. One explanation for these disorders is that children with SSD fail to form stable phonological representations when acquiring the speech sound system of their language due to poor phonological memory (PM). The goal of this study was to examine PM in individuals with histories of SSD employing functional MR imaging (fMRI). Participants were six right-handed adolescents with a history of early childhood SSD and seven right-handed matched controls with no history of speech and language disorders. We performed an fMRI study using an overt non-word repetition (NWR). Right lateralized hypoactivation in the inferior frontal gyrus and middle temporal gyrus was observed. The former suggests a deficit in the phonological processing loop supporting PM, while the later may indicate a deficit in speech perception. Both are cognitive processes involved in speech production. Bilateral hyperactivation observed in the pre and supplementary motor cortex, inferior parietal, supramarginal gyrus and cerebellum raised the possibility of compensatory increases in cognitive effort or reliance on the other components of the articulatory rehearsal network and phonologic store. These findings may be interpreted to support the hypothesis that individuals with SSD may have a deficit in PM and to suggest the involvement of compensatory mechanisms to counteract dysfunction of the normal network.