Introduction
The development of language skills is one of the most important achievements of early childhood. Language acquisition leads to new opportunities in other developmental areas, such as cognitive, social, and emotional development and is a cornerstone for successful outcomes later in life (Schlichting et al.
1995). Although language ability usually develops spontaneously, it does not progress typically for a substantial number of children. Children with autism spectrum disorder (ASD) show impairments in language development (Tager-Flusberg et al.
2005; Eigsti et al.
2011). The development of functional language serves an important prognostic function (Mahwood et al.
2000). Thus, gaining a better understanding of language impairments in ASD is important for development and improvement of interventions.
In order to provide a context in which we can evaluate the language impairments in ASD, typical language development will be described briefly. Bates (
1979) indicated two critical transitions in early childhood which precede language development: (1) development of communicative intentionality, and (2) symbol formation. The first transition is the onset of communicative intentionality, when children become aware that their signals influence behavior of others. In typical development, intentionality emerges when children start to use proto-imperative and proto-declarative behaviors, i.e., to obtain a desired object/event or to share attention/interest between persons on a common focus, by the end of their first year of life (9–13 months) (Baron-Cohen
1989; Bates
1979; Camaioni
1997).
The second transition concerns the emergence of symbol understanding and use. According to Bates (
1979) symbol formation requires an understanding of the relationship between a sign and its referent. The sign can replace that referent in a variety of situations, even if the referent is not currently present. During the first years of life children gradually learn to understand and use symbols, which is reflected in the development of conventional gestures, words, and symbolic play (Wetherby et al.
1998). Language is a complex symbolic communication system, for which symbol-referent relations cannot be inferred through observations, but have to be learned step by step. Children may use first words before they have any explicit symbolic understanding. Only when a word is generalized beyond the initial learning environment, it may be granted symbol use and understanding (Namy and Waxman
2005). Next to the verbal domain, the process of symbol development can also be examined in the nonverbal domain. Symbolic play requires the ability to symbolize, and awareness of the relation between the present play object or action and its absent referent in daily life (Jarrold et al.
1997). Symbolic play has been linked to language development in typical children and young children with ASD (e.g., Lewis et al.
2000; McCune
1995; Toth et al.
2006). Other symbolic abilities, such as understanding of pictures, have not been studied in relation to language abilities yet. Stephenson and Linfoot (
1996) argued that the use of pictures as symbols has clear parallels to the use of words as symbols.
In typical semantic development, language comprehension always precedes production (Fenson et al.
2000a). Word comprehension emerges around 9 months of age and by the first birthday toddlers usually say their first recognizable words. By this age, they can understand many more words and even some simple phrases (Fenson et al.
1994). Initially, meaning is linked to a specific context. Between 12 and 18 months of age, receptive and expressive vocabulary gradually increases and becomes less context-bound (Tager-Flusberg et al.
2009). The production of phrases starts between 18 and 24 months of age (Fenson et al.
1994). During the preschool period, the vocabulary and complexity of grammar expand rapidly (Tager-Flusberg et al.
2005).
Limited intentionality and symbol formation are considered to be core deficits in communication of individuals with ASD (Noens and Van Berckelaer-Onnes
2005; Travis and Sigman
2001; Wetherby et al.
2000). Young children with ASD exhibit intentional communication less frequently than typically developing children or children with developmental delays (e.g., Chiang et al.
2008; Shumway and Wetherby
2009). A reduced amount of intentional communication is also seen in older children with autistic disorder and associated ID (Maljaars et al.
2011). The most striking impairments in ASD are found in proto-declarative communication or joint attention (e.g., Mundy and Burnette
2005). Several studies found a predictive relationship between joint attention and expressive language development for young children with ASD (e.g., Charman et al.
2003; McDuffie et al.
2005; Smith et al.
2007; Watt et al.
2006). Symbolic impairments have already long been associated with ASD (Hammes and Langdel
1981; Ricks and Wing
1975), but it remains unclear whether a more general impairment in symbol formation is related to language impairments in ASD.
Children with ASD vary widely in their language abilities and features. Only a few studies analyzed differences in expressive versus receptive language abilities based on a within-group design (Tager-Flusberg et al.
2005). In toddlers with ASD, lower levels of both expressive and receptive language compared to typical control or norm groups were found. In contrast to typical development, language comprehension is often even more delayed relative to production (Charman et al.
2003; Luyster et al.
2008; Hudry et al.
2010; Weismer et al.
2010). Generally, in older, high-functioning children with ASD the discrepancy between receptive and expressive language decreases (Rapin and Dunn
2003; Tager-Flusberg et al.
2005). Jarrold et al. (
1997) described relatively uniform language profiles in children and adolescents with ASD with equal levels of expressive and receptive language. Another study from Kjelgaard and Tager-Flusberg (
2001) found that receptive skills were comparable to expressive skills on vocabulary tests.
Research into ASD has focused on high-functioning, rather than low-functioning children with ASD. In case of ASD with co-occurring intellectual disability (ID), children are ‘double impaired’ with respect to language abilities. A large proportion of low-functioning individuals with ASD do not develop language or acquires at most a few functional words or signs (Boucher et al.
2007; Rapin and Dunn
2003). Language comprehension is always impaired in low-functioning individuals with ASD, although severity varies (Boucher et al.
2007). A higher level of expressive language compared to receptive language can for example be due to echolalia or the ability to label or to reproduce memorized language in specific contexts (Tager-Flusberg et al.
2005). In this way, true level of language understanding is concealed. Not surprisingly, impairments in receptive language are strongly associated with severe behavior problems (Sigafoos
2000). Language abilities and characteristics in children with ASD are closely related to nonverbal cognitive level of functioning (Luyster et al.
2008; Weismer et al.
2010).
Identifying factors that are associated with concurrent language abilities and possibly influence differential outcomes is necessary for improving treatments. General intellectual level of functioning is probably the most important factor associated with language development in children with ASD (Luyster et al.
2008; Weismer et al.
2010), but this factor does not explain all variance in language abilities. Other developmental factors, such as social and cognitive abilities, are also crucial (Thurm et al.
2007; Prizant
1996). Low-functioning children with ASD often exhibit more severe delays in language development relative to their nonverbal cognitive level. The question is whether limitations in language abilities in older children with ASD and ID are associated with social aspects, such as the ability to initiate and respond to joint attention, or with cognitive aspects, such as the understanding of symbols. Both limited intentionality and symbol formation are considered to be core deficits in communication development of individuals with ASD (Travis and Sigman
2001). Joint attention cannot be the only explaining factor for language impairment in children with ASD, because older children with ASD sometimes have impaired joint attention skills but adequate levels of language (Bloom
2000). Indirect evidence for an association between symbol formation and language abilities is that individuals with ASD are impaired in their development of different skills that require symbolic abilities, such as language and symbolic play. Furthermore, in contrast with for example specific language impairment, problems in language development seem to be amodal in ASD (Boucher et al.
2007). Nonverbal children with ASD often fail to compensate their lack of verbal communication with other modalities, such as gestures (Mundy et al.
1994). These findings support the hypothesis of a relationship between language development and symbolic capacities in the nonverbal domain.
Research into specific language profiles and the relation with other developmental aspects in low-functioning children with ASD is important from the perspective of clinical practice, but also for research. First, a lack of sufficient functional language can result in challenging behavior, in particular in low-functioning nonverbal individuals (Chiang
2008; McClintock et al.
2003), and form a major stressor for parents (Lecavalier et al.
2006). Language abilities are closely related to later prognosis (Mawhood et al.
2000). Therefore, more insight into developmental factors which are associated with language development in low-functioning children with ASD is important. Better understanding of language impairments in low-functioning children is necessary in order to improve interventions and care. Second, the results can be useful for genetic and neurobiological studies. Individuals with ASD and associated ID are more often included in genetic studies than in behavioral or cognitive studies. Subsetting by language phenotypes has proven to be useful in genetic research, and also led to identification of genome-wide significant loci (Abrahams and Geschwind
2008). Therefore, the main purposes of this study are: (1) to study differences and similarities between receptive and expressive language abilities in children with autistic disorder and ID compared to children with ID without ASD and typically developing children; and (2) to examine whether and how precursors of language (joint attention and symbol understanding) are related to concurrent receptive and expressive language abilities in these groups.
Discussion
There is growing interest in studying early language acquisition in toddlers with ASD. Although language is often extremely impaired in older children with ASD and associated intellectual disability (ID), less is known about language profiles and related factors in this low-functioning group with ASD. The first purpose of our study was to examine within- and between-group differences regarding receptive and expressive language in children with autistic disorder (AD) and ID, children with ID without ASD, and typically developing children. Nonverbal mental age was comparable across groups. The second purpose of this study was to examine the contribution of nonverbal cognitive level of functioning and two important precursors of language, joint attention and symbol formation, to concurrent language abilities.
Several key findings emerged from this study of language profiles in low-functioning children with autistic disorder. First, lower scores for both receptive and expressive language were found in the AD group compared to the other groups, while groups scored the same on nonverbal mental age. These results confirm the presence of severe communication problems in low-functioning children with ASD (Noens and Van Berckelaer-Onnes
2004). Second, a substantial discrepancy between expressive and receptive language skills on the one hand and nonverbal cognitive level on the other was found in the group of low-functioning children with autistic disorder. In contrast, in both comparison groups language levels approximated the nonverbal cognitive level, except for expressive language in the ID group. Hence, developmental profiles in low-functioning children with ASD were more uneven than in both comparison groups. Third, language profiles differed significantly across the three groups. Children with AD and ID achieved an expressive language level above their receptive language level, whereas children in the ID and the TD groups showed the opposite profile. However, not in all cases with ASD and ID receptive language was relatively more impaired than expressive language. Approximately half of the group achieved equal levels for receptive as for expressive language, but a substantial part showed a considerably lower level for comprehension than for production. All findings about language profiles are in line with the results for toddlers with ASD (e.g., Hudry et al.
2010; Weismer et al.
2010). However, studies in older high functioning children with ASD found no differences between expressive and receptive language abilities (Jarrold et al.
1997; Kjelgaard and Tager-Flusberg
2001). These previous results, combined with the results of this study, suggest a discrepancy in language profiles between high and low-functioning children with ASD.
The second purpose of this study was to examine the relation between nonverbal mental age, social and symbolic skills with concurrent language abilities. The results suggest that joint attention, symbol understanding, and nonverbal mental age share a substantial amount of variance in the TD and ID groups. However, in the low-functioning AD group other skills made a significant and unique contribution to language outcome. In the TD and ID groups nonverbal mental age was the most robust concurrent predictor of both language comprehension and production. Although nonverbal mental age was correlated with expressive and receptive language skills in the AD group, it did not remain significant in the final regression models, where joint attention and symbol understanding were the major predictors. In general, the role of joint attention in predicting language skills is consistent with previous findings in toddlers with ASD (e.g., Charman et al.
2003; McDuffie et al.
2005). However, the literature reported mixed findings regarding the association between joint attention and language abilities, with several studies indicating joint attention as a stronger predictor for expressive than receptive language (e.g., Watt et al.
2006), but other studies reporting opposite results (e.g., Luyster et al.
2008).
The current data suggest that symbol understanding plays an important role in both expressive and receptive language development in low-functioning children with AD. Nonverbal children with ASD also have difficulties in understanding the symbolic nature of pictures. Therefore, impairments in symbol formation could be a fundamental deficit underlying problems in language development in children with ASD (Bates
1979; Travis and Sigman
2001). The nature of the relationship between joint attention and symbol formation and the relative contribution of both precursors to language development remain unclear. Joint attention is supportive, but not necessary for the acquisition of symbols (Travis and Sigman
2001). On the basis of a longitudinal study in young children with ASD, Toth and colleagues (
2006) suggest that joint attention seems to be an important skill for the start of language development, while symbolic play—which refers to more representational or symbolic skills—is associated with the continued development and the expansion of language abilities in later years.
A limitation of our study is the use of different instruments for assessing receptive and expressive language abilities, involving both direct assessment and parent questionnaires. The parental reports were used to correct the floor effects of the direct assessment measures in the lowest functioning children in our study. This strategy was chosen, because several previous studies have suggested that there is a very close agreement among these different language measures (e.g., Fenson et al.
1994; Luyster et al.
2008; Weismer et al.
2010; Zwaigenbaum et al.
2005). No instruments were available which evaluate language comprehension and production for the whole age range of the participants included in this study. The Reynell/Schlichting and the D-CDI have the same measurement goal, i.e. assessment of receptive and expressive language, but for a different age range, and all instruments result in an age equivalent score.
The results of our study have clinical relevance with respect to assessment and intervention regarding language abilities in low-functioning children with ASD. Regarding language assessment, our findings indicate that all children with ASD and ID demonstrate significant delays in receptive and expressive language abilities, even relative to their nonverbal cognitive level. Assessment should always include language measurements, in order to elicit strengths and weaknesses so that intervention targets may be adjusted to individual needs. Moreover, both expressive and receptive language abilities need attention separately, because substantial and varying discrepancies might be present. Many low-functioning children with ASD have receptive skills that are more impaired than their expressive language skills. Particularly in the case of more verbal children, communication partners will be misled by the level of expressive language abilities, assuming a comparable level of comprehensions skills (Hudry et al.
2010; Noens and Van Berckelaer-Onnes
2004). In typical development, comprehension always precedes language production. From a developmental perspective, parents, teachers, and caregivers are used to simplifying their language. They often use words and sentence structures just above the expressive language level of children in order to encourage development. Consequently, language comprehension in low-functioning children with ASD is often overestimated, when they are approached at the level of their expressive language (Hudry et al.
2010). Several previous studies demonstrated a significant relation between communication difficulties and the severity of challenging behavior in low-functioning individuals (Bott et al.
1997; Chamberlain et al.
1993; McClintock et al.
2003). The study of Sigafoos (
2000) showed that more severe deficits in receptive language compared to expressive language are associated with challenging behavior. Problem behaviors may serve different functions (Day et al.
1994; Hanley et al.
2003; Reese et al.
2005). Some specific behavior problems might be related to receptive communication problems (e.g., to escape too difficult demands) and other to expressive communication problem (e.g., to get a desirable object which is not within reach). Unfortunately, little attention has so far been given to comprehension problems in interventions for children with ASD (Kevan
2003; Sigafoos
2000). Interventions and approaches should have improvement and support of language comprehension as a key target; interventions which only aim for expressive language improvement will widen the gap between comprehension and production. Further, the findings from our study have implications for designing interventions to improve language comprehension. Pictures or objects are often used in a symbolic and representational way to support communication in children with ASD (Wendt
2009). This strategy is problematic when language impairments are accompanied by impairments in symbolic understanding of pictures. For these children, augmentative communication adapted to their level of sense making is a recommended treatment strategy (Noens and Van Berckelaer-Onnes
2004). Facilitating joint attention development is also an important target for intervention in low-functioning children with ASD. Several studies have described techniques that may be effective (e.g. Kasari et al.
2008; Siller and Sigman
2002). Future research will need to examine the relation between symbolic understanding and language development more closely in younger children with ASD, as well as in ASD children with different levels of ID. It is important to differentiate between factors contributing to the emergence and expansion of language skills.