There is now a growing body of literature showing that children with a variety of neurodevelopmental disorders are ‘at risk’ of motor impairments. One such disorder is specific language impairment (SLI) (see Hill (Reference Hill2001) for a review and Rechetnikov & Maitra (Reference Rechetnikov and Maitra2009) for a meta-analysis). This is despite the fact that SLI is diagnosed on the basis of language difficulties: (i) significantly below age level; (ii) out of keeping with other aspects of development (usually based on a substantial discrepancy between language test scores and non-verbal IQ); and (iii) not explained by physical causes such as hearing loss, abnormal structure of the articulators or by recognized syndromes associated with language delay such as Pervasive Developmental Disorder (American Psychiatric Association, 1994). Thus motor difficulties are not included in the SLI diagnosis. While there has been much debate concerning accurate definition of this disorder (see Bishop (Reference Bishop1997) for a review), this discussion has not included whether motor difficulties should be a part of the diagnostic criteria. Nonetheless, a relatively large proportion of children with SLI have been reported to show motor impairment profiles similar to those seen in children with developmental coordination disorder (DCD; sometimes referred to as dyspraxia). DCD is diagnosed in those whose motor skill: (i) is out of keeping with chronological age and general ability; (ii) interferes with activities of daily living and/or educational achievement; and (iii) can not be accounted for by learning disability or other medical condition (e.g. muscular dystrophy or cerebral palsy) (American Psychiatric Association, 1994). In research samples, between 40% and 91% of children with SLI have been suggested to have motor difficulties similar to those seen in DCD (see Hill, Reference Hill2001). There is also some evidence of a genetic overlap between motor and language impairment within an SLI sample (Bishop, Reference Bishop2002). Overall, however, this has not been an area of much focused research and it remains unclear whether those children who do not appear to meet criteria for DCD exhibit subtle movement difficulties relative to their peers, or whether movement skill is totally preserved in this group. Since SLI is not a rare disorder, with an estimated prevalence of around 7% (Tomblin, Records, Buckwalter, Zhang, Smith & O'Brien, Reference Tomblin, Records, Buckwalter, Zhang, Smith and O'Brien1997), it is important to characterize the precise range of difficulties experienced by individuals with SLI. This will aid therapeutic interventions, educational support and good outcome across the lifespan.

Despite the relative paucity of research data on the extent and nature of motor difficulties in SLI, as well as the possible relationship between language and motor difficulties, some focus has been placed recently on the question of co-occurring difficulties across motor and cognitive domains within neurodevelopmental disorders, where the difficulties under investigation fall outside the scope of a specific disorder's diagnostic classification: For example, motor difficulties in autism spectrum disorder (see Mari, Castiello, Marks, Marraffa & Prior (Reference Mari, Castiello, Marks, Marraffa and Prior2003) for a review) and dyslexia (e.g. Ramus, Pidgeon & Frith, Reference Ramus, Pidgeon and Frith2003) as well as working memory difficulties in DCD (Alloway & Archibald, Reference Alloway and Archibald2008). Other recent, related evidence comes from the literature on typical development where views on the importance of emerging skill in one domain impacting on another in early development are gaining ground (cf., for example, Iverson's (this issue) timely and stimulating account; Bremner, Holmes & Spence, Reference Bremner, Holmes and Spence2008; Bremner & Spence, Reference Bremner and Spence2008).

The remainder of this commentary will focus on motor difficulties in SLI. In the past, the presence of motor difficulties in this disorder has been related to shared cognitive processes, as well as to neuroanatomical and neuromaturational accounts (see Hill (Reference Hill2001) for a review). However, no one explanation appears to account fully for the observed data. More recently, there have been suggestions of a genetic association (Bishop, Reference Bishop2002). This is a promising aspect for future research, although it is likely that a complex interaction of genetic and environmental effects is at play. As Iverson notes, her interpretation of the literature on language and motor development provides a possible developmental mechanism for the observed co-occurrence of motor and language difficulties in many individuals with SLI. This is particularly appealing in a field that has often drawn parallels between developmental and acquired disorders. Such comparisons are useful up to a point, but fail to account for the crucial influence of abnormality of a developing system and the dynamic impact of impairment in one domain on development of another (see, for example, Bishop, Reference Bishop1997; Karmiloff-Smith, Reference Karmiloff-Smith2007).

Iverson's account allows for motor deficits to be observed in many, but not all, individuals with language impairment. Moreover, her account allows for (in my view) the presence of severe motor difficulties in some individuals with SLI and more subtle difficulties in others. Finally, a broad range of motor skills may be affected to a greater or lesser degree across individuals. This account allows for, and is supported by, the SLI subgroups reported in our work on motor skill in SLI (see below).

Since motor development is described by Iverson as being ‘normally participatory’ in the emergence of language, it may also be possible that at least some children diagnosed with DCD – whose diagnosis depends on typical development in other domains – might show a degree of language difficulty. This might be striking in some cases, and subtler in others. Given the focus of most societies on language for communication, it would be most likely that the former group of children would be diagnosed with a language impairment and then referred for occupational therapy on the basis of poor motor skill, rather than the other way round. However, the latter group – those with DCD and subtle language difficulties – might be identified first on the basis of their motor difficulties. At the point of writing, there appears to be only one published study investigating language profiles in children diagnosed with DCD but not SLI (Archibald & Alloway, Reference Archibald and Alloway2008). In this language comparison study of eleven children with SLI and eleven children with DCD, a wide variety of standardized and non-standardized language tests including vocabulary, grammatical skill, non-word repetition, sentence recall, story retelling and articulation rate were administered. It is reported that language impairment co-occurred frequently with DCD. For the most part, the language profile of the two groups was similar.

One question that will be important for future research is whether there are differences in the language impairment profiles of children with and without diagnosed motor difficulties (and vice versa). From the limited literature to date, it appears that motor difficulties are more similar than different in children with SLI who do versus do not meet criteria for DCD. Specifically, in our earlier research we compared children with SLI who met criteria for DCD (including falling below the 15th percentile cut-offs on the Movement ABC (Henderson & Sugden, Reference Henderson and Sugden1992), a standardized movement test battery commonly used in the UK as part of a diagnosis of DCD) with a matched group of children with SLI who did not meet criteria for DCD. Both SLI groups had motor profiles that were not significantly different from an age and non-verbal ability matched group of children with DCD. This was the case in both speeded and non-speeded motor tasks. While speeded performance might be expected to be impaired in SLI (in line with Tallal's temporal processing deficit theory of the disorder; see, for example, Fitch & Tallal, Reference Fitch and Tallal2003), difficulties on non-speeded movements such as gesture imitation and arm placing without vision would not be predicted by this account (Hill, Reference Hill1998; Hill & Bishop, Reference Hill and Bishop1998; Hill, Bishop & Nimmo-Smith, Reference Hill, Bishop and Nimmo-Smith1998). Subtyping studies (in which one is aiming to identify endophenotypesFootnote ¹) will be crucial to unpack the nature of the relationship between motor and language difficulties in neurodevelopmental disorders.

How might our understanding of subgroups evolve? Prospective, longitudinal studies that focus on subgroups of children with neurodevelopmental difficulties, as well as evaluation of the association between specific components of motor and language skills in large studies of typical development, will further understanding of this issue. It will also be important to model the role of IQ, as different mechanisms may scaffold the existing abilities of at least some individuals with neurodevelopmental disorders. Indeed, subtyping studies are essential for the improvement of our understanding of co-occurring behavioural features that are increasingly being reported across neurodevelopmental disorders. One useful method will be to apply Morton's (Reference Morton2004; see also Morton & Frith, Reference Morton, Frith, Cichetti and Cohen1995) causal modelling approach to this question. The causal modelling approach is a graphical tool for thinking about neurodevelopmental disorders. It has four constituent parts: biology, cognition, behaviour and the environment (see Figure 1). In turn, these factors may themselves be broken down. In the biological domain membrane proteins, receptors and neurons, for example, may all play a role. Likewise, cognitive processes will be broken down into their constituent parts; for example, phonological processing, verbal short-term memory and skill automatization. Behaviours include word naming, digit span and peg-moving. Finally, aspects of the environment may have an influence at any level of the model. These influences may be protective or destructive and might include intra-uterine environment, parenting style, diet or relationships. Cognition is crucial in the causal modelling approach, as it acts as a mediator between biological and behavioural features. Finally, it is likely that there will be interactions between different levels of the model, such that relationships between components will be bi-directional, having a developmental impact on one another over time.

Fig. 1. Morton's (Reference Morton2004; Morton & Frith, Reference Morton, Frith, Cichetti and Cohen1995) causal modelling approach.

From the point of view of the co-occurrence of motor and language difficulties in neurodevelopmental disorders, Iverson's account of the relationship between language and motor development is worthy of detailed investigation. I believe that it could form the basis of a more complex ‘normally participating’ account where other domains come on-line as the infant grows from their first year, through the toddler and preschool years, into middle childhood and beyond. Social interaction, for example, would be a candidate by this point, as would executive function skills whose protracted development stems from infancy but develops gradually through adolescence and beyond. Lack of sophisticated motor skill (for a child's age) will logically lead to all sorts of potential difficulties in addition to those in the language domain outlined by Iverson. For example, locomotion in the environment allows communication but also social interaction. This allows, for example, experience with turn-taking and executive functions. Of course, these are not the only situations in which such skills can be recruited and practised, but they arguably have an impact and may lead to reduced practice and reinforcement at the cognitive and behavioural levels as well as altered neuronal connectivity, pruning, etc. at the biological level of Morton's model. There is certainly evidence for structural differences in neuroanatomy in individuals with SLI (e.g. Soriano-Mas, Pujol, Ortiz, Deus, López-Sala & Sans, Reference Soriano-Mas, Pujol, Ortiz, Deus, López-Sala and Sans2009), although this has yet to be investigated in DCD. It should be noted that this is a rather speculative suggestion at this point, and needs to be confirmed through substantial future research programmes.

To conclude, I quote from Iverson's own conclusion where she states that ‘there is a relationship between motor development and language development, but it is complex and multi-faceted rather than simple and directional (Iverson, this issue, p. 000). The account that she presents accords well with data from neurodevelopmental disorders showing increased motor difficulties in those with language difficulties (and here I have focused on specific language impairment, although the same arguments would follow for other disorders which include language difficulties and show increased motor difficulties, such as autism spectrum disorder). For a long time, a critical challenge for researchers in this field has been the nature and cause of this co-occurrence. Iverson's account presents an invaluable base on which to build future research in this area.