Comparison of motor deficits in autism spectrum disorder and developmental coordination disorder
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Autism Spectrum Disorder (ASD) is an umbrella term for disorders involving deficits in social interaction, stereotyped behaviours and communication dificulties. A growing area of research has recently focused on motor deficits in ASD, which have been noted in clinical observations and diagnostic criteria since autism was first described. However, motor deficits have traditionally carried little weight in the diagnostic procedure. Until recent changes to diagnostic criteria (Diagnostic and Statistical Manual 5th edition: DSM-5), a comorbid diagnosis of Developmental Coordination Disorder (DCD: a neurodevelopmental disorder affecting motor development) was not possible for those with ASD and motor deficits. This exclusion criterion prompted an investigation of the nature of motor deficits in ASD, questioning whether they are characteristically different from motor deficits in DCD. Previous literature suggested a possible double dissociation in the use of vision and proprioception to guide movement and perception in ASD and DCD, with a reliance on proprioception in ASD, and an over-reliance on vision in DCD. Motor deficits were first investigated by looking at high-level motor skills, and then more basic sensory processing associated with movement to investigate this possible dissociation. There was no significant difference between ASD and DCD on a standardised motor battery (Movement Assessment Battery for Children 2nd edition: MABC-2), with 70% of children with ASD showing motor difficulties within the clinical range on tasks such as timed manual dexterity tasks and balance. Similarly, children with ASD and poor motor skills were indistinguishable from children with DCD on a number of basic motor tasks manipulating visual and proprioceptive cues. These tests included spatial location matching, reaching, goal-directed movements towards proprioceptively-defined targets, and the rubber hand illusion. Children with poor motor skills with a diagnosis of either ASD or DCD seemed to either rely more heavily on visual cues, or behaved in a similar way to typically developing (TD) children. In the spatial location matching task, children with ASD and spared motor skills showed a tendency to give more weight to proprioceptive cues, however too few children with ASD and spared motor skills took part in other tasks to fully investigate cue weighting in this subgroup. Mirroring the overlap in social and motor skills in the clinical groups, a study of the relationship between perceived social and motor ability in a large sample of TD children highlighted the related nature of these developmental domains in typical development. It is concluded that motor deficits in ASD are not ASD-specific but are instead indicative of an additional diagnosis of DCD. This is supported by the recent change to diagnostic criteria.