An investigation of ocular motor and fine motor control in autism and Asperger's disorder: furthering neurobehavioural definitions

Autistic disorder (henceforth referred to as autism) and Asperger’s Disorder (AD) are neurodevelopmental disorders that share characteristic social deficits and repetitive and stereotyped behaviours and interests. In the Diagnostic and Statistical Manual of MentalDisorders - Fourth edition, text revision (DSM-IV-TR), AD is differentiated from autism on the basis of early language acquisition. However there has been considerable debate as to whether clinical separation of the two conditions is justified, particularly given the considerable overlap between those with autism who have normal intellectual ability (high functioning autism; HFA) and AD. It has been proposed that in the next revision of the Diagnostic and Statistical Manual of Mental Disorders, due to be released in 2013, that the two disorders be merged into a single autism spectrum disorder diagnostic category. Based on recent studies by Gibbs et al (2012), the proposed ASD diagnostic criteria will result in approximately 20-40% of children who currently meet DSM-IV-TR criteria for autism or Asperger’s disorder not meeting diagnosis, despite having significant impairments. Therefore, ongoing research to more clearly define and understand the behavioural and neurobiological phenotypes of autism and AD will be important for creating an evidence base to contribute to future revisions of the DSM. Motor coordination difficulties are increasingly regarded as a commonly associated symptom of HFA and AD. Characterising and elucidating the mechanisms of motor coordination difficulties HFA and AD is essential to developing comprehensive definitions of these disorders, as well as tailoring of appropriate interventions. Furthermore, recent evidence has found subtle dissociation of the two groups based on a profile of motor function and neuroanatomical abnormalities. Behavioural and neuroimaging studies have implicated greater involvement of the cerebellum in the pathology of autism than in AD. Impairment of the cerebellar and fronto-striatal circuitry results in distinct changes to motor profiles. Therefore, such research has the potential to shed light on the neurobiological distinctiveness between autism and AD on the basis of a unique cerebellar-related motor profile. Accordingly, the central aims of this thesis were: 1. to use ocular motor performance and kinematic assessments of handwriting movements in order to further characterise motor abnormalities in HFA and AD 2. elucidate the contributions of the cerebellum and fronto-striatal circuitry in motor impairment in HFA and AD, and 3. to determine whether any differences exist between the two groups. Experiment 1 comprehensively characterized the profile of visually guided saccades. It was revealed that those with HFA showed greater eye movement inaccuracy across both small and large movement amplitudes, which was not observed in AD. In Experiment 2, a saccade adaptation paradigm was used to further investigate motor learning, through the refinement of eye movement precision and the role of visual feedback. This experiment confirmed slower refinement of eye movement accuracy in response to visual error in HFA. Furthermore, both HFA and AD were slower to respond to visual feedback, which was more pronounced in HFA than AD. Together the findings from Experiment 1 and 2 are consistent with greater disruption of the cerebellum in HFA than AD. Saccade accuracy (Experiment 1) and saccade correction time (Experiment 2) were significantly correlated with manual dexterity as measured by the Movement Assessment Battery for Children – 2nd edition (MABC-2). Findings from Experiments 3 and 4, which kinematically examined the control of handwriting, found that both children with HFA and AD are heavily reliant on visual cues in modulating the size and consistency of their movements. Experiment 3 revealed that while both children with HFA and AD were successfully able to scaled the size and trajectory of their handwriting movements, HFA showed an additional increase in movement velocity. Although handwriting movements remained accurate despite the increase in movement velocity, movement velocity was negatively correlated with manual dexterity and total motor performance as assessed by the MABC-2. In Experiment 4, in which no visual guides were presented, both HFA and AD were unable to maintain the size and consistency of handwriting movements, and handwriting became macrographic (abnormally large). There were a greater number of perseverative errors and higher movement velocities in the HFA group compared to TD children. ADHD is another neurodevelopmental disorder that shares fronto-striatal abnormalities. However ADHD has a different clinical profile to HFA, characterised primarily by hyperactivity and inattention. Moreover, the motor profiles of the two disorders are also distinct: while HFA shows higher movement variability and difficulties shifting motor plans (as seen in greater numbers of perseverative errors), ADHD show impaired motoric inhibition and decreased cognitive control of motor behaviours. Both HFA and ADHD are known to have handwriting impairments, but standard treatment of handwriting difficulties currently uses a ‘one size fits all’ approach. Experiment 5, which used a naturalistic handwriting task, contrasted the profile of handwriting difficulties between HFA and ADHD. Experiment 5 revealed no difficulties with spelling and corrections in the HFA group, but rather with the spatial arrangement of letters and words within a sentence. By contrast ADHD showed increase spelling and correction errors, but no impairments in spatial arrangement. These findings help to further delineate the boundaries of the profile HFA and neurodevelopmental disorders. A summary of overall findings across both ocular motor and handwriting studies revealed that motor difficulties in both autism and AD may be underpinned by an overreliance on visual cues for controlling movement size, and impaired visual feedback mechanisms. Children with HFA were distinguishable from those with AD by greater inaccuracy of eye movements during simple, visually guided eye movements, slower movement correction time following to visual feedback and protracted ocular motor learning. These findings fit within the overarching hypothesis that subtle motor abnormalities may dissociate HFA and AD. Moreover, they are consistent with the hypothesis that while both conditions share deficits suggestive of frontal-basal ganglia abnormalities, HFA is characterised by additional motor abnormalities suggestive of additional disruption to cerebellar circuitry.