Motor System Integrity in Older Adults with Autism Spectrum Disorder

Background:  Gait disturbance, clumsiness, and other mild movement problems are often observed in persons with autism spectrum disorder (ASD; Maurer, 1982). This study focused on brain differences that may indicate the neural basis for these motor symptoms which are common, although not ubiquitous, among individuals with ASD.

Objectives:  Using magnetic resonance imaging (MRI), we examined cortical and white matter integrity in brain regions associated with motor function (motor cortex, cerebellum, basal ganglia) in a cross-sectional study comparing middle-aged (40-65 years) adults with ASD and age-matched typically developing (TD) controls. We hypothesized gray matter and white matter tracts associated with motor functioning would be smaller in middle-aged adults with ASD, as compared to TD controls. We also expected to observe reduced motor performance (slower finger tapping speed) in the ASD group as compared to their TD counterparts.

Methods: Thirteen right-handed men with ASD (Age: 49.2 (7.3); Education: 14.9 (2.7); IQ: 109.5 (14.5)) and 17 matched-TD men (Age: 49.4 (6.9); Education: 15.8 (2.6); IQ: 110.1 (11.1)) were recruited. Motor performance was measured using a finger tapping task. 3D T1 structural and diffusion tensor images were obtained using a Philips 3T scanner. FreeSurfer was used to determine gray and white matter volumes. White matter integrity was assessed via fractional anisotropy (FA), masked with each person’s white matter map via voxel based morphometry in SPM8.

Results:  The mean number of right finger taps in the allotted time was less in the ASD (47.2 (13.0)) group compared to TDs (51.6 (7.0)), but was not statistically significant. However, this is likely due to the much greater degree of variability in the ASD group compared to controls. This finding was also observed for the left finger tapping. The ASD group had smaller white matter volume for bilateral cerebellum (left; p=0.05, right; p=0.04), and brainstem white matter volume (p=0.04) compared to the TD group. There were no significant differences between ASD and TD participants for cortical or subcortical (gray matter) volume.

Conclusions:  Smaller cerebellar white matter volume may account for the slower motor differences observed in older adults with ASD. Previous studies have found anatomical differences in the cerebella of younger ASD subjects (Courchesne et al., 2001), consistent with our findings of cerebellar differences. This cross-sectional study aims to extend those findings to aging adults. Interestingly, finger tapping speed was extremely variable among the ASDs, and it is possible that other factors beyond the structures examined here play a role in sustained fine motor movements.