Manual Motor Performance Related to Autistic Traits, Daily Living Skills, and White Matter Microstructure in Autism Spectrum Disorder

Background: Mounting evidence suggests that individuals with Autism Spectrum Disorder (ASD) exhibit poorer motor skills and that these poorer motor skills may be related to more severe core ASD symptomatology and poorer adaptive functioning (Hilton et al., 2007; MacDonald, Lord, & Ulrich, 2013; Radonovich et al., 2013;Travers et al., 2013). However, the causal nature of this relation is unknown. Because motor function, social communication, and adaptive functions likely require an integration of information across the entire brain, it is possible that decreased white matter microstructural integrity is a third variable that could be affecting both motor skills and core ASD symptoms.

Objectives:  1) To examine the relation between manual motor performance (i.e., grip strength and finger tapping speed) and ASD traits/adaptive functioning, while controlling for age and IQ. 2) To examine whether the relation between motor function and ASD symptomatology/adaptive functions was mediated by whole-brain white matter microstructural integrity, as measured by Diffusion Tensor Imaging (DTI).

Methods: Seventy males with ASD and 41 males with typical development between the ages of 5 and 33 years participated in this study (Time 1 data of a broader longitudinal study). Participants completed bimanual measures of grip strength and finger tapping speed. The Social Responsiveness Scale measured autistic traits, and the Vineland Adaptive Behavior Scale measured adaptive functioning. Participants completed a DTI scan (12 directions, b = 1000 s/mm², one b0 image, 60 contiguous axial slices, matrix= 128 x 128, FOV = 256mm, resolution= 2 x 2 x 2.5 mm, averages = 4, TR = 7000 ms, TE = 84 ms). Average whole-brain white matter fractional anisotropy (FA) was calculated across the deep white matter tracts. Independent-samples t-tests examined group differences in manual motor performance. Partial correlations examined the relation among manual motor performance, ASD traits/adaptive functioning, and whole brain FA, while controlling for age and IQ.  

Results: The ASD group demonstrated weaker grip strength, t(109) = 2.14, p = .04, and slower finger tapping, t(109)=2.03, p=.046, compared to the typically developing group. After controlling for age and IQ, weaker grip strength, r=-.20, p<.05, and slower finger tapping, r=-.20, p<.05, were found to relate to more severe autistic traits across the combined sample. Similarly, weaker grip strength was found to relate to more difficulty with daily living skills, r=+.28, p=.007. Grip strength, r=+.23, p=.01, and finger tapping speed, r=+.35, p<.001, were also found to relate to whole-brain white matter FA across both groups combined. However, there were no reliable relations between whole-brain white matter FA and autistic traits or daily living skills (p’s>.48). 

Conclusions:  The present results suggest that manual motor performance is associated with autistic traits, daily living skills, and white-matter microstructural integrity in individuals with ASD. However, after controlling for age and IQ, white matter microstructure was not related to autistic traits or daily living skills, which suggests that whole-brain white matter microstructure is not a mediating variable between motor function and autism symptomatology.