Objectives: Given the importance of brain connectivity for cognitive and social functioning, we wished to examine whether there were already differences at 6 months of age in the degree of maturation in white matter tracts between infants at low and high risk for ASD.
Methods: In addition to undergoing structural MRI, infants who participated in the IBIS study were also scanned using a 25 direction Diffusion Tensor Imaging Sequence. Data collected using this sequence enabled us to investigate the diffusion properties of white matter fiber tracts in 79 infants at high risk for ASD and 27 infants at low risk for ASD (Age 6 months -1 wk/ +3 wks) . Diffusion properties including fractional anisotropy, Frobenius Norm, and radial and axial diffusivity were calculated. Tract statistics were computed for the genu, splenium, mid corpus callosum, inferior longitudinal fasciculus, uncinate, and the motor and sensory tracts. Several of these tracts were chosen because previous work has demonstrated that the support social cognition, an area in which individuals with ASD are impaired.
Results: Results from multivariate functional tract analyses revealed that there were no differences in tensor shape, as measured by FA, or tensor volume, as measured by FRO, between the two groups in any of the tracts of interest. However, a further analysis, in which infants were split into two narrower age groups (Infants below 203 days, Mean 190d, std 7.8d and infants above 203 days: Mean 227d, std 19.7d), revealed a reduction in diffusion in all of the fiber tracts in the group of older infants. This age effect confirmed that DTI is sensitive to differences in white matter maturation even within a narrow time window at this young age and indicated that the lack of difference found between the HR and LR groups was not due to a lack of sensitivity of our method.
Conclusions: The results from this ongoing study do not reveal any differences between infants at high risk for ASD and those at low risk for ASD in the diffusion properties of several major fiber tracts in the brain. However, these children are being followed longitudinally until 24 months old, so future work will examine the fiber tract diffusivity across time, to see if developmental trajectories differ between those children who receive a diagnosis of ASD and those who do not.
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