Cognition and behavior: White matter shows delays in autism
A difference in how auditory nerve fibers develop may explain why children with autism process sounds a fraction of a second more slowly than typically developing children do. The result, published in September in Brain Research, also suggests that a widely used method for assessing nerve fiber structure may not be appropriate for autism research1.
The brain’s response to sound speeds up as children age. This boost in speed is known to be accompanied by the maturation of white matter — the nerve fibers that connect brain regions. Specifically, there is an age-related change in myelin, the fatty coating that helps neurons transmit signals.
In children with autism, however, the brain’s response to sound is known to be delayed. To determine whether this delay is associated with abnormal white matter, the researchers examined the structure of the nerve fibers in 53 children with autism and 39 age-matched controls, all aged 5 to 17 years.
The researchers used a technique called magnetoencephalography, which maps magnetic fields generated by neurons to map brain activity patterns. They found that although response time in children with autism is on average slower than in the controls, it increases at the same rate with age in both groups.
The researchers then looked at the structure of the white matter using diffusion tensor imaging, which measures how water molecules diffuse along nerve fibers. They found that, based on a widely used measure called fractional anisotropy, white matter in the brains of children with autism does not mature with age as it does in controls.
However, a closer look at the data reveals that this interpretation is incorrect: White matter in children with autism does mature, but differently than it does in controls.
Fractional anisotropy is a composite of two individual measurements: axial diffusivity, which assesses how water diffuses along nerve fibers, and radial diffusivity, which measures how it diffuses across the fibers. Radial diffusivity decreases with age in both groups, but more slowly in the children with autism. Meanwhile, axial diffusivity decreases only in the children with autism, in whom the fibers are still immature.
Taken together for children with autism, however, these two measures cancel each other out, incorrectly suggesting that the children’s nerve fibers do not mature over time.
The findings suggest that fractional anisotropy may not be a good way to assess white matter structure in autism, the authors say. They add that because both the response to sound and white matter structure vary widely, even among the children within each group, unpacking the exact relationship between them will require following individual children over long periods of time.
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1. Roberts T.P. et al. Brain Res. 1537, 79-85 (2013) PubMed