Functional Tests of Atypical Connectivity in the Autistic Brain using Magnetoencephalography
Coskun, Mehmet A. 1985-
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Emerging evidence for differences between individuals with autism and neurotypical controls in tactile processing suggests the somatosensory cortex as a promising substrate for identifying differences in functional brain connectivity. Identifying a neural biomarker of ASD can spur novel biological and pharmacological treatments. We tested four candidate neural biomarkers of autism using magnetoencephalography (MEG) to examine the cortical response to passive tactile stimulation of the thumb and index finger of the dominant hand and lip of individuals with and without autism. The following observations can be made: 1) Individuals with autism have bigger brains early in development. We found the distance between the cortical representations of thumb and lip was significantly larger in the autism group, but not for the representations of the index finger and lip. The inhomogeneity in cortical topography is not a straightforward consequence of a bigger brain but does demonstrate abnormality in sensory organization of the autistic brain. 2) Autistic brains have noisy synapses. The hypothesis predicts increased variability in the response to touch. We did not find higher variability in the evoked response to tactile stimulation in autism, arguing against the noisy synapse hypothesis. 3) Lower level of inhibition in brain circuits of autism is a leading biomarker candidate. The amplitude of cortical response to the stimulation of adjacent fingers is governed by the level of cortical inhibition, and is a physiological test. A comparison of the two groups found a difference in the direction opposite that of prediction. We also examined neural adaptation to prolonged stimulation as cortical inhibition levels, at least in part, controls its extent. Contrary to prediction, the somatosensory cortex of individuals with autism adapts to touch to the same extent as control. Thus, our findings do not support reduced cortical inhibition in autism. 4) Another leading candidate biomarker is local overconnectivity. When a digit is stimulated, i.e., touched, its representation in cortex is directly activated; local intracortical connections indirectly activate non-primary cortical representations corresponding to adjacent digits. Local overconnectivity in autism implies higher nonprimary/primary response ratios, which we did not observe. The results were more consistent with local underconnectivity instead.