Power and Coherence Abnormalities in Autistic Children: Insights into Local Processing and Long Distance Functional Connectivity

Background: Autistics show a diminished ability to integrate complex information but enhanced perceptual processing abilities for a variety of simple sensory stimuli. fcMRI scans have shown diminished long distance functional connectivity between visual and frontal cortices in autistics but increased functional connectivity in thalamocortical circuits involving visual system. EEG power spectrum measures the amount of electrocortical activation in regional neural networks across brain wave frequencies. EEG coherence measures electrocortical synchrony of oscillatory brain rhythms between neural networks, which is hypothesized as a mechanism for functional connectivity. We sought to test the hypothesis that autistics would show increased power in primary sensory cortices during sensory stimulation but that the increase in power would be associated with decreased long distance coherence, and hence functional connectivity, between primary sensory cortices in right and left hemispheres. Objectives: To collect long latency flash visual evoked potentials in age-matched autistic and control subjects and to compare power spectrum and coherence patterns for the two groups in the primary visual cortex. Methods: Flash visual evoked potentials (VEP) were recorded from children (ages 5 to 8 yr) with autism spectrum disorders (ASD, n = 6) and from age-matched controls (n = 8) using high-density EEG recording. VEP power spectrum was determined from averaged wave forms over a 1000 ms time frame after the stimulus and coherence was computed between all channel pairs for each frequency band over the 1000 ms time frame after the stimulus. Analysis focused on primary visual areas. Results: Group comparisons showed increased spectral power in the ASD group, predominately over left occipital region, across a range of frequencies including the delta, alpha, beta, and gamma bands as well as the high gamma band above 80 Hz, suggesting increased activation of visual cortex in children with ASD over controls. Simultaneously, coherence between left and right visual areas was reduced in children with ASD in the delta and beta bands, with a trend toward reduction in the alpha band, suggesting reduced cross-hemispheric functional connectivity in children with ASD compared to controls. The fact that local EEG power increased at the same time that cross-hemispheric coherence was reduced argues strongly that our coherence result indexes neural connectivity and is not a mere consequence of volume conduction. Conclusions: Taken together, these results support the view that children with ASD have hyper-responsive primary sensory cortices in the brain with reduced long distance inter-hemispheric functional connectivity and hence decreased inter-hemispheric integration within sensory processing systems. Hyper-responsiveness of the primary visual cortex may relate to fcMRI findings of increased thalamocortical connectivity in autism.