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Weaker Neural Suppression during Visual Perception in Autism Spectrum Disorder
It has been proposed that autism spectrum disorder (ASD) is associated with an imbalance of excitation and inhibition. While this is supported by specific animal models of ASD, there is little direct experimental support for this hypothesis from studies in humans.
Objectives:
Using a visual motion paradigm, we tested whether mechanisms that regulate neural activity may be disrupted among individuals with ASD.
Methods:
Within the visual system, a powerful suppressive regulatory effect occurs in neurons when a stimulus is presented that is larger than that neuron’s classical receptive field – information from the surround suppresses the neural response to the stimulus in the center. We used a well-known behavioral paradigm that reflects this spatial suppression by measuring the amount of time required to perceive the motion direction of stimuli at various sizes. In addition, we used fMRI to measure neural suppression in the motion sensitive brain area called human MT complex (hMT+). We further tested the mechanism of this suppression by measuring levels of the inhibitory neurotransmitter GABA in the hMT+ region using MR spectroscopy. Participants included 28 young adults with ASD (10 male, 8 female) with diagnoses confirmed through ADOS, ADI and clinical judgment using DSM-5 criteria, as well as 35 age-, gender-, and IQ- matched neurotypical controls (21 male, 14 female).
Results:
In both the behavioral and fMRI measurements, we found strong evidence of reduced suppression among young adults with ASD compared to neurotypical controls (Supplemental Figures 1 & 2). However, MR spectroscopy measurements in the region of hMT+ showed no difference in GABA levels between ASD and control groups.
Conclusions:
Our results suggest that differences in suppressive regulatory mechanisms in ASD reflect changes in neural activity, but are not driven by a specific difference in GABA concentration.