Atypical Motion Sensitivity Characterized By Larger Receptive Fields in Autism Spectrum Disorder

Background: Individuals with autism spectrum disorder (ASD) demonstrate atypical visual processing abilities across a wide range of tasks. However, we still have a limited understanding of mechanisms contributing to such perceptual abnormalities. Recent studies have identified two possible mechanistic explanations: differences in 1) contrast gain control, a mechanism responsible for regulating the amplitude of neural responses in relation to stimulus contrast, and 2) visual receptive field (RF) sizes, which can affect how the brain responds to stimuli of different sizes. Notably, both of these mechanisms are important for maintaining the balance between excitatory and inhibitory (E/I) neural responses. This is of specific interest given growing evidence for E/I imbalance in ASD.

Objectives: To investigate the integrity of contrast gain control and RF size, and their effects on visual motion perception in ASD.

Methods: Participants were 20 children and adolescents with ASD and 20 age- and IQ- matched typically developing (TD) controls. A visual motion discrimination task was employed, where participants judged the motion directions (left or right) of briefly presented moving gratings. Contrast and size of stimuli were manipulated in three conditions. In two Size conditions, stimulus size varied (1-8 deg in radius) while the contrast was fixed at either low (2.3%) or high (99%) contrast. In the Contrast condition, contrast varied (2-99%) with a fixed stimulus size (1 deg). Duration thresholds (i.e., minimum stimulus duration required to reliably judge motion direction) were estimated to evaluate motion perception. A computational model, based on divisive normalization of E/I responses with parameters representing gain control and RF sizes, was fitted to the thresholds, thus assessing the underlying mechanisms contributing to differences in performance. Group differences in thresholds were assessed using a Mixed Model ANOVA, and a non-parametric bootstrap analysis was used to compare the model parameters between groups. 

Results: Individuals with ASD had higher thresholds (worse performance) across all contrast levels in the Contrast condition (F(1,38) = 5.49, p < 0.05). No significant group differences in thresholds were observed in either of the Size conditions (p’s > 0.05), but there was a trend for higher thresholds overall in the Low-contrast Size condition, and for the smallest size in the High-contrast Size condition in ASD. The model analysis revealed this pattern of results can be explained by a significantly larger excitatory RFs in ASD compared to TD (p< 0.05). 

Conclusions: Our threshold results reveal decreased perceptual sensitivity to motion directions in ASD for small stimuli across all contrast levels. This motion perception difference was best explained by larger size of excitatory RF in ASD, consistent with a previous fMRI study showing larger population RFs in ASD (Schwarzkopf et al., 2014). Our findings suggest that differences in RF sizes may disrupt the E/I balance in ASD; reduction in excitatory responses to stimuli smaller than the RF size may lead to changes in the E/I response ratio and perceptual sensitivities to stimuli with different sizes. Future studies can further investigate the association between RF size differences and other behavioral and sensory symptoms of ASD.