A Visual Perceptual Task Provides Evidence for an Excitatory:Inhibitory Imbalance in Adults with Autism

Background: Sensory processing abnormalities are a key symptom of autism spectrum disorders (ASDs). In line with the emerging excitatory:inhibitory imbalance (E:I) hypothesis of ASD symptoms, it has been hypothesized that these sensory traits may be related to an imbalance in GABA and glutamate neurotransmission. However, this has not been directly tested in adults with an ASD..

Objectives: In order to investigate the functional balance between E:I neurotransmission in the visual cortex, we utilized the paradoxical motion perception task (PMPT). In this paradigm, high visual contrast normatively produces an impairment in perception of the direction of motion. This effect is referred to as paradoxical because, under most conditions, higher contrast stimuli are easier to perceive. The PMP effect reflects GABAergic lateral inhibition between neurons in visual cortex.

Methods: Medication-free adult males with ASD were recruited through a specialist clinic. All ASD participants were diagnosed using ADI-R and ADOS. Healthy control adult males were recruited via community advertisements. The two groups did not differ on age or full-scale IQ (t-test, p > 0.4). Participants age ranged from 18-56 years (mean 30.6) and IQ ranged from 95 to 139 (mean 119). Total n=30. During the PMPT, participants viewed vertical greyscale sine wave gratings of varying size (small vs. large) and contrast (bright vs. faint). On each trial, stimuli drifted either left or right at a constant speed. The task was to indicate the direction of motion, based on a brief presentation. The difficulty (duration of stimulus presentation) varied from trial to trial according to an adaptive 2-up-1-down staircase design with one staircase for each of the four stimulus conditions. In line with previous work, contrast impairment on motion perception is defined as the ratio between the estimated threshold (in milliseconds) for large-bright compared to large-faint stimuli.

Results: Adults with an ASD showed significantly reduced contrast-induced perceptual impairment (p=0.022, t₂₈ = 2.487). In other words, in the high contrast condition, individuals with an ASD selectively performed better than the controls. This effect was not accounted for by overall difference in ability to perceive motion or to follow task instructions. Also, performance was not correlated with age (p=0.89) or IQ (p=0.2).

Conclusions: Adults with ASD have a selective enhancement of motion perception under conditions of high contrast. These data - the first results using this task in adults with ASD - are consistent with GABAergic abnormalities and an E:I imbalance model of the condition.