Atypical Maturation of Oculomotor and Manual Motor Response Inhibition In ASD

Background:  Eye movement studies of individuals with ASD have demonstrated compromised abilities to voluntarily inhibit prepotent behavioral responses that may be related to repetitive behaviors.  Yet, little is known about whether this deficit is specific to the oculomotor system, how it is expressed across development, and whether it is related to repetitive behaviors in ASD.

Objectives:  To examine age-related changes in manual motor and oculomotor response inhibition deficits in individuals with ASD, and to identify whether these deficits underlie repetitive behavior subtypes.

Methods:  Forty-one individuals with ASD and 36 healthy controls matched on age (range 6-38 years) and Performance IQ were administered manual motor and oculomotor stop-signal tasks (SST) and blocks of 100% go trials to determine mean RT in that context.  During the SST, subjects were instructed to either press a button (manual version) or make a saccade (oculomotor version) when a peripheral target appeared ('go' trials), or inhibit these responses when a central stop signal appeared at varying times following the appearance of the peripheral cue ('stop' trials).  Subjects' reaction times and rate of incorrect stop trials (i.e., pressing a button or looking towards the peripheral target) were examined.  The Repetitive Behavior Scale-Revised (RBS-R) was used to assess repetitive behaviors. 

Results:  Subjects with ASD made more errors on stop trials than healthy controls on the manual motor and oculomotor tasks (p’s<.05).  Stop trial error rate was associated with the degree to which subjects slowed their reaction times from baseline trials to SST task go trials, such that increased slowing during the interspersed go and stop trial conditions was associated with fewer stop trial errors (p<.01).  Subjects with ASD did not slow their reaction times as much as controls (p<.05).  Increased age was associated with fewer stop trial errors and increased slowing of reaction times in healthy controls (p’s<.01), but not in subjects with ASD.  The increased stop trial error rates in ASD participants were related to increased rates of compulsive behaviors on the RBS-R (r = -0.432, p<.05).

Conclusions:  Inhibiting learned or prepotent responses involves suppression of motor pathways via input from PFC and striatum.  These frontostriatal systems show significant maturation into adulthood.  Our findings suggest that this dysmaturation of frontostriatal brain systems may underlie some repetitive compulsive behaviors that are evident in many individuals with ASD.  Treatments aimed at enhancing cognitive control and frontostriatal function may reduce these compulsive behaviors.