Response Inhibition in Manual and Oculomotor Systems in ASD

Background: Studies of response inhibition in ASD have yielded inconsistent findings, although considerable evidence now documents increased error rates on oculomotor inhibition tasks. In particular, inconsistencies in findings with manual and oculomotor paradigms raise questions about the task or neural system specificity of response inhibition deficits in ASD. 

Objectives: To examine manual and oculomotor response inhibition in individuals with ASD. 

Methods: Thirty-one individuals with ASD and 27 healthy controls matched on age (range 6-38 years) and Performance IQ were administered one manual motor and one oculomotor stop-signal task (SST). During SST’s, 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 following the appearance of the peripheral cue (‘stop’ trials).  Subjects’ baseline manual and saccade latencies (i.e., reaction times) also were examined. 

Results: During the manual (p=.011) and oculomotor (p<.001) SSTs, individuals with ASD made more errors than controls.  For both tasks, adults (≥18 years) with ASD were relatively more impaired than children and adolescents with ASD suggesting diminished improvement in this cognitive ability with age in adult patients.  For both tasks, subjects increased their response latencies from the baseline measures to ‘go’ trials of the SSTs.  The amount of slowing in reaction times on the SST task relative to the control task was associated with the percentage of correct ‘stop’ trials for manual (r= .71) and oculomotor (r= .57) tasks.  This suggests that the more individuals slowed their reaction times, the better they were at suppressing responses when stop signals were presented.  Individuals with ASD did not increase their reaction times as much as controls for either the manual (p=.001) or oculomotor (p=.001) SST.

Conclusions: These results indicate that individuals with ASD show response inhibition deficits that affect both manual and oculomotor systems, and both of these deficits appear to reflect a reduced ability to adaptively and strategically slow reaction times to enhance the ability to suppress responses on stop trials.  Individuals with ASD are limited in their ability to utilize this strategic bias and thus are less able to voluntarily suppress reflexive/automatic responses in a context appropriate fashion.  Results also indicated that the developmental trajectory of manual and oculomotor inhibitory control may follow distinct developmental time courses in ASD.  For both tasks, performance improves beyond early adolescence for healthy individuals but less so for adults with ASD.  Studies of SST performance in ASD therefore may offer insight into the timing and nature of atypical brain development affecting the voluntary control of manual and oculo- motor systems.  The pathophysiology of these alterations has yet to be elucidated in ASD, but our results indicate that studies of SST performance may yield important insights into cognitive and neural mechanisms associated with these disorders.