Neural Correlates of Unsuccessful Inhibition and Selective Attention in Young Adults with ASD, ADHD and Comorbid Presentation during the Modified Stop-Signal Task

Background:

Individuals with attention-deficit/hyperactivity disorder (ADHD) or with autism spectrum disorder (ASD) often demonstrate motor response inhibition deficits. Neurobehavioural findings in the ASD population suggest that these deficits could be associated with co-occurring ADHD symptoms.^{1, 2} A recent study showed that boys with ASD showed over-activation in the bilateral inferior frontal gyri (IFG) in contrast to age-matched ADHD boys who demonstrate under-activation in the IFG instead.³ To what extent the neural correlates of motor inhibition in individuals with combined presentation (i.e., ASD+ADHD) are similar or different to the “pure” condition is presently unknown, however.

Objectives:

This study investigated the neural correlates of motor response inhibition and selective attention in young adult males with ASD, ADHD, ASD+ADHD and typical development (TD) using the functional magnetic resonance imaging (fMRI).

Methods:

We tested 91 adult males (age 20-27 years, FSIQ ≥ 70) with ASD (n = 21), ADHD (n = 25), ASD+ADHD (n = 23) against TD controls (n = 22) on a modified stop-signal task, that included oddball trials to control for selective attention. Medication intake and ADHD subtypes were balanced across groups with ADHD diagnoses. The task comprised 200 “Go”, 40 “Oddball” and 60 “Stop” trials and required participants to cancel responses already initiated upon the arrival of an infrequent stop signal. A tracking algorithm was implemented to adjust the stop-signal arrival time to ensure that task performance converged to 50% success rate. Analyses were undertaken using the Statistical Parametric Mapping (SPM8) software. The correlates of successful/unsuccessful response inhibition, controlling for selective attention, was studied by contrasting the successful/unsuccessful Stop vs. Oddball trials. Selective attention was examined by contrasting the Oddball vs. Go trials. Whole-brain comparison was conducted with univariate ANCOVAs (peak threshold p < .001 and extent threshold p <.05, family-wise error corrected) on these contrasts, covarying for volume-to-volume total movement. Significant clusters of activation across groups were extracted using MarsBaR toolbox for post-hoc pairwise comparison, Dunn-Sidak corrected.

Results:

Univariate ANCOVAs revealed significant group effects during unsuccessful response inhibition in left insula/IFG/superior temporal/middle temporal gyri (p < .001) and right insula/IFG/thalamus/parahippocampal gyrus (p < .001) with post-hoc pairwise comparison showing that the ASD+ADHD group was significantly under-activated (all ps ≤ .001) in these clusters compared to all other groups, which did not differ from one another. A group effect was also found during selective attention in right precuneus with the ASD and ASD+ADHD groups demonstrating over-activation relative to the TD and ADHD groups (all ps ≤ .01). The neural correlates of successful inhibition did not differ across groups. All results remained after covarying for IQ.

Conclusions:

During unsuccessful inhibition, young adults with comorbid ASD+ADHD demonstrate impairments in key regions of motor response inhibition typically found in boys with ADHD, which was neurofunctionally associated with error monitoring or late-arriving motor inhibition activation. The over-activation found in right precuneus in the ASD and ASD+ADHD groups could signify reduced de-activation of the default mode network, which is typically suppressed during task positive condition.