Inhibitory control of memory retrieval and motor processing associated with the right lateral prefrontal cortex: evidence from deficits in individuals with ADHD

Abstract

Studies of inhibitory control have focused on inhibition of motor responses. Individuals with ADHD consistently show reductions in inhibitory control and exhibit reduced activity of rLPFC activity compared to controls when performing such tasks. Recently these same brain regions have been implicated in the inhibition of memory retrieval. The degree to which inhibition of motor responses and inhibition of memory retrieval might involve overlapping systems has been relatively unexplored. The current study examined whether inhibitory difficulties in ADHD extend to inhibitory control over memory retrieval. During fMRI 16 individuals with ADHD and 16 controls performed the Think/No-Think (TNT) task. Behaviorally, the Stop Signal Reaction Time task (SSRT) was used to assess inhibitory control over motor responses. To link both of these measures to behavior, the severity of inattentive and hyperactive symptomatology was also assessed. Behaviorally, ADHD individuals had specific difficulty in inhibiting, but not in elaborating/increasing memory retrieval, which was correlated with symptom severity and longer SSRT. Additionally, ADHD individuals showed reduced activity in rLPFC during the TNT, as compared to control individuals. Moreover, unlike controls, in whom the correlation between activity of the rMFG and hippocampus predicts inhibitory success, no such correlation was observed for ADHD individuals. Moreover, decreased activity in rIFG in individuals with ADHD predicted a decrease in the ability to inhibit motor responses. These results suggest that inhibitory functions of rLPFC include control over both memory and motoric processes. They also suggest that inhibitory deficits in individuals with ADHD extend to the memory domain.

Published by Elsevier Ltd.

Figures

Fig. 1

Behavioral results from both ADHD and control individuals. Ω,* indicates statistically significant within group comparisons (p<.10 p while indicates statistically significant between group comparisons adhd individuals baseline recall="58%," think trials="64.7%" and no-think for control errors bars represent standard error.>

Fig. 2

Brain regions showing group differences during NT>T trials. Orange indicates greater activity in ADHD individuals, while blue identifies greater activity in control individuals (Z>2.81, p<.005 cluster threshold of p except for hippocampus amygdala and thalamus>

Fig. 3

Comparison of percent signal change for the contrast of NT>baseline in ADHD (red) and controls (blue) for the critical ROIs involved in inhibition of emotional memory retrieval. * indicates statistically significant within group comparisons, while + indicates between group (p<.05 error bars represent standard error.>

Fig. 4

Correlations between activity in brain regions (ROIs) and inhibition of memory. Bonferroni corrected correlations between brain regions’ mean percent signal change across individuals (inter-individual) is shown next to arrows connecting those brain region (i.e., next to orange and green arrows). Bonferroni corrected correlations between mean percentage signal change for a given brain region (inter-individual) and retrieval inhibition (measured as a percentage of NT trials correctly suppressed) are shown in blue boxes (connected to the retrieval inhibition rectangle by blue arrows). Correlations between the time series of activity across brain regions (intra-individual) within an individual and retrieval inhibition (correlations in small circles, connected to the retrieval inhibition rectangle by brown arrows). Results for the control group are shown in blue, while those for the ADHD group are shown in red. * indicates statistically significant within group comparisons, while + indicates between group determined by Fisher’s Z (p<.05>