Motivational context for response inhibition influences proactive involvement of attention

Motoric inhibition is ingrained in human cognition and implicated in pervasive neurological diseases and disorders. The present electroencephalographic (EEG) study investigated proactive motivational adjustments in attention during response inhibition. We compared go-trial data from a stop-signal task, in which infrequently presented stop-signals required response cancellation without extrinsic incentives ("standard-stop"), to data where a monetary reward was posted on some stop-signals ("rewarded-stop"). A novel EEG analysis was used to directly model the covariation between response time and the attention-related N1 component. A positive relationship between response time and N1 amplitudes was found in the standard-stop context, but not in the rewarded-stop context. Simultaneously, average go-trial N1 amplitudes were larger in the rewarded-stop context. This suggests that down-regulation of go-signal-directed attention is dynamically adjusted in the standard-stop trials, but is overridden by a more generalized increase in attention in reward-motivated trials. Further, a diffusion process model indicated that behavior between contexts was the result of partially opposing evidence accumulation processes. Together these analyses suggest that response inhibition relies on dynamic and flexible proactive adjustments of low-level processes and that contextual changes can alter their interplay. This could prove to have ramifications for clinical disorders involving deficient response inhibition and impulsivity

The Effect of Colour on Response Execution and Inhibition in the Stop Signal Paradigm

Recent investigations have shown that smooth pursuit target selection is biased according to a hierarchy of red, green, yellow, and blue (e.g. red is always selected over green). This implies that colours higher on the hierarchy have greater attentional salience. Using the stop signal task, we conducted experiments in which go signal colour was manipulated (exps. 1 and 2) and in which stop signal colour was manipulated (exp. 3) to determine whether the hierarchy also applied to response execution and inhibition. When colour was either irrelevant (exp. 1) or relevant (exp. 2) to response execution there was no effect on reaction times or individual RT variance. When colour was relevant to response inhibition (exp. 3) estimated stop signal reaction times were significantly faster for red (~225ms) relative to green (~250ms) stop signals. This suggests that response inhibition, but not execution, networks are sensitive to differences in colour salience

Impulsivity in Parkinson’s disease and Tourette syndrome, and human motor decision making

Motor response inhibition pertains to the ability to inhibit motor actions. It is hypothesised that a breakdown in motor response inhibition might underlie impulsivity in Parkinson’s disease and tics in Tourette syndrome. This thesis outlines how motor response inhibition is modulated in these clinical disorders by first characterising them in healthy subjects. We use TMS to show that one set of inputs to the motor cortex are inhibited during motor preparation whilst the other inputs reflect uncertainty about potential stopping. In the next chapter, we challenged an assumption that movement preparation during proactive inhibition always preceded movement execution and found that movement preparation and execution are two independent processes. With this in mind, we investigated features of motor response inhibition and movement preparation and execution in patients with Tourette syndrome, finding that these were remarkably similar to healthy controls, suggesting that volitional features of movement and inhibition are normal in Tourette syndrome. However, we did find a specific impairment of automatic inhibition in Tourette syndrome, which correlated with motor tic severity. As dopamine agonists are implicated as triggers for impulsivity in Parkinson’s disease, we first investigated the influence of ropinirole on motor response inhibition in healthy control subjects, finding that motor response inhibition was globally impaired. This was accompanied by analyses suggesting that ropinirole impaired the ability to adjust the decision threshold when stopping might be required. However, investigation of motor inhibition in Parkinson’s disease patients on dopamine agonists showed unremarkable effects compared to patients without dopamine agonist use. Our data provide a novel insight into the basic mechanisms of voluntary movement and propose a new theory for tic generation in Tourette syndrome