Inhibitory Control Processes During the Preparation and Initiation of Motor Responses

The ability to stop ongoing movements or prevent unwanted movements is fundamental to behavioural control. This thesis addresses the neural processes underlying inhibitory control and how initiation and stop processes interact to control behaviour. We conducted four studies, employing various behavioural tasks that require humans to prepare to initiate a response with the possibility that it may have to be prevented or stopped from being initiated. In the first experiment we sought to determine whether the increase in reaction time (RT) during the performance of traditional stop-signal task was due to a decreased the amount of go-related preparatory activation. We used a startling acoustic stimulus (SAS) to determine whether the go-response could be triggered involuntarily, and investigated whether the latency of SAS responses were delayed when participants were instructed that they might have to stop their response compared to when they knew they would never stop (i.e., simple RT task). We found that the go-response was prepared in advance during the stop-signal task, but to a lesser degree as indicated by the slower SAS response latency, compared to when go trials were completed in the simple RT task. Thus, even the possibility of having to stop a response reduces the level of preparatory go-activation.\ud The second experiment tested the hypothesis that behavioural control during a stop-signal task is determined by an independent race between go- and stop-processes. In this experiment we used a SAS to manipulate initiation and inhibition by probing the go- and stop-response prior to and after the stop-signal respectively in a stop-signal task. We found that the go-response could be triggered by the SAS even 200 ms following the stop-signal suggesting that behavioural control during a stop-signal task is not determined by an independent race between go- and stop-activations, but rather by an interaction between go-activation and stop processes. \ud The third experiment investigated the effect of advance preparation on the ability to proactively and selectively inhibit a single limb in a bimanual response that had been cued to maybe stop. TMS was used to measure the excitability of the limb that was cued to maybe stop in comparison to the limb that was to continue with its response. In addition, a SAS was used to probe the preparatory state of the go-response in each limb. We found that increased preparatory go-activation of responses in both limbs overshadowed the neurophysiological evidence of proactive selective inhibition, while processes related to the selective stopping task appeared to suppress subcortical motor structures and the ability of the SAS to involuntarily trigger the prepared responses.\ud The fourth experiment sought to determine the role of the right inferior frontal gyrus (rIFG) and the pre-supplementary motor area (preSMA) in the inhibition of response initiation during a go/no-go task. We temporarily deactivated rIFG or preSMA using continuous theta burst stimulation (cTBS) and examined changes in inhibition, voluntary initiation, and the ability of a SAS to involuntary trigger the initiation of the response. We found that stimulation to both cortical sites impaired participant’s ability to withhold movements during no-go trials. Notably, deactivating rIFG and preSMA did not affect voluntary initiation and did not enable the SAS to involuntarily trigger the response. These findings implicate the rIFG and the preSMA in the ability to inhibit responses during a go/no-go task, and suggests that preparation and initiation of the go-response occurs in response to the imperative stimulus, with inhibition only applied once the stimulus is identified as a no-go signal.\ud Taken together, these studies show that i) modulation of preparatory go-activation contributes to the ability to inhibit a motor response, ii) motor response inhibition is achieved by initiation activation being prevented from reaching threshold, iii) preparatory go-activation overshadows proactive inhibition, iv) inhibitory control depends on the integrity and recruitment of top-down inhibitory control to suppress initiation activation once a no-go stimulus is identified. This research speaks to the interaction between initiation and inhibition processes and provides novel insight and evidence in support of an interactive model of inhibitory control