Motor preparation and execution are independent processes

Behavioural analysis from experiments 1 and 2 showed that the decision-making strategy used to slow down in the face of stopping can change depending on task demands. In experiment 1, motor preparation was the same between the SST and Go-only task, as only one response needed to be prepared. Hence, the reaction time differences between tasks represent a difference in motor initiation, as motor preparation is equal between tasks.

This observation shows that motor preparation and initiation are not inevitably coupled, as a real-time delay can be incorporated into the response. We noticed that the rise in CSE was either delayed (SST) or slower (critical go trials) when stopping might be expected compared to when it is not. Since non-decision time mediates the slowing down in the SST, it is no surprise that no differences are observed in the response-locked analysis, when the reaction times are controlled, between SST and Go-only go trials. That is, the decision to respond is made at the same time in both conditions, but the point at which that movement is initiated and executed by M1, is different. This difference in movement initiation is what mediates the reaction time difference between conditions. A delay in the initiation of saccades has been shown to be present in neurones recorded in the frontal eye fields and superior colliculus of macaque monkeys performing the stop-signal task (199). Our results extend this notion that movement generation under apprehension can occur via a method of delayed initiation, outside of the ocular system.

In contrast, reaction time distribution and DDM analysis of CSST behavioural data revealed that the slowing down during critical go trials was mediated by an increase in

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the boundary separation and decrease in drift rate. Consequently, we expected there to be differences in CSE between critical and non-critical go trials during movement execution in the response-locked analysis. However, as in experiment 1, there was no difference in M1 activity during response execution, between stopping conditions.

These results together show that M1 executes the same process, regardless of differences in movement preparation and hence favour a model whereby movement preparation and execution are independent; motor execution does not necessarily occur when the perceptual decision threshold has been reached. The independence between movement preparation and movement initiation has recently been reported using free and forced reaction time paradigms, showing that accurate responses can be made when movement preparation has not been completed (121). Evidence for a physiological distinction between the decision-related component of an action and the execution is sparse, although it has been reported that variations in evidence accumulation impact parietal delta oscillations and lateralised beta-band power integrate the sensory evidence as a response preparation signal (186). In monkeys, changes in decision-related neural activity rather than changes in threshold can mediate reaction time during the speed-accuracy tradeoff (200).

It is likely that once the decision to move is resolved, a signal is sent to M1 to execute the necessary action. The neural correlate of this signal is currently unknown, although a candidate could be dorsal premotor cortex (201,202); variability of activity in the dorsal premotor cortex has been shown to correlate with motor execution, and has been proposed to be a signature of motor preparation (203). Alternatively, the supplementary motor area is a region that has long been implicated in the triggering of volitional movements (202,204–207). The subthalamic nucleus may also be implicated in the delay period between the command to move and the execution of the motor command; lesion studies and deep brain stimulation in patients with Parkinson’s disease of the subthalamic nucleus have shown deficits in the ability to pause when stopping may be required (11,41,208).

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It is known that M1 excitability can vary as a function of the functional state of the motor system, reflecting inputs to the motor system regarding decision-making or processes involving action selection. For example, effort (94), contextual uncertainty and surprise (95), value (96) and spatial attention (97) are reflected in changes in CSE before movements are executed. In fact, Klein-Flugge and Bestmann have shown that the MEP amplitude before action execution can differentiate between the selected and unselected effectors (181). Furthermore, the subjective value of such choices was also reflected in motor cortex excitability before the choice was expressed thereby reflecting incoming evidence for one option over another (96). Thus, it is tempting to assume that the rate of accumulation of evidence or change in boundary separation in the drift-diffusion model will also be reflected as a rise in CSE. However, the response-locked analyses in our experiment could not differentiate between go trials when stopping may be required, despite a difference in DDM parameters. Although seemingly contradictory, this finding speaks to the role of the M1 as a binary executor of motor commands; here we assayed during movement execution and found no differences between critical and non-critical go trials, whereas in previous TMS studies reflecting higher order cognitive processes as aforementioned, TMS was delivered in the preparatory phase. Perhaps functional states are expressed in the preparatory phase, which are not expressed during movement execution. However, there is evidence from Chapter 3 of this thesis that CSE can reflect elements of expectancy of stopping. To reconcile this, we propose that on receipt of the go signal, a “decision centre” detects the signal and queries whether it is correct or not. If it is correct, the decision centre queries M1 and the state of motor preparation – that is, it asks whether the appropriate movement plan is in place. If it is, then movement is triggered and M1 executes the corresponding movement. In this scenario, M1 excitability is a marker of how ready M1 is to receive the motor command from the decision variable to execute the movement. Hence, it can be understood that in Chapter 3 when CSE reflected the probability of subsequent stopping, that M1 in these cases was not ready to receive the motor command.

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4.5 Conclusions

In this chapter, we questioned whether voluntary movements to a stimulus are made in accordance with a rise-to-threshold model or if, according to recent findings, the processes of movement preparation and execution are independent. Our data show that whilst movement preparation can be altered between different stopping tasks, the execution of the motor command is the same, regardless of stopping requirements. This suggests that movement preparation and execution are two independent processes and that response to a cue does not always occur in a rise-to-threshold manner. Having investigated this in healthy controls, we next sought to investigate this independence was retained in a disorder where movement is generated spontaneously, sometimes without preparation – Tourette syndrome.

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