Discussion

The present study investigated how the time course of premovement suppression of corticospinal excitability (CE) was affected by the type of reaction time (RT) task being performed (simple RT, choice RT, and go/no‐go RT). In contrast to previous studies examining RT paradigms involving different limbs (e.g., Leocani et al. 2000; Quoilin et al. 2019), all three tasks required the participant to perform a similar wrist extension movement on some trials, but under different cueing conditions. Because the level of certainty regarding the required response differs between these tasks, it has been argued that differences exist in the level of advance preparation that is afforded by each task (Leuthold et al. 2004), as well as the required level of inhibitory activation to restrain unwanted movement (Greenhouse et al. 2015). These differences appeared to be reflected in the RT results, as shown by significantly shorter response latency for the simple RT task (Figure 2), as this condition allowed for (1) more advance preparation than choice RT and (2) no need to inhibit responses on “no‐go” trials. Most importantly, the type of RT task also resulted in differences in the timeline of CE suppression, which was assessed at six distinct intervals during the foreperiod. As compared to rest, corticospinal excitability was similarly elevated at 500 ms prior to the go‐signal in all tasks, indicating the presence of general preparatory task‐related neural activity (Figure 3). Although suppression of this excitability was observed in all three RT tasks beginning at approximately 150 ms prior to the go‐signal, for the simple RT and go/no‐go RT tasks, CE was further suppressed as the go‐signal approached, whereas no further suppression was observed in the choice RT task (Figure 4). These results indicate that RT task requirements, including the ability to prepare a response in advance and the potential requirement to inhibit a response, affect the timeline and magnitude of premovement suppression observed in the delay period leading up to the go‐signal in different RT tasks.

The Effect of Task Requirements and TMS on Reaction Time

As has been demonstrated for over a century (Donders 1969), the different RT tasks used here exhibited differences in mean RT with simple RT being significantly shorter than choice RT and go/no‐go RT (Figure 2). The shorter RT latencies observed in simple RT tasks are typically attributed to the fact that the required response is certain, and thus no response selection need occur during the RT interval. In this case, given sufficient time, the response can be fully prepared in advance, and the only requirement for the participant is to detect that the imperative stimulus has occurred and to initiate the prepared action (Niemi and Näätänen 1981). On the other hand, choice RT incurs additional processing time, ostensibly due to the requirement for response selection and response preparation prior to initiation. For a go/no‐go RT task, the response on go‐trials is certain, but additional processing must be undertaken during the RT interval to discriminate between stimuli in order to determine whether to initiate or withhold the planned response. These additional processing steps lead to longer go/no‐go RT latencies compared to simple RT, as seen here. Additionally, when there are multiple response options such as in choice and go/no‐go RT tasks, proactive inhibitory control may be necessary in order to stop an unwanted response (see Aron 2011; van den Wildenberg et al. 2022 for reviews), which may have also contributed to delayed RT observed in these tasks. Although choice RT tasks have often been reported to exhibit longer RT than go/no‐go tasks (Brebner and Welford 1980), this was not the case in the present experiment (Figure 2), and previous studies have also reported similar negligible RT differences in RT between these tasks in young adults (Gomez et al. 2007; Perea et al. 2016).

As a secondary RT finding, the application of TMS also had a small (~10 ms) but consistent effect of reducing RT when TMS was applied at 150 ms prior to the go‐signal. Similar RT speeding on TMS trials has been reported previously (Pascual‐Leone et al. 1992; Leocani et al. 2000), and because this effect was significant only at 150 ms prior to the go‐signal (and to a lesser extent at 300 ms prior to the go, p = 0.100), it likely reflects either cross‐model/intersensory facilitation or a cueing effect whereby the physical sensation associated with the TMS predicted the upcoming go‐signal. This effect was not different between RT tasks, and at later stimulation times, the sound/tactile stimulation from the TMS would likely have occurred too late for it to be processed as a cue.

The Effect of Task Requirements on Corticospinal Suppression

Given the RT latency differences between conditions, it was predicted that the task‐specific timeline of preparation, inhibition, and initiation processes would be reflected in the magnitude of MEP changes. The general level of corticospinal activity (as indexed by MEP amplitude) was elevated at the warning signal, as compared to baseline for all three RT tasks (Figure 3), suggesting that this increased excitability is nonspecific and not strictly related to the level of preparation for a specific response. If this activity was directly related to the level of response‐specific preparation, then excitability would be expected to be highest for the simple RT task (where the required response is certain) and lowest for the choice RT task (where a lack of advance knowledge precludes response‐specific preparation). However, it should be noted that partial advance preparation can occur in a choice RT task, depending on the specific task requirements, as well as the number and nature of the potential response alternatives (Cisek 2006). Here, response alternatives included flexion or extension with the right wrist, and as such, it is possible that at least partial preparation specific to the involved limb was able to be undertaken, as compared to a “true” choice in which no aspect of the required response is known in advance (e.g., Rosenbaum 1980).

The increase in activation from the baseline condition was maintained until the −300‐ms time point, at which time CE suppression occurred in all conditions, as shown by a significant decrease in MEP amplitude at the −150‐ms time point (~11%–18%; Figure 4). Although this suppression prior to the go‐signal replicates many previous studies (Touge et al. 1998; Greenhouse et al. 2015; e.g., Duque et al. 2017), the lack of difference between tasks during this timeframe, even though RTs differed greatly, suggests that this “early” (−300 to −150 ms) suppression is not due to preparatory or inhibitory processes associated with the specific upcoming response. Instead, it is likely related to the anticipation of initiation processes following the go‐signal, such as increasing the signal‐to‐noise ratio for motor output pathways (i.e., “spotlight control”; Greenhouse et al. 2015). This assertion is also consistent with studies that concluded CE suppression relates to initiation process, due to its presence in task‐irrelevant muscles (Quoilin et al. 2019), as well as during self‐initiated responses (Ibáñez et al. 2020).

Although the initial CE suppression was not task dependent, in the final 150 ms prior to the go‐signal, differences occurred between the RT tasks (Figure 4). Specifically, the conditions that allowed for advance response preparation (simple and go/no‐go RT tasks) showed further reductions in MEP amplitude (9%–14%) as compared to the choice RT condition (in which the largest additional suppression was 0.8%, p > 0.999). This “late” suppression (−150 to 0 ms) in the simple and go/no‐go tasks may be associated with inhibitory processes that serve to prevent the prepared response from being released prior to the go‐signal (Duque and Ivry 2009; e.g., “impulse control”; Duque et al. 2010), yet an impulse control explanation cannot fully account for some previous findings. For example, Ibáñez et al. (2020) demonstrated that suppression was observed even in self‐initiated responses, where impulse control would be unnecessary. Instead, the late suppression observed here in the simple RT and go/no‐go tasks may be more consistent with the explanation that the suppression reflects a system that is closer to a state that is ready for a response to be executed (Hannah et al. 2018; Ibáñez et al. 2020). That is, both the simple RT task and the go/no‐go task enabled a greater state of preparation of the specific motor task (if needed), leading to larger observed suppression. This conclusion is also supported by the secondary analysis, which showed that shorter RTs were associated with a greater amount of MEP suppression (Figure 5). Although not measured in the current study, it would be predicted that further suppression would occur after the go‐signal for the choice RT task, once the response is specified, as the spotlight hypothesis proposes that inhibition levels are similar between simple and choice task contexts, once a selected response is fully prepared (Greenhouse et al. 2015).

It may seem unexpected that a similar level of CE suppression was seen between the simple RT and go/no‐go tasks, as the RTs were very different (with significantly longer RT latency in the go/no‐go conditions; Figure 3). However, the RT difference is not surprising because although both tasks likely involve advance preparation and some inhibitory processes to prevent false starts, the go/no‐go task likely requires additional processes once the imperative stimulus occurs. Specifically, the individual must determine whether a “go” signal occurred or a “no‐go” signal occurred (and thus release, maintain, or increase any response‐related inhibitory processes), which may not be reflected in the delay interval CE activation level. Importantly, this RT difference also suggests that the suppression seen in these two conditions may originate from different mechanisms: It is possible that in the go/no‐go task, a greater proportion of the observed suppression was related to “impulse control” whereby suppression acts to prevent unwanted premature or incorrect release of the prepared motor response (Duque and Ivry 2009), whereas in the simple RT task a greater amount of the suppression was related to an increased preparatory state (Ibáñez et al. 2020). Nevertheless, further study would be needed to determine the relative contributions of each of these mechanisms to the observed suppression.

It is worth considering whether the weaker preparatory CE suppression observed in the choice RT task may relate to the use of two possible responses within the same limb, as MEP amplitude of the ECR was assessed on both flexion and extension trials. The role of ECR would be one of agonist in the extension movement and antagonist in the flexion movement, and there is also research showing differences in CE suppression between task relevant and task‐irrelevant muscles (van Elswijk et al. 2008). However, in the current study, the TMS application occurred in the foreperiod, prior to knowledge of the required response, and thus was likely not related to processes associated with specific musculature. In order to confirm that using a within‐limb choice RT task did not contribute to the observed differences in the CE suppression timeline, future studies may consider a similar experimental protocol that includes examination of MEP amplitude in task‐irrelevant musculature or a choice RT task that is not confounded by the same muscle involved in a different role in the two response alternatives.

Another potential confound involves the magnitude of background EMG preceding the application of TMS, which could influence MEP amplitude. The analysis showed that background EMG was larger in the simple RT task compared to choice RT, which was likely due to increased average preparatory activation when the response was known in advance. Furthermore, the experimental design resulted in there being approximately twice the number of eventual wrist extension movements in the simple RT task as opposed to the choice and go/no‐go tasks, which may have also contributed to this difference. However, it is unlikely that this background EMG difference affected the MEP results, as early TMS application times (−500 and −300 ms) showed no difference between the RT task conditions. Furthermore, later TMS time points (−50 and 0 ms) showed reduced MEP amplitude for simple RT relative to choice RT, which is in the opposite direction to the difference observed in the background ratio. That is, if the background EMG influenced MEP amplitude, the effect would have been to reduce the task differences seen at the late stimulation points.