A Double-Coil TMS Method to Assess Corticospinal Excitability Changes at a Near-Simultaneous Time in the Two Hands during Movement Preparation

Corticospinal excitability reductions during action preparation and action stopping in humans: Different sides of the same inhibitory coin?

Motor functions and cognitive processes are closely associated with each other. In humans, this linkage is reflected in motor system state changes both when an action must be prepared and stopped. Single-pulse transcranial magnetic stimulation showed that both action preparation and action stopping are accompanied by a reduction of corticospinal excitability, referred to as preparatory and response inhibition, respectively. While previous efforts have been made to describe both phenomena extensively, an updated and comprehensive comparison of the two phenomena is lacking. To ameliorate such deficit, this review focuses on the role and interpretation of single-coil (single-pulse and paired-pulse) and dual-coil TMS outcome measures during action preparation and action stopping in humans. To that effect, it aims to identify commonalities and differences, detailing how TMS-based outcome measures are affected by states, traits, and psychopathologies in both processes. Eventually, findings will be compared, and open questions will be addressed to aid future research.

Relationship between transcranial magnetic stimulation markers of motor control and clinical recovery in obsessive compulsive disorder/Gilles de la Tourette syndrome: a proof of concept case study

Background

Obsessive compulsive disorder (OCD) and Gilles de la Tourette syndrome (GTS) are neurodevelopmental disorders characterized by difficulties in controlling intrusive thoughts (obsessions) and undesired actions (tics), respectively. Both conditions have been associated with abnormal inhibition but a tangible deficit of inhibitory control abilities is controversial in GTS.

Methods

Here, we examined a 25 years-old male patient with severe OCD symptoms and a mild form of GTS, where impairments in motor control were central. Transcranial magnetic stimulation (TMS) was applied over the primary motor cortex (M1) to elicit motor-evoked potentials (MEPs) during four experimental sessions, allowing us to assess the excitability of motor intracortical circuitry at rest as well as the degree of MEP suppression during action preparation, a phenomenon thought to regulate movement initiation.

Results

When tested for the first time, the patient presented a decent level of MEP suppression during action preparation, but he exhibited a lack of intracortical inhibition at rest, as evidenced by reduced short-interval intracortical inhibition (SICI) and long-interval intracortical inhibition (LICI). Interestingly, the patient's symptomatology drastically improved over the course of the sessions (reduced obsessions and tics), coinciding with feedback given on his good motor control abilities. These changes were reflected in the TMS measurements, with a significant strengthening of intracortical inhibition (SICI and LICI more pronounced than previously) and a more selective tuning of MEPs during action preparation; MEPs became even more suppressed, or selectively facilitated depending on the behavioral condition in which they we probed.

Conclusion

This study highlights the importance of better understanding motor inhibitory mechanisms in neurodevelopmental disorders and suggests a biofeedback approach as a potential novel treatment.

PMd and action preparation: bridging insights between TMS and single neuron research

Transcranial magnetic stimulation (TMS) research has furthered understanding of human dorsal premotor cortex (PMd) function due to its unrivalled ability to measure the inhibitory and facilitatory influences of PMd over the primary motor cortex (M1) in a temporally precise manner. TMS research indicates that PMd transiently modulates inhibitory output to effector representations within M1 during motor preparation, with the direction of modulation depending on which effectors are selected for response, and the timing of modulations co-varying with task selection demands. In this review, we critically assess this literature in the context of a dynamical systems approach used to model nonhuman primate (NHP) PMd/M1 single-neuron recordings during action preparation. Through this process, we identify gaps in the literature and propose future experiments.

Exploring the links between gut microbiota and excitatory and inhibitory brain processes in alcohol use disorder: A TMS study

While the impact of the gut microbiota on brain and behavior is increasingly recognized, human studies examining this question are still scarce. The primary objective of the current study was to explore the potential relationships between the gut microbiota composition, motor cortical excitability at rest and during inhibitory control, as well as behavioral inhibition, in healthy volunteers and in patients suffering from alcohol use disorder. Motor cortical excitability was examined using a range of transcranial magnetic stimulation (TMS) measures probed at rest, including the recruitment curve, short and long intracortical inhibition, and intracortical facilitation within the primary motor cortex. Moreover, TMS was applied during a choice reaction time task to assess changes in motor excitability associated with inhibitory control. Finally, behavioral inhibition was investigated using a neuropsychological task (anti-saccade). Overall, our results highlight several interesting correlations between microbial composition and brain measures. Hence, higher bacterial diversity, as well as higher relative abundances of UGC-002 and Christensenellaceae R-7 group were correlated with stronger changes in motor excitability associated with inhibitory control. Also, higher abundance of Anaerostipes was associated with higher level of corticospinal excitability. Finally, relative abundances of Bifidobacterium and Faecalibacterium were positively related to performance in the neuropsychological task, suggesting that they might have a positive impact on behavioral inhibition. Although correlation is not causation, the present study suggests that excitatory and inhibitory brain processes might be related to gut microbiota composition. This article is part of the Special Issue on 'Microbiome & the Brain: Mechanisms & Maladies'.

Investigating the role of contextual cues and interhemispheric inhibitory mechanisms in response-selective stopping: a TMS study

Response-selective stopping requires cancellation of only one component of a multicomponent action. While research has investigated how delays to the continuing action components ("stopping interference") can be attenuated by way of contextual cues of the specific stopping demands ("foreknowledge"), little is known of the underlying neural mechanisms. Twenty-seven, healthy, young adults undertook a multicomponent stop-signal task. For two thirds of trials, participants responded to an imperative (go) stimulus (IS) with simultaneous button presses using their left and right index fingers. For the remaining one third of trials, the IS was followed by a stop-signal requiring cancellation of only the left, or right, response. To manipulate foreknowledge of stopping demands, a cue preceded the IS that informed participants which hand might be required to stop (proactive) or provided no such information (reactive). Transcranial magnetic stimulation (TMS) assessed corticospinal excitability (CSE) as well as short- and long-interval interhemispheric inhibition (SIHI, LIHI) between the primary motor cortices. Proactive cues reduced, but did not eliminate, stopping interference relative to the reactive condition. Relative to TMS measures at cue onset, decreases in CSE (both hands and both cue conditions) and LIHI (both hands, proactive condition only) were observed during movement preparation. During movement cancellation, LIHI reduction in the continuing hand was greater than that in the stopping hand and greater than LIHI reductions in both hands during execution of multicomponent responses. Our results indicate that foreknowledge attenuates stopping interference and provide evidence for a novel role of LIHI, mediated via prefrontal regions, in facilitating continuing action components.

Corticospinal Suppression Underlying Intact Movement Preparation Fades in Parkinson's Disease

BACKGROUND

In Parkinson's disease (PD), neurophysiological abnormalities within the primary motor cortex (M1) have been shown to contribute to bradykinesia, but exact modalities are still uncertain. We propose that such motor slowness could involve alterations in mechanisms underlying movement preparation, especially the suppression of corticospinal excitability-called "preparatory suppression"-which is considered to propel movement execution by increasing motor neural gain in healthy individuals.

METHODS

On two consecutive days, 29 PD patients (on and off medication) and 29 matched healthy controls (HCs) underwent transcranial magnetic stimulation over M1, eliciting motor-evoked potentials (MEPs) in targeted hand muscles, while they were either at rest or preparing a left- or right-hand response in an instructed-delay choice reaction time task. Preparatory suppression was assessed by expressing MEP amplitudes during movement preparation relative to rest.

RESULTS

Contrary to HCs, PD patients showed a lack of preparatory suppression when the side of the responding hand was analyzed, especially when the latter was the most affected one. This deficit, which did not depend on dopamine medication, increased with disease duration and also tended to correlate with motor impairment, as measured by the Movement Disorder Society Unified Parkinson's Disease Rating Scale, Part III (both total and bradykinesia scores).

CONCLUSIONS

Our novel findings indicate that preparatory suppression fades in PD, in parallel with worsening motor symptoms, including bradykinesia. Such results suggest that an alteration in this marker of intact movement preparation could indeed cause motor slowness and support its use in future studies on the relation between M1 alterations and motor impairment in PD. © 2022 International Parkinson and Movement Disorder Society.

Action Selection and Motor Decision Making: Insights from Transcranial Magnetic Stimulation

In everyday life, goal-oriented motor behaviour relies on the estimation of the rewards/costs associated with alternative actions and on the appropriate selection of movements. Motor decision making is defined as the process by which a motor plan is chosen among a set of competing actions based on the expected value. In the present literature review we discuss evidence from transcranial magnetic stimulation (TMS) studies of motor control. We focus primarily on studies of action selection for instructed movements and motor decision making. In the first section, we delve into the usefulness of various TMS paradigms to characterise the contribution of motor areas and distributed brain networks to cued action selection. Then, we address the influence of motivational information (e.g., reward and biomechanical cost) in guiding action choices based on TMS findings. Finally, we conclude that TMS represents a powerful tool for elucidating the neurophysiological mechanisms underlying action choices in humans.

Response bias reveals the role of interhemispheric inhibitory networks in movement preparation and execution

Human movement is influenced by various cognitive processes, such as bias, that dynamically shape competing movement representations. However, the neurophysiological mechanisms underlying the effects of bias on movement selection across the lifespan remains poorly understood. Healthy young (n = 21) and older (n = 20) adults completed a choice reaction-time task necessitating left- or right-hand responses to imperative stimuli (IS). Response bias was manipulated via a cue that informed participants a particular response was 70% likely (i.e., the IS was either congruent, or incongruent, with the cue); biasing was either fixed for blocks of trials (block-wise bias) or varied from trial-to-trial (trial-wise bias). As well as assessing the behavioural manifestations of bias, we used transcranial magnetic stimulation to determine changes in corticospinal excitability (CSE) and short- and long-interval interhemispheric inhibition (SIHI, LIHI) during movement preparation and execution. Participants responded more quickly, and accurately, in congruent compared to incongruent trials. CSE decreases occurred in both hands following the cue, consistent with the 'inhibition for impulse control' hypothesis of preparatory inhibition. In contrast, IHI modulations occurred in a hand-specific manner. Greater SIHI was observed during movement preparation in the hand biased away from, compared to the hand biased towards, the cue; furthermore, greater SIHI was observed during movement execution in the hand biased towards the cue when it was not required to respond (i.e., incongruent trial) compared to when it was required to respond (congruent trial). Additionally, during the movement preparation period, the LIHI ratio of the hand biased towards, compared to the hand biased away from, the cue was greatest when the cue varied trial-by-trial. Overall, the IHI results provide support for the 'inhibition for competition resolution' hypothesis, with hand specific modulation of inhibition during movement preparation and execution.

Long-Term Wu Qin Xi Exercise on Response Inhibition and Cortical Connectivity in Parkinson's Disease: Design and Implementation of a Randomized Controlled Clinical Trial

Background: Motor symptom disorders in patients with Parkinson disease (PD) are closely related to reduced inhibitory ability. Although exercise has been shown to improve this ability in patients with PD, its effects on proactive and reactive inhibition have not been determined. Most previous studies of inhibitory control disorder in people with PD have been behavioral, and little attention has been paid to functional cortical connectivity. Wu Qin Xi, a low–medium-intensity qigong exercise that is safe and easy to do for elderly individuals, can support physical well-being and help prevent and alleviate disease. In this study, our aims were to explore the effects of a long-term Wu Qin Xi intervention on response inhibition and to examine how improved inhibition control relates to cortical connectivity using dual-site paired-pulse transcranial magnetic stimulation (ppTMS), in patients with mild–moderate PD.

Methods: A single-blind randomized controlled trial will be conducted. A total of 90 elderly subjects will be recruited and allocated randomly to Wu Qin Xi, balance exercise, and healthy control groups. The exercise interventions will be implemented in three 90-min sessions per week for 24 weeks; the healthy control group will receive no intervention. The primary assessments will be response inhibition metrics and task-based ppTMS. The secondary outcomes will include motor symptom severity, mobility, balance, emotional state, and quality of life. Assessments will be conducted at baseline, at the conclusion of the intervention period (week 24), and a few months after the intervention (week 36 follow-up).

Discussion: This study is designed to provide insights into the effects of practicing Wu Qin Xi on response inhibition function in people with PD. The results will provide evidence on the value of traditional Chinese exercise as a therapeutic rehabilitation option for these patients. They will also provide data addressing how brain function–related cortical connectivity is related to reactive vs. proactive inhibition in people with PD participating in an exercise intervention.

Clinical Trial Registration: This study has been registered prospectively in the Chinese Clinical Trial Registry (ChiCTR2000038517, 18 January 2021).

Augmented tendency to act and altered impulse control in alcohol use disorders

Action preparation relies on the operation of control processes that modulate the excitability of the corticospinal tract. On the one hand, excitatory processes prepare the motor system for the forthcoming response; the stronger these influences, the stronger the tendency to act. On the other hand, inhibitory influences allow to suppress inappropriate actions and, more generally, to ensure some sort of impulse control. Because an impairment in these processes could foster inappropriate drinking behavior, the present study aimed at evaluating the motor correlates of such excitatory and inhibitory influences in non-treatment seeking heavy drinkers (HDs) and inpatients suffering from severe alcohol use disorder (SAUDs). Besides, as cue-elicited craving might further alter these processes, we also assessed the impact of an alcohol-related exposure. To do so, 15 healthy controls (HCs), 15 HDs and 15 SAUDs performed a choice reaction time task after having been immersed in a neutral or an alcohol-related environment, using virtual reality videos. Importantly, single-pulse transcranial magnetic stimulation was applied over the left and the right primary motor cortex during the task to elicit motor-evoked potentials in a set of hand muscles allowing us to specifically probe the impact of excitatory and inhibitory processes on motor activity. Our data indicate that excitatory influences are particularly high in both HDs and SAUDs, especially in the dominant hand, an effect that was not observed in HCs. By contrast, inhibitory influences were found to be perfectly normal in HDs, while they were lacking in SAUDs. Furthermore, the alcohol-related exposure enhanced the level of self-reported craving, but this effect only arose in HDs and did not significantly alter the strength of excitatory and inhibitory influences. Overall, although these results have to be taken with caution due to the small sample sizes, this study suggests that enhanced excitatory processes characterize both HDs and SAUDs, while weaker inhibitory influences only concern SAUDs. Hence, an abnormally strong tendency to act could represent a common feature of hazardous drinking, leading individuals to excessive alcohol consumption, whereas deficient impulse control would be a hallmark of more severe forms of AUD, potentially due to the chronic neurotoxic effects of alcohol. Finally, although an alcohol-related exposure does not seem to affect excitatory and inhibitory processes at play during action preparation per se, future works should evaluate changes in corticospinal excitability during the preparation of responses specifically targeting alcohol-related cues.

Neural bases of inhibitory control: Combining transcranial magnetic stimulation and magnetic resonance imaging in alcohol-use disorder patients

Inhibitory control underlies the ability to inhibit inappropriate responses and involves processes that suppress motor excitability. Such motor modulatory effect has been largely described during action preparation but very little is known about the neural circuit responsible for its implementation. Here, we addressed this point by studying the degree to which the extent of preparatory suppression relates to brain morphometry. We investigated this relationship in patients suffering from severe alcohol use disorder (AUD) because this population displays an inconsistent level of preparatory suppression and major structural brain damage, making it a suitable sample to measure such link. To do so, 45 detoxified patients underwent a structural magnetic resonance imaging (MRI) and performed a transcranial magnetic stimulation (TMS) experiment, in which the degree of preparatory suppression was quantified. Besides, behavioral inhibition and trait impulsivity were evaluated in all participants. Overall, whole-brain analyses revealed that a weaker preparatory suppression was associated with a decrease in cortical thickness of a medial prefrontal cluster, encompassing parts of the anterior cingulate cortex and superior-frontal gyrus. In addition, a negative association was observed between the thickness of the supplementary area (SMA)/pre-SMA and behavioral inhibition abilities. Finally, we did not find any significant correlation between preparatory suppression, behavioral inhibition and trait impulsivity, indicating that they represent different facets of inhibitory control. Altogether, the current study provides important insight on the neural regions underlying preparatory suppression and allows highlighting that the excitability of the motor system represents a valuable read-out of upstream cognitive processes.

A TMS study of preparatory suppression in binge drinkers

Binge drinking consists in a pattern of consumption characterised by the repeated alternation between massive alcohol intakes and abstinence periods. A continuum hypothesis suggests that this drinking endeavour represents an early stage of alcohol dependence rather than a separate phenomenon. Among the variety of alterations in alcohol-dependent individuals (ADIs), one has to do with the motor system, which does not show a normal pattern of activity during action preparation. In healthy controls (HCs), motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) over primary motor cortex (M1) show both facilitation and suppression effects, depending on the time and setting of TMS during action preparation. A recent study focusing on the suppression component revealed that this aspect of preparatory activity is abnormally weak in ADIs and that this defect scales with the risk of relapse. In the present study, we tested whether binge drinkers (BDs) present a similar deficit. To do so, we recorded MEPs in a set of hand muscles applying TMS in 20 BDs and in 20 matched HCs while they were preparing index finger responses in an instructed-delay choice reaction time task. Consistent with past research, the MEP data in HCs revealed a strong MEP suppression in this task. This effect was evident in all hand muscles, regardless of whether they were relevant or irrelevant in the task. BDs also showed some preparatory suppression, yet this effect was less consistent, especially in the prime mover of the responding hand. These findings suggest abnormal preparatory activity in BDs, similar to alcohol-dependent patients, though some of the current results also raise new questions regarding the significance of these observations.

Considering Motor Excitability During Action Preparation in Gambling Disorder: A Transcranial Magnetic Stimulation Study

A lack of inhibitory control appears to contribute to the development and maintenance of addictive disorders. Among the mechanisms thought to assist inhibitory control, an increasing focus has been drawn on the so-called preparatory suppression, which refers to the drastic suppression observed in the motor system during action preparation. Interestingly, deficient preparatory suppression has been reported in alcohol use disorders. However, it is currently unknown whether this deficit also concerns behavioral, substance-free, addictions, and thus whether it might represent a vulnerability factor common to both substance and behavioral addictive disorders. To address this question, neural measures of preparatory suppression were obtained in gambling disorder patients (GDPs) and matched healthy control subjects. To do so, single-pulse transcranial magnetic stimulation was applied over the left and the right motor cortex to elicit motor-evoked potentials (MEPs) in both hands when participants were performing a choice reaction time task. In addition, choice and rapid response impulsivity were evaluated in all participants, using self-report measures and neuropsychological tasks. Consistent with a large body of literature, the MEP data revealed that the activity of the motor system was drastically reduced during action preparation in healthy subjects. Surprisingly, though, a similar MEP suppression was observed in GDPs, indicating that those subjects do not globally suffer from a deficit in preparatory suppression. By contrast, choice impulsivity was higher in GDPs than healthy subjects, and a higher rapid response impulsivity was found in the more severe forms of GD. Altogether, those results demonstrated that although some aspects of inhibitory control are impaired in GDPs, these alterations do not seem to concern preparatory suppression. Yet, the profile of individuals suffering of a GD is very heterogeneous, with only part of them presenting an impulsive disposition, such as in patients with alcohol use disorders. Hence, a lack of preparatory suppression may be only shared by this sub-type of addicts, an interesting issue for future investigation.

Tuning the Corticospinal System: How Distributed Brain Circuits Shape Human Actions

Interactive behaviors rely on the operation of several processes allowing the control of actions, including their selection, withholding, and cancellation. The corticospinal system provides a unique route through which multiple brain circuits can exert control over bodily motor acts. In humans, the influence of these modulatory circuits on the corticospinal system can be probed using various transcranial magnetic stimulation (TMS) protocols. Here, we review neural data from TMS studies at the basis of our current understanding of how diverse pathways—including intra-cortical, trans-cortical, and subcortico-cortical circuits—contribute to action control by tuning the activity of the corticospinal system. Critically, when doing so, we point out important caveats in the field that arise from the fact that these circuits, and their impact on the corticospinal system, have not been considered equivalently for action selection, withholding, and cancellation. This has led to the misleading view that some circuits or regions are specialized in specific control processes and that they produce particular modulatory changes in corticospinal excitability (e.g., generic vs. specific modulation of corticospinal excitability). Hence, we point to the need for more transversal research approaches in the field of action control.

Preparatory inhibition: Impact of choice in reaction time tasks

By applying transcranial magnetic stimulation (TMS) over primary motor cortex (M1) to elicit motor-evoked potentials (MEPs) in muscles of the contralateral hand during reaction time (RT) tasks, many studies have reported a strong global suppression of motor excitability during action preparation, a phenomenon called preparatory inhibition. Several hypotheses have been put forward regarding the role of this broad suppression, with the predominant view that it reflects inhibitory processes assisting action selection. However, this assumption is still a matter of debate. Here, we aimed at directly addressing this idea by comparing MEPs in a task that required subjects to select a finger response within a set of predefined options (choice RT task: left or right index finger abduction) or when subjects simply had to provide the same finger response on every trial, in the absence of choice (simple RT task). Moreover, we minimized any effect that could be associated with other forms of inhibition. In both versions of the task, TMS was applied on both M1 (double-coil protocol) at several time points between the go signal and the left or right index finger response, eliciting MEPs bilaterally in the prime mover (index finger agonist) and in an irrelevant muscle (pinky agonist). Overall, MEP suppression was moderate in this study compared to past research; it was only found for the irrelevant muscle. As such, MEPs in the index agonist were facilitated when elicited in a responding hand (e.g. left MEPs preceding left responses) and remained mostly unchanged in a non-responding hand (e.g. left MEPs preceding right responses). In contrast, MEPs were almost always suppressed in the pinky muscle when elicited in the non-responding hand. This finding contrasts with previous studies where preparatory inhibition usually concerns both relevant and irrelevant muscles. Yet importantly, the suppression was more consistent in the choice than in the simple RT task, supporting the view that preparatory inhibition may assist action selection.

Planning face, hand, and leg movements: anatomical constraints on preparatory inhibition

Motor-evoked potentials (MEPs), elicited by transcranial magnetic stimulation (TMS) over the motor cortex, are reduced during the preparatory period in delayed response tasks. In this study we examined how MEP suppression varies as a function of the anatomical organization of the motor cortex. MEPs were recorded from a left index muscle while participants prepared a hand or leg movement in experiment 1 or prepared an eye or mouth movement in experiment 2. In this manner, we assessed if the level of MEP suppression in a hand muscle varied as a function of the anatomical distance between the agonist for the forthcoming movement and the muscle targeted by TMS. MEP suppression was attenuated when the cued effector was anatomically distant from the hand (e.g., leg or facial movement compared with finger movement). A similar effect was observed in experiment 3 in which MEPs were recorded from a muscle in the leg and the forthcoming movement involved the upper limb or face. These results demonstrate an important constraint on preparatory inhibition: it is sufficiently broad to be manifest in a muscle that is not involved in the task, but it is not global, showing a marked attenuation when the agonist muscle belongs to a different segment of the body. NEW & NOTEWORTHY Using transcranial magnetic stimulation, we examined changes in corticospinal excitability as people prepared to move. Consistent with previous work, we observed a reduction in excitability during the preparatory period, an effect observed in both task-relevant and task-irrelevant muscles. However, this preparatory inhibition is anatomically constrained, attenuated in muscles belonging to a different body segment than the agonist of the forthcoming movement.

The supplementary motor area modulates interhemispheric interactions during movement preparation

The execution of coordinated hand movements requires complex interactions between premotor and primary motor areas in the two hemispheres. The supplementary motor area (SMA) is involved in movement preparation and bimanual coordination. How the SMA controls bimanual coordination remains unclear, although there is evidence suggesting that the SMA could modulate interhemispheric interactions. With a delayed-response task, we investigated interhemispheric interactions underlying normal movement preparation and the role of the SMA in these interactions during the delay period of unimanual or bimanual hand movements. We used functional MRI and transcranial magnetic stimulation in 22 healthy volunteers (HVs), and then in two models of SMA dysfunction: (a) in the same group of HVs after transient disruption of the right SMA proper by continuous transcranial magnetic theta-burst stimulation; (b) in a group of 22 patients with congenital mirror movements (CMM), whose inability to produce asymmetric hand movements is associated with SMA dysfunction. In HVs, interhemispheric connectivity during the delay period was modulated according to whether or not hand coordination was required for the forthcoming movement. In HVs following SMA disruption and in CMM patients, interhemispheric connectivity was modified during the delay period and the interhemispheric inhibition was decreased. Using two models of SMA dysfunction, we showed that the SMA modulates interhemispheric interactions during movement preparation. This unveils a new role for the SMA and highlights its importance in coordinated movement preparation.

Investigating the effect of anticipating a startling acoustic stimulus on preparatory inhibition

OBJECTIVES

Motor-evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) show a profound suppression when elicited during the instructed-delay of reaction time (RT) tasks. One predominant hypothesis is that this phenomenon, called "preparatory inhibition", reflects the operation of processes that suppress motor activity to withhold prepared (but delayed) responses, a form of impulse control. In addition, a startling acoustic stimulus (SAS) - a loud and narrow sound - can trigger the release of prepared responses in RT tasks. We predicted that, if such premature release is clearly forbidden, then anticipating a SAS during delay periods may be associated with increased preparatory inhibition for greater impulse control.

METHODS

Subjects performed a behavioural (n=16) and TMS (n=11) experiment. Both used a choice RT task that required subjects to choose a response based on a preparatory cue but to only release it after an imperative signal. SAS and TMS pulses were elicited at the end of the delay period and subjects were asked to do their best to only release their response after the imperative signal, even in the presence of SAS. SAS could be either rare or frequent, in separate blocks.

RESULTS

Consistent with the literature, SAS shortened RTs, especially when they occurred frequently. Moreover, MEPs were suppressed when subjects delayed prepared responses but this preparatory inhibition did not depend on whether SAS were frequent or rare.

DISCUSSION

The stronger RT shortening with frequent rather than rare SAS may be due to increased attention and/or reduced reactive inhibition to SAS, leaving preparatory inhibition unaffected.

Deficient inhibition in alcohol-dependence: let's consider the role of the motor system!

Impaired inhibitory control contributes to the development, maintenance, and relapse of alcohol-dependence, but the neural correlates of this deficit are still unclear. Because inhibitory control has been labeled as an executive function, most studies have focused on prefrontal areas, overlooking the contribution of more "primary" structures, such as the motor system. Yet, appropriate neural inhibition of the motor output pathway has emerged as a central aspect of healthy behavior. Here, we tested the hypothesis that this motor inhibition is altered in alcohol-dependence. Neural inhibitory measures of motor activity were obtained in 20 detoxified alcohol-dependent (AD) patients and 20 matched healthy subjects, using a standard transcranial magnetic stimulation procedure whereby motor-evoked potentials (MEPs) are elicited in a choice reaction time task. Moreover, behavioral inhibition and trait impulsivity were evaluated in all participants. Finally, the relapse status of patients was assessed 1 year after the experiment. As expected, AD patients displayed poorer behavioral inhibition and higher trait impulsivity than controls. More importantly, the MEP data revealed a considerable shortage of neural motor inhibition in AD patients. Interestingly, this neural defect was strongest in the patients who ended up relapsing during the year following the experiment. Our data suggest a strong motor component in the neural correlates of altered inhibitory control in AD patients. They also highlight an intriguing relationship with relapse and the perspective of a new biomarker to follow strategies aiming at reducing relapse in AD patients.

Using a Double-Coil TMS Protocol to Assess Preparatory Inhibition Bilaterally

Transcranial magnetic stimulation (TMS) applied over the primary motor cortex (M1), elicits motor-evoked potentials (MEPs) in contralateral limb muscles which are valuable indicators of corticospinal excitability (CSE) at the time of stimulation. So far, most studies have used single-coil TMS over one M1, yielding MEPs in muscles of a single limb-usually the hand. However, tracking CSE in the two hands simultaneously would be useful in many contexts. We recently showed that, in the resting state, double-coil stimulation of the two M1 with a 1 ms inter-pulse interval (double-coil_{1 ms} TMS) elicits MEPs in both hands that are comparable to MEPs obtained using single-coil TMS. To further evaluate this new technique, we considered the MEPs elicited by double-coil_{1 ms} TMS in an instructed-delay choice reaction time task where a prepared response has to be withheld until an imperative signal is displayed. Single-coil TMS studies have repetitively shown that in this type of task, the motor system is transiently inhibited during the delay period, as evident from the broad suppression of MEP amplitudes. Here, we aimed at investigating whether a comparable inhibitory effect can be observed with MEPs elicited using double-coil_{1 ms} TMS. To do so, we compared the amplitude as well as the coefficient of variation (CV) of MEPs produced by double-coil_{1 ms} or single-coil TMS during action preparation. We observed that MEPs were suppressed (smaller amplitude) and often less variable (smaller CV) during the delay period compared to baseline. Importantly, these effects were equivalent whether single-coil or double-coil_{1 ms} TMS was used. This suggests that double-coil_{1 ms} TMS is a reliable tool to assess CSE, not only when subjects are at rest, but also when they are involved in a task, opening new research horizons for scientists interested in the corticospinal correlates of human behavior.

Towards assessing corticospinal excitability bilaterally: Validation of a double-coil TMS method

BACKGROUND

For several decades, Transcranial magnetic stimulation (TMS) has been used to monitor corticospinal excitability (CSE) changes in various contexts. Habitually, single-coil TMS is applied over one primary motor cortex (M1), eliciting motor-evoked potentials (MEPs) in a contralateral limb muscle, usually a hand effector. However, in many situations, it would be useful to obtain MEPs in both hands simultaneously, to track CSE bilaterally. Such an approach requires stimulating both M1 concurrently while avoiding interference between the two descending stimuli.

NEW METHOD

We examined MEPs obtained at rest using a double-coil TMS approach where the two M1 are stimulated with a 1ms inter-pulse interval (double-coil_1ms). MEPs were acquired using double-coil_1ms (MEP_double) or single-coil (MEP_single) TMS, at five different intensities of stimulation (100, 115, 130, 145 or 160% of the resting motor threshold, rMT). Given the 1ms inter-pulse interval in double-coil_1ms trials, MEP_double were either evoked by a 1st (MEP_double-1) or a 2nd (MEP_double-2) TMS pulse.

RESULTS

All MEP_TYPE (MEP_TYPE=MEP_single, MEP_double-1 and MEP_double-2) were equivalent, regardless of the hand within which they were elicited, the intensity of stimulation or the pulse order.

COMPARISON WITH EXISTING METHOD

This method allows one to observe state-related CSE changes for the two hands simultaneously on a trial-by-trial basis.

CONCLUSION

These results infer the absence of any neural interactions between the two cortico-spinal volleys with double-coil_1ms TMS. Hence, this technique can be reliably used to assess CSE bilaterally, opening new research perspectives for scientists interested in physiological markers of activity in the motor output system.

Learning stage-dependent effect of M1 disruption on value-based motor decisions

The present study aimed at characterizing the impact of M1 disruption on the implementation of implicit value information in motor decisions, at both early stages (during reinforcement learning) and late stages (after consolidation) of action value encoding. Fifty subjects performed, over three consecutive days, a task that required them to select between two finger responses according to the color (instruction) and to the shape (implicit, undisclosed rule) of an imperative signal: considering the implicit rule in addition to the instruction allowed subjects to earn more money. We investigated the functional contribution of M1 to the implementation of the implicit rule in subjects' motor decisions. Continuous theta burst stimulation (cTBS) was applied over M1 either on Day 1 or on Day 3, producing a temporary lesion either during reinforcement learning (cTBS_Learning group) or after consolidation of the implicit rule, during decision-making (cTBS_Decision group), respectively. Interestingly, disrupting M1 activity on Day 1 improved the reliance on the implicit rule, plausibly because M1 cTBS increased dopamine release in the putamen in an indirect way. This finding corroborates the view that cTBS may affect activity in unstimulated areas, such as the basal ganglia. Notably, this effect was short-lasting; it did not persist overnight, suggesting that the functional integrity of M1 during learning is a prerequisite for the consolidation of implicit value information to occur. Besides, cTBS over M1 did not impact the use of the implicit rule when applied on Day 3, although it did so when applied on Day 2 in a recent study where the reliance on the implicit rule declined following cTBS (Derosiere et al., 2017). Overall, these findings indicate that the human M1 is functionally involved in the consolidation and implementation of implicit value information underlying motor decisions. However, M1 contribution seems to vanish as subjects become more experienced in using the implicit value information to make their motor decisions.

Physiological Markers of Motor Inhibition during Human Behavior

Transcranial magnetic stimulation (TMS) studies in humans have shown that many behaviors engage processes that suppress excitability within the corticospinal tract. Inhibition of the motor output pathway has been extensively studied in the context of action stopping, where a planned movement needs to be abruptly aborted. Recent TMS work has also revealed markers of motor inhibition during the preparation of movement. Here, we review the evidence for motor inhibition during action stopping and action preparation, focusing on studies that have used TMS to monitor changes in the excitability of the corticospinal pathway. We discuss how these physiological results have motivated theoretical models of how the brain selects actions, regulates movement initiation and execution, and switches from one state to another.

Effect of Aging on Motor Inhibition during Action Preparation under Sensory Conflict

Motor behaviors often require refraining from selecting options that may be part of the repertoire of natural response tendencies but that are in conflict with ongoing goals. The presence of sensory conflict has a behavioral cost but the latter can be attenuated in contexts where control processes are recruited because conflict is expected in advance, producing a behavioral gain compared to contexts where conflict occurs in a less predictable way. In the present study, we investigated the corticospinal correlates of these behavioral effects (both conflict-driven cost and context-related gain). To do so, we measured motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) of young and healthy older adults performing the Eriksen Flanker Task. Subjects performed button-presses according to a central arrow, flanked by irrelevant arrows pointing in the same (congruent trial) or opposite direction (incongruent trial). Conflict expectation was manipulated by changing the probability of congruent and incongruent trials in a given block. It was either high (mostly incongruent blocks, MIB, 80% incongruent trials) or low (mostly congruent blocks, MCB, 80% congruent). The MEP data indicate that the conflict-driven behavioral cost is associated with a strong increase in inappropriate motor activity regardless of the age of individuals, as revealed by larger MEPs in the non-responding muscle in incongruent than in congruent trials. However, this aberrant facilitation disappeared in both groups of subjects when conflict could be anticipated (i.e., in the MIBs) compared to when it occurred in a less predictably way (MCBs), probably allowing the behavioral gain observed in both the young and the older individuals. Hence, the ability to overcome and anticipate conflict was surprisingly preserved in the older adults. Nevertheless, some control processes are likely to evolve with age because the behavioral gain observed in the MIB context was associated with an attenuated suppression of MEPs at the time of the imperative signal (i.e., before conflict is actually detected) in older individuals, suggesting altered motor inhibition, compared to young individuals. In addition, the behavioral analysis suggests that young and older adults rely on different strategies to cope with conflict, including a change in speed-accuracy tradeoff.

Comparison of Motor Inhibition in Variants of the Instructed-Delay Choice Reaction Time Task

Using instructed-delay choice reaction time (RT) paradigms, many previous studies have shown that the motor system is transiently inhibited during response preparation: motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) over the primary motor cortex are typically suppressed during the delay period. This effect has been observed in both selected and non-selected effectors, although MEP changes in selected effectors have been more inconsistent across task versions. Here, we compared changes in MEP amplitudes in three different variants of an instructed-delay choice RT task. All variants required participants to choose between left and right index finger movements but the responses were either provided “in the air” (Variant 1), on a regular keyboard (Variant 2), or on a response device designed to control from premature responses (Variant 3). The task variants also differed according to the visual layout (more concrete in Variant 3) and depending on whether participants received a feedback of their performance (absent in Variant 1). Behavior was globally comparable between the three variants of the task although the propensity to respond prematurely was highest in Variant 2 and lowest in Variant 3. MEPs elicited in a non-selected hand were similarly suppressed in the three variants of the task. However, significant differences emerged when considering MEPs elicited in the selected hand: these MEPs were suppressed in Variants 1 and 3 whereas they were often facilitated in Variant 2, especially in the right dominant hand. In conclusion, MEPs elicited in selected muscles seem to be more sensitive to small variations to the task design than those recorded in non-selected effectors, probably because they reflect a complex combination of inhibitory and facilitatory influences on the motor output system. Finally, the use of a standard keyboard seems to be particularly inappropriate because it encourages participants to respond promptly with no means to control for premature responses, probably increasing the relative amount of facilitatory influences at the time motor inhibition is probed.