Role of corticospinal suppression during motor preparation

StartReact effects are dependent on engagement of startle reflex circuits: support for a subcortically mediated initiation pathway

The "StartReact" effect refers to the rapid involuntary triggering of a prepared movement in response to a loud startling acoustic stimulus (SAS). This effect is typically confirmed by the presence of short-latency electromyographic activity in startle reflex-related muscles such as the sternocleidomastoid (SCM); however, there is debate regarding the specific neural pathways involved in the StartReact effect. Some research has implicated a subcortically mediated pathway, which would predict different response latencies depending on the presence of a startle reflex. Alternatively, other research has suggested that this effect involves the same pathways responsible for voluntary response initiation and simply reflects higher preparatory activation levels, and thus faster voluntary initiation. To distinguish between these competing hypotheses, the present study assessed preparation level during a simple reaction time (RT) task involving wrist extension in response to a control tone or a SAS. Premotor RT and startle circuitry engagement (as measured by SCM activation) were determined for each trial. Additionally, preparation level at the go signal on each trial was measured using motor-evoked potentials (MEP) elicited by transcranial magnetic stimulation (TMS). Results showed that SAS trial RTs were significantly shorter (P = 0.009) in the presence of startle-related SCM activity. Nevertheless, preparation levels (as indexed by MEP amplitude) were statistically equivalent between trials with and without SCM activation. These results indicate that the StartReact effect relates to engagement of the startle reflex circuitry rather than simply being a result of an increased level of preparatory activation.NEW & NOTEWORTHY The neural mechanism underlying the early triggering of goal-directed actions by a startling acoustic stimulus (SAS) is unclear. We show that although significant reaction time differences were evident depending on whether the SAS elicited a startle reflex, motor preparatory activation was the same. Thus, in a highly prepared state, the short-latency responses associated with the StartReact effect appear to be related to engagement of startle reflex circuitry, not differences in motor preparatory level.

Corticospinal excitability related to reciprocal muscles during the motor preparation period: effect of movement repetition

OBJECTIVES

Voluntary movements require a preparatory phase before action initiation to elaborate optimal motor commands. This study investigated the time course of change in corticospinal excitability related to each agonist or antagonist muscle during the motor preparation period and the influence of movement repetition on time course change in corticospinal excitability.

METHODS

The participants were instructed to perform wrist flexion as quickly as possible in response to the appearance of a red circle. One hundred fifty milliseconds after the presentation of the red circle cue, we delivered transcranial magnetic stimulation at the primary motor cortex related to the agonist flexor and the antagonist extensor carpi radialis muscles.

RESULTS

The motor evoked potential amplitudes for both the flexor and extensor carpi radialis during the motor preparation period which were regressed on the logarithm of time, and there were no significant differences in the slope of the logarithmic curves in the early half trials of movement repetition. However, the slope of the logarithmic curve of the flexor carpi radialis was steeper than that of the extensor carpi radialis in the late half trials of movement repetition.

CONCLUSION

These results implied that corticospinal excitability for agonist muscles during the motor preparation period is consistently modulated by movement repetition, whereas that for antagonist muscles is weakened by movement repetition.

Between-hand coupling during response inhibition

Response inhibition reflects the process of terminating inappropriate preplanned or ongoing movements. When one hand is cued to stop after preparing a bimanual response (Partial trial), there is a substantial delay on the responding side. This delay is termed the interference effect and identifies a constraint that limits selective response inhibition. γ-Aminobutyric acid (GABA)-mediated networks within primary motor cortex (M1) may have distinct roles during response inhibition. In this study we examined whether the interference effect is the consequence of between-hand "coupling" into a unitary response and whether this is reflected in GABAergic intracortical inhibition within M1. Eighteen healthy right-handed participants performed a bimanual synchronous and asynchronous anticipatory response inhibition task. Electromyographic recordings were obtained from the first dorsal interosseous muscle bilaterally. Motor evoked potentials were elicited by single- and paired-pulse transcranial magnetic stimulation over right M1. As expected, Go trial performance was better with the synchronous compared with the asynchronous version of the task. Paradoxically, response delays during Partial trials were longer with the synchronous compared with the asynchronous task. Although task difficulty did not modulate GABAergic intracortical inhibition, there was a trend for between-hand coupling on asynchronous trials to be associated with greater GABA_B receptor-mediated inhibition and lesser recruitment of GABA_A receptor-mediated inhibition. The novel findings indicate that the interference effect is in part a consequence of between-hand coupling into a unitary response during movement preparation. The ability to respond independently with the two hands may rely on modulation of distinct inhibitory processes.NEW & NOTEWORTHY The temporal dynamics of an anticipated response task were manipulated to effect the difficulty of behavioral stopping and the underlying effects on motor neurophysiology. There were large response delays during trials where a subcomponent of an upcoming bimanual response was cued to stop in conditions where the anticipated action of the hands were synchronous, but not when asynchronous. Response delays reflected the integration of actions of both hands into a unitary response.

Reward anticipation changes corticospinal excitability during task preparation depending on response requirements and time pressure

The preparation of an action is accompanied by transient corticospinal (CS) excitability changes. Motivation can modulate these changes. Specifically, when a cue indicates that a reward can be obtained, CS excitability initially increases, followed by a pronounced decrease. This dynamic could reflect processes related to reward expectancy, processes related to action preparation, or a combination of both. Here we set up two experiments to dissociate these accounts. A rewarded choice reaction time task was used in which individuals were cued at the beginning of each trial whether or not a response would be required at target onset and whether or not a reward could be obtained. We used single-pulse transcranial magnetic stimulation (spTMS) over the left primary motor cortex (M1) early (shortly after cue onset) or late (shortly before target onset) preceding target onset to examine CS excitability during motivated action preparation. Electromyography (EMG) was obtained from the right first dorsal interosseous (FDI) muscle. In the first experiment, we used a lenient response deadline, whereas a strict response time-out procedure was employed in the second experiment. Reward modulated CS excitability differentially only in the second experiment: CS excitability was highest during reward anticipation for the early stimulation epoch and was reduced for the late stimulation epoch when individuals were required to prepare a response, while CS excitability remained unchanged during non-reward anticipation. Our findings suggest that the reward effect on CS excitability is dependent on the actual implementation of effort to attain reward (i.e., the preparation of an actual action), as well as on temporal requirements (i.e., time pressure) invoked by the task.

Time in the motor cortex: Motor evoked potentials track foreperiod duration without concurrent movement

Transcranial magnetic stimulation (TMS) allows for the monitoring of motor cortex dynamics in preparation for response. Using this method, it has previously been shown that motor evoked potentials (MEPs) are suppressed as a response approaches. In the current article, we applied TMS while participants either relaxed or contracted their first dorsal interosseous muscle. We varied the time at which TMS was applied, however, unlike previous studies, no participant response was required. Using this method, we provide evidence that MEPs systematically decrease with the duration of the trial, while inhibition is not similarly affected. Further, we found some evidence that MEPs are inversely proportional to the duration of the prior trial. These findings have ramifications for other research interested in the application of TMS, especially when used across multiple possible points in a trial. Further, this finding shows a role for the motor cortex in timing more broadly.

Motor Preparation for Action Inhibition: A Review of Single Pulse TMS Studies Using the Go/NoGo Paradigm

Human behavior must be flexible to respond to environmental and social demands, and to achieve these goals, it requires control. For instance, inhibitory control is used to refrain from executing unwanted or anticipated responses to environmental stimuli. When inhibitory mechanisms are inefficient due to some pathological conditions, such as attention-deficit hyperactivity disorder (ADHD) or pathological gambling, patients show a reduced capability of refraining from executing actions. When planning to execute an action, various inhibitory control mechanisms are activated to prevent the unwanted release of impulses and to ensure that the correct response is produced. A great body of research has used various cognitive tasks to isolate one or more components of inhibitory control (e.g., response selectivity) and to investigate their neuronal underpinnings. However, inter-individual differences in behavior are rarely properly considered, although they often represent a considerable source of noise in the data. In the present review, we will address this issue using the specific case of action inhibition, presenting the results of studies that coupled the so-called Go/NoGo paradigm with non-invasive brain stimulation to directly test the effects of motor inhibition on the excitability of the corticospinal system (CSE). Motor preparation is rarely measured in action inhibition studies, and participants’ compliancy to the task’s requests is often assumed rather than tested. Single pulse transcranial magnetic stimulation (TMS) is a powerful tool to directly measure CSE, whose responsivity depends on both excitatory and inhibitory processes. However, when motor preparation is not measured and the task design does not require participants to prepare responses in advance, fluctuations in CSE levels can be mistaken for active inhibition. One way to isolate motor preparation is to use a carefully designed task that allows to control for excessive variability in the timing of activation of inhibitory control mechanisms. Here, we review single pulse TMS studies that have used variants of the Go/NoGo task to investigate inhibitory control functions in healthy participants. We will identify the specific strategies that likely induced motor preparation in participants, and their results will be compared to current theories of action inhibition.

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.

Motor Cortex Excitability Reflects the Subjective Value of Reward and Mediates Its Effects on Incentive-Motivated Performance

Performance-based incentives tend to increase an individual's motivation, resulting in enhancements in behavioral output. While much work has focused on understanding how the brain's reward circuitry influences incentive-motivated performance, fewer studies have investigated how such reward representations act on the motor system. Here we measured motor cortical excitability with transcranial magnetic stimulation while female and male human participants performed a motoric incentive motivation task for prospective monetary gains and losses. We found that individuals' performance increased for increasing prospective gains and losses. While motor cortical excitability appeared insensitive to prospective loss, temporal features of motor cortical excitability for prospective gains were modulated by an independent measure of an individual's subjective preferences for incentive (i.e., loss aversion). Those individuals that were more loss averse had a greater motor cortical sensitivity to prospective gain, closer to movement onset. Critically, behavioral sensitivity to incentive and motor cortical sensitivity to prospective gains were both predicted by loss aversion. Furthermore, causal modeling indicated that motor cortical sensitivity to incentive mediated the relationship between subjective preferences for incentive and behavioral sensitivity to incentive. Together, our findings suggest that motor cortical activity integrates information about the subjective value of reward to invigorate incentive-motivated performance.SIGNIFICANCE STATEMENT Increasing incentives tend to increase motivation and effort. Using a motoric incentive motivation task and transcranial magnetic stimulation, we studied the motor cortical mechanisms responsible for incentive-motivated motor performance. We provide experimental evidence that motor cortical sensitivity to incentive mediates the relationship between subjective preferences for incentive and incentive-motivated performance. These results indicate that, rather than simply being a reflection of motor output, motor cortical physiology integrates information about reward value to motivate performance.

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.

Corticospinal excitability is modulated by temporal feedback gaps

The integration of sensorimotor information is important for accurate goal-directed movement and affects corticospinal excitability (CE). This study investigated CE during the motor preparation period in a goal-directed movement task with temporal feedback gaps. Each trial began with a pair of first-informative and second-response beeps presented successively as cues. Trials with temporal feedback gaps showed that virtual hand movements lagged 400 ms behind actual performed movements. The participants were instructed to prepare for movement in accordance with the first beep, start the movement upon hearing the second beep, and perform movements that were both fast and accurate to the virtual target. We delivered a single-pulse of transcranial magnetic stimulation to the first dorsal interosseous muscle 250 ms before the presentation of the response beep. Motor-evoked potential amplitudes with temporal feedback gaps were significantly higher than those without temporal feedback gaps. Moreover, motor-evoked potential amplitudes with temporal feedback gaps gradually decreased over the course of the trials, whereas those without temporal feedback gaps did not change. In summary, CE during the motor preparation period was increased by temporal feedback gaps, and this excitation decreased in accordance with adaptation to temporal feedback gaps.

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.

In Standing, Corticospinal Excitability Is Proportional to COP Velocity Whereas M1 Excitability Is Participant-Specific

Reductions in the base of support (BOS) make standing difficult and require adjustments in the neural control of sway. In healthy young adults, we determined the effects of reductions in mediolateral (ML) BOS on peroneus longus (PL) motor evoked potential (MEP), intracortical facilitation (ICF), short interval intracortical inhibition (SICI) and long interval intracortical inhibition (LICI) using transcranial magnetic stimulation (TMS). We also examined whether participant-specific neural excitability influences the responses to increasing standing difficulty. Repeated measures ANOVA revealed that with increasing standing difficulty MEP size increased, SICI decreased (both p < 0.05) and ICF trended to decrease (p = 0.07). LICI decreased only in a sub-set of participants, demonstrating atypical facilitation. Spearman's Rank Correlation showed a relationship of ρ = 0.50 (p = 0.001) between MEP size and ML center of pressure (COP) velocity. Measures of M1 excitability did not correlate with COP velocity. LICI and ICF measured in the control task correlated with changes in LICI and ICF, i.e., the magnitude of response to increasing standing difficulty. Therefore, corticospinal excitability as measured by MEP size contributes to ML sway control while cortical facilitation and inhibition are likely involved in other aspects of sway control while standing. Additionally, neural excitability in standing is determined by an interaction between task difficulty and participant-specific neural excitability.

The Way We Do the Things We Do: How Cognitive Contexts Shape the Neural Dynamics of Motor Areas in Humans

In spontaneously triggered movements the nature of the executed response has a prominent effect on the intensity and the dynamics of motor areas recruitment. Under time pressure, the time course of motor areas recruitment is necessarily shorter than that of spontaneously triggered movements because RTs may be extremely short. Moreover, different classes of RT tasks allow examining the nature and the dynamics of motor areas activation in different cognitive contexts. In the present article, we review experimental results obtained from high temporal resolution methods (mainly, but not exclusively EEG ones), during voluntary movements; these results indicate that the activity of motor areas not only depends on the nature of the executed movement but also on the cognitive context in which these movements have to be executed.

Unilateral movement preparation causes task-specific modulation of TMS responses in the passive, opposite limb

KEY POINTS

Activity in the primary motor cortices of both hemispheres increases during unilateral movement preparation, but the functional role of ipsilateral motor cortex activity is unknown. Ipsilateral motor cortical activity could represent subliminal 'motor planning' for the passive limb. Alternatively, it could represent the state of the active limb, to support coordination between the limbs should a bimanual movement be required. Here we assessed how preparation of forces toward different directions, with the left wrist, alters evoked responses to transcranial magnetic stimulation of left motor cortex. Preparation of a unilateral movement caused excitability increases in ipsilateral motor cortex that reflected forces produced with the active limb in an intrinsic (body-centred), rather than an extrinsic (world-centred), coordinate system. These results suggest that ipsilateral motor cortical activity prior to unilateral action reflects the state of the active limb, rather than subliminal motor planning for the passive limb.

ABSTRACT

Corticospinal excitability is modulated for muscles on both sides of the body during unilateral movement preparation. For the effector, there is a progressive increase in excitability, and a shift in direction of muscle twitches evoked by transcranial magnetic stimulation (TMS) toward the impending movement. By contrast, the directional characteristics of excitability changes in the opposite (passive) limb have not been fully characterized. Here we assessed how preparation of voluntary forces towards four spatially distinct visual targets with the left wrist alters muscle twitches and motor-evoked potentials (MEPs) elicited by TMS of left motor cortex. MEPs were facilitated significantly more in muscles homologous to agonist rather than antagonist muscles in the active limb, from 120 ms prior to voluntary EMG onset. Thus, unilateral motor preparation has a directionally specific influence on pathways projecting to the opposite limb that corresponds to the active muscles rather than the direction of movement in space. The directions of TMS-evoked twitches also deviated toward the impending force direction of the active limb, according to muscle-based coordinates, following the onset of voluntary EMG. The data indicate that preparation of a unilateral movement increases task-dependent excitability in ipsilateral motor cortex, or its downstream projections, that reflects the forces applied by the active limb in an intrinsic (body-centred), rather than an extrinsic (world-centred), coordinate system. The results suggest that ipsilateral motor cortical activity prior to unilateral action reflects the state of the active limb, rather than subliminal motor planning for the passive limb.

Mackintosh lecture—: Association and cognition: Two processes, one system

This article argues that the dual-process position can be a useful first approximation when studying human mental life, but it cannot be the whole truth. Instead, we argue that cognition is built on association, in that associative processes provide the fundamental building blocks that enable propositional thought. One consequence of this position is to suggest that humans are able to learn associatively in a similar fashion to a rat or a pigeon, but another is that we must typically suppress the expression of basic associative learning in favour of rule-based computation. This stance conceptualises us as capable of symbolic computation but acknowledges that, given certain circumstances, we will learn associatively and, more importantly, be seen to do so. We present three types of evidence that support this position: The first is data on human Pavlovian conditioning that directly support this view. The second is data taken from task-switching experiments that provide convergent evidence for at least two modes of processing, one of which is automatic and carried out “in the background.” And the last suggests that when the output of propositional processes is uncertain, the influence of associative processes on behaviour can manifest.

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.

Response inhibition activates distinct motor cortical inhibitory processes

We routinely cancel preplanned movements that are no longer required. If stopping is forewarned, proactive processes are engaged to selectively decrease motor cortex excitability. However, without advance information there is a nonselective reduction in motor cortical excitability. In this study we examined modulation of human primary motor cortex inhibitory networks during response inhibition tasks with informative and uninformative cues using paired-pulse transcranial magnetic stimulation. Long- (LICI) and short-interval intracortical inhibition (SICI), indicative of GABAB- and GABAA-receptor mediated inhibition, respectively, were examined from motor evoked potentials obtained in task-relevant and task-irrelevant hand muscles when response inhibition was preceded by informative and uninformative cues. When the participants (10 men and 8 women) were cued to stop only a subcomponent of the bimanual response, the remaining response was delayed, and the extent of delay was greatest in the more reactive context, when cues were uninformative. For LICI, inhibition was reduced in both muscles during all types of response inhibition trials compared with the pre-task resting baseline. When cues were uninformative and left-hand responses were suddenly canceled, task-relevant LICI positively correlated with response times of the responding right hand. In trials where left-hand responding was highly probable or known (informative cues), task-relevant SICI was reduced compared with that when cued to rest, revealing a motor set indicative of responding. These novel findings indicate that the GABAB-receptor-mediated pathway may set a default inhibitory tone according to task context, whereas the GABAA-receptor-mediated pathways are recruited proactively with response certainty.

NEW & NOTEWORTHY We examined how informative and uninformative cues that trigger both proactive and reactive processes modulate GABAergic inhibitory networks within human primary motor cortex. We show that GABAB inhibition was released during the task regardless of cue type, whereas GABAA inhibition was reduced when responding was highly probable or known compared with rest. GABAB-receptor-mediated inhibition may set a default inhibitory tone, whereas GABAA circuits may be modulated proactively according to response certainty.

Motor-evoked potentials reveal functional differences between dominant and non-dominant motor cortices during response preparation

Transcranial magnetic stimulation (TMS) of the motor cortex produces motor-evoked potentials (MEPs) in contralateral muscles. The amplitude of these MEPs can be used to measure the excitability of the corticospinal tract during motor planning. In two experiments, we investigated learning-related changes in corticospinal excitability as subjects prepared to respond in a choice reaction-time task. Subjects responded with their left or right hand to a left or right arrow, and on some trials the arrow was immediately preceded by a warning cue that signaled which response would be required. TMS was applied to the motor cortex during the warning cues, and MEPs were measured in the dominant or non-dominant hand. We observed changes in corticospinal excitability during the warning cue, but these depended on which hand the subject was preparing to respond with, and how experienced they were with the task. When subjects prepared to respond with the non-dominant hand, excitability increased in the non-dominant hemisphere and decreased in the dominant hemisphere. These changes became stronger with task experience, and were accompanied by behavioral improvements in the task. When subjects were preparing a dominant-hand response, the non-dominant hemisphere was suppressed, but this effect disappeared as subjects gained experience with the task. There were no changes in the dominant hemisphere before dominant-hand responses. We conclude that preparing to respond with the non-dominant hand involves temporarily reversing an asymmetry in excitability that normally favors the dominant hemisphere, and that this pattern is enhanced by learning during the task.

Selective Suppression of Local Interneuron Circuits in Human Motor Cortex Contributes to Movement Preparation

Changes in neural activity occur in the motor cortex before movement, but the nature and purpose of this preparatory activity is unclear. To investigate this in the human (male and female) brain noninvasively, we used transcranial magnetic stimulation (TMS) to probe the excitability of distinct sets of excitatory inputs to corticospinal neurons during the warning period of various reaction time tasks. Using two separate methods (H-reflex conditioning and directional effects of TMS), we show that a specific set of excitatory inputs to corticospinal neurons are suppressed during motor preparation, while another set of inputs remain unaffected. To probe the behavioral relevance of this suppression, we examined whether the strength of the selective preparatory inhibition in each trial was related to reaction time. Surprisingly, the greater the amount of selective preparatory inhibition, the faster the reaction time was. This suggests that the inhibition of inputs to corticospinal neurons is not involved in preventing the release of movement but may in fact facilitate rapid reactions. Thus, selective suppression of a specific set of motor cortical neurons may be a key aspect of successful movement preparation.

SIGNIFICANCE STATEMENT Movement preparation evokes substantial activity in the motor cortex despite no apparent movement. One explanation for the lack of movement is that motor cortical output in this period is gated by an inhibitory mechanism. This notion was supported by previous noninvasive TMS studies of human motor cortex indicating a reduction of corticospinal excitability. On the contrary, our data support the idea that there is a coordinated balance of activity upstream of the corticospinal output neurons. This includes a suppression of specific local circuits that supports, rather than inhibits, the rapid generation of prepared movements. Thus, the selective suppression of local circuits appears to be an essential part of successful movement preparation instead of an external control mechanism.

Modulation of corticospinal output in agonist and antagonist proximal arm muscles during motor preparation

Previous studies have shown modulation of corticospinal output of the agonist muscle when a known-movement is prepared but withheld until a response signal appearance, reflecting motor preparation processes. However, modulation in the antagonist muscles has not been described, despite the fact that reaching movements require precise coordination between the activation of agonist and antagonist muscles. In this study, participants performed an instructed-delay reaction time (RT) task, with randomized elbow flexion and extension movements. The aim was to assess the time course modulation of corticospinal output in two antagonist muscles, by simultaneously quantified the amplitude of motor evoked potentials (MEPs) in biceps brachii and triceps brachii, and the amplitude and direction of elbow movements evoked by transcranial magnetic stimulation (TMS). Depending on the prepared movement direction, a specific modulation of corticospinal output was observed, MEPs and TMS-evoked movements amplitude being relatively greater for extension compared to flexion. At the end of motor preparation, a decrease in MEPs amplitude was observed for both biceps brachii and triceps brachii, regardless of the prepared movement direction. In contrast, the probability of evoking movement in the flexion direction and the amplitude of TMS-evoked movement decreased at the end of preparation for flexion, but not for extension. Together, these results confirm the existence of inhibitory processes at the end of the motor preparation, probably to avoid a premature motor response. Moreover, they provide evidence of differences in the corticospinal control of elbow flexor and extensor muscles with patterns of modulation that are not necessarily reciprocal during motor preparation.

Oculomotor measures reveal the temporal dynamics of preparing for search

Theories of visual search assume that selection is driven by an active template representation of the target object. Earlier studies suggest that template activation occurs prior to search, but the temporal dynamics of such preactivation remain unclear. Two experiments employed microsaccades to track both general preparation (i.e., anticipation of the search task as such) and template-specific preparation (i.e., anticipation of target selection) of visual search. Participants memorized a target color (i.e., the template) for an upcoming search task. During the delay period, we presented an irrelevant rapid serial visual presentation (RSVP) of lateralized colored disks. Crucially, at different time points into the RSVP, the template color was inserted, allowing us to measure attentional biases toward the template match as a function of time. Results showed a general suppression of saccades: the closer in time to the search display, the less saccades were produced. This suppression was stronger when a template-matching color was present compared to when absent. However, when microsaccades occurred, they were biased toward the template-matching color and more so just prior to the search display. We conclude that observers adapt search template activation to the anticipated moment of search, and that microsaccades reflect general as well as target-specific preparation effects.

Testing the physiological plausibility of conflicting psychological models of response inhibition: A forward inference fMRI study

The neural mechanisms underlying response inhibition and related disorders are unclear and controversial for several reasons. First, it is a major challenge to assess the psychological bases of behaviour, and ultimately brain-behaviour relationships, of a function which is precisely intended to suppress overt measurable behaviours. Second, response inhibition is difficult to disentangle from other parallel processes involved in more general aspects of cognitive control. Consequently, different psychological and anatomo-functional models coexist, which often appear in conflict with each other even though they are not necessarily mutually exclusive. The standard model of response inhibition in go/no-go tasks assumes that inhibitory processes are reactively and selectively triggered by the stimulus that participants must refrain from reacting to. Recent alternative models suggest that action restraint could instead rely on reactive but non-selective mechanisms (all automatic responses are automatically inhibited in uncertain contexts) or on proactive and non-selective mechanisms (a gating function by which reaction to any stimulus is prevented in anticipation of stimulation when the situation is unpredictable). Here, we assessed the physiological plausibility of these different models by testing their respective predictions regarding event-related BOLD modulations (forward inference using fMRI). We set up a single fMRI design which allowed for us to record simultaneously the different possible forms of inhibition while limiting confounds between response inhibition and parallel cognitive processes. We found BOLD dynamics consistent with non-selective models. These results provide new theoretical and methodological lines of inquiry for the study of basic functions involved in behavioural control and related disorders.

Motor Cortical Visuomotor Feedback Activity Is Initially Isolated from Downstream Targets in Output-Null Neural State Space Dimensions

Neural circuits must transform new inputs into outputs without prematurely affecting downstream circuits while still maintaining other ongoing communication with these targets. We investigated how this isolation is achieved in the motor cortex when macaques received visual feedback signaling a movement perturbation. To overcome limitations in estimating the mapping from cortex to arm movements, we also conducted brain-machine interface (BMI) experiments where we could definitively identify neural firing patterns as output-null or output-potent. This revealed that perturbation-evoked responses were initially restricted to output-null patterns that cancelled out at the neural population code readout and only later entered output-potent neural dimensions. This mechanism was facilitated by the circuit's large null space and its ability to strongly modulate output-potent dimensions when generating corrective movements. These results show that the nervous system can temporarily isolate portions of a circuit's activity from its downstream targets by restricting this activity to the circuit's output-null neural dimensions.

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.

Individual Differences in Resting Corticospinal Excitability Are Correlated with Reaction Time and GABA Content in Motor Cortex

Individuals differ in the intrinsic excitability of their corticospinal pathways and, perhaps more generally, their entire nervous system. At present, we have little understanding of the mechanisms underlying these differences and how variation in intrinsic excitability relates to behavior. Here, we examined the relationship between individual differences in intrinsic corticospinal excitability, local cortical GABA levels, and reaction time (RT) in a group of 20 healthy human adults. We measured corticospinal excitability at rest with transcranial magnetic stimulation, local concentrations of basal GABA with magnetic resonance spectroscopy, and RT with a behavioral task. All measurements were repeated in two separate sessions, and tests of reliability confirmed the presence of stable individual differences. There was a negative correlation between corticospinal excitability and RT, such that larger motor-evoked potentials (MEPs) measured at rest were associated with faster RTs. Interestingly, larger MEPs were associated with higher levels of GABA in M1, but not in three other cortical regions. Together, these results suggest that individuals with more excitable corticospinal pathways are faster to initiate planned responses and have higher levels of GABA within M1, possibly to compensate for a more excitable motor system.SIGNIFICANCE STATEMENT This study brings together physiological, behavioral, and neurochemical evidence to examine variability in the excitability of the human motor system. Previous work has focused on state-based factors (e.g., preparedness, uncertainty), with little attention given to the influence of inherent stable characteristics. Here, we examined how the excitability of the motor system relates to reaction time and the regional content of the inhibitory neurotransmitter GABA. Importantly, motor pathway excitability and GABA concentrations were measured at rest, outside a task context, providing assays of intrinsic properties of the individuals. Individuals with more excitable motor pathways had faster reaction times and, paradoxically, higher concentrations of GABA. We propose that greater GABA capacity in the motor cortex counteracts an intrinsically more excitable motor system.

Time-Based Expectancy for Task Relevant Stimulus Features

When a particular target stimulus appears more frequently after a certain interval than after another one, participants adapt to such regularity, as evidenced by faster responses to frequent interval-target combinations than to infrequent ones. This phenomenon is known as time-based expectancy. Previous research has suggested that time-based expectancy is primarily motor-based, in the sense that participants learn to prepare a particular response after a specific interval. Perceptual time-based expectancy — in the sense of learning to perceive a certain stimulus after specific interval — has previously not been observed. We conducted a Two-Alternative-Forced-Choice experiment with four stimuli differing in shape and orientation. A subset of the stimuli was frequently paired with a certain interval, while the other subset was uncorrelated with interval. We varied the response relevance of the interval-correlated stimuli, and investigated under which conditions time-based expectancy transfers from trials with interval-correlated stimuli to trials with interval-uncorrelated stimuli. Transfer was observed only where transfer of perceptual expectancy and transfer of response expectancy predicted the same behavioral pattern, not when they predicted opposite patterns. The results indicate that participants formed time-based expectancy for stimuli as well as for responses. However, alternative interpretations are also discussed.

A switching cost for motor planning

Movement planning consists of choosing the intended endpoint of the movement and selecting the motor program that will bring the effector on the endpoint. It is widely accepted that movement endpoint is updated on a trial-by-trial basis with respect to the observed errors and that the motor program for a given movement follows the rules of optimal feedback control. In this article, we show clear limitations of these theories. First, participants in the current study could not tune their motor program appropriately for each individual trial. This was true even when the participants selected the width of the target that they reached toward or when they had learned the appropriate motor program previously. These data are compatible with the existence of a switching cost for motor planning, which relates to the drop in performance due to an imposed switch of motor programs. This cost of switching shares many features of costs reported in cognitive task switching experiments and, when tested in the same participants, was correlated with it. Second, we found that randomly changing the width of a target over the course of a reaching experiment prevents the motor system from updating the endpoint of movements on the basis of the performance on the previous trial if the width of the target has changed. These results provide new insights into the process of motor planning and how it relates to optimal control theory and to an action selection based on the reward consequences of the motor program rather than that based on the observed error.

Acute aerobic activity enhances response inhibition for less than 30min

Acute exercise appears to facilitate certain aspects of cognitive processing. The possibility that exercise may lead to more efficient inhibitory processes is of particular interest, owing to the wide range of cognitive and motor functions that inhibition may underlie. The purpose of the present study was to examine the immediate and the delayed effect of acute aerobic exercise on response inhibition, motor planning, and eye-hand coordination in healthy active adults. Forty healthy and active participants (10 females) with a mean age of 51.88±8.46years performed the Go-NoGo test (response inhibition) and the Catch Game (motor planning and eye-hand coordination) before, immediately after, and following a 30-min recovery period in two conditions: a moderate-intensity aerobic session and a control session. In 2-way repeated measures ANOVAs (2 treatments×3 times) followed by contrast comparisons for post hoc analyses, significant pre-post interactions - indicating improvements immediately following exercise but not following the control condition - were observed in the Go-NoGo measures: Accuracy, Reaction Time, and Performance Index, but not in the Catch Game. In the post-follow-up interaction a deterioration was observed in Performance Index, and a trend of deterioration in Accuracy and Reaction Time. The conclusion was that a single session of moderate-intensity aerobic exercise facilitates response inhibition, but not motor planning or eye-hand coordination, in middle-aged healthy active adults. On the other hand, the improvement does not last 30min following a recovery period. Further studies are needed to examine the duration of the inhibitory control benefits and the accumulative effect of a series of acute exercise bouts, as well as to determine the brain networks and/or neurotransmitter systems most affected by the intervention.

Neural and behavioral mechanisms of proactive and reactive inhibition

Response inhibition is an important component of adaptive behavior. Substantial prior research has focused on reactive inhibition, which refers to the cessation of a motor response that is already in progress. More recently, a growing number of studies have begun to examine mechanisms underlying proactive inhibition, whereby preparatory processes result in a response being withheld before it is initiated. It has become apparent that proactive inhibition is an essential component of the overall ability to regulate behavior and has implications for the success of reactive inhibition. Moreover, successful inhibition relies on learning the meaning of specific environmental cues that signal when a behavioral response should be withheld. Proactive inhibitory control is mediated by stopping goals, which reflect the desired outcome of inhibition and include information about how and when inhibition should be implemented. However, little is known about the circuits and cellular processes that encode and represent features in the environment that indicate the necessity for proactive inhibition or how these representations are implemented in response inhibition. In this article, we will review the brain circuits and systems involved in implementing inhibitory control through both reactive and proactive mechanisms. We also comment on possible cellular mechanisms that may contribute to inhibitory control processes, noting that substantial further research is necessary in this regard. Furthermore, we will outline a number of ways in which the temporal dynamics underlying the generation of the proactive inhibitory signal may be particularly important for parsing out the neurobiological correlates that contribute to the learning processes underlying various aspects of inhibitory control.

Revisiting the Corticomotor Plasticity in Low Back Pain: Challenges and Perspectives

Chronic low back pain (CLBP) is a recurrent debilitating condition that costs billions to society. Refractoriness to conventional treatment, lack of improvement, and associated movement disorders could be related to the extensive brain plasticity present in this condition, especially in the sensorimotor cortices. This narrative review on corticomotor plasticity in CLBP will try to delineate how interventions such as training and neuromodulation can improve the condition. The review recommends subgrouping classification in CLBP owing to brain plasticity markers with a view of better understanding and treating this complex condition.

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.

Proactive modulation of long-interval intracortical inhibition during response inhibition

Daily activities often require sudden cancellation of preplanned movement, termed response inhibition. When only a subcomponent of a whole response must be suppressed (required here on Partial trials), the ensuing component is markedly delayed. The neural mechanisms underlying partial response inhibition remain unclear. We hypothesized that Partial trials would be associated with nonselective corticomotor suppression and that GABAB receptor-mediated inhibition within primary motor cortex might be responsible for the nonselective corticomotor suppression contributing to Partial trial response delays. Sixteen right-handed participants performed a bimanual anticipatory response inhibition task while single- and paired-pulse transcranial magnetic stimulation was delivered to elicit motor evoked potentials in the left first dorsal interosseous muscle. Lift times, amplitude of motor evoked potentials, and long-interval intracortical inhibition were examined across the different trial types (Go, Stop-Left, Stop-Right, Stop-Both). Go trials produced a tight distribution of lift times around the target, whereas those during Partial trials (Stop-Left and Stop-Right) were substantially delayed. The modulation of motor evoked potential amplitude during Stop-Right trials reflected anticipation, suppression, and subsequent reinitiation of movement. Importantly, suppression was present across all Stop trial types, indicative of a "default" nonselective inhibitory process. Compared with blocks containing only Go trials, inhibition increased when Stop trials were introduced but did not differ between trial types. The amount of inhibition was positively correlated with lift times during Stop-Right trials. Tonic levels of inhibition appear to be proactively modulated by task context and influence the speed at which unimanual responses occur after a nonselective "brake" is applied.

Changing ideas about others' intentions: updating prior expectations tunes activity in the human motor system

Predicting intentions from observing another agent's behaviours is often thought to depend on motor resonance - i.e., the motor system's response to a perceived movement by the activation of its stored motor counterpart, but observers might also rely on prior expectations, especially when actions take place in perceptually uncertain situations. Here we assessed motor resonance during an action prediction task using transcranial magnetic stimulation to probe corticospinal excitability (CSE) and report that experimentally-induced updates in observers' prior expectations modulate CSE when predictions are made under situations of perceptual uncertainty. We show that prior expectations are updated on the basis of both biomechanical and probabilistic prior information and that the magnitude of the CSE modulation observed across participants is explained by the magnitude of change in their prior expectations. These findings provide the first evidence that when observers predict others' intentions, motor resonance mechanisms adapt to changes in their prior expectations. We propose that this adaptive adjustment might reflect a regulatory control mechanism that shares some similarities with that observed during action selection. Such a mechanism could help arbitrate the competition between biomechanical and probabilistic prior information when appropriate for prediction.

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

Background: Many previous transcranial magnetic stimulation (TMS) studies have investigated corticospinal excitability changes occurring when choosing which hand to use for an action, one of the most frequent decision people make in daily life. So far, these studies have applied single-pulse TMS eliciting motor-evoked potential (MEP) in one hand when this hand is either selected or non-selected. Using such method, hand choices were shown to entail the operation of two inhibitory mechanisms, suppressing MEPs in the targeted hand either when it is non-selected (competition resolution, CR) or selected (impulse control, IC). However, an important limitation of this “Single-Coil” method is that MEPs are elicited in selected and non-selected conditions during separate trials and thus those two settings may not be completely comparable. Moreover, a more important problem is that MEPs are computed in relation to the movement of different hands. The goal of the present study was to test a “Double-Coil” method to evaluate IC and CR preceding the same hand responses by applying Double-Coil TMS over the two primary motor cortices (M1) at a near-simultaneous time (1 ms inter-pulse interval).

Methods: MEPs were obtained in the left (MEP_LEFT) and right (MEP_RIGHT) hands while subjects chose between left and right hand key-presses in blocks using a Single-Coil or a Double-Coil method; in the latter blocks, TMS was either applied over left M1 first (TMS_LRM1 group, n = 12) or right M1 first (TMS_RLM1 group, n = 12).

Results: MEP_LEFT were suppressed preceding both left (IC) and right (CR) hand responses whereas MEP_RIGHT were only suppressed preceding left (CR) but not right (IC) hand responses. This result was observed regardless of whether Single-Coil or Double-Coil TMS was applied in the two subject groups. However, in the TMS_LRM1 group, the MEP suppression was attenuated in Double-Coil compared to Single-Coil blocks for both IC and CR, when probed with MEP_LEFT (elicited by the second pulse).

Conclusions: Although Double-Coil TMS may be a reliable method to assess bilateral motor excitability provided that a RM1-LM1 pulse order is used, further experiments are required to understand the reduced MEP_LEFT changes in Double-Coil blocks when the LM1-RM1 pulse order was used.

Does response inhibition have pre- and postdiagnostic utility in Parkinson's disease?

Parkinson's disease (Pd) is the second most prevalent degenerative neurological condition worldwide. Improving and sustaining quality of life is an important goal for Parkinson's patients. Key areas of focus to achieve this goal include earlier diagnosis and individualized treatment. In this review the authors discuss impulse control in Pd and examine how measures of impulse control from a response inhibition task may provide clinically useful information (a) within an objective test battery to aid earlier diagnosis of Pd and (b) in postdiagnostic Pd, to better identify individuals at risk of developing impulse control disorders with dopaminergic medication.

Nonspecific Inhibition of the Motor System during Response Preparation

Motor system excitability is transiently inhibited during the preparation of responses. Previous studies have attributed this inhibition to the operation of two mechanisms, one hypothesized to help resolve competition between alternative response options, and the other to prevent premature response initiation. By this view, inhibition should be restricted to task-relevant muscles. Although this prediction is supported in one previous study (Duque et al., 2010), studies of stopping ongoing actions suggest that some forms of motor inhibition may be widespread (Badry et al., 2009). This motivated us to conduct a series of transcranial magnetic stimulation (TMS) experiments to examine in detail the specificity of preparatory inhibition in humans. Motor-evoked potentials were inhibited in task-irrelevant muscles during response preparation, even when the muscles were contralateral and not homologous to the responding effector. Inhibition was also observed in both choice and simple response task conditions, with and without a preparatory interval. Control experiments ruled out that this inhibition is due to expectancy of TMS or a possible need to cancel the prepared response. These findings suggest that motor inhibition during response preparation broadly influences the motor system and likely reflects a process that occurs whenever a response is selected. We propose a reinterpretation of the functional significance of preparatory inhibition, one by which inhibition reduces noise to enhance signal processing and modulates the gain of a selected response.

SIGNIFICANCE STATEMENT

Motor preparation entails the recruitment of excitatory and inhibitory neural mechanisms. The current experiments address the specificity of inhibitory mechanisms, asking whether preparatory inhibition affects task-irrelevant muscles. Participants prepared a finger movement to be executed at the end of a short delay period. Transcranial magnetic stimulation over primary motor cortex provided an assay of corticospinal excitability. Consistent with earlier work, the agonist muscle for the forthcoming response was inhibited during the preparatory period. Moreover, this inhibition was evident in task-irrelevant muscles, although the magnitude of inhibition depended on whether the response was fixed or involved a choice. These results implicate a broadly tuned inhibitory mechanism that facilitates response preparation, perhaps by lowering background activity before response initiation.

Comparison of the two cerebral hemispheres in inhibitory processes operative during movement preparation

Neuroimaging and neuropsychological studies suggest that in right-handed individuals, the left hemisphere plays a dominant role in praxis, relative to the right hemisphere. However hemispheric asymmetries assessed with transcranial magnetic stimulation (TMS) has not shown consistent differences in corticospinal (CS) excitability of the two hemispheres during movements. In the current study, we systematically explored hemispheric asymmetries in inhibitory processes that are manifest during movement preparation and initiation. Single-pulse TMS was applied over the left or right primary motor cortex (M1LEFT and M1RIGHT, respectively) to elicit motor-evoked potentials (MEPs) in the contralateral hand while participants performed a two-choice reaction time task requiring a cued movement of the left or right index finger. In Experiments 1 and 2, TMS probes were obtained during a delay period following the presentation of the preparatory cue that provided partial or full information about the required response. MEPs were suppressed relative to baseline regardless of whether they were elicited in a cued or uncued hand. Importantly, the magnitude of these inhibitory changes in CS excitability was similar when TMS was applied over M1LEFT or M1RIGHT, irrespective of the amount of information carried by the preparatory cue. In Experiment 3, there was no preparatory cue and TMS was applied at various time points after the imperative signal. When CS excitability was probed in the cued effector, MEPs were initially inhibited and then rose across the reaction time interval. This function was similar for M1LEFT and M1RIGHT TMS. When CS excitability was probed in the uncued effector, MEPs remained inhibited throughout the RT interval. However, MEPs in right FDI became more inhibited during selection and initiation of a left hand movement, whereas MEPs in left FDI remained relatively invariant across RT interval for the right hand. In addition to these task-specific effects, there was a global difference in CS excitability across experiments between the two hemispheres. When the intensity of stimulation was set to 115% of the resting threshold, MEPs were larger when the TMS probe was applied over the M1LEFT than over M1RIGHT. In summary, while the latter result suggests that M1LEFT is more excitable than M1RIGHT, the recruitment of preparatory inhibitory mechanisms is similar within the two cerebral hemispheres.

Transcranial Magnetic Stimulation

Preparing actions requires the operation of several cognitive control processes that influence the state of the motor system to ensure that the appropriate behavior is ultimately selected and executed. For example, some form of competition resolution ensures that the right action is chosen among alternatives, often in the presence of conflict; at the same time, impulse control ought to be deployed to prevent premature responses. Here we review how state-changes in the human motor system during action preparation can be studied through motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation over the contralateral primary motor cortex (M1). We discuss how the physiological fingerprints afforded by MEPs have helped to decompose some of the dynamic and effector-specific influences on the motor system during action preparation. We focus on competition resolution, conflict and impulse control, as well as on the influence of higher cognitive decision–related variables. The selected examples demonstrate the usefulness of MEPs as physiological readouts for decomposing the influence of distinct, but often overlapping, control processes on the human motor system during action preparation.

One hand clapping: lateralization of motor control

Lateralization of motor control refers to the ability to produce pure unilateral or asymmetric movements. It is required for a variety of coordinated activities, including skilled bimanual tasks and locomotion. Here we discuss the neuroanatomical substrates and pathophysiological underpinnings of lateralized motor outputs. Significant breakthroughs have been made in the past few years by studying the two known conditions characterized by the inability to properly produce unilateral or asymmetric movements, namely human patients with congenital “mirror movements” and model rodents with a “hopping gait”. Whereas mirror movements are associated with altered interhemispheric connectivity and abnormal corticospinal projections, abnormal spinal cord interneurons trajectory is responsible for the “hopping gait”. Proper commissural axon guidance is a critical requirement for these mechanisms. Interestingly, the analysis of these two conditions reveals that the production of asymmetric movements involves similar anatomical and functional requirements but in two different structures: (i) lateralized activation of the brain or spinal cord through contralateral silencing by cross-midline inhibition; and (ii) unilateral transmission of this activation, resulting in lateralized motor output.

Influence of Delay Period Duration on Inhibitory Processes for Response Preparation

In this study, we examined the dynamics of inhibitory preparatory processes, using a delayed response task in which a cue signaled a left or right index finger (Experiment 1) or hand (Experiment 2) movement in advance of an imperative signal. In Experiment 1, we varied the duration of the delay period (200, 500, and 900 ms). When transcranial magnetic stimulation (TMS) was applied 100 ms before the imperative, motor evoked potentials (MEPs) elicited in the first dorsal interosseous were strongly inhibited. For delays of 500 ms or longer, this inhibition was greater when the targeted muscle was selected compared with when it was not selected. In contrast, the magnitude of inhibition just after the cue was inversely related to the duration of the delay period, and the difference between the selected and nonselected conditions was attenuated. In Experiment 2, TMS and peripheral nerve stimulation procedures were used during a 300-ms delay period. MEPs in the flexor carpi radialis for both selected and nonselected conditions were inhibited, but without any change in the H-reflex. Taken together, these results reveal the dual influence of temporal constraints associated with anticipation and urgency on inhibitory processes recruited during response preparation.

Controlling your impulses: electrical stimulation of the human supplementary motor complex prevents impulsive errors

To err is human. However, an inappropriate urge does not always result in error. Impulsive errors thus entail both a motor system capture by an urge to act and a failed inhibition of that impulse. Here we show that neuromodulatory electrical stimulation of the supplementary motor complex in healthy humans leaves action urges unchanged but prevents them from turning into overt errors. Subjects performed a choice reaction-time task known to trigger impulsive responses, leading to fast errors that can be revealed by analyzing accuracy as a function of poststimulus time. Yet, such fast errors are only the tip of the iceberg: electromyography (EMG) revealed fast subthreshold muscle activation in the incorrect response hand in an even larger proportion of overtly correct trials, revealing covert response impulses not discernible in overt behavior. Analyzing both overt and covert response tendencies enables to gauge the ability to prevent these incorrect impulses from turning into overt action errors. Hyperpolarizing the supplementary motor complex using transcranial direct current stimulation (tDCS) preserves action impulses but prevents their behavioral expression. This new combination of detailed behavioral, EMG, and tDCS techniques clarifies the neurophysiology of impulse control, and may point to avenues for improving impulse control deficits in various neurologic and psychiatric disorders.

Signs of timing in motor cortex during movement preparation and cue anticipation

The capacity to accurately anticipate the timing of predictable events is essential for sensorimotor behavior. Motor cortex holds an established role in movement preparation and execution. In this chapter we review the different ways in which motor cortical activity is modulated by event timing in sensorimotor delay tasks. During movement preparation, both single neuron and population responses reflect the temporal constraints of the task. Anticipatory modulations prior to sensory cues are also observed in motor cortex when the cue timing is predictable. We propose that the motor cortical activity during cue anticipation and movement preparation is embedded in a timing network that facilitates sensorimotor processing. In this context, the pre-cue and post-cue activity may reflect a presetting mechanism, complementing processing during movement execution, while prohibiting premature responses in situations requiring delayed motor output.

Neural processes mediating the preparation and release of focal motor output are suppressed or absent during imagined movement

Movements that are executed or imagined activate a similar subset of cortical regions, but the extent to which this activity represents functionally equivalent neural processes is unclear. During preparation for an executed movement, presentation of a startling acoustic stimulus (SAS) evokes a premature release of the planned movement with the spatial and temporal features of the tasks essentially intact. If imagined movement incorporates the same preparatory processes as executed movement, then a SAS should release the planned movement during preparation. This hypothesis was tested using an instructed-delay cueing paradigm during which subjects were required to rapidly release a handheld weight while maintaining the posture of the arm or to perform first-person imagery of the same task while holding the weight. In a subset of trials, a SAS was presented at 1500, 500, or 200 ms prior to the release cue. Task-appropriate preparation during executed and imagined movements was confirmed by electroencephalographic recording of a contingent negative variation waveform. During preparation for executed movement, a SAS often resulted in premature release of the weight with the probability of release progressively increasing from 24 % at -1500 ms to 80 % at -200 ms. In contrast, the SAS rarely (<2 % of trials) triggered a release of the weight during imagined movement. However, the SAS frequently evoked the planned postural response (suppression of bicep brachii muscle activity) irrespective of the task or timing of stimulation (even during periods of postural hold without preparation). These findings provide evidence that neural processes mediating the preparation and release of the focal motor task (release of the weight) are markedly attenuated or absent during imagined movement and that postural and focal components of the task are prepared independently.