Online Maintenance of Sensory and Motor Representations: Effects on Corticospinal Excitability

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.

EEG responses induced by cerebellar TMS at rest and during visuomotor adaptation

BACKGROUND

Connections between the cerebellum and the cortex play a critical role in learning and executing complex behaviours. Dual-coil transcranial magnetic stimulation (TMS) can be used non-invasively to probe connectivity changes between the lateral cerebellum and motor cortex (M1) using the motor evoked potential as an outcome measure (cerebellar-brain inhibition, CBI). However, it gives no information about cerebellar connections to other parts of cortex.

OBJECTIVES

We used electroencephalography (EEG) to investigate whether it was possible to detect activity evoked in any areas of cortex by single-pulse TMS of the cerebellum (cerebellar TMS evoked potentials, cbTEPs). A second experiment tested if these responses were influenced by the performance of a cerebellar-dependent motor learning paradigm.

METHODS

In the first series of experiments, TMS was applied over either the right or left cerebellar cortex, and scalp EEG was recorded simultaneously. Control conditions that mimicked auditory and somatosensory inputs associated with cerebellar TMS were included to identify responses due to non-cerebellar sensory stimulation. We conducted a follow-up experiment that evaluated whether cbTEPs are behaviourally sensitive by assessing individuals before and after learning a visuomotor reach adaptation task.

RESULTS

A TMS pulse over the lateral cerebellum evoked EEG responses that could be distinguished from those caused by auditory and sensory artefacts. Significant positive (P80) and negative peaks (N110) over the contralateral frontal cerebral area were identified with a mirrored scalp distribution after left vs. right cerebellar stimulation. The P80 and N110 peaks were replicated in the cerebellar motor learning experiment and changed amplitude at different stages of learning. The change in amplitude of the P80 peak was associated with the degree of learning that individuals retained following adaptation. Due to overlap with sensory responses, the N110 should be interpreted with caution.

CONCLUSIONS

Cerebral potentials evoked by TMS of the lateral cerebellum provide a neurophysiological probe of cerebellar function that complements the existing CBI method. They may provide novel insight into mechanisms of visuomotor adaptation and other cognitive processes.

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.

Motor Conflict: Revealing Involuntary Conditioned Motor Preparation Using Transcranial Magnetic Stimulation

Preparing actions to achieve goals, overriding habitual responses, and substituting actions that are no longer relevant are aspects of motor control often assumed to be driven by deliberate top-down processes. In the present study, we investigated whether motor control could come under involuntary control of environmental cues that have been associated with specific actions in the past. We used transcranial magnetic stimulation (TMS) to probe corticospinal excitability as an index of motor preparation, while participants performed a Go/No-Go task (i.e., an action outcome or no action outcome task) and rated what trial was expected to appear next (Go or No-Go). We found that corticospinal excitability during a warning cue for the upcoming trial closely matched recent experience (i.e., cue–outcome pairings), despite conflicting with what participants expected would appear. The results reveal that in an action–outcome task, neurophysiological indices of motor preparation show changes that are consistent with participants learning to associate a preparatory warning cue with a specific action, and are not consistent with the action that participants explicitly anticipate making. This dissociation with conscious expectancy ratings reveals that conditioned responding and motor preparation can operate independently of conscious expectancies about having to act.

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.

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.

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.

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.

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.

Inhibition during response preparation is sensitive to response complexity

Motor system excitability is transiently suppressed during the preparation of movement. This preparatory inhibition is hypothesized to facilitate response selection and initiation. Given that demands on selection and initiation processes increase with movement complexity, we hypothesized that complexity would influence preparatory inhibition. To test this hypothesis, we probed corticospinal excitability during a delayed-response task in which participants were cued to prepare right- or left-hand movements of varying complexity. Single-pulse transcranial magnetic stimulation was applied over right primary motor cortex to elicit motor evoked potentials (MEPs) from the first dorsal interosseous (FDI) of the left hand. MEP suppression was greater during the preparation of responses involving coordination of the FDI and adductor digiti minimi relative to easier responses involving only the FDI, independent of which hand was cued to respond. In contrast, this increased inhibition was absent when the complex responses required sequential movements of the two muscles. Moreover, complexity did not influence the level of inhibition when the response hand was fixed for the trial block, regardless of whether the complex responses were performed simultaneously or sequentially. These results suggest that preparatory inhibition contributes to response selection, possibly by suppressing extraneous movements when responses involve the simultaneous coordination of multiple effectors.

Interaction of temporal and ordinal representations in movement sequences

The production of movement sequences requires an accurate control of muscle activation in time. How does the nervous system encode the precise timing of these movements? One possibility is that the timing of movements (temporal sequence) is an emergent property of the dynamic state of the nervous system and therefore intimately linked to a representation of the sequence of muscle commands (ordinal sequence). Alternatively, timing may be represented independently of the motor effectors and would be transferable to a new ordinal sequence. Some studies have found that a learned temporal sequence cannot be transferred to a new ordinal sequence, thus arguing for an integrated representation. Others have observed temporal transfer across movement sequences and have advocated an independent representation of temporal information. Using a modified serial reaction time task, we tested alternative models of the representation of temporal structure and the interaction between the output of separate ordinal and temporal sequence representations. Temporal transfer depended on whether a novel ordinal sequence was fixed within each test block. Our results confirm the presence of an independent representation of temporal structure and advocate a nonlinear multiplicative neural interaction of temporal and ordinal signals in the production of movements.

Influence of cueing on the preparation and execution of untrained and trained complex motor responses

This study investigated the influence of cueing on the performance of untrained and trained complex motor responses. Healthy adults responded to a visual target by performing four sequential movements (complex response) or a single movement (simple response) of their middle finger. A visual cue preceded the target by an interval of 300, 1000, or 2000 ms. In Experiment 1, the complex and simple responses were not previously trained. During the testing session, the complex response pattern varied on a trial-by-trial basis following the indication provided by the visual cue. In Experiment 2, the complex response and the simple response were extensively trained beforehand. During the testing session, the trained complex response pattern was performed in all trials. The latency of the untrained and trained complex responses decreased from the short to the medium and long cue-target intervals. The latency of the complex response was longer than that of the simple response, except in the case of the trained responses and the long cue-target interval. These results suggest that the preparation of untrained complex responses cannot be completed in advance, this being possible, however, for trained complex responses when enough time is available. The duration of the 1st submovement, 1st pause and 2nd submovement of the untrained and the trained complex responses increased from the short to the long cue-target interval, suggesting that there is an increase of online programming of the response possibly related to the degree of certainty about the moment of target appearance.

Evidence for two concurrent inhibitory mechanisms during response preparation

Inhibitory mechanisms are critically involved in goal-directed behaviors. To gain further insight into how such mechanisms shape motor representations during response preparation, motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and H-reflexes were recorded from left hand muscles during choice reaction time tasks. The imperative signal, which indicated the required response, was always preceded by a preparatory cue. During the postcue delay period, left MEPs were suppressed when the left hand had been cued for the forthcoming response, suggestive of a form of inhibition specifically directed at selected response representations. H-reflexes were also suppressed on these trials, indicating that the effects of this inhibition extend to spinal circuits. In addition, left MEPs were suppressed when the right hand was cued, but only when left hand movements were a possible response option before the onset of the cue. Notably, left hand H-reflexes were not modulated on these trials, consistent with a cortical locus of inhibition that lowers the activation of task-relevant, but nonselected responses. These results suggest the concurrent operation of two inhibitory mechanisms during response preparation: one decreases the activation of selected responses at the spinal level, helping to control when selected movements should be initiated by preventing their premature release; a second, upstream mechanism helps to determine what response to make during a competitive selection process.

Coding of visual space during motor preparation: Approaching objects rapidly modulate corticospinal excitability in hand-centered coordinates

Defensive behaviors, such as withdrawing your hand to avoid potentially harmful approaching objects, rely on rapid sensorimotor transformations between visual and motor coordinates. We examined the reference frame for coding visual information about objects approaching the hand during motor preparation. Subjects performed a simple visuomanual task while a task-irrelevant distractor ball rapidly approached a location either near to or far from their hand. After the distractor ball appearance, single pulses of transcranial magnetic stimulation were delivered over the subject's primary motor cortex, eliciting motor evoked potentials (MEPs) in their responding hand. MEP amplitude was reduced when the ball approached near the responding hand, both when the hand was on the left and the right of the midline. Strikingly, this suppression occurred very early, at 70-80 ms after ball appearance, and was not modified by visual fixation location. Furthermore, it was selective for approaching balls, since static visual distractors did not modulate MEP amplitude. Together with additional behavioral measurements, we provide converging evidence for automatic hand-centered coding of visual space in the human brain.

Changing the "when" and "what" of intended actions

Humans often have to modify the timing and/or type of their planned actions on the basis of new sensory information. In the present experiments, participants planned to make a right index finger keypress 3 s after a warning stimulus but on some trials were interrupted by a temporally unpredictable auditory tone prompting the same action (experiment 1) or a different action (experiment 2). In experiment 1, by comparing the reaction time (RT) to tones presented at different stages of the preparatory period to RT in a simple reaction time condition, we determined the cost of switching from an internally generated mode of response production to an externally triggered mode in situations requiring only a change in when an action is made (i.e., when the tone prompts the action at a different time from the intended time of action). Results showed that the cost occurred for interruption tones delivered 200 ms after a warning stimulus and remained relatively stable throughout most of the preparatory period with a reduction in the magnitude of the cost during the last 200 ms prior to the intended time of movement. In experiment 2, which included conditions requiring a change in both when and what action is produced on the tone, results show a larger cost when the switched to action is different from the action being prepared. We discuss our results in the light of neurophysiological experiments on motor preparation and suggest that intending to act is accompanied by a general inhibitory mechanism preventing premature motor output and a specific excitatory process pertaining to the intended movement. Interactions between these two mechanisms could account for our behavioral results.

Transition from rest to movement: brain correlates revealed by functional connectivity

It is suggested that resting state networks reflecting correlated neural regional activities participate significantly in brain functioning. A fundamental issue is to understand how these networks interact and how their activities change during behavioral transitions. Our aim was to understand better with functional MRI connectivity how the brain switched from a "resting" to a movement-related state by exploring the transitory readiness state for an intended movement of the right hand. Our study does not address movement preparation occurring in a time scale of milliseconds before movement which has been widely studied but movement-readiness which can last longer. At rest, in the absence of overt goal-directed behavior, a "default-mode" network, whose main areas are the posterior cingulate cortex and precuneus (PCC/Pcu), shows high activity interpreted as day dreaming, free association, stream of consciousness, and inner rehearsal. We found that, during rest, the "default-mode" network and the sensorimotor network were not functionally correlated. During movement-readiness, the two networks were functionally correlated through an interaction between the PCC/Pcu and the medial superior parietal cortex in the upper precuneus. The complex PCC/Pcu has been shown to be involved in retrieval and/or setting up spatial attributes for motor imagery, and thus, would be a key region in the movement-readiness phase. It might functionally connect to the medial superior parietal cortex to initiate the movement programming through retrieval of suited movement parameters. The anterior cingulum, functionally correlated to the primary sensorimotor cortex during movement-readiness would have a motivational role or could generate predictions about the movement.

Role of corticospinal suppression during motor preparation

Behavior arises from a constant competition between potential actions. For example, movements performed unimanually require selecting one hand rather than the other. Corticospinal (CS) excitability of the nonselected hand is typically decreased prior to movement initiation, suggesting that response selection may involve mechanisms that inhibit nonselected candidate movements. To examine this hypothesis, participants performed a reaction time task, responding with the left, right, or both indexes. Transcranial magnetic stimulation was applied over the right primary motor cortex (M1) to induce motor-evoked potentials (MEPs) in a left hand muscle at various stages during response preparation. To vary the time of response selection, an imperative signal was preceded by a preparatory cue that was either informative or uninformative. Left MEPs decreased following the cue. Surprisingly, this decrease was greater when an informative cue indicated that the response might require the left hand than when it indicated a right hand response. In the uninformative condition, we did not observe additional attenuation of left MEP after an imperative indicating a right hand response. These results argue against the "deselection" hypothesis. Rather, CS suppression seems to arise from "impulse control" mechanisms that ensure that responses associated with potentially selected actions are not initiated prematurely.

Changes in corticospinal excitability and the direction of evoked movements during motor preparation: a TMS study

BACKGROUND

Preparation of the direction of a forthcoming movement has a particularly strong influence on both reaction times and neuronal activity in the primate motor cortex. Here, we aimed to find direct neurophysiologic evidence for the preparation of movement direction in humans. We used single-pulse transcranial magnetic stimulation (TMS) to evoke isolated thumb-movements, of which the direction can be modulated experimentally, for example by training or by motor tasks. Sixteen healthy subjects performed brisk concentric voluntary thumb movements during a reaction time task in which the required movement direction was precued. We assessed whether preparation for the thumb movement lead to changes in the direction of TMS-evoked movements and to changes in amplitudes of motor-evoked potentials (MEPs) from the hand muscles.

RESULTS

When the required movement direction was precued early in the preparatory interval, reaction times were 50 ms faster than when precued at the end of the preparatory interval. Over time, the direction of the TMS-evoked thumb movements became increasingly variable, but it did not turn towards the precued direction. MEPs from the thumb muscle (agonist) were differentially modulated by the direction of the precue, but only in the late phase of the preparatory interval and thereafter. MEPs from the index finger muscle did not depend on the precued direction and progressively decreased during the preparatory interval.

CONCLUSION

Our data show that the human corticospinal movement representation undergoes progressive changes during motor preparation. These changes are accompanied by inhibitory changes in corticospinal excitability, which are muscle specific and depend on the prepared movement direction. This inhibition might indicate a corticospinal braking mechanism that counteracts any preparatory motor activation.

Delay-related cerebral activity and motor preparation

Flexible goal-oriented behavior requires the ability to carry information across temporal delays. This ability is associated with sustained neural firing. In cognitive terms, this ability has often been associated with the maintenance of sensory material online, as during short-term memory tasks, or with the retention of a motor code, as during movement preparation tasks. The general issue addressed in this paper is whether short-term storage of sensory information and preparation of motor responses rely on different anatomical substrates. We used functional magnetic resonance imaging (fMRI) to measure sustained and time-varying delay-related cerebral activity evoked during performance of a delay non-match to sample (DNMS) task, where task contingencies rather than explicit instructions ensured that either sensory or motor representations were used to cross the delay period on each trial. This approach allowed us to distinguish sensory from motor characteristics of delay-related activity evoked by task contingencies, rather than differences in the control of short-term storage driven by verbal instructions. Holding sensory material online evoked both sustained and time-varying delay-related activity in prefrontal regions, whereas movement preparation evoked delay-related responses in precentral areas. Intraparietal cortex was sensitive to the presence of memoranda, but indifferent to the type of information that was retained in memory. Our findings indicate that short-term storage of sensory information and preparation of motor responses rely on partially segregated cerebral circuits. In the frontal lobe, these circuits are organized along a rostro-caudal dimension, corresponding to the sensory or motor nature of the stored material.

Effects of motor preparation and spatial attention on corticospinal excitability in a delayed-response paradigm

The preparation of motor responses during the delay period of an instructed delay task is associated with sustained neural firing in the primate premotor cortex. It remains unclear how and when such preparation-related premotor activity influences the motor output system. In this study, we tested modulation of corticospinal excitability using single-pulse transcranial magnetic stimulation (TMS) during a delayed-response task. At the beginning of the delay interval participants were either provided with no information, spatial attentional information concerning location but not identity of an upcoming imperative stimulus, or information regarding the upcoming response. Behavioral data indicate that participants used all information available to them. Only when information concerning the upcoming response was provided did corticospinal excitability show differential modulation for the effector muscle compared to other task-unrelated muscles. We conclude that modulation of corticospinal excitability reflects specific response preparation, rather than non-specific event preparation.