Neurophysiological mechanisms involved in transfer of procedural knowledge

Altered bidirectional plasticity and reduced implicit motor learning in concussed athletes

Persistent motor/cognitive alterations and increased prevalence of Alzheimer's disease are known consequences of recurrent sports concussions, the most prevalent cause of mild traumatic brain injury (TBI) among youth. Animal models of TBI demonstrated that impaired learning was related to persistent synaptic plasticity suppression in the form of long-term potentiation (LTP) and depression (LTD). In humans, single and repeated concussive injuries lead to lifelong and cumulative enhancements of gamma-aminobutyric acid (GABA)-mediated inhibition, which is known to suppress LTP/LTD plasticity. To test the hypothesis that increased GABAergic inhibition after repeated concussions suppresses LTP/LTD and contributes to learning impairments, we used a paired associative stimulation (PAS) protocol to induce LTP/LTD-like effects in primary motor cortex (M1) jointly with an implicit motor learning task (serial reaction time task, SRTT). Our results indicate that repeated concussions induced persistent elevations of GABA(B)-mediated intracortical inhibition in M1, which was associated with suppressed PAS-induced LTP/LTD-like synaptic plasticity. This synaptic plasticity suppression was related to reduced implicit motor learning on the SRTT task relative to normal LTP/LTD-like synaptic plasticity in unconcussed teammates. These findings identify GABA neurotransmission alterations after repeated concussions and suggest that impaired learning after multiple concussions could at least partly be related to compromised GABA-dependent LTP/LTD synaptic plasticity.

Structural integrity of callosal midbody influences intermanual transfer in a motor reaction-time task

Training one hand on a motor task results in performance improvements in the other hand, also when stimuli are randomly presented (nonspecific transfer). Corpus callosum (CC) is the main structure involved in interhemispheric information transfer; CC pathology occurs in patients with multiple sclerosis (PwMS) and is related to altered performance of tasks requiring interhemispheric transfer of sensorimotor information. To investigate the role of CC in nonspecific transfer during a pure motor reaction-time task, we combined motor behavior with diffusion tensor imaging analysis in PwMS. Twenty-two PwMS and 10 controls, all right-handed, were asked to respond to random stimuli with appropriate finger opposition movements with the right (learning) and then the left (transfer) hand. PwMS were able to improve motor performance reducing response times with practice with a trend similar to controls and preserved the ability to transfer the acquired motor information from the learning to the transfer hand. A higher variability in the transfer process, indicated by a significantly larger standard deviation of mean nonspecific transfer, was found in the PwMS group with respect to the control group, suggesting the presence of subtle impairments in interhemispheric communication in some patients. Then, we correlated the amount of nonspecific transfer with mean fractional anisotropy (FA) values, indicative of microstructural damage, obtained in five CC subregions identified on PwMS's FA maps. A significant correlation was found only in the subregion including posterior midbody (Pearson's r = 0.74, P = 0.003), which thus seems to be essential for the interhemispheric transfer of information related to pure sensorimotor tasks.

Brain lateralisation and motor learning: selective effects of dominant and non-dominant hand practice on the early acquisition of throwing skills

Findings from neurosciences indicate that the two brain hemispheres are specialised for the processing of distinct movement features. How this knowledge can be useful in motor learning remains unclear. Two experiments were conducted to investigate the influence of initial practice with the dominant vs non-dominant hand on the acquisition of novel throwing skills. Within a transfer design two groups practised a novel motor task with the same amount of practice on each hand, but in opposite hand-order. In Experiment 1, participants acquired the position throw in basketball, which places high demands on throwing accuracy. Participants practising this task with their non-dominant hand first, before changing to the dominant hand, showed better skill acquisition than participants practising in opposite order. In Experiment 2 participants learned the overarm throw in team handball, which requires great throwing strength. Participants initially practising with their dominant hand benefited more from practice than participants beginning with their non-dominant hand. These results indicate that spatial accuracy tasks are learned better after initial practice with the non-dominant hand, whereas initial practice with the dominant hand is more efficient for maximum force production tasks. The effects are discussed in terms of brain lateralisation and bilateral practice schedules.

Motor sequence learning: acquisition of explicit knowledge is concomitant to changes in motor strategy of finger opposition movements

Motor sequence learning is not a unitary phenomenon, but involves optimizing different components that include declarative and procedural aspects. In this work we designed an experimental approach that allows monitoring all the aspects of sequence learning using a finger opposition task and a movement-by-movement analysis. Subjects performed a visuomotor sequence learning paradigm with (Explicit) or without (Implicit) instructions and we measured response time (RT) and touch duration (TD) for each finger opposition movement of the sequence. Our results indicated that sequence learning induced a double-faced effect on motor performance: a decrease of RT and an increase of TD. However, the above changes manifested differently among subjects: all subjects that, by the end of session, had complete recall of the sequence order, reached an equal level of performance by the last sequence block while in those who had on average only a poor recall of the sequence order, learning was evident only as a slight decrease of RT across sequence blocks, while no kinematic changes (i.e., changes in TD) occurred. Our results indicate that, in the absence of specific instructions, learning evolves from an early stage in which only small decreases of RT are observed to a phase in which progressive knowledge of the sequential structure allows for dramatic changes of RT, together with a progressive change of motor performance (i.e., changes in TD).

Using repetitive transcranial magnetic stimulation to study the underlying neural mechanisms of human motor learning and memory

In the last two decades, there has been a rapid development in the research of the physiological brain mechanisms underlying human motor learning and memory. While conventional memory research performed on animal models uses intracellular recordings, microfusion of protein inhibitors to specific brain areas and direct induction of focal brain lesions, human research has so far utilized predominantly behavioural approaches and indirect measurements of neural activity. Repetitive transcranial magnetic stimulation (rTMS), a safe non-invasive brain stimulation technique, enables the study of the functional role of specific cortical areas by evaluating the behavioural consequences of selective modulation of activity (excitation or inhibition) on memory generation and consolidation, contributing to the understanding of the neural substrates of motor learning. Depending on the parameters of stimulation, rTMS can also facilitate learning processes, presumably through purposeful modulation of excitability in specific brain regions. rTMS has also been used to gain valuable knowledge regarding the timeline of motor memory formation, from initial encoding to stabilization and long-term retention. In this review, we summarize insights gained using rTMS on the physiological and neural mechanisms of human motor learning and memory. We conclude by suggesting possible future research directions, some with direct clinical implications.

The effect of bilateral isometric forces in different directions on motor cortical function in humans

The activity in the primary motor cortex (M1) reflects the direction of movements, but little is known about physiological changes in the M1 during generation of bilateral isometric forces in different directions. Here, we used transcranial magnetic stimulation to examine motor evoked potentials (MEPs), short-interval intracortical inhibition (SICI), and interhemispheric inhibition (IHI) in the left first dorsal interosseous (FDI) during isometric index finger abduction while the right index finger remained at rest or performed isometric forces in different directions (abduction or adduction) and in different postures (prone and supine). Left FDI MEPs were suppressed during bilateral compared with unilateral forces, with a stronger suppression when the right index finger force was exerted in the adduction direction regardless of hand posture. IHI targeting the left FDI increased during bilateral compared with unilateral forces and this increase was stronger during right index finger adduction despite the posture of the right hand. SICI decreased to a similar extent during both bilateral forces in both hand postures. Thus generation of index finger isometric forces away from the body midline (adduction direction), regardless of the muscle engaged in the task, down-regulates corticospinal output in the contralateral active hand to a greater extent than forces exerted toward the body midline (abduction direction). Transcallosal inhibition, but not GABAergic intracortical circuits, was modulated by the direction of the force. These findings suggest that during generation of bimanual isometric forces the M1 is driven by "extrinsic" parameters related to the hand action.

Anodal transcranial direct current stimulation enhances procedural consolidation

The primary motor cortex (M1) area recruitment enlarges while learning a finger tapping sequence. Also M1 excitability increases during procedural consolidation. Our aim was to investigate whether increasing M1 excitability by anodal transcranial DC stimulation (AtDCS) when procedural consolidation occurs was able to induce an early consolidation improvement. Forty-seven right-handed healthy participants were trained in a nine-element serial finger tapping task (SFTT) executed with the left hand. Random series blocks were interspersed with training series blocks. Anodal or sham tDCS was administered over the right M1 after the end of the training session. After stimulation, the motor skills of both trained and a new untrained sequential series blocks were tested again. For each block, performance was estimated as the median execution time of correct series. Early consolidation of the trained series, assessed by the performance difference between the first block after and the last block before stimulation normalized by the random, was enhanced by anodal and not by sham tDCS. Stimulation did not affect random series execution. No stimulation effect was found on the on-line learning of the trained and new untrained series. Our results suggest that AtDCS applied on M1 soon after training improves early consolidation of procedural learning. Our data highlight the importance of neuromodulation procedures for understanding learning processes and support their use in the motor rehabilitation setting, focusing on the timing of the application.

Changes in cerebello-motor connectivity during procedural learning by actual execution and observation

The cerebellum is involved in motor learning of new procedures both during actual execution of a motor task and during observational training. These processes are thought to depend on the activity of a neural network that involves the lateral cerebellum and primary motor cortex (M1). In this study, we used a twin-coil TMS technique to investigate whether execution and observation of a visuomotor procedural learning task is related to modulation of cerebello-motor connectivity. We observed that, at rest, a magnetic conditioning pulse applied over the lateral cerebellum reduced the motor-evoked potentials obtained by stimulating the contralateral M1, indicating activation of a cerebello-motor connection. Furthermore, during procedural learning, cerebellar stimulation resulted in selective facilitation, not inhibition, of contralateral M1 excitability. The effects were evident when motor learning was obtained by actual execution of the task or by observation, but they disappeared if procedural learning had already been acquired by previous observational training. These results indicate that changes in cerebello-motor connectivity occur in relation to specific phases of procedural learning, demonstrating a complex pattern of excitatory and inhibitory drives modulated across time.

The ipsilateral motor cortex contributes to cross-limb transfer of performance gains after ballistic motor practice

Although it has long been known that practicing a motor task with one limb can improve performance with the limb opposite, the mechanisms remain poorly understood. Here we tested the hypothesis that improved performance with the untrained limb on a fastest possible (i.e. ballistic) movement task depends partly on cortical circuits located ipsilateral to the trained limb. The idea that crossed effects, which are important for the learning process, might occur in the 'untrained' hemisphere following ballistic training is based on the observation that tasks requiring strong descending drive generate extensive bilateral cortical activity. Twenty-one volunteers practiced a ballistic index finger abduction task with their right hand, and corticospinal excitability was assessed in two hand muscles (first dorsal interosseus, FDI; adductor digiti minimi, ADM). Eight control subjects did not train. After training, repetitive transcranial magnetic stimulation (rTMS; 15 min at 1 Hz) was applied to the left (trained) or right (untrained) motor cortex to induce a 'virtual lesion'. A third training group received sham rTMS, and control subjects received rTMS to the right motor cortex. Performance and corticospinal excitability (for FDI) increased in both hands for training but not control subjects. rTMS of the left, trained motor cortex specifically reduced training-induced gains in motor performance for the right, trained hand, and rTMS of the right, untrained motor cortex specifically reduced performance gains for the left, untrained hand. Thus, cortical processes within the untrained hemisphere, ipsilateral to the trained hand, contribute to early retention of ballistic performance gains for the untrained limb.

Mechanisms controlling motor output to a transfer hand after learning a sequential pinch force skill with the opposite hand

OBJECTIVE

Training to perform a serial reaction-time task (procedural motor learning) with one hand results in performance improvements in the untrained as well as in the trained hand, a phenomenon referred to as intermanual transfer. The aim of this study was to investigate the neurophysiological changes associated with intermanual transfer associated with learning to perform an eminently different task involving fine force control within the primary motor cortex (M1). We hypothesized that intermanual transfer of learning such a task would reveal intracortical changes within M1.

METHODS

Speed (time to complete each sequence) and accuracy (% of accuracy errors) of motor performance were measured in both hands before and after right (dominant) hand practice. Transcranial magnetic stimulation (TMS) was used to characterize recruitment curves (RC), short intracortical inhibition (SICI), intracortical facilitation (ICF) and interhemispheric inhibition (IHI) from the left to the right M1.

RESULTS

Practice resulted in significant improvements in both speed and accuracy in the right trained hand and in the left untrained hand. RC increased in the left M1, SICI decreased in both M1s, and IHI from the left to the right M1 decreased. No changes were identified in ICF nor in RC in the right M1.

CONCLUSIONS

Our results suggest that some neurophysiological mechanisms operating in the M1 controlling performance of an untrained hand may contribute to optimize the procedure for selecting and implementing correct pinch force levels.

SIGNIFICANCE

These results raise the hypothesis of a contribution of modulation of SICI and IHI, or an interaction between both to intermanual transfer after learning a sequential pinch force task.

Effect of drawing hand and age on figural fluency: a graphomotor study with the five-point test in children

The aim of the study was to assess conceptual thinking in children in relation to age and motor dominance. We investigated the effect of the right and the left hand in a fluency task in four groups of 127 healthy right-handed children (age 5-12 years) and an adult control group. They performed the Five-Point Test twice, once with their dominant right and once with their nondominant left hand. The number of items and errors were analyzed with respect to age, drawing hand, and motor transfer. The performance of boys and girls did not differ. There was a significant effect for age and a prominent interaction between age, hand, and order (right hand or left hand first). Performance improved linearly with age. The dominant right hand performed generally better and there was a learning effect for both hands, but there was a learning advantage for the dominant hand, which increased with age. The influence of motor dominance in this fluency task seems to establish before conceptual maturity (around age 7, respectively 9 to 10).

M1 contributes to the intrinsic but not the extrinsic components of motor-skills

Procedural skills consist of several components that can be simultaneously acquired. During a motor-learning task we can distinguish between how a "movement" is performed (intrinsic component) and the spatial-related (extrinsic) component of this movement. The intrinsic movement component is thought to be supported by motor loops, including primary motor cortex (M1) as assessed with neuroimaging studies. Here we want to test further whether M1 makes a critical contribution to the movement rather than spatial-related component of skill-learning. To this purpose, we used repetitive Transcranial Magnetic Stimulation (rTMS) and the serial reaction time (SRT) task. Twenty right-handed participants performed the SRT-task starting with their left or right hand. After this learning session, participants switched to the untrained hand by performing original (spatial-related) and mirror-ordered (movement-based) sequences. rTMS was applied to M1 ipsi- or contralateral to the transfer-hand and both sequences were retested. Results revealed rTMS-interference with motor-skill transfer of mirror-ordered but not original sequences, showing that M1 is critically involved in the retrieval/transformation of the intrinsic but not the extrinsic movement coordinates. rTMS-interference in the mirror-condition consisted of both (i) disruption and (ii) release of motor-skill transfer depending on the stimulated hemisphere and on transfer-hand. The pattern of results suggests (i) contralateral (right) M1 involvement in retrieval/transformation of motor information during left-hand reproduction of previously acquired right-hand motor-skills; and (ii) modulatory interactions of inhibitory nature from the dominant (left) to the non-dominant (right) M1 in the same transfer-condition. These results provide further evidence that M1 is essential to intrinsic movement-based skill-learning and novel insight on models of motor-learning and hemispheric specialization, suggesting the involvement of interhemispheric inhibition.

Unilateral grip fatigue reduces short interval intracortical inhibition in ipsilateral primary motor cortex

OBJECTIVE

This study was designed to examine whether exhaustive grip exercise of the left hand affected intracortical excitability in ipsilateral motor cortex.

METHODS

Ten healthy male subjects (aged 21-24 years) participated in experiment 1 in which paired-pulse transcranial magnetic stimulation (TMS) was used to test corticospinal and corticocortical excitability in right (relaxed) first dorsal interosseous (FDI) muscle during the recovery period after exhaustive forceful grip exercise of the left hand. Seven of the same subjects participated in experiment 2, in which the intensity of the test stimulus was adjusted so that the amplitude of motor evoked potential (MEP(TEST)) was kept constant throughout the measurement.

RESULTS

In experiment 1, MEP(TEST) was slightly reduced from 5 to 15min after exercise whilst short interval intracortical inhibition (SICI) at interstimulus interval (ISI) of 2 and 3ms became less effective. Intracortical facilitation (ICF) was unchanged. In experiment 2 when the MEP(TEST) was maintained at a constant size there was again no change in ICF, and the reduction in SICI was still present at the same intervals.

CONCLUSIONS

We conclude that unilateral exhaustive grip exercise reduced the excitability of the corticospinal output of the ipsilateral motor cortex whilst simultaneously reducing the excitability of SICI. These results would be compatible with the idea that fatigue increases the tonic level of interhemispheric inhibition from the fatigued to the non-fatigued cortex.

SIGNIFICANCE

Muscle fatigue to the point of exhaustion has lasting effects on the excitability of intracortical circuits in the non-exercised hemisphere, perhaps via changes in the tonic levels of activity in transcallosal pathways.

Time-specific contribution of the supplementary motor area to intermanual transfer of procedural knowledge

The supplementary motor area (SMA) makes a crucial contribution to intermanual transfer: the ability to use one hand to perform a skill practiced and learned with the other hand. However, the timing of this contribution relative to movement remains unknown. Here, 33 healthy volunteers performed a 12 item sequence in the serial reaction time task. During training, each participant responded to a sequence of visual cues presented at 1 Hz by pressing one of four keys with their right hand. The measure of intermanual transfer was response time (RT) during repetition of the trained sequence with the left hand, which was at rest during learning. Participants were divided into three groups, which did not differ in their learning rates or amounts. In two groups, 1 Hz repetitive transcranial magnetic stimulation induced transient virtual lesions of the SMA during training, either 100 ms before each cue (the premovement group) or during each key press (the movement group). The third group received sham stimulation (the sham group). After training with the right hand, RTs for performance with the left (transfer) hand were longer for the premovement group than for the movement or sham groups. Thus, the most crucial contribution of SMA to intermanual transfer occurs in the interval between movements, when the memory of a previous movement plays a role in encoding specific sequences. These results provide insight into frontal lobe contributions to procedural knowledge.

Divergence of explicit and implicit processing speed during associative memory retrieval

Consolidation theory assumes that as time passes, some memories are strengthened and become resistant to change while other memories are weakened and forgotten. Recent demonstrations that implicit, or procedural, memories are retrieved more efficiently after learning and retention are consistent with the idea that these particular memory traces have strengthened with time, and therefore may be accessed faster. However, it is not clear whether the process of explicit memory retrieval also becomes more efficient with time. In two experiments, we explored 1) how much time is required for retrieval of separate explicit and implicit components of hippocampal-dependent visuomotor associative memories after variable retention intervals, and 2) how the explicit and implicit processing times change when the associations are rehearsed after initial retrieval. We found that after learning and retention, explicit and implicit processing times diverged: 1) the time taken to retrieve successfully the explicit component increased relative to a pre-retention baseline but, after spaced rehearsal, decreased, although not to a level significantly below that obtained at the end of learning, and 2) the implicit, or procedural, component processing times continued to gradually decrease after retention, and with continued rehearsal, reached a level significantly below the pre-retention baseline. We conclude that the observed divergence in post-retention reaction times suggests that explicit and implicit memory systems may reorganize differently after learning, and that as a consequence, different amounts of processing time may be required for retrieval of these different memory components.

Influence of somatosensory input on interhemispheric interactions in patients with chronic stroke

BACKGROUND

Ischemia-induced cutaneous anesthesia of the healthy hand in patients with chronic stroke elicits transient improvements of motor performance in the contralateral, paretic hand.

OBJECTIVE

The present study was designed to investigate one of the possible mechanisms underlying this effect.

METHODS

The authors evaluated the effects of transient ischemic cutaneous anesthesia of the healthy hand (target intervention) and healthy foot (control intervention) on transcranial magnetic stimulation-induced interhemispheric inhibition from the contralesional onto the ipsilesional primary motor cortex (M1). Ten subjects with chronic, predominantly subcortical stroke with motor impairment were assessed.

RESULTS

Cutaneous anesthesia of the intact hand but not the intact leg resulted in reduction of the inhibitory drive from the contralesional to the ipsilesional M1 both at rest and immediately preceding movements of the paretic hand. Changes in premovement interhemispheric inhibition showed a trend for correlation with improvements in finger-tapping speed in the paretic hand.

CONCLUSION

The findings suggest that modulation of interhemispheric inhibitory interactions between the contralesional and ipsilesional M1, either primarily or secondary to intrahemispheric excitability changes in either hemisphere, may contribute to performance improvements with cutaneous anesthesia of the intact hand. The present study provides additional insight into the mechanisms by which rehabilitative interventions focused on training one hand and restraining the other may operate after chronic stroke.

Degree of handedness affects intermanual transfer of skill learning

Intermanual transfer of skill learning has often been used as a paradigm to study functional specialization and hemispheric interactions in relation to handedness. This literature has not evaluated whether degree of handedness impacts learning and intermanual transfer. Because handedness scores are related to factors that might influence intermanual transfer, such as engagement of the ipsilateral hemisphere during movement (Dassonville et al. in Proc Natl Acad Sci USA 94:14015-14018, 1997) and corpus callosum volume (Witelson in Science 229:665-668, 1985; Brain 112:799-835, 1989), we tested whether degree of handedness is correlated with transfer magnitude. We had groups of left and right handed participants perform a sensorimotor adaptation task and a sequence learning task. Following learning with either the dominant or nondominant hand, participants transferred to task performance with the other hand. We evaluated whether the magnitude of learning and intermanual transfer were influenced by either direction and/or degree of handedness. Participants exhibited faster sensorimotor adaptation with the right hand, regardless of whether they were right or left handed. In addition, less strongly left handed individuals exhibited better intermanual transfer of sensorimotor adaptation, while less strongly right handed individuals exhibited better intermanual transfer of sequence learning. The findings suggest that involvement of the ipsilateral hemisphere during learning may influence intermanual transfer magnitude.

Unilateral practice of a ballistic movement causes bilateral increases in performance and corticospinal excitability

It has long been known that practicing a task with one limb can result in performance improvements with the opposite, untrained limb. Hypotheses to account for cross-limb transfer of performance state that the effect is mediated either by neural adaptations in higher order control centers that are accessible to both limbs, or that there is a “spillover” of neural drive to the opposite hemisphere that results in bilateral adaptation. Here we address these hypotheses by assessing performance and corticospinal excitability in both hands after unilateral practice of a ballistic finger movement. Participants ( n = 9) completed 300 practice trials of a ballistic task with the right hand, the aim of which was to maximize the peak abduction acceleration of the index finger. Practice caused a 140% improvement in right-hand performance and an 82% improvement for the untrained left hand. There were bilateral increases in the amplitude of responses to transcranial magnetic stimulation, but increased corticospinal excitability was not correlated with improved performance. There were no significant changes in corticospinal excitability or task performance for a control group that did not train ( n = 9), indicating that performance testing for the left hand alone did not induce performance or corticospinal effects. Although the data do not provide conclusive evidence whether increased corticospinal excitability in the untrained hand is causally related to the cross-transfer of ballistic performance, the finding that ballistic practice can induce bilateral corticospinal adaptations may have important clinical implications for movement rehabilitation.

Mechanisms underlying functional changes in the primary motor cortex ipsilateral to an active hand

Performance of a unimanual hand motor task results in functional changes in both primary motor cortices (M1(ipsilateral) and M1(contralateral)). The neuronal mechanisms controlling the corticospinal output originated in M1(ipsilateral) and the resting hand during a unimanual task remain unclear. Here, we assessed functional changes within M1(ipsilateral) and in interhemispheric inhibition (IHI) associated with parametric increases in unimanual force. We measured motor-evoked potential (MEP) recruitment curves (RCs) and short-interval intracortical inhibition (SICI) in M1(ipsilateral), IHI from M1(contralateral) to M1(ipsilateral), and the influence of IHI over SICI using transcranial magnetic stimulation at rest and during 10, 30, and 70% of maximal right wrist flexion force. EMG from the left resting flexor carpi radialis (FCR) muscle was comparable across conditions. Left FCR MEP RCs increased, and SICI decreased with increasing right wrist force. Activity-dependent (rest and 10, 30, and 70%) left FCR maximal MEP size correlated with absolute changes in SICI. IHI decreased with increasing force at matched conditioned MEP amplitudes. IHI and SICI were inversely correlated at increasing forces. In the presence of IHI, SICI decreased at rest and 70% force. In summary, we found activity-dependent changes in (1) SICI in M1(ipsilateral), (2) IHI from M1(contralateral) to M1(ipsilateral), and (3) the influence of IHI over SICI in the left resting hand during force generation by the right hand. Our findings indicate that interactions between GABAergic intracortical circuits mediating SICI and interhemispheric glutamatergic projections between M1s contribute to control activity-dependent changes in corticospinal output to a resting hand during force generation by the opposite hand.

Motor learning: passing a skill from one hand to the other

A recent study has provided surprising new insights into the neural mechanisms underlying our ability to transfer a learned motor skill from one hand to the other.

Contribution of transcranial magnetic stimulation to the understanding of cortical mechanisms involved in motor control

Transcranial magnetic stimulation (TMS) was initially used to evaluate the integrity of the corticospinal tract in humans non-invasively. Since these early studies, the development of paired-pulse and repetitive TMS protocols allowed investigators to explore inhibitory and excitatory interactions of various motor and non-motor cortical regions within and across cerebral hemispheres. These applications have provided insight into the intracortical physiological processes underlying the functional role of different brain regions in various cognitive processes, motor control in health and disease and neuroplastic changes during recovery of function after brain lesions. Used in combination with neuroimaging tools, TMS provides valuable information on functional connectivity between different brain regions, and on the relationship between physiological processes and the anatomical configuration of specific brain areas and connected pathways. More recently, there has been increasing interest in the extent to which these physiological processes are modulated depending on the behavioural setting. The purpose of this paper is (a) to present an up-to-date review of the available electrophysiological data and the impact on our understanding of human motor behaviour and (b) to discuss some of the gaps in our present knowledge as well as future directions of research in a format accessible to new students and/or investigators. Finally, areas of uncertainty and limitations in the interpretation of TMS studies are discussed in some detail.

The Olympic brain. Does corticospinal plasticity play a role in acquisition of skills required for high-performance sports?

Non-invasive electrophysiological and imaging techniques have recently made investigation of the intact behaving human brain possible. One of the most intriguing new research areas that have developed through these new technical advances is an improved understanding of the plastic adaptive changes in neuronal circuitries underlying improved performance in relation to skill training. Expansion of the cortical representation or modulation of corticomotor excitability of specific muscles engaged in task performance is required for the acquisition of the skill. These changes at cortical level appear to be paralleled by changes in transmission in spinal neuronal circuitries, which regulate the contribution of sensory feedback mechanisms to the execution of the task. Such adaptive changes also appear to be essential for the consolidation of a memory of performance of motor tasks and thus for the lasting ability of performing highly skilled movements such as those required for Olympic sports.