Mechanisms influencing acquisition and recall of motor memories

Intermittent practice facilitates stable motor memories

Humans adaptively control reaching movements to maintain good performance in the presence of novel forces acting on the arm. A recent study suggested that motor memories of different force conditions are not transformed from fragile to stable states, but rather are always vulnerable to interference from newly learned conditions (Caithness et al., 2004). This is contrary to the results of previous studies (Brashers-Krug et al., 1996; Shadmehr and Brashers-Krug, 1997), although all of these studies followed similar methods. Here, we show that a seemingly insignificant and inconsistently applied methodological detail may reconcile this discrepancy. Catch trials, in which the novel forces are removed, may be randomly interspersed among the more frequent force trials to assess how a subject is learning to predict the pattern of forces. In the absence of an interfering condition, subjects retained their learning until retest a day later regardless of whether they experienced catch trials. But in the presence of an interfering condition, only the subjects who had experienced forces intermittently retained their learning and thereby showed resistance to the interference. Thus, intermittent rather than constant practice conditions appear to be critical for dynamic motor memory stabilization.

Cross‐modal plasticity of the motor cortex while listening to a rehearsed musical piece

Learning a musical piece requires the development of a strong linkage between sensory and motor representations. Audition plays a central role and a tight cortical auditory-motor corepresentation is a characteristic feature of music processing. Recent works have indicated the establishment of a functional connection between auditory and motor cortices during the learning of a novel piece, although no causal relation has yet been demonstrated. Here transcranial magnetic stimulation of the cortical motor representation involved in musical performance was used to test excitability changes in piano players during auditory presentation of a rehearsed and a non-rehearsed piece. Results showed an increased motor excitability for the rehearsed but not for the non-rehearsed piece. Moreover, we observed an increase of excitability over time as intracortical facilitation was already present after 30 min of training whereas cortico-spinal facilitation increased after a longer training period (5 days).

Translational Studies in Neurorehabilitation: From Bench to Bedside

Stroke is the leading cause of adult disability in the western world. Consensus has built over the last few years regarding the usefulness of training to improve motor disability resulting from stroke. Until recently, there were no accepted strategies to enhance the beneficial effects of training. However, the combination of basic and clinical science data over the last few years is changing this picture, and is highly relevant to the field of neurorehabilitation. Human studies in both healthy individuals and patients after brain damage demonstrate as a proof of principle that somatosensory input, cortical stimulation, interhemispheric interactions, and pharmacologic interventions can modulate cortical plasticity in neurorehabilitation after stroke. These findings strongly suggest directions in the development of novel strategies to enhance training effects on motor recovery. The intent of this review is to describe these strategies, the basic science principles on which they are based, and the clinical applications that have emerged so far.

Impaired motor control in patients with benign focal epilepsy of childhood

Upper extremity motor function was quantitatively assessed in 6 children (age 7-11 years) treated with antiepileptic drugs for benign focal epilepsies of childhood and compared with that of 30 age-matched normal children. Both motor performance and adaptation to perturbing mechanical constraints imposed by a robotic device were significantly impaired in children with benign focal epilepsies of childhood. Our findings thus question whether certain "benign" epilepsies are truly benign and whether pharmacologic treatment might contribute to motor impairment.

Consolidation of motor memory

An issue of great recent interest is whether motor memory consolidates in a manner analogous to declarative memory--that is, with the formation of a memory that progresses over time from a fragile state, which is susceptible to interference by a lesion or a conflicting motor task, to a stabilized state, which is resistant to such interference. Here, we first review studies that examine the anatomical basis for motor consolidation. Evidence implicates cerebellar circuitry in two types of associative motor learning--eyelid conditioning and vestibulo-ocular reflex adaptation--and implicates primary motor cortex in skilled finger movements. We also review evidence for and against a consolidation process for adaptation of arm movements. We propose that contradictions have arisen because consolidation can be masked by inhibition of memory retrieval.

Effects of human cerebellar thalamus disruption on adaptive control of reaching

Lesion or degeneration of the cerebellum can profoundly impair adaptive control of reaching in humans. Computational models have proposed that internal models that help control movements form in the cerebellum and influence planned motor output through the cerebello-thalamo-cortical pathway. However, lesion studies of the cerebellar thalamus have not consistently found impairment in reaching or adaptation of reaching. To elucidate the role of the cerebellar thalamus in humans, we studied a group of essential tremor (ET) patients with deep brain stimulation (DBS) electrodes placed in the cerebellar thalamus. The stimulation can be turned on or off remotely and is thought to reduce tremor by blocking the spread of the pathological output from the cerebellum. We studied the effect of thalamic DBS on the ability to adapt arm movements to novel force fields. Although thalamic DBS resulted in a dramatic and significant reduction of tremor in ET, it also impaired motor adaptation: the larger the stimulation voltage, the greater the reduction in rates of adaptation. We next examined ET patients that had undergone unilateral thalamotomy in the cerebellar thalamus and found that adaptation with the contralateral arm was impaired compared with the ipsilateral arm. Therefore, although both lesion and electrical stimulation of the cerebellar thalamus are highly effective in reducing tremor, they significantly impair the ability of the brain to form internal models of action. Adaptive control of reaching appears to depend on the integrity of the cerebello-thalamo-cortical pathway.

Influence of the inter-reach-interval on motor learning

Previous studies have demonstrated changes in motor memories with the passage of time on the order of hours. We sought to further this work by determining the influence that time on the order of seconds has on motor learning by changing the duration between successive reaches (inter-reach-interval (IRI)). Human subjects made reaching movements to visual targets while holding onto a robotic manipulandum that presented a viscous curl field. We tested four experimental groups that differed with respect to the IRI (0.5, 5, 10 or 20 s). The 0.5 s IRI group performed significantly worse with respect to a learning index than the other groups over the first set of 192 reaches. Each group demonstrated significant learning during the first set. There was no significant difference with respect to the learning index between the 5, 10 and 20 s IRI groups. During the second and third set of 192 reaches the 0.5 s IRI group's performance became indistinguishable from the other groups indicating that with continued training the initial deficit in performance could be overcome.

Impedance control and internal model use during the initial stage of adaptation to novel dynamics in humans

This study investigated the neuromuscular mechanisms underlying the initial stage of adaptation to novel dynamics. A destabilizing velocity-dependent force field (VF) was introduced for sets of three consecutive trials. Between sets a random number of 4-8 null field trials were interposed, where the VF was inactivated. This prevented subjects from learning the novel dynamics, making it possible to repeatedly recreate the initial adaptive response. We were able to investigate detailed changes in neural control between the first, second and third VF trials. We identified two feedforward control mechanisms, which were initiated on the second VF trial and resulted in a 50% reduction in the hand path error. Responses to disturbances encountered on the first VF trial were feedback in nature, i.e. reflexes and voluntary correction of errors. However, on the second VF trial, muscle activation patterns were modified in anticipation of the effects of the force field. Feedforward cocontraction of all muscles was used to increase the viscoelastic impedance of the arm. While stiffening the arm, subjects also exerted a lateral force to counteract the perturbing effect of the force field. These anticipatory actions indicate that the central nervous system responds rapidly to counteract hitherto unfamiliar disturbances by a combination of increased viscoelastic impedance and formation of a crude internal dynamics model.

Temporary Occlusion of Associative Motor Cortical Plasticity by Prior Dynamic Motor Training

A novel Hebbian stimulation paradigm was employed to examine physiological correlates of motor memory formation in humans. Repetitive pairing of median nerve stimulation with transcranial magnetic stimulation over the contralateral motor cortex (paired associative stimulation, PAS) may decrease human motor cortical excitability at interstimulus intervals of 10 ms (PAS10) or increase excitability at 25 ms (PAS25). The properties of this plasticity have previously been shown to resemble associative timing-dependent long-term depression (LTD) and long-term potentiation (LTP) as established in vitro. Immediately after training a novel dynamic motor task, the capacity of the motor cortex to undergo plasticity in response to PAS25 was abolished. PAS10-induced plasticity remained unchanged. When retested after 6 h, PAS25-induced plasticity recovered to baseline levels. After training, normal PAS25-induced plasticity was observed in the contralateral training-naive motor cortex. Motor training did not reduce the efficacy of PAS25 to enhance cortical excitability when PAS10 was interspersed between the training and application of the PAS25 protocol. This indicated that the mechanism supporting PAS25-induced plasticity had remained intact immediately after training. Behavioral evidence was obtained for continued optimization of force generation at a time when PAS25-induced plasticity was blocked in the training motor cortex. Application of the PAS protocols after motor training did not prevent the consolidation of motor skills evident as performance gains at later retesting. The results are consistent with a concept of temporary suppression of associative cortical plasticity by neuronal mechanisms involved in motor training. Although it remains an open question exactly which element of motor training was responsible for this effect, our findings may link dynamic properties of LTP formation, as established in animal experiments, with human motor memory formation and possibly dynamic motor learning.

Reorganization and plasticity in the adult brain during learning of motor skills

On the basis of brain imaging studies, Doyon and Ungerleider recently proposed a model describing the cerebral plasticity that occurs in both cortico-striatal and cortico-cerebellar systems of the adult brain during learning of new motor skilled behaviors. This theoretical framework makes several testable predictions with regards to the contribution of these neural systems based on the phase (fast, slow, consolidation, automatization, and retention) and nature of the motor learning processes (motor sequence versus motor adaptation) acquired through repeated practice. There has been recent behavioral, lesion and additional neuroimaging studies that have addressed the assumptions made in this theory that will help in the revision of this model.

Skill Learning: Putting Procedural Consolidation in Context

Information acquired during skill learning continues to be processed long after practice has ceased. An important aspect of this processing is thought to be the transformation of a memory from a fragile to a stable state: a concept challenged by a recent study.

Neurobiology of rehabilitation

Rehabilitation aims to lessen the physical and cognitive impairments and disabilities of patients with stroke, multiple sclerosis, spinal cord or brain injury, and other neurologic diseases. Conventional approaches beyond compensatory adjustments to disability may be augmented by applying some of the myriad experimental results about mechanisms of intrinsic biological changes after injury and the effects of extrinsic manipulations on spared neuronal assemblies. The organization and inherent adaptability of the anatomical nodes within distributed pathways of the central nervous system offer a flexible substrate for treatment strategies that drive activity-dependent plasticity. Opportunities for a new generation of approaches are manifested by rodent and non-human primate studies that reveal morphologic and physiologic adaptations induced by injury, by learning-associated practice, by the effects of pharmacologic neuromodulators, by the behavioral and molecular bases for enhancing activity-dependent synaptic plasticity, and by cell replacement, gene therapy, and regenerative biologic strategies. Techniques such as functional magnetic resonance imaging and transcranial magnetic stimulation will help determine the most optimal physiologic effects of interventions in patients as the cortical representations for skilled movements and cognitive processes are modified by the combination of conventional and biologic therapies. As clinicians digest the finer details of the neurobiology of rehabilitation, they will translate laboratory data into controlled clinical trials. By determining how much they can influence neural reorganization, clinicians will extend the opportunities for neurorestoration.

Memory consolidation in sleep; dream or reality

We discuss several lines of evidence refuting the hypothesis that procedural or declarative memories are processed/consolidated in sleep. One of the strongest arguments against a role for sleep in declarative memory involves the demonstration that the marked suppression or elimination of REM sleep in subjects on antidepressant drugs or with brainstem lesions produces no detrimental effects on cognition. Procedural memory, like declarative memory, undergoes a slow, time-dependent period of consolidation. A process has recently been described wherein performance on some procedural tasks improves with the mere passage of time and has been termed "enhancement." Some studies, but not others, have reported that the consolidation/enhancement of perceptual and motor skills is dependent on sleep. We suggest that consolidation or enhancement, initiated in waking with task acquisition, could in some instances extend to sleep, but sleep would serve no unique role in these processes. In sum, there is no compelling evidence to support a relationship between sleep and memory consolidation.

Sleep-dependent learning and memory consolidation

While the functions of sleep remain largely unknown, one of the most exciting and contentious hypotheses is that sleep contributes importantly to memory. A large number of studies offer a substantive body of evidence supporting this role of sleep in what is becoming known as sleep-dependent memory processing. This review will provide evidence of sleep-dependent memory consolidation and sleep-dependent brain plasticity and is divided into five sections: (1) an overview of sleep stages, memory categories, and the distinct stages of memory development; (2) a review of the specific relationships between sleep and memory, both in humans and animals; (3) a survey of evidence describing sleep-dependent brain plasticity, including human brain imaging studies as well as animal studies of cellular neurophysiology and molecular biology. We close (4) with a consideration of unanswered questions as well as existing arguments against the role of sleep in learning and memory and (5) a concluding summary.

Role of voluntary drive in encoding an elementary motor memory

Motor training consisting of repetitive thumb movements results in encoding of motor memories in the primary motor cortex. It is not known if proprioceptive input originating in the training movements is sufficient to produce this effect. In this study, we compared the ability of training consisting of voluntary (active) and passively-elicited (passive) movements to induce this form of plasticity. Active training led to successful encoding accompanied by characteristic changes in corticomotor excitability, while passive training did not. These results support a pivotal role for voluntary motor drive in coding motor memories in the primary motor cortex.

Strategies for stroke rehabilitation

Rehabilitation after hemiplegic stroke has typically relied on the training of patients in compensatory strategies. The translation of neuroscientific research into care has led to new approaches and renewed promise for better outcomes. Improved motor control can progress with task-specific training incorporating increased use of proximal and distal movements during intensive practice of real-world activities. Functional gains are incorrectly said to plateau by 3-6 months. Many patients retain latent sensorimotor function that can be realised any time after stroke with a pulse of goal-directed therapy. The amount of practice probably best determines gains for a given level of residual movement ability. Clinicians should encourage patients to build greater strength, speed, endurance, and precision of multijoint movements on tasks that increase independence and enrich daily activity. Imaging tools may help clinicians determine the capacity of residual networks to respond to a therapeutic approach and help establish optimal dose-response curves for training. Promising adjunct approaches include practice with robotic devices or in a virtual environment, electrical stimulation to increase cortical excitability during training, and drugs to optimise molecular mechanisms for learning. Biological strategies for neural repair may augment rehabilitation in the next decade.

Consolidation of human motor cortical neuroplasticity by D-cycloserine

D-Cycloserine (CYC), a partial N-methyl-D-aspartate (NMDA) agonist, has been shown to improve cognitive functions in humans. However, the neurophysiological basis of this effect is unclear so far. We studied the impact of this drug on long-lasting after-effects of transcranial direct current (tDCS)-generated motor cortical excitability shifts, as revealed by transcranial magnetic stimulation-elicited motor-evoked potentials. While anodal tDCS enhances motor cortical excitability, cathodal tDCS diminishes it. Both effects seem to be NMDA receptor dependent. D-CYC selectively potentiated the duration of motor cortical excitability enhancements induced by anodal tDCS. D-CYC alone did not modulate excitability. The potency of this drug to consolidate neuronal excitability enhancements, most probably by stabilizing the strengthening of NMDA receptors, which is a probable neurophysiological derivate of learning processes, makes it an interesting substance to improve cognitive functions.

Motor-learning impairment by amantadine in healthy volunteers

NMDA receptor antagonists impair learning and memory in animal models, presumably by inhibiting long-term potentiation in the motor cortex. Human studies are limited and restricted by the paucity of safe NMDA antagonists. Here, we investigated the contribution of glutamatergic neurotransmission to the capacity of acquiring motor-adaptation learning in humans. In a double-blind design, 200 mg of amantadine (a low-affinity NMDA receptor channel blocker) or a matching placebo were given orally to groups of 14 and 13 human healthy young volunteers, respectively. Blood samples were collected 3 h after treatment to assay plasma concentrations, and the subjects were then tested using a motor-adaptation paradigm consisting of an eight-target-pointing task. To rule out drug-related generalized impairments such sedation, tests measuring motor dexterity and attention were also administered pre- and post-treatment. Comparison of the mean performance levels on the motor-adaptation task revealed that subjects in the amantadine group performed at a lower level than those in the placebo group, but this difference did not reach significance. Interestingly, however, despite plasma amantadine concentrations being relatively low, ranging from 2.09 to 4.74 microM (mean=3.3 microM), they nevertheless correlated negatively with motor learning. Furthermore, when the amantadine group was divided into low-performance and high-performance subgroups, subjects in the former subgroup displayed mean amantadine concentrations 36% higher than the latter subgroup, and performed significantly worser than the placebo group. No change in performance was found on the motor-dexterity and attention tests. Altogether, our results lend support to the hypothesis that normal NMDA receptor function is necessary for the acquisition of motor adaptation.

Age-dependent changes in the ability to encode a novel elementary motor memory

In healthy individuals, motor training elicits cortical plasticity that encodes the kinematic details of the practiced movements and is thought to underlie recovery of function after stroke. The influence of age on this form of plasticity is incompletely understood. We studied 55 healthy subjects and identified a substantial decrease in training-dependent plasticity as a function of age in the absence of differences in training kinematics. These results suggest that the ability of the healthy aging motor cortex to reorganize in response to training decreases with age.

Motor learning and prediction in a variable environment

Traditional studies of motor learning and prediction have focused on how subjects perform a single task. Recent advances have been made in our understanding of motor learning and prediction by investigating the way we learn variable tasks, which change either predictably or unpredictably over time. Similarly, studies have examined how variability in our own movements affects motor learning.

Neocortical mechanisms in motor learning

The ability to learn novel motor skills has fundamental importance for adaptive behavior. Neocortical mechanisms support human motor skill learning, from simple practice to adaptation and arbitrary sensory-motor associations. Behavioral and neural manifestations of motor learning evolve in time and involve multiple structures across the neocortex. Modifications of neural properties, synchrony and synaptic efficacy are all related to the development and maintenance of motor skill.