In Search of the Motor Engram: Motor Map Plasticity as a Mechanism for Encoding Motor Experience

Activity-dependent neural plasticity from bench to bedside

Much progress has been made in understanding how behavioral experience and neural activity can modify the structure and function of neural circuits during development and in the adult brain. Studies of physiological and molecular mechanisms underlying activity-dependent plasticity in animal models have suggested potential therapeutic approaches for a wide range of brain disorders in humans. Physiological and electrical stimulations as well as plasticity-modifying molecular agents may facilitate functional recovery by selectively enhancing existing neural circuits or promoting the formation of new functional circuits. Here, we review the advances in basic studies of neural plasticity mechanisms in developing and adult nervous systems and current clinical treatments that harness neural plasticity, and we offer perspectives on future development of plasticity-based therapy.

Recovery after brain injury: mechanisms and principles

The past 20 years have represented an important period in the development of principles underlying neuroplasticity, especially as they apply to recovery from neurological injury. It is now generally accepted that acquired brain injuries, such as occur in stroke or trauma, initiate a cascade of regenerative events that last for at least several weeks, if not months. Many investigators have pointed out striking parallels between post-injury plasticity and the molecular and cellular events that take place during normal brain development. As evidence for the principles and mechanisms underlying post-injury neuroplasticity has been gleaned from both animal models and human populations, novel approaches to therapeutic intervention have been proposed. One important theme has persisted as the sophistication of clinicians and scientists in their knowledge of neuroplasticity mechanisms has grown: behavioral experience is the most potent modulator of brain plasticity. While there is substantial evidence for this principle in normal, healthy brains, the injured brain is particularly malleable. Based on the quantity and quality of motor experience, the brain can be reshaped after injury in either adaptive or maladaptive ways. This paper reviews selected studies that have demonstrated the neurophysiological and neuroanatomical changes that are triggered by motor experience, by injury, and the interaction of these processes. In addition, recent studies using new and elegant techniques are providing novel perspectives on the events that take place in the injured brain, providing a real-time window into post-injury plasticity. These new approaches are likely to accelerate the pace of basic research, and provide a wealth of opportunities to translate basic principles into therapeutic methodologies.

Rehabilitation for Subacromial Impingement Starts at the Scapula

Subacromial impingement, especially secondary subacromial impingement, is a common malady of athletes and non-athletes alike. Although several pathologies may lead to impingement, they all relate back to poor posture. Over time, postural changes increase stress to soft tissue structures to change both alignment and performance. Injury results as low-level stresses impact weakening tissues to the point of overload. Crucial to effective treatment of secondary subacromial impingement is the identification and correction of all causes. Basic to successful treatment is correction of posture, including scapular posture and muscles which control, stabilize, and move the scapula. An evidence-based approach to not only identifying the causes but also creating a treatment regimen to effectively resolve secondary subacromial impingement is presented.

Pathophysiological mechanisms of impaired limb use and repair strategies for motor systems after unilateral injury of the developing brain

The corticospinal tract (CST) is important for limb control. In humans, it begins developing prenatally but CST connections do not have a mature pattern until about 6 months of age and its capacity to evoke muscle contraction does not mature until mid-adolescence. An initially bilateral projection is subsequently refined, so that most ipsilateral CST connections are eliminated. Unilateral brain damage during refinement leads to bilateral developmental impairments. The damaged side develops sparse and weak contralateral spinal connections and the non-involved hemisphere maintains its ipsilateral projection to develop an aberrant bilateral spinal projection. In a kitten model of unilateral spastic cerebral palsy, we replicate key features of the CST circuit changes: robust bilateral CST projections from the non-involved hemisphere, sparse contralateral connections from the affected hemisphere, and motor impairments. We discuss the role of activity-dependent synaptic competition in development of bilateral CSTs and consider several experimental strategies for restoring a more normal pattern of CST connections from the damaged and non-involved sides. We highlight recent results stressing the importance of combined repair of CST axons, restoration of a more normal motor cortex motor representation, and key involvement of spinal cholinergic interneurons in restoring skilled motor function.

Abnormal movement preparation in task-specific focal hand dystonia

Electrophysiological and behavioral studies in primary dystonia suggest abnormalities during movement preparation, but this crucial phase preceding movement onset has not yet been studied specifically with functional magnetic resonance imaging (fMRI). To identify abnormalities in brain activation during movement preparation, we used event-related fMRI to analyze behaviorally unimpaired sequential finger movements in 18 patients with task-specific focal hand dystonia (FHD) and 18 healthy subjects. Patients and controls executed self-initiated or externally cued prelearnt four-digit sequential movements using either right or left hands. In FHD patients, motor performance of the sequential finger task was not associated with task-related dystonic posturing and their activation levels during motor execution were highly comparable with controls. On the other hand reduced activation was observed during movement preparation in the FHD patients in left premotor cortex / precentral gyrus for all conditions, and for self-initiation additionally in supplementary motor area, left mid-insula and anterior putamen, independent of effector side. Findings argue for abnormalities of early stages of motor control in FHD, manifesting during movement preparation. Since deficits map to regions involved in the coding of motor programs, we propose that task-specific dystonia is characterized by abnormalities during recruitment of motor programs: these do not manifest at the behavioral level during simple automated movements, however, errors in motor programs of complex movements established by extensive practice (a core feature of FHD), trigger the inappropriate movement patterns observed in task-specific dystonia.

Dopaminergic meso-cortical projections to m1: role in motor learning and motor cortex plasticity

Although the architecture of a dopaminergic (DA) system within the primary motor cortex (M1) was well characterized anatomically, its functional significance remained obscure for a long time. Recent studies in rats revealed that the integrity of DA fibers in M1 is a prerequisite for successful acquisition of motor skills. This essential contribution of DA for motor learning is plausible as it modulates M1 circuitry at multiple levels thereby promoting plastic changes that are required for information storage: at the network level, DA increases cortical excitability and enhances the stability of motor maps. At the cellular level, DA induces the expression of learning-related genes via the transcription factor c-Fos. At the level of synapses, DA is required for the formation of long-term potentiation, a mechanism that likely is a fingerprint of a motor memory trace within M1. DA fibers innervating M1 originate within the midbrain, precisely the ventral tegmental area (VTA) and the medial portion of substantia nigra (SN). Thus, they could be part of the meso-cortico-limbic pathway – a network that provides information about saliency and motivational value of an external stimulus and is commonly referred as “reward system.” However, the behavioral triggers of the release of dopamine in M1 are not yet identified. As alterations in DA transmission within M1 occur under various pathological conditions such as Parkinson disease or ischemic and traumatic brain injury, a deeper understanding of the interaction of VTA/SN and M1 may reveal a deeper insight into a large spectrum of neurological disorders.

Rapid and persistent impairments of the forelimb motor representations following cervical deafferentation in rats

Skilled motor control is regulated by the convergence of somatic sensory and motor signals in brain and spinal motor circuits. Cervical deafferentation is known to diminish forelimb somatic sensory representations rapidly and to impair forelimb movements. Our focus was to determine what effect deafferentation has on the motor representations in motor cortex, knowledge of which could provide new insights into the locus of impairment following somatic sensory loss, such as after spinal cord injury or stroke. We hypothesized that somatic sensory information is important for cortical motor map topography. To investigate this we unilaterally transected the dorsal rootlets in adult rats from C4 to C8 and mapped the forelimb motor representations using intracortical microstimulation, immediately after rhizotomy and following a 2-week recovery period. Immediately after deafferentation we found that the size of the distal representation was reduced. However, despite this loss of input there were no changes in motor threshold. Two weeks after deafferentation, animals showed a further distal representation reduction, an expansion of the elbow representation, and a small elevation in distal movement threshold. These changes were specific to the forelimb map in the hemisphere contralateral to deafferentation; there were no changes in the hindlimb or intact-side forelimb representations. Degradation of the contralateral distal forelimb representation probably contributes to the motor control deficits after deafferentation. We propose that somatic sensory inputs are essential for the maintenance of the forelimb motor map in motor cortex and should be considered when rehabilitating patients with peripheral or spinal cord injuries or after stroke.

Motor maps and the cortical control of movement

The brain's cortical maps serve as a macroscopic framework upon which additional levels of detail can be overlaid. Unlike sensory maps generated by measuring the brain's responses to incoming stimuli, motor maps are made by directly stimulating the brain itself. To understand the significance of motor maps and the functions they represent, it is necessary to consider the relationship between the natural operation of the motor system and the pattern of activity evoked in it by artificial stimulation. We review recent findings from the study of the cortical motor system and new insights into the control of movement based on its mapping within cortical space.

Improved methods for chronic light-based motor mapping in mice: automated movement tracking with accelerometers, and chronic EEG recording in a bilateral thin-skull preparation

Optogenetic stimulation of the mouse cortex can be used to generate motor maps that are similar to maps derived from electrode-based stimulation. Here we present a refined set of procedures for repeated light-based motor mapping in ChR2-expressing mice implanted with a bilateral thinned-skull chronic window and a chronically implanted electroencephalogram (EEG) electrode. Light stimulation is delivered sequentially to over 400 points across the cortex, and evoked movements are quantified on-line with a three-axis accelerometer attached to each forelimb. Bilateral maps of forelimb movement amplitude and movement direction were generated at weekly intervals after recovery from cranial window implantation. We found that light pulses of ~2 mW produced well-defined maps that were centered approximately 0.7 mm anterior and 1.6 mm lateral from bregma. Map borders were defined by sites where light stimulation evoked EEG deflections, but not movements. Motor maps were similar in size and location between mice, and maps were stable over weeks in terms of the number of responsive sites, and the direction of evoked movements. We suggest that our method may be used to chronically assess evoked motor output in mice, and may be combined with other imaging tools to assess cortical reorganization or sensory-motor integration.

Region and task-specific activation of Arc in primary motor cortex of rats following motor skill learning

Motor learning requires protein synthesis within the primary motor cortex (M1). Here, we show that the immediate early gene Arc/Arg3.1 is specifically induced in M1 by learning a motor skill. Arc mRNA was quantified using a fluorescent in situ hybridization assay in adult Long-Evans rats learning a skilled reaching task (SRT), in rats performing reaching-like forelimb movement without learning (ACT) and in rats that were trained in the operant but not the motor elements of the task (controls). Apart from M1, Arc expression was assessed within the rostral motor area (RMA), primary somatosensory cortex (S1), striatum (ST) and cerebellum. In SRT animals, Arc mRNA levels in M1 contralateral to the trained limb were 31% higher than ipsilateral (p<0.001), 31% higher than in the contralateral M1 of ACT animals (p<0.001) and 48% higher than in controls (p<0.001). Arc mRNA expression in SRT was positively correlated with learning success between two sessions (r=0.52; p=0.026). For RMA, S1, ST or cerebellum no significant differences in Arc mRNA expression were found between hemispheres or across behaviors. As Arc expression has been related to different forms of cellular plasticity, these findings suggest a link between M1 Arc expression and motor skill learning in rats.

Motor system plasticity in stroke models: intrinsically use-dependent, unreliably useful

Background and Purpose

The natural response to disability in one limb is to learn new ways of using the other limb. This compensatory behavioral strategy after stroke has long been thought to contribute to persistent dysfunction in the paretic limb by encouraging its disuse. Our recent findings suggest that it goes beyond the encouragement of disuse to disrupt neural substrates of paretic limb functional improvements.

Methods

We overview recent findings from rodent models of chronic upper extremity impairments in which precise control and manipulation of forelimb experiences were used to understand bilateral and interhemispheric contributions to motor functional outcome.

Results

Skill learning with the less-affected (nonparetic) forelimb promotes neural plasticity in the contralesional motor cortex that subserves its function. At the same time, it exacerbates dysfunction and limits the efficacy of rehabilitative training in the paretic limb. The maladaptive effects of skill learning with the nonparetic forelimb are dependent on callosal connections and contralesional motor cortex, and linked with reduced neural activation of peri-infarct motor cortex during rehabilitative training.

Conclusions

These findings suggest that learning to rely on the nonparetic body side has the capacity to disrupt functionality in a region of the injured hemisphere that contributes to outcome of the paretic limb. Whether this effect generalizes across injury loci and functional modalities remains to be tested.

Pilot fMRI investigation of representational plasticity associated with motor skill learning and its functional consequences

Complex skill learning at a joint initiates competition between its representation in the primary motor cortex (M1) and that of the neighboring untrained joint. This process of representational plasticity has been mapped by cortically-evoking simple movements. We investigated, following skill learning at a joint, 1) whether comparable processes of representational plasticity are observed when mapping is based on volitionally produced complex movements and 2) the consequence on the skill of the adjacent untrained joint. Twenty-four healthy subjects were assigned to either finger- or elbow-skill training or no-training control group. At pretest and posttest, subjects performed complex skill movements at finger, elbow and ankle concurrent with functional magnetic resonance imaging (fMRI) to define learning and allow mapping of corresponding activation-based representations in M1. Skill following both finger- and elbow- training transferred to the ankle (remote joint) (p = 0.05 and 0.05); however, finger training did not transfer to the elbow and elbow training did not transfer to the finger. Following finger training, location of the trained finger representation showed a trend (p = 0.08) for medial shift towards the representation of adjacent untrained elbow joint; the change in intensity of the latter representation was associated with elbow skill (Spearman's ρ = -0.71, p = 0.07). Following elbow training, the trained elbow representation and the adjacent untrained finger representation increased their overlap (p = 0.02), which was associated with finger skill (Spearman's ρ = -0.83, p = 0.04). Thus, our pilot study reveals comparable processes of representational plasticity with fMRI mapping of complex skill movements as have been demonstrated with cortically-evoked methods. Importantly, these processes may limit the degree of transfer of skill between trained and adjacent untrained joints. These pilot findings that await confirmation in large-scale studies have significant implications for neuro-rehabilitation. For instance, techniques, such as motor cortical stimulation, that can potentially modulate processes of representational plasticity between trained and adjacent untrained representations, may optimize transfer of skill.

A robotic platform to assess, guide and perturb rat forelimb movements

Animal models are widely used to explore the mechanisms underlying sensorimotor control and learning. However, current experimental paradigms allow only limited control over task difficulty and cannot provide detailed information on forelimb kinematics and dynamics. Here we propose a novel robotic device for use in motor learning investigations with rats. The compact, highly transparent, three degree-of-freedom manipulandum is capable of rendering nominal forces of 2 N to guide or perturb rat forelimb movements, while providing objective and quantitative assessments of endpoint motor performance in a 50×30 mm(2) planar workspace. Preliminary experiments with six healthy rats show that the animals can be familiarized with the experimental setup and are able to grasp and manipulate the end-effector of the robot. Further, dynamic perturbations and guiding force fields (i.e., haptic tunnels) rendered by the device had significant influence on rat motor behavior (ANOVA, ). This approach opens up new research avenues for future characterizations of motor learning stages, both in healthy and in stroke models.

Dual-tDCS Enhances Online Motor Skill Learning and Long-Term Retention in Chronic Stroke Patients

BACKGROUND

Since motor learning is a key component for stroke recovery, enhancing motor skill learning is a crucial challenge for neurorehabilitation. Transcranial direct current stimulation (tDCS) is a promising approach for improving motor learning. The aim of this trial was to test the hypothesis that dual-tDCS applied bilaterally over the primary motor cortices (M1) improves online motor skill learning with the paretic hand and its long-term retention.

METHODS

Eighteen chronic stroke patients participated in a randomized, cross-over, placebo-controlled, double bind trial. During separate sessions, dual-tDCS or sham dual-tDCS was applied over 30 min while stroke patients learned a complex visuomotor skill with the paretic hand: using a computer mouse to move a pointer along a complex circuit as quickly and accurately as possible. A learning index involving the evolution of the speed/accuracy trade-off was calculated. Performance of the motor skill was measured at baseline, after intervention and 1 week later.

RESULTS

After sham dual-tDCS, eight patients showed performance worsening. In contrast, dual-tDCS enhanced the amount and speed of online motor skill learning compared to sham (p < 0.001) in all patients; this superiority was maintained throughout the hour following. The speed/accuracy trade-off was shifted more consistently after dual-tDCS (n = 10) than after sham (n = 3). More importantly, 1 week later, online enhancement under dual-tDCS had translated into superior long-term retention (+44%) compared to sham (+4%). The improvement generalized to a new untrained circuit and to digital dexterity.

CONCLUSION

A single-session of dual-tDCS, applied while stroke patients trained with the paretic hand significantly enhanced online motor skill learning both quantitatively and qualitatively, leading to successful long-term retention and generalization. The combination of motor skill learning and dual-tDCS is promising for improving post-stroke neurorehabilitation.

Decoding hindlimb movement for a brain machine interface after a complete spinal transection

Stereotypical locomotor movements can be made without input from the brain after a complete spinal transection. However, the restoration of functional gait requires descending modulation of spinal circuits to independently control the movement of each limb. To evaluate whether a brain-machine interface (BMI) could be used to regain conscious control over the hindlimb, rats were trained to press a pedal and the encoding of hindlimb movement was assessed using a BMI paradigm. Off-line, information encoded by neurons in the hindlimb sensorimotor cortex was assessed. Next neural population functions, or weighted representations of the neuronal activity, were used to replace the hindlimb movement as a trigger for reward in real-time (on-line decoding) in three conditions: while the animal could still press the pedal, after the pedal was removed and after a complete spinal transection. A novel representation of the motor program was learned when the animals used neural control to achieve water reward (e.g. more information was conveyed faster). After complete spinal transection, the ability of these neurons to convey information was reduced by more than 40%. However, this BMI representation was relearned over time despite a persistent reduction in the neuronal firing rate during the task. Therefore, neural control is a general feature of the motor cortex, not restricted to forelimb movements, and can be regained after spinal injury.

The BDNF val(66)met polymorphism is not related to motor function or short-term cortical plasticity in elderly subjects

The brain derived neurotrophic factor (BDNF) val(66)met polymorphism affects function of the motor system in young subjects, but little is known about motor system effects in the elderly. The current study assessed motor system physiology and behavior, plus a measure of short-term motor cortex plasticity using transcranial magnetic stimulation, in 38 elderly subjects, then examined whether findings varied in relation to BDNF genotype. Baseline data were also collected from 14 young subjects. At baseline, elderly subjects had poorer motor performances, larger motor cortex maps, and smaller motor evoked potentials compared to young subjects. Degree of age-related differences in neurophysiology correlated inversely with motor performance, for example, larger map area correlated with weaker pinch grip force (r=-0.42, P=0.01). In elderly subjects, baseline behavior and neurophysiology did not differ in relation to BDNF genotype. In addition, although map area increased significantly (P=0.03) across 30 min of exercise, this change did not vary according to BDNF genotype. Aging is associated with changes in neurophysiology that might represent a compensatory response. The data do not support an association between BDNF genotype and behavior, neurophysiology, or short-term cortical plasticity in the motor system of healthy elderly subjects.

Distal forelimb representations in primary motor cortex are redistributed after forelimb restriction: a longitudinal study in adult squirrel monkeys

Primary motor cortex (M1) movement representations reflect acquired motor skills. Representations of muscles and joints used in a skilled task expand. However, it is unknown whether motor restriction in healthy individuals results in complementary reductions in M1 representations. With the use of intracortical microstimulation techniques in squirrel monkeys, detailed maps of movement representations in M1 were derived before and up to 35 wk after restriction of the preferred distal forelimb (DFL) by use of a soft cast. Although total DFL area and movement threshold remained constant, casting resulted in a redistribution of digit and wrist/forearm representations. Digit representations progressively decreased, whereas wrist/forearm representations progressively increased in areal extent. In three of four monkeys, hand preference returned to normal by the end of the postcast recovery period, and postrecovery maps demonstrated reversal of restriction-induced changes. However, in one monkey, a chronic motor impairment occurred in the casted limb. Rehabilitation via a forced-use paradigm resulted in recovery in use and skill of the impaired limb, as well as restoration of normal motor maps. These results demonstrate that plasticity in motor representations can be induced by training or restricting movements of the limb. Physiological changes induced by restriction appear to be reversible, even in the case of adverse motor outcomes. The respective contributions of both disuse and lost motor skills are discussed. These results have relevance for clinical conditions requiring forelimb casting as well as interpreting the differential effects of injury and disuse that are necessarily intertwined after cortical injury, as occurs in stroke.

Does treatment with bone marrow mononuclear cells recover skilled motor function after focal cortical ischemia? Analysis with a forelimb skilled motor task in rats

Previous studies have shown sensorimotor recovery by treatment with bone marrow mononuclear cells (BMMCs) after focal brain ischemia. However, sensorimotor tests commonly used are designed to examine motor patterns that do not involve skill or training. We evaluated whether BMMCs treatment was able to recover forelimb skilled movements. Reaching chamber/pellet retrieval (RCPR) task was used, in which animals had to learn to grasp a single food pellet and lead it to its mouth. We also evaluated therapeutic effect of this training on unskilled sensorimotor function. Adult male Wistar rats suffered unilateral cortical ischemia by thermocoagulation in motor and somesthetic primary areas. A day later, they received i.v. injection of 3×10(7) BMMCs or vehicle (saline), forming four experimental groups: BMMCs+RCPR; saline+RCPR; BMMCs and saline. Cylinder and adhesive tests were applied in all experimental groups, and all behavioral tests were performed before and along post-ischemic weeks after induction of ischemia. Results from RCPR task showed no significant difference between BMMCs+RCPR and saline+RCPR groups. In cylinder test, BMMCs-treated groups showed significant recovery, but no significant effect of RCPR training was observed. In adhesive test, BMMCs treatment promoted significant recovery. Synergistic effect was found since only together they were able to accelerate recovery. The results showed that BMMCs treatment promoted increased recovery of unsophisticated sensorimotor function, but not of skilled forepaw movements. Thus, BMMCs might not be able to recover all aspects of sensorimotor functions, although further studies are still needed to investigate this treatment in ischemic lesions with different locations and extensions.

Directed functional connectivity matures with motor learning in a cortical pattern generator

Sequential motor skills may be encoded by feedforward networks that consist of groups of neurons that fire in sequence (Abeles 1991; Long et al. 2010). However, there has been no evidence of an anatomic map of activation sequence in motor control circuits, which would be potentially detectable as directed functional connectivity of coactive neuron groups. The proposed pattern generator for birdsong, the HVC (Long and Fee 2008; Vu et al. 1994), contains axons that are preferentially oriented in the rostrocaudal axis (Nottebohm et al. 1982; Stauffer et al. 2012). We used four-tetrode recordings to assess the activity of ensembles of single neurons along the rostrocaudal HVC axis in anesthetized zebra finches. We found an axial, polarized neural network in which sequential activity is directionally organized along the rostrocaudal axis in adult males, who produce a stereotyped song. Principal neurons fired in rostrocaudal order and with interneurons that were rostral to them, suggesting that groups of excitatory neurons fire at the leading edge of travelling waves of inhibition. Consistent with the synchronization of neurons by caudally travelling waves of inhibition, the activity of interneurons was more coherent in the orthogonal mediolateral axis than in the rostrocaudal axis. If directed functional connectivity within the HVC is important for stereotyped, learned song, then it may be lacking in juveniles, which sing a highly variable song. Indeed, we found little evidence for network directionality in juveniles. These data indicate that a functionally directed network within the HVC matures during sensorimotor learning and may underlie vocal patterning.

Skill learning induced plasticity of motor cortical representations is time and age-dependent

Movement representations in the motor cortex can reorganize to support motor skill learning during young adulthood. However, little is known about how motor representations change during aging or whether their change is influenced by continued practice of a skill after it is learned. We used intracortical microstimulation to characterize the organization of the forelimb motor cortex in young and aged C57/BL6 mice after short (2-4 weeks) or long (8 weeks) durations of training on a skilled reaching task or control procedures. In young mice, a short duration of reach training increased the area of proximal forelimb movement representations at the expense of distal representations. Following a longer training duration, ratios of proximal to distal movements returned to baseline, even with ongoing practice and skill maintenance. However, lingering changes were evident in thresholds for eliciting distal forelimb movements, which declined over the longer training period. In aged mice, movement representations and movement thresholds failed to change after either duration of training. Furthermore, there was an age-related loss of digit representations and performance decrements on other sensorimotor tests. Nevertheless, in quantitative measures of reaching success, aged mice learned and performed the skilled reaching task at least as well as younger mice. These results indicate that experience-driven topographical reorganization of motor cortex varies with age, as well as time, and is partially dissociable from behavioral performance. They also support an enduring capacity to learn new manual skills during aging, even as more youthful forms of cortical plasticity and sensorimotor function are lost.

Using motor behavior during an early critical period to restore skilled limb movement after damage to the corticospinal system during development

This study investigated the requirements for restoring motor function after corticospinal (CS) system damage during early postnatal development. Activity-dependent competition between the CS tracts (CSTs) of the two hemispheres is imperative for normal development. Blocking primary motor cortex (M1) activity unilaterally during a critical period [postnatal week 5 (PW5) to PW7] produces permanent contralateral motor skill impairments, loss of M1 motor map, aberrant CS terminations, and decreases in CST presynaptic sites and spinal cholinergic interneuron numbers. To repair these motor systems impairments and restore function, we manipulated motor experience in three groups of cats after this CST injury produced by inactivation. One group wore a jacket restraining the limb ipsilateral to inactivation, forcing use of the contralateral, impaired limb, for the month after M1 inactivation (PW8-PW13; "restraint alone"). A second group wore the restraint during PW8-PW13 and was also trained for 1 h/d in a reaching task with the contralateral forelimb ("early training"). To test the efficacy of intervention during adolescence, a third group wore the restraint and received reach training during PW20-PW24 ("delayed training"). Early training restored CST connections and the M1 motor map, increased cholinergic spinal interneurons numbers on the contralateral, relative to ipsilateral, side, and abrogated limb control impairments. Delayed training restored CST connectivity and the M1 motor map but not contralateral spinal cholinergic cell counts or motor performance. Restraint alone only restored CST connectivity. Our findings stress the need to reestablish the integrated functions of the CS system at multiple hierarchical levels in restoring skilled motor function after developmental injury.

Modulation of proprioceptive integration in the motor cortex shapes human motor learning

Sensory and motor systems interact closely during movement performance. Furthermore, proprioceptive feedback from ongoing movements provides an important input for successful learning of a new motor skill. Here, we show in humans that attention to proprioceptive input during a purely sensory task can influence subsequent learning of a novel motor task. We applied low-amplitude vibration to the abductor pollicis brevis (APB) muscle of eight healthy volunteers for 15 min while they discriminated either a small change in vibration frequency or the presence of a simultaneous weak cutaneous stimulus. Before and after the sensory attention tasks, we evaluated the following in separate experiments: (1) sensorimotor interaction in the motor cortex by testing the efficacy of proprioceptive input to reduce GABA(A)ergic intracortical inhibition using paired-pulse transcranial magnetic stimulation, and (2) how well the same subjects learned a ballistic thumb abduction task using the APB muscle. Performance of the vibration discrimination task increased the interaction of proprioceptive input with motor cortex excitability in the APB muscle, whereas performance in the cutaneous discrimination task had the opposite effect. There was a significant correlation between the integration of proprioceptive input in the motor cortex and the motor learning gain: increasing the integration of proprioceptive input from the APB increased the rate of motor learning and reduced performance variability, while decreasing proprioceptive integration had opposite effects. These findings suggest that the sensory attention tasks transiently change how proprioceptive input is integrated into the motor cortex and that these sensory changes drive subsequent learning behavior in the human motor cortex.

Spatial organization of cortical and spinal neurons controlling motor behavior

A major task of the central nervous system (CNS) is to control behavioral actions, which necessitates a precise regulation of muscle activity. The final components of the circuitry controlling muscles are the motorneurons, which settle into pools in the ventral horn of the spinal cord in positions that mirror the musculature organization within the body. This 'musculotopic' motor-map then becomes the internal CNS reference for the neuronal circuits that control motor commands. This review describes recent progress in defining the neuroanatomical organization of the higher-order motor circuits in the cortex and spinal cord, and our current understanding of the integrative features that contribute to complex motor behaviors. We highlight emerging evidence that cortical and spinal motor command centers are loosely organized with respect to the musculotopic spatial-map, but these centers also incorporate organizational features that associate with the function of different muscle groups during commonly enacted behaviors.

Development of motor maps in rats and their modulation by experience

While a substantial literature demonstrates the effect of differential experience on development of mammalian sensory cortices and plasticity of adult motor cortex, characterization of differential experience on the functional development of motor cortex is meager. We first determined when forelimb movement representations (motor maps) could be detected in rats during postnatal development and then whether their motor map expression could be altered with rearing in an enriched environment consisting of group housing and novel toys or skilled learning by training on the single pellet reaching task. All offspring had high-resolution intracortical microstimulation (ICMS)-derived motor maps using methodologies previously optimized for the adult rat. First, cortical GABA-mediated inhibition was depressed by bicuculline infusion directly into layer V of motor cortex and ICMS-responsive points were first reliably detected on postnatal day (PND) 13. Without relying on bicuculline disinhibition of cortex, motor maps emerged on PND 35 and then increased in size until PND 60 and had progressively lower movement thresholds. Second, environmental enrichment did not affect initial detection of responsive points and motor maps in non-bicuculline-treated pups on PND 35. However, motor maps were larger on PND 45 in enriched rat pups relative to pups in the standard housing condition. Rats in both conditions had similar map sizes on PNDs 60, 75, and 90. Third, reach training in rat pups resulted in an internal reorganization of the map in the hemisphere contralateral, but not ipsilateral, to the trained forelimb. The map reorganization was expressed as proportionately more distal (digit and wrist) representations on PND 45. Our data indicate that both environmental enrichment and skilled reach training experience can differentially modify expression of motor maps during development.

Neuroplasticity in rehabilitation after central nervous system damages – computational models

Increasing survival rates in severe illnesses and traumatic injuries can lead to an increase in the number of disabled people with central nervous system (CNS) damages. Motor training after CNS damage is an important part of neurorehabilitation. It can partially reverse the loss of cortical representation after lesion thanks to neuroplasticity. Patients may regain some motor functions in the months following damage due both to spontaneous recovery and physical therapy interventions targeted at further improvement of function. The neural correlates of motor training after CNS damage have been investigated in animals with motor cortex lesions and in humans using fMRI, TMS, etc. However it is hard to fully explain all mechanisms of neuroplasticity. One of ways to increase knowledge and clinical experience is developing of computational models. To refine a lot of hypotheses existing in the area of CNS neuroplasticity there are useful computational models of lesions and following recovery due to neurorehabilitation. The models based on artificial neural networks are novel solution, but in some cases can provide effectivity and biological plausibility.

This article aims at investigating the extent to which the available opportunities are being exploited, including models as a first step in the development of adaptive and cost-effective rehabilitation methods tailored to individuals with CNS deficits.

The muscle engram: the reflex that limits conventional occlusal treatment

The engram (the masticatory "muscle memory") is shown to be a conditionable reflex whose muscle conditioning lasts less than two minutes, far shorter than previously thought. This reflex, reinforced and stored in the masticatory muscles at every swallow, adjusts masticatory muscle activity to guide the lower arch unerringly into its ICP. These muscle adjustments compensate for the continually changing intemal and external factors that affect the mandible's entry into the ICP. A simple quick experiment described in this article isolates the engram, enabling the reader to see its action clearly for the first time. It is urged that every reader perform this experiment. This experiment shows how the engram, by hiding the masticatory muscles' reaction (the hit-and-slide), limits the success of the therapist in achieving occlusion-muscle compatibility. This finding has major clinical implications. It means that, as regards the muscle aspect of treating occlusion, the dentist treating occlusion conventionally is working blind, a situation the neuromuscular school of occlusal thought seeks to correct. The controversy over occlusion continues.

Neuroplasticity subserving motor skill learning

Recent years have seen significant progress in our understanding of the neural substrates of motor skill learning. Advances in neuroimaging provide new insight into functional reorganization associated with the acquisition, consolidation, and retention of motor skills. Plastic changes involving structural reorganization in gray and white matter architecture that occur over shorter time periods than previously thought have been documented as well. Data from experimental animals provided crucial information on plausible cellular and molecular substrates contributing to brain reorganization underlying skill acquisition in humans. Here, we review findings demonstrating functional and structural plasticity across different spatial and temporal scales that mediate motor skill learning while identifying converging areas of interest and possible avenues for future research.

Cortical plasticity during motor learning and recovery after ischemic stroke

The motor system has the ability to adapt to environmental constraints and injury to itself. This adaptation is often referred to as a form of plasticity allowing for livelong acquisition of new movements and for recovery after stroke. We are not sure whether learning and recovery work via same or similar neural mechanisms. But, all these processes require widespread changes within the matrix of the brain. Here, basic mechanisms of these adaptations on the level of cortical circuitry and networks are reviewed. We focus on the motor cortices because their role in learning and recovery has been investigated more thoroughly than other brain regions.

Parallel contributions of cerebellar, striatal and M1 mechanisms to motor sequence learning

When learning a new motor sequence, we must execute the correct order of movements while simultaneously optimizing sensorimotor parameters such as trajectory, timing, velocity and force. Neurophysiological studies in animals and humans have identified the major brain regions involved in sequence learning, including the motor cortex (M1), basal ganglia (BG) and cerebellum. Current models link these regions to different stages of learning (early vs. late) or different components of performance (spatial vs. sensorimotor). At the same time, research in motor control has given rise to the concept that internal models at different levels of the motor system may contribute to learning. The goal of this review is to develop a new framework for motor sequence learning that combines stage and component models within the context of internal models. To do this, we review behavioral and neuroimaging studies in humans and neurophysiological studies in animals. Based on this evidence, we present a model proposing that sequence learning is underwritten by parallel, interacting processes, including internal model formation and sequence representation, that are instantiated in specific cerebellar, BG or M1 mechanisms depending on task demands and the stage of learning. The striatal system learns predictive stimulus-response associations and is critical for motor chunking. The role of the cerebellum is to acquire the optimal internal model for sequence performance in a particular context, and to contribute to error correction and control of on-going movement. M1 acts to store the representation of a learned sequence, likely as part of a distributed network including the parietal lobe and premotor cortex.

Person-specific changes in motor performance accompany upper extremity functional gains after stroke

In animal models, hundreds of repetitions of upper extremity (UE) task practice promote neural adaptation and functional gain. Recently, we demonstrated improved UE function following a similar intervention for people after stroke. In this secondary analysis, computerized measures of UE task performance were used to identify movement parameters that changed as function improved. Ten people with chronic poststroke hemiparesis participated in high-repetition UE task-specific training 3 times per week for 6 weeks. Before and after training, we assessed UE function with the Action Research Arm Test (ARAT), and evaluated motor performance using computerized motion capture during a reach-grasp-transport-release task. Movement parameters included the duration of each movement phase, trunk excursion, peak aperture, aperture path ratio, and peak grip force. Group results showed an improvement in ARAT scores (p = .003). Although each individual changed significantly on at least one movement parameter, across the group there were no changes in any movement parameter that reached or approached significance. Changes on the ARAT were not closely related to changes in movement parameters. Since aspects of motor performance that contribute to functional change vary across individuals, an individualized approach to upper extremity motion analysis appears warranted.

Neuroplasticity of the sensorimotor cortex during learning

We will discuss some of the current issues in understanding plasticity in the sensorimotor (SM) cortices on the behavioral, neurophysiological, and synaptic levels. We will focus our paper on reaching and grasping movements in the rat. In addition, we will discuss our preliminary work utilizing inhibition of protein kinase Mζ (PKMζ), which has recently been shown necessary and sufficient for the maintenance of long-term potentiation (LTP) (Ling et al., 2002). With this new knowledge and inhibitors to this system, as well as the ability to overexpress this system, we can start to directly modulate LTP and determine its influence on behavior as well as network level processing dependent at least in part due to this form of LTP. We will also briefly introduce the use of brain machine interface (BMI) paradigms to ask questions about sensorimotor plasticity and discuss current analysis techniques that may help in our understanding of neuroplasticity.

Reversal of TMS-induced motor twitch by training is associated with a reduction in excitability of the antagonist muscle

Background

A single session of isolated repetitive movements of the thumb can alter the response to transcranial magnetic stimulation (TMS), such that the related muscle twitch measured post-training occurs in the trained direction. This response is attributed to transient excitability changes in primary motor cortex (M1) that form the early part of learning. We investigated; (1) whether this phenomenon might occur for movements at the wrist, and (2) how specific TMS activation patterns of opposing muscles underlie the practice-induced change in direction.

Methods

We used single-pulse suprathreshold TMS over the M1 forearm area, to evoke wrist movements in 20 healthy subjects. We measured the preferential direction of the TMS-induced twitch in both the sagittal and coronal plane using an optical goniometer fixed to the dorsum of the wrist, and recorded electromyographic (EMG) activity from the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles. Subjects performed gentle voluntary movements, in the direction opposite to the initial twitch for 5 minutes at 0.2 Hz. We collected motor evoked potentials (MEPs) elicited by TMS at baseline and for 10 minutes after training.

Results

Repetitive motor training was sufficient for TMS to evoke movements in the practiced direction opposite to the original twitch. For most subjects the effect of the newly-acquired direction was retained for at least 10 minutes before reverting to the original. Importantly, the direction change of the movement was associated with a significant decrease in MEP amplitude of the antagonist to the trained muscle, rather than an increase in MEP amplitude of the trained muscle.

Conclusions

These results demonstrate for the first time that a TMS-twitch direction change following a simple practice paradigm may result from reduced corticospinal drive to muscles antagonizing the trained direction. Such findings may have implications for training paradigms in neurorehabilitation.

Reflections of experience-expectant development in repair of the adult damaged brain

Behavioral experience has long been known to influence functional outcome after brain injury, but only recently has its pervasive role in the reorganization of the adult brain after damage become appreciated. We briefly review findings from animal models on the role of experience in shaping neuronal events after stroke-like injury. Experience-dependent neural plasticity can be enhanced or impaired by brain damage, depending upon injury parameters and timing. The neuronal growth response to some experiences is heightened due to interactions with denervation-induced plasticity. This includes compensatory behavioral strategies developed in response to functional impairments. Early behavioral experiences can constrain later experience-dependent plasticity, leading to suboptimal functional outcome. Time dependencies and facets of neural growth patterns are reminiscent of experience-expectant processes that shape brain development. As with sensitive periods in brain development, this process may establish behavioral patterns early after brain injury which are relatively resistant to later change.

Emotion regulation and brain plasticity: expressive suppression use predicts anterior insula volume

Expressive suppression is an emotion regulation strategy that requires interoceptive and emotional awareness. These processes both recruit the anterior insula. It is not known, however, whether increased use of expressive suppression is associated with increased anterior insula volume. In the present study, high-resolution anatomical MRI images were used to calculate insula volumes in a set of 50 healthy female subjects (mean 21.9 years) using both region of interest (ROI) and voxel-based morphometry (VBM) approaches. Participants also completed trait measures of expressive suppression usage, cognitive reappraisal usage, and negative emotional reactivity (the latter two served as control measures). As predicted, both ROI and VBM methods found that expressive suppression usage, but not negative affect and cognitive reappraisal, was positively related to anterior insula volume. These findings are consistent with the idea that trait patterns of emotion processing are related to brain structure.

Optimal parameters for microstimulation derived forelimb movement thresholds and motor maps in rats and mice

Intracortical microstimulation (ICMS) is a technique that was developed to derive movement representations (motor maps) of the motor cortex, and was originally used in cats and the capuchin monkey. In more modern experiments, ICMS has been used in rats and mice to assess and interpret plasticity of motor maps in response to experimental manipulation; however, a systematic determination of the optimal ICMS parameters necessary to derive baseline motor maps in rats and mice has not been published. In the present manuscript, we describe two experiments. We first determined the optimal stimulation frequency, pulse number, neocortical depth, and current polarity to achieve the minimum current intensity (movement threshold) to elicit forelimb movements in rats and mice. We show that experimentally naïve rats and mice differ on several of these ICMS parameters. In the second experiment, we measured movement thresholds and map size in states of enhanced neocortical inhibition by the administration of diazepam, as well as neocortical sensitization as the result of repeated seizures. We conclude that movement thresholds are inversely related to motor map size, and that treatments result in a widespread shift the balance between excitation and inhibition in motor neocortical layer 5 influences both movement thresholds and map size.

Distributed versus focal cortical stimulation to enhance motor function and motor map plasticity in a rodent model of ischemia

BACKGROUND

Motor rehabilitation after cerebral ischemia can enhance motor performance and induce motor map reorganization. Electrical stimulation of the cortex (CS) during rehabilitative training (CS/RT) augments motor map plasticity and confers gains in motor function beyond those observed with motor rehabilitation alone. However, it is unclear how the distribution of electrical stimulation across the cortex accomplishes these changes. This study examined the behavioral and neurophysiological effects of delivering CS/RT through a distributed versus focal arrangement of electrical contacts.

METHODS

Adult male rats were given rehabilitative training on a skilled forelimb reaching task following induction of focal ischemic damage within motor cortex. Intracortical microstimulation was used to derive high-resolution maps of forelimb movement representations within motor cortex contralateral to the trained/impaired paw before and after rehabilitation.

RESULTS

All animals that received rehabilitation showed greater increases in motor map area and reaching accuracy than animals that received no training. Animals with the distributed configuration performed significantly greater reaching accuracy than animals in both the CS/RT with focused contact arrangement and rehabilitative training alone (RT) conditions on days 3 to 4 and on day 6 through the remainder of the study (P < .05). However, both CS/RT groups exhibited larger motor maps than the RT condition (E1-CS/RT, 4.71 ± 0.66 mm(2); E2-CS/RT, 4.64 ± 0.46 mm(2); RT, 2.99 ± 0.28 mm(2)).

CONCLUSION

The results indicate that although both focal and distributed forms of CS/RT promote motor map reorganization only the distributed form of CS/RT enhances motor performance with rehabilitation.

Feed-forward control of preshaping in the rat is mediated by the corticospinal tract

Rats are used to model human corticospinal tract (CST) injury and repair. We asked whether rats possess the ability to orient their paw to the reaching target and whether the CST mediates this skill, as it does in primates. To test this ability, called preshaping, we trained rats to reach for pieces of pasta oriented either vertically or horizontally. We measured paw angle relative to the target and asked whether rats used target information attained before contact to preshape the paw, indicating feed-forward control. We also determined whether preshaping improved with practice. We then selectively lesioned the CST in the medullary pyramid contralateral to the reaching forepaw to test whether preshaping relies on the CST. Rats significantly oriented their paw to the pasta orientation before contact, demonstrating feed-forward control. Both preshaping and reaching efficiency improved with practice, while selective CST lesion abrogated both. The loss of preshaping was greatest for pasta oriented vertically, suggesting loss of supination, as seen with human CST injury. The degree of preshaping loss strongly correlated with the amount of skill acquired at baseline, suggesting that the CST mediates the learned component of preshaping. Finally, the amount of preshaping lost after injury strongly correlated with reduced retrieval success, showing an important functional consequence for preshaping. We have thus demonstrated, for the first time, preshaping in the rat and dependence of this skill on the CST. Understanding the basis for this skill and measuring its recovery after injury will be important for studying higher-level motor control in rats.

Nonword repetition in children and adults: effects on movement coordination

Hearing and repeating novel phonetic sequences, or novel nonwords, is a task that taps many levels of processing, including auditory decoding, phonological processing, working memory, speech motor planning and execution. Investigations of nonword repetition abilities have been framed within models of psycholinguistic processing, while the motor aspects, which also are critical for task performance, have been largely ignored. We focused our investigation on both the behavioral and speech motor performance characteristics of this task as performed in a learning paradigm by 9- and 10-year-old children and young adults. Behavioral (percent correct productions) and kinematic (movement duration, lip aperture variability - an index of the consistency of inter-articulator coordination on repeated trials) measures were obtained in order to investigate the short-term (Day 1, first five vs. next five trials) and longer-term (Day 1 vs. Day 2, first five vs. next five trials) changes associated with practice within and between sessions. Overall, as expected, young adults showed higher levels of behavioral accuracy and greater levels of coordinative consistency than the children. Both groups, however, showed a learning effect, such that in general, later Day 1 trials and Day 2 trials were shorter in duration and more consistent in coordination patterns than Day 1 early trials. Phonemic complexity of the nonwords had a profound effect on both the behavioral and speech motor aspects of performance. The children showed marked learning effects on all nonwords that they could produce accurately, while adults' performance improved only when challenged by the more complex nonword stimuli in the set. The findings point to a critical role for speech motor processes within models of nonword repetition and suggest that young adults, similar to children, show short- and longer-term improvements in coordinative consistency with repeated production of complex nonwords. There is also a clear developmental change in nonword production performance, such that less complex novel sequences elicit changes in speech motor performance in children, but not in adults.

The organization of the forelimb representation of the C57BL/6 mouse motor cortex as defined by intracortical microstimulation and cytoarchitecture

The organization of forelimb representation areas of the monkey, cat, and rat motor cortices has been studied in depth, but its characterization in the mouse lags far behind. We used intracortical microstimulation (ICMS) and cytoarchitectonics to characterize the general organization of the C57BL/6 mouse motor cortex, and the forelimb representation in more detail. We found that the forelimb region spans a large area of frontal cortex, bordered primarily by vibrissa, neck, shoulder, and hindlimb representations. It included a large caudal forelimb area, dominated by digit representation, and a small rostral forelimb area, containing elbow and wrist representations. When the entire motor cortex was mapped, the forelimb was found to be the largest movement representation, followed by head and hindlimb representations. The ICMS-defined motor cortex spanned cytoarchitecturally identified lateral agranular cortex (AGl) and also extended into medial agranular cortex. Forelimb and hindlimb representations extended into granular cortex in a region that also had cytoarchitectural characteristics of AGl, consistent with the primary motor-somatosensory overlap zone (OL) characterized in rats. Thus, the mouse motor cortex has homologies with the rat in having 2 forelimb representations and an OL but is distinct in the predominance of digit representations.