Stimulation of the posterior parietal cortex interferes with arm trajectory adjustments during the learning of new dynamics

Organization and evolution of parieto-frontal processing streams in macaque monkeys and humans

The functional organization of the parieto-frontal system is crucial for understanding cognitive-motor behavior and provides the basis for interpreting the consequences of parietal lesions in humans from a neurobiological perspective. The parieto-frontal connectivity defines some main information streams that, rather than being devoted to restricted functions, underlie a rich behavioral repertoire. Surprisingly, from macaque to humans, evolution has added only a few, new functional streams, increasing however their complexity and encoding power. In fact, the characterization of the conduction times of parietal and frontal areas to different target structures has recently opened a new window on cortical dynamics, suggesting that evolution has amplified the probability of dynamic interactions between the nodes of the network, thanks to communication patterns based on temporally-dispersed conduction delays. This might allow the representation of sensory-motor signals within multiple neural assemblies and reference frames, as to optimize sensory-motor remapping within an action space characterized by different and more complex demands across evolution.

Population interactions between parietal and primary motor cortices during reach

Neural interactions between parietal area 2/5 and primary motor cortex (M1) were examined to determine the timing and behavioral correlates of cortico-cortical interactions. Neural activity in areas 2/5 and M1 was simultaneously recorded with 96-channel microelectrode arrays in three rhesus monkeys performing a center-out reach task. We introduce a new method to reveal parietal-motor interactions at a population level using partial spike-field coherence (PSFC) between ensembles of neurons in one area and a local field potential (LFP) in another. PSFC reflects the extent of phase locking between spike times and LFP, after removing the coherence between LFPs in the two areas. Spectral analysis of M1 LFP revealed three bands: low, medium, and high, differing in power between movement preparation and performance. We focus on PSFC in the 1-10 Hz band, in which coherence was strongest. PSFC was also present in the 10-40 Hz band during movement preparation in many channels but generally nonsignificant in the 60-200 Hz band. Ensemble PSFC revealed stronger interactions than single cell-LFP pairings. PSFC of area 2/5 ensembles with M1 LFP typically rose around movement onset and peaked ∼500 ms afterward. PSFC was typically stronger for subsets of area 2/5 neurons and M1 LFPs with similar directional bias than for those with opposite bias, indicating that area 2/5 contributes movement direction information. Together with linear prediction of M1 LFP by area 2/5 spiking, the ensemble-LFP pairing approach reveals interactions missed by single neuron-LFP pairing, demonstrating that cortico-cortical communication can be more readily observed at the ensemble level.

Spatial effects of shifting prisms on properties of posterior parietal cortex neurons

The posterior parietal cortex contains neurons that respond to visual stimulation and motor behaviour. The objective of the current study was to test short-term adaptation in neurons in macaque area 7a and the dorsal prelunate during visually guided reaching using Fresnel prisms that displaced the visual field. The visual perturbation shifted the eye position and created a mismatch between perceived and actual reach location. Two non-human primates were trained to reach to visual targets before, during and after prism exposure while fixating the reach target in different locations. They were required to reach to the physical location of the reach target and not the perceived, displaced location. While behavioural adaptation to the prisms occurred within a few trials, the majority of neurons responded to the distortion either with substantial changes in spatial eye position tuning or changes in overall firing rate. These changes persisted even after prism removal. The spatial changes were not correlated with the direction of induced prism shift. The transformation of gain fields between conditions was estimated by calculating the translation and rotation in Euler angles. Rotations and translations of the horizontal and vertical spatial components occurred in a systematic manner for the population of neurons suggesting that the posterior parietal cortex retains a constant representation of the visual field remapping between experimental conditions.

Sensorimotor adaptation: multiple forms of plasticity in motor circuits

One of the most striking properties of the adult central nervous system is its ability to undergo changes in function and/or structure. In mammals, learning is a major inducer of adaptive plasticity. Sensorimotor adaptation is a type of procedural--motor--learning that allows maintaining accurate movements in the presence of environmental or internal perturbations by adjusting motor output. In this work, we will review experimental evidence gathered from rodents and human and nonhuman primates pointing to possible sites of adaptation-related plasticity at different levels of organization of the nervous system.

Brain representations for acquiring and recalling visual-motor adaptations

Humans readily learn and remember new motor skills, a process that likely underlies adaptation to changing environments. During adaptation, the brain develops new sensory-motor relationships, and if consolidation occurs, a memory of the adaptation can be retained for extended periods. Considerable evidence exists that multiple brain circuits participate in acquiring new sensory-motor memories, though the networks engaged in recalling these and whether the same brain circuits participate in their formation and recall have less clarity. To address these issues, we assessed brain activation with functional MRI while young healthy adults learned and recalled new sensory-motor skills by adapting to world-view rotations of visual feedback that guided hand movements. We found cerebellar activation related to adaptation rate, likely reflecting changes related to overall adjustments to the visual rotation. A set of parietal and frontal regions, including inferior and superior parietal lobules, premotor area, supplementary motor area and primary somatosensory cortex, exhibited non-linear learning-related activation that peaked in the middle of the adaptation phase. Activation in some of these areas, including the inferior parietal lobule, intra-parietal sulcus and somatosensory cortex, likely reflected actual learning, since the activation correlated with learning after-effects. Lastly, we identified several structures having recall-related activation, including the anterior cingulate and the posterior putamen, since the activation correlated with recall efficacy. These findings demonstrate dynamic aspects of brain activation patterns related to formation and recall of a sensory-motor skill, such that non-overlapping brain regions participate in distinctive behavioral events.

Long-lasting changes in brain activation induced by a single REAC technology pulse in Wi-Fi bands. Randomized double-blind fMRI qualitative study

The aim of this randomized double-blind study was to evaluate in healthy adult subjects, with functional magnetic resonance imaging (fMRI), long lasting changes in brain activation patterns following administration of a single, 250 milliseconds pulse emitted with radio-electric asymmetric conveyer (REAC) technology in the Wi-Fi bands. The REAC impulse was not administered during the scan, but after this, according to a protocol that has previously been demonstrated to be effective in improving motor control and postural balance, in healthy subjects and patients. The study was conducted on 33 healthy volunteers, performed with a 1.5 T unit while operating a motor block task involving cyclical and alternating flexion and extension of one leg. Subsequently subjects were randomly divided into a treatment and a sham treatment control group. Repeated fMRI examinations were performed following the administration of the REAC pulse or sham treatment. The Treated group showed cerebellar and ponto-mesencephalic activation components that disappeared in the second scan, while these activation components persisted in the Sham group. This study shows that a very weak signal, such as 250 milliseconds Wi-Fi pulse, administered with REAC technology, could lead to lasting effects on brain activity modification.

Temporal analysis of reference frames in parietal cortex area 5d during reach planning

The neural encoding of spatial and postural reference frames in posterior parietal cortex has traditionally been studied during fixed epochs, but the temporal evolution of these representations (or lack thereof) can provide insight into the underlying computations and functions of this region. Here we present single-unit data recorded from two rhesus macaques during a reach planning task. We found that area 5d coded the position of the hand relative to gaze before presentation of the reach target, but switched to coding the target location relative to hand position soon after target presentation. In the pretarget period the most relevant information for success in the task is the position of the hand relative to gaze; however, after target onset, the most task-relevant spatial relationship is the location of the target relative to the hand. The switch in coding suggests that population activity in area 5d may represent postural and spatial information in the reference frame that is most pertinent at each stage of the task. Moreover, although target-hand coding was dominant from soon after the reach target onset, this representation was not static but built in strength as movement onset approached, which we speculate could reflect a role for this region in building an accurate state estimate for the limb. We conclude that representations in area 5d are more flexible and dynamic than previously reported.

rTMS-induced virtual lesion of the posterior parietal cortex (PPC) alters the control of reflexive shifts of social attention triggered by pointing hands

In highly social groups like human and non-human primates, gaze and pointing cues are fundamentally important for directing the attention of conspecifics. Although neuroimaging studies indicate that shifts of attention triggered by observation of social cues activate the onlookers׳ fronto-parietal cortices, information on whether these regions play a causative role in orienting and re-orienting of social attention is lacking. To advance our understanding of this, we used event-related repetitive dual pulse transcranial magnetic stimulation to interfere with neural activity in the right frontal eye field (rFEF) and posterior parietal cortex (rPPC). This procedure allowed us to explore how inhibiting rFEF and rPPC influences shifts of attention triggered by the observation of body-related (gaze and hand) and non body-related (arrow) directional distractors. Participants were asked to perform a leftward or rightward pointing movement according to the color change of a central imperative signal while ignoring a distractor, which was either a gaze, a pointing hand or an arrow. Stimulation of rPPC in a region supposedly linked to attentional re-orienting and to planning and execution of upper limb movements increased the reflexive tendency to follow distracting pointing hands but not oriented gaze or arrows. These findings suggest that inhibition of cortical structures that control attentional shifts triggered by social stimuli brings forth an increase of the cost of attentional re-orienting. Moreover, our results provide the first causative evidence that reflexive social attention in humans may be coded according to body-part-centered frames of reference.

Two different motor learning mechanisms contribute to learning reaching movements in a rotated visual environment

Practice of movement in virtual-reality and other artificially altered environments has been proposed as a method for rehabilitation following neurological injury and for training new skills in healthy humans.  For such training to be useful, there must be transfer of learning from the artificial environment to the performance of desired skills in the natural environment.  Therefore an important assumption of such methods is that practice in the altered environment engages the same learning and plasticity mechanisms that are required for skill performance in the natural environment.  We test the hypothesis that transfer of learning may fail because the learning and plasticity mechanism that adapts to the altered environment is different from the learning mechanism required for improvement of motor skill.  In this paper, we propose that a model that separates skill learning and environmental adaptation is necessary to explain the learning and aftereffects that are observed in virtual reality experiments.  In particular, we studied the condition where practice in the altered environment should lead to correct skill performance in the original environment. Our 2-mechanism model predicts that aftereffects will still be observed when returning to the original environment, indicating a lack of skill transfer from the artificial environment to the original environment. To illustrate the model prediction, we tested 10 healthy participants on the interaction between a simple overlearned motor skill (straight hand movements to targets in different directions) and an artificially altered visuomotor environment (rotation of visual feedback of the results of movement).  As predicted by the models, participants show adaptation to the altered environment and after-effects on return to the baseline environment even when practice in the altered environment should have led to correct skill performance.  The presence of aftereffect under all conditions that involved changes in environment demonstrates separation of environmental adaptation and skill learning. Our results support the existence of two distinct learning modules with different adaptation properties.  Therefore we suggest that adaptation to an altered environment may not be useful for training new skills.

The Relative Influence of Goal and Kinematics on Corticospinal Excitability Depends on the Information Provided to the Observer

Viewing a person perform an action activates the observer's motor system. Whether this phenomenon reflects the action's kinematics or its final goal remains a matter of debate. One alternative to this apparent controversy is that the relative influence of goal and kinematics depends on the information available to the observer. Here, we addressed this possibility. For this purpose, we measured corticospinal excitability (CSE) while subjects viewed 3 different grasping actions with 2 goals: a large and a small object. Actions were directed to the large object, the small object, or corrected online in which case the goal switched during the movement. We first determined the kinematics and dynamics of the 3 actions during execution. This information was used in 2 other experiments to measure CSE while observers viewed videos of the same actions. CSE was recorded prior to movement onset and at 3 time points during the observed action. To discern between goal and kinematics, information about the goal was manipulated across experiments. We found that the goal influenced CSE only when its identity was known before movement onset. In contrast, a kinematic modulation of CSE was observed whether or not information regarding the goal was provided.

Hand position-dependent modulation of errors in vibrotactile temporal order judgments: the effects of transcranial magnetic stimulation to the human posterior parietal cortex

The ability to decide which of the two stimuli is presented first can be probed using a temporal order judgment (TOJ) task. When the stimuli are delivered to the fingers, TOJ decisions can be confounded by the fact that the hands can be moved to different locations in space. How and where this confounded information is processed in the brain is poorly understood. In the present set of experiments, we addressed this knowledge gap by using single-pulse transcranial magnetic stimulation (TMS) to disrupt processing in the right or left posterior parietal cortex (PPC) during a vibrotactile TOJ task with stimuli applied to the right and left index fingers. In the first experiment, participants held their hands in an uncrossed configuration, and we found that when the index finger contralateral to the site of TMS was stimulated first, there was a significant increase in TOJ errors. This increase did not occur when stimuli were delivered to the ipsilateral finger first. In the second experiment, participants held their hands in a crossed configuration and the pattern of errors was reversed relative to the first experiment. In both the first two experiments, significant increases in TOJ error were present with TMS over either hemisphere, regardless of arm configuration; however, they were larger overall following TMS over the right PPC. Control experiments using sham TMS indicated the systematic modulation in error was not due to nonspecific effects of the stimulation. Additionally, we showed that these TMS-induced changes in TOJ errors were not due to a reduced ability to detect the timing of the vibrotactile stimuli. Taken together, these results demonstrate that both the right and left PPC contribute to the processing underlying vibrotactile TOJs by integrating vibrotactile information and proprioceptive information related to arm position in space.

Using in vivo probabilistic tractography to reveal two segregated dorsal 'language-cognitive' pathways in the human brain

Primate studies have recently identified the dorsal stream as constituting multiple dissociable pathways associated with a range of specialized cognitive functions. To elucidate the nature and number of dorsal pathways in the human brain, the current study utilized in vivo probabilistic tractography to map the structural connectivity associated with subdivisions of the left supramarginal gyrus (SMG). The left SMG is a prominent region within the dorsal stream, which has recently been parcellated into five structurally-distinct regions which possess a dorsal-ventral (and rostral-caudal) organisation, postulated to reflect areas of functional specialisation. The connectivity patterns reveal a dissociation of the arcuate fasciculus into at least two segregated pathways connecting frontal-parietal-temporal regions. Specifically, the connectivity of the inferior SMG, implicated as an acoustic-motor speech interface, is carried by an inner/ventro-dorsal arc of fibres, whilst the pathways of the posterior superior SMG, implicated in object use and cognitive control, forms a parallel outer/dorso-dorsal crescent.

A key region in the human parietal cortex for processing proprioceptive hand feedback during reaching movements

Seemingly effortless, we adjust our movements to continuously changing environments. After initiation of a goal-directed movement, the motor command is under constant control of sensory feedback loops. The main sensory signals contributing to movement control are vision and proprioception. Recent neuroimaging studies have focused mainly on identifying the parts of the posterior parietal cortex (PPC) that contribute to visually guided movements. We used event-related TMS and force perturbations of the reaching hand to test whether the same sub-regions of the left PPC contribute to the processing of proprioceptive-only and of multi-sensory information about hand position when reaching for a visual target. TMS over two distinct stimulation sites elicited differential effects: TMS applied over the posterior part of the medial intraparietal sulcus (mIPS) compromised reaching accuracy when proprioception was the only sensory information available for correcting the reaching error. When visual feedback of the hand was available, TMS over the anterior intraparietal sulcus (aIPS) prolonged reaching time. Our results show for the first time the causal involvement of the posterior mIPS in processing proprioceptive feedback for online reaching control, and demonstrate that distinct cortical areas process proprioceptive-only and multi-sensory information for fast feedback corrections.

The temporal evolution of feedback gains rapidly update to task demands

Recent theoretical frameworks such as optimal feedback control suggest that feedback gains should modulate throughout a movement and be tuned to task demands. Here we measured the visuomotor feedback gain throughout the course of movements made to "near" or "far" targets in human subjects. The visuomotor gain showed a systematic modulation over the time course of the reach, with the gain peaking at the middle of the movement and dropping rapidly as the target is approached. This modulation depends primarily on the proportion of the movement remaining, rather than hand position, suggesting that the modulation is sensitive to task demands. Model-predictive control suggests that the gains should be continuously recomputed throughout a movement. To test this, we investigated whether feedback gains update when the task goal is altered during a movement, that is when the target of the reach jumped. We measured the visuomotor gain either simultaneously with the jump or 100 ms after the jump. The visuomotor gain nonspecifically reduced for all target jumps when measured synchronously with the jump. However, the visuomotor gain 100 ms later showed an appropriate modulation for the revised task goal by increasing for jumps that increased the distance to the target and reducing for jumps that decreased the distance. We conclude that visuomotor feedback gain shows a temporal evolution related to task demands and that this evolution can be flexibly recomputed within 100 ms to accommodate online modifications to task goals.

Role of posterior parietal cortex in reaching movements in humans: clinical implication for 'optic ataxia'

OBJECTIVE

To clarify the spatio-temporal profile of cortical activity related to reaching movement in the posterior parietal cortex (PPC) in humans.

METHODS

Four patients with intractable partial epilepsy who underwent subdural electrode implantation were studied as a part of pre-surgical evaluation. We investigated the Bereitschaftspotential (BP) associated with reaching and correlated the findings with the effect of electrical stimulation of the same cortical area.

RESULTS

BPs specific for reaching, as compared with BPs for simple movements by the hand or arm contralateral to the implanted hemisphere, were recognized in all patients, mainly around the intraparietal sulcus (IPS), the superior parietal lobule (SPL) and the precuneus. BPs near the IPS had the earlier onset than BPs in the SPL. Electrical stimulation of a part of the PPC, where the reach-specific BPs were recorded, selectively impaired reaching.

CONCLUSIONS

Intracranial BP recording and cortical electrical stimulation delineated human reach-related areas in the PPC.

SIGNIFICANCE

The present study for the first time by direct cortical recording in humans demonstrates that parts of the cortices around the IPS and SPL play a crucial role in visually-guided reaching.

The effects of brain lateralization on motor control and adaptation

Lateralization of mechanisms mediating functions such as language and perception is widely accepted as a fundamental feature of neural organization. Recent research has revealed that a similar organization exists for the control of motor actions, in that each brain hemisphere contributes unique control mechanisms to the movements of each arm. The authors review present research that addresses the nature of the control mechanisms that are lateralized to each hemisphere and how they impact motor adaptation and learning. In general, the studies suggest an enhanced role for the left hemisphere during adaptation, and the learning of new sequences and skills. The authors suggest that this specialization emerges from a left hemisphere specialization for predictive control-the ability to effectively plan and coordinate motor actions, possibly by optimizing certain cost functions. In contrast, right hemisphere circuits appear to be important for updating ongoing actions and stopping at a goal position, through modulation of sensorimotor stabilization mechanisms such as reflexes. The authors also propose that each brain hemisphere contributes its mechanism to the control of both arms. They also discuss the potential advantages of such a lateralized control system.

Neuromagnetic brain activity associated with anticipatory postural adjustments for bimanual load lifting

During bimanual load lifting, the brain must anticipate the effects of unloading upon the load-bearing arm. Little is currently known about the neural networks that coordinate these anticipatory postural adjustments. We measured neuromagnetic brain activity with whole-head magnetoencephalography while participants performed a bimanual load-lifting task. Anticipatory adjustments were associated with reduction in biceps brachii muscle activity of the load-bearing arm and pre-movement desynchronization of the cortical beta rhythm. Beamforming analyses localized anticipatory brain activity to the precentral gyrus, basal ganglia, supplementary motor area, and thalamus, contralateral to the load-bearing arm. To our knowledge this is the first human neuroimaging study to directly investigate anticipatory postural adjustments and to explicitly partition the anticipatory and volitional aspects of brain activity in bimanual load lifting. These data contribute to our understanding of the neural systems supporting anticipatory postural adjustments in healthy adults.

A role for the parietal cortex in sensorimotor adaptation of saccades

Sensorimotor adaptation ensures movement accuracy despite continuously changing environment and body. Adaptation of saccadic eye movements is a classical model of sensorimotor adaptation. Beside the well-established role of the brainstem-cerebellum in the adaptation of reactive saccades (RSs), the cerebral cortex has been suggested to be involved in the adaptation of voluntary saccades (VSs). Here, we provide direct evidence for a causal involvement of the parietal cortex in saccadic adaptation. First, the posterior intraparietal sulcus (pIPS) was identified in each subject using functional magnetic resonance imaging (fMRI). Then, a saccadic adaptation paradigm was used to progressively reduce the amplitude of RSs and VSs, while single-pulse transcranial magnetic stimulation (spTMS) was applied over the right pIPS. The perturbations of pIPS resulted in impairment for the adaptation of VSs, selectively when spTMS was applied 60 ms after saccade onset. In contrast, the adaptation of RSs was facilitated by spTMS applied 90 ms after saccade initiation. The differential effect of spTMS relative to saccade types suggests a direct interference with pIPS activity for the VS adaptation and a remote interference with brainstem-cerebellum activity for the RS adaptation. These results support the hypothesis that the adaptation of VSs and RSs involves different neuronal substrates.

Poststroke hemiparesis impairs the rate but not magnitude of adaptation of spatial and temporal locomotor features

BACKGROUND

Persons with stroke and hemiparesis walk with a characteristic pattern of spatial and temporal asymmetry that is resistant to most traditional interventions. It was recently shown in nondisabled persons that the degree of walking symmetry can be readily altered via locomotor adaptation. However, it is unclear whether stroke-related brain damage affects the ability to adapt spatial or temporal gait symmetry.

OBJECTIVE

Determine whether locomotor adaptation to a novel swing phase perturbation is impaired in persons with chronic stroke and hemiparesis.

METHODS

Participants with ischemic stroke (14) and nondisabled controls (12) walked on a treadmill before, during, and after adaptation to a unilateral perturbing weight that resisted forward leg movement. Leg kinematics were measured bilaterally, including step length and single-limb support (SLS) time symmetry, limb angle center of oscillation, and interlimb phasing, and magnitude of "initial" and "late" locomotor adaptation rates were determined.

RESULTS

All participants had similar magnitudes of adaptation and similar initial adaptation rates both spatially and temporally. All 14 participants with stroke and baseline asymmetry temporarily walked with improved SLS time symmetry after adaptation. However, late adaptation rates poststroke were decreased (took more strides to achieve adaptation) compared with controls.

CONCLUSIONS

Mild to moderate hemiparesis does not interfere with the initial acquisition of novel symmetrical gait patterns in both the spatial and temporal domains, though it does disrupt the rate at which "late" adaptive changes are produced. Impairment of the late, slow phase of learning may be an important rehabilitation consideration in this patient population.

Towards a healthy human model of neural disorders of movement

A quantitative approach to virtual-lesion physiology is presented which integrates event-related fMRI, image-guided, repetitive, transcranial magnetic stimulation (irTMS), and simultaneous recording of 3-D movement kinematics. By linking motor neuroscience with clinical disorders of motor function, our method allows development of a healthy, human system model of disorders of skilled action.

Sensorimotor integration for speech motor learning involves the inferior parietal cortex

Sensorimotor integration is important for motor learning. The inferior parietal lobe, through its connections with the frontal lobe and cerebellum, has been associated with multisensory integration and sensorimotor adaptation for motor behaviors other than speech. In the present study, the contribution of the inferior parietal cortex to speech motor learning was evaluated using repetitive transcranial magnetic stimulation (rTMS) prior to a speech motor adaptation task. Subjects' auditory feedback was altered in a manner consistent with the auditory consequences of an unintended change in tongue position during speech production, and adaptation performance was used to evaluate sensorimotor plasticity and short-term learning. Prior to the feedback alteration, rTMS or sham stimulation was applied over the left supramarginal gyrus (SMG). Subjects who underwent the sham stimulation exhibited a robust adaptive response to the feedback alteration whereas subjects who underwent rTMS exhibited a diminished adaptive response. The results suggest that the inferior parietal region, in and around SMG, plays a role in sensorimotor adaptation for speech. The interconnections of the inferior parietal cortex with inferior frontal cortex, cerebellum and primary sensory areas suggest that this region may be an important component in learning and adapting sensorimotor patterns for speech.

Critical neural substrates for correcting unexpected trajectory errors and learning from them

Our proficiency at any skill is critically dependent on the ability to monitor our performance, correct errors and adapt subsequent movements so that errors are avoided in the future. In this study, we aimed to dissociate the neural substrates critical for correcting unexpected trajectory errors and learning to adapt future movements based on those errors. Twenty stroke patients with focal damage to frontal or parietal regions in the left or right brain hemispheres and 20 healthy controls performed a task in which a novel mapping between actual hand motion and its visual feedback was introduced. Only patients with frontal damage in the right hemisphere failed to correct for this discrepancy during the ongoing movement. However, these patients were able to adapt to the distortion such that their movement direction on subsequent trials improved. In contrast, only patients with parietal damage in the left hemisphere showed a clear deficit in movement adaptation, but not in online correction. Left frontal or right parietal damage did not adversely impact upon either process. Our findings thus identify, for the first time, distinct and lateralized neural substrates critical for correcting unexpected errors during ongoing movements and error-based movement adaptation.

Principles of sensorimotor learning

The exploits of Martina Navratilova and Roger Federer represent the pinnacle of motor learning. However, when considering the range and complexity of the processes that are involved in motor learning, even the mere mortals among us exhibit abilities that are impressive. We exercise these abilities when taking up new activities - whether it is snowboarding or ballroom dancing - but also engage in substantial motor learning on a daily basis as we adapt to changes in our environment, manipulate new objects and refine existing skills. Here we review recent research in human motor learning with an emphasis on the computational mechanisms that are involved.

One week of motor adaptation induces structural changes in primary motor cortex that predict long-term memory one year later

The neural bases of motor adaptation have been extensively explored in human and nonhuman primates. A network including the cerebellum, primary motor cortex, and posterior parietal cortex appears to be crucial for this type of learning. Yet, to date, it is unclear whether these regions contribute directly or indirectly to the formation of motor memories. Here we trained subjects on a complex visuomotor rotation associated with long-term memory (in the order of months) to identify potential sites of structural plasticity induced by adaptation. One week of training led to (1) an increment in local gray matter concentration over the hand area of the contralateral primary motor cortex and (2) an increase in fractional anisotropy in an area underneath this region that correlated with the speed of learning. Moreover, the change in gray matter concentration measured immediately after training predicted improvements in the speed of learning during readaptation 1 year later. Our study suggests that motor adaptation induces structural plasticity in primary motor circuits. In addition, it provides the first piece of evidence indicating that early structural changes induced by motor learning may impact on behavior up to 1 year after training.

Error correction, sensory prediction, and adaptation in motor control

Motor control is the study of how organisms make accurate goal-directed movements. Here we consider two problems that the motor system must solve in order to achieve such control. The first problem is that sensory feedback is noisy and delayed, which can make movements inaccurate and unstable. The second problem is that the relationship between a motor command and the movement it produces is variable, as the body and the environment can both change. A solution is to build adaptive internal models of the body and the world. The predictions of these internal models, called forward models because they transform motor commands into sensory consequences, can be used to both produce a lifetime of calibrated movements, and to improve the ability of the sensory system to estimate the state of the body and the world around it. Forward models are only useful if they produce unbiased predictions. Evidence shows that forward models remain calibrated through motor adaptation: learning driven by sensory prediction errors.

Online control of hand trajectory and evolution of motor intention in the parietofrontal system

The frontal mechanisms of motor intention were studied in dorsal premotor and motor cortex of monkeys making direct reaches to visual targets and online corrections of hand trajectory, whenever a change of the target's location occurred. This study and our previous one of parietal cortex (Archambault et al., 2009) provide a picture on the evolution of motor intention and online control of movement in the parietofrontal system. In frontal cortex, significant relationships were found between neural activity and hand kinematics (position, speed, and movement direction). When a change of motor intention occurred, the activity typical of the movement to the first target smoothly evolved into that associated with the movement toward the second one, as observed during direct reaches. Under these conditions, parietal cells remained a more accurate predictor of hand trajectory than frontal ones. The time lags of neural activity with hand kinematics showed that motor, premotor, and parietal cortex were activated sequentially. After the first target's presentation and its change of location, the population activity signaled the change of motor plan before the hand moved to the initial target's position. This signaling occurred earlier in premotor than in motor and parietal cortex. Thus, premotor cortex encodes a higher-order command for the correction of motor intention, while parietal cortex seems responsible for estimating the kinematics of the motor periphery, an essential step to allow motor cortex to modify the hand trajectory. This indicates that the parietofrontal system can update an original and not-yet-accomplished motor plan during its execution.

Contributions of the motor cortex to adaptive control of reaching depend on the perturbation schedule

During adaptation, motor commands tend to repeat as performance plateaus. It has been hypothesized that this repetition produces plasticity in the motor cortex (M1). Here, we considered a force field reaching paradigm, varied the perturbation schedule to potentially alter the amount of repetition, and quantified the interaction between disruption of M1 using transcranial magnetic stimulation (TMS) and the schedule of perturbations. In the abrupt condition (introduction of the perturbation on a single trial followed by constant perturbation), motor output adapted rapidly and was then followed by significant repetition as performance plateaued. TMS of M1 had no effect on the rapid adaptation phase but reduced adaptation at the plateau. In the intermediate condition (introduction of the perturbation over 45 trials), disruption of M1 had no effect on the phase in which motor output changed but again impaired adaptation when performance had plateaued. Finally, when the perturbation was imposed gradually (over 240 trials), the motor commands continuously changed during adaptation and never repeated, and disruption of M1 had no effect on performance. Therefore, TMS of M1 appeared to reduce adaptation of motor commands during a specific phase of learning: when motor commands tended to repeat.

Rapid Online Correction Is Selectively Suppressed During Movement With a Visuomotor Transformation

Reaching movements to visual targets are under fast feedback control, which can rapidly correct an ongoing movement for errors. This study investigates how this online correction is affected by the application of a new visuomotor transformation. Thirty-two subjects made planar pointing movements to visual targets. Vision of the arm was prevented, and hand position was represented by a cursor displayed in the movement plane. In some trials, the target abruptly changed location at the onset of arm movement, which required a rapid correction of movement direction. After performing baseline trials, some subjects were required to adapt to a mirror-image transformation that inverted the visual feedback of their hand position across the body midline, whereas others were not familiarized with the transformation. Afterward, subjects' online correction was tested with target jumps in the presence of the mirror transformation. Results show that after short-term motor adaptation to the mirror transformation there was a selective suppression of the rapid non-mirror correction in the direction of visual target displacement but no mirror reversal. The suppression occurred within the first few trials after the introduction of the mirror transformation, and it was strongest for the movements in which the transformation caused the largest dissociation between the target location and hand movement. Finally, whether or not the short-latency non-mirror correction was suppressed in a given trial, the mirror correction occurred at the same latency as the onset time of voluntary correction in subjects who had not experienced the mirror transformation.

Reduction in learning rates associated with anterograde interference results from interactions between different timescales in motor adaptation

Prior experiences can influence future actions. These experiences can not only drive adaptive changes in motor output, but they can also modulate the rate at which these adaptive changes occur. Here we studied anterograde interference in motor adaptation – the ability of a previously learned motor task (Task A) to reduce the rate of subsequently learning a different (and usually opposite) motor task (Task B). We examined the formation of the motor system's capacity for anterograde interference in the adaptive control of human reaching-arm movements by determining the amount of interference after varying durations of exposure to Task A (13, 41, 112, 230, and 369 trials). We found that the amount of anterograde interference observed in the learning of Task B increased with the duration of Task A. However, this increase did not continue indefinitely; instead, the interference reached asymptote after 15–40 trials of Task A. Interestingly, we found that a recently proposed multi-rate model of motor adaptation, composed of two distinct but interacting adaptive processes, predicts several key features of the interference patterns we observed. Specifically, this computational model (without any free parameters) predicts the initial growth and leveling off of anterograde interference that we describe, as well as the asymptotic amount of interference that we observe experimentally (R² = 0.91). Understanding the mechanisms underlying anterograde interference in motor adaptation may enable the development of improved training and rehabilitation paradigms that mitigate unwanted interference.

The act of learning one task can not only have direct effects on the performance of other tasks, but it can also affect the ability to learn other tasks. One example of the latter is the phenomenon of anterograde interference in motor adaptation, in which the learning of one adaptation can substantially reduce the rate at which the opposite adaptation can be learned. Here we show that the amount of anterograde interference depends systematically on the strength of a particular component of the initial adaptation rather than on the total amount of adaptation that is achieved. This component of the motor memory evolves more slowly than the overall learning and acts in combination with a quickly evolving component of memory to produce the observed improvement in task performance. We proceed to show that a simple computational model of the interactions between these adaptive processes predicts greater than 90% of the variance in the observed interference patterns, suggesting that this quantitative model may enable the development of improved training and rehabilitation paradigms that mitigate unwanted interference.

Functional magnetic resonance imaging reveals the neural substrates of arm transport and grip formation in reach-to-grasp actions in humans

Picking up a cup requires transporting the arm to the cup (transport component) and preshaping the hand appropriately to grasp the handle (grip component). Here, we used functional magnetic resonance imaging to examine the human neural substrates of the transport component and its relationship with the grip component. Participants were shown three-dimensional objects placed either at a near location, adjacent to the hand, or at a far location, within reach but not adjacent to the hand. Participants performed three tasks at each location as follows: (1) touching the object with the knuckles of the right hand; (2) grasping the object with the right hand; or (3) passively viewing the object. The transport component was manipulated by positioning the object in the far versus the near location. The grip component was manipulated by asking participants to grasp the object versus touching it. For the first time, we have identified the neural substrates of the transport component, which include the superior parieto-occipital cortex and the rostral superior parietal lobule. Consistent with past studies, we found specialization for the grip component in bilateral anterior intraparietal sulcus and left ventral premotor cortex; now, however, we also find activity for the grasp even when no transport is involved. In addition to finding areas specialized for the transport and grip components in parietal cortex, we found an integration of the two components in dorsal premotor cortex and supplementary motor areas, two regions that may be important for the coordination of reach and grasp.

Understanding the parietal lobe syndrome from a neurophysiological and evolutionary perspective

In human and nonhuman primates parietal cortex is formed by a multiplicity of areas. For those of the superior parietal lobule (SPL) there exists a certain homology between man and macaques. As a consequence, optic ataxia, a disturbed visual control of hand reaching, has similar features in man and monkeys. Establishing such correspondence has proven difficult for the areas of the inferior parietal lobule (IPL). This difficulty depends on many factors. First, no physiological information is available in man on the dynamic properties of cells in the IPL. Second, the number of IPL areas identified in the monkey is paradoxically higher than that so far described in man, although this issue will probably be reconsidered in future years, thanks to comparative imaging studies. Third, the consequences of parietal lesions in monkeys do not always match those observed in humans. This is another paradox if one considers that, in certain cases, the functional properties of neurons in the monkey's IPL would predict the presence of behavioral skills, such as construction capacity, that however do not seem to emerge in the wild. Therefore, constructional apraxia, which is well characterized in man, has never been described in monkeys and apes. Finally, only certain aspects, i.e. hand directional hypokinesia and gaze apraxia (Balint's psychic paralysis of gaze), of the multifaceted syndrome hemispatial neglect have been described in monkeys. These similarities, differences and paradoxes, among many others, make the study of the evolution and function of parietal cortex a challenging case.

Examining procedural consolidation with visuomotor learning in the lower limb

To investigate the effects of interference tasks in the early stages of motor learning, we used a computerized motor skill analyzer (CMSA) for the lower limb that evaluates the accuracy and speed of movements in tracking tasks. Forty-five healthy subjects were randomly divided into three groups: a control group without an interference task, a mirror image interference group, and a point symmetry interference group. The practice and measurement of motor-learning trials were carried out as follows: (1) 10 practice trials followed by 10 measurement trials in the morning of the first day. (2) 10 measurement trials 6-7 awake hours after the first measurement. (3) 10 measurement trials on the morning of day 2. In the learning interference groups, 10 trials of the mirror image or point symmetry task were performed immediately after the first measurement. Although motor skills of the control group improved, those of the learning interference groups did not. This suggests that a second task performed immediately after the first task interferes with the consolidation of motor skill learning demanding complex visuomotor transformation.

Visuospatial perspective taking in a dynamic environment: perceiving moving objects from a first-person-perspective induces a disposition to act

Spatial perspective taking is an everyday cognitive process that is involved in predicting the outcome of goal directed behavior. We used dynamic virtual stimuli and fMRI to investigate at the neural level whether motion perception interacts with spatial perspective taking in a life-like design. Subjects were asked to perform right-left-decisions about the position of either a motionless, hovering (STATic) or a flying ball (DYNamic), either from their own (1PP) or from the perspective of a virtual character (avatar, 3PP). Our results showed a significant interaction of STIMULUS TYPE and PERSPECTIVE with significantly increased activation in right posterior intraparietal sulcus (IPS) for 1PPDYN condition. As the IPS is critically involved in the computation of object-directed action preparation, we suppose that the simple perception of potentially action-relevant dynamic objects induces a 'readiness for (re)action', restricted to the 1PP. Results are discussed against the background of current theories on embodiment and enactive perception.

Selective regions of the visuomotor system are related to gain-induced changes in force error

When humans perform movements and receive on-line visual feedback about their performance, the spatial qualities of the visual information alter performance. The spatial qualities of visual information can be altered via the manipulation of visual gain and changes in visual gain lead to changes in force error. The current study used functional magnetic resonance imaging during a steady-state precision grip force task to examine how cortical and subcortical brain activity can change with visual gain induced changes in force error. Small increases in visual gain < 1° were associated with a substantial reduction in force error and a small increase in the spatial amplitude of visual feedback. These behavioral effects corresponded with an increase in activation bilaterally in V3 and V5 and in left primary motor cortex and left ventral premotor cortex. Large increases in visual gain > 1° were associated with a small change in force error and a large change in the spatial amplitude of visual feedback. These behavioral effects corresponded with increased activity bilaterally in dorsal and ventral premotor areas and right inferior parietal lobule. Finally, activity in the left and right lobule VI of the cerebellum and left and right putamen did not change with increases in visual gain. Together, these findings demonstrate that the visuomotor system does not respond uniformly to changes in the gain of visual feedback. Instead, specific regions of the visuomotor system selectively change in activity related to large changes in force error and large changes in the spatial amplitude of visual feedback.

Cognitive neural prosthetics

The cognitive neural prosthetic (CNP) is a very versatile method for assisting paralyzed patients and patients with amputations. The CNP records the cognitive state of the subject, rather than signals strictly related to motor execution or sensation. We review a number of high-level cortical signals and their application for CNPs, including intention, motor imagery, decision making, forward estimation, executive function, attention, learning, and multi-effector movement planning. CNPs are defined by the cognitive function they extract, not the cortical region from which the signals are recorded. However, some cortical areas may be better than others for particular applications. Signals can also be extracted in parallel from multiple cortical areas using multiple implants, which in many circumstances can increase the range of applications of CNPs. The CNP approach relies on scientific understanding of the neural processes involved in cognition, and many of the decoding algorithms it uses also have parallels to underlying neural circuit functions.

Intention, action planning, and decision making in parietal-frontal circuits

The posterior parietal cortex and frontal cortical areas to which it connects are responsible for sensorimotor transformations. This review covers new research on four components of this transformation process: planning, decision making, forward state estimation, and relative-coordinate representations. These sensorimotor functions can be harnessed for neural prosthetic operations by decoding intended goals (planning) and trajectories (forward state estimation) of movements as well as higher cortical functions related to decision making and potentially the coordination of multiple body parts (relative-coordinate representations).

Dual adaptation supports a parallel architecture of motor memory

Although our understanding of the mechanisms underlying motor adaptation has greatly benefited from previous computational models, the architecture of motor memory is still uncertain. On one hand, two-state models that contain both a fast-learning-fast-forgetting process and a slow-learning-slow-forgetting process explain a wide range of data on motor adaptation, but cannot differentiate whether the fast and slow processes are arranged serially or in parallel and cannot account for learning multiple tasks simultaneously. On the other hand, multiple parallel-state models learn multiple tasks simultaneously but cannot account for a number of motor adaptation data. Here, we investigated the architecture of human motor memory by systematically testing possible architectures via a combination of simulations and a dual visuomotor adaptation experimental paradigm. We found that only one parsimonious model can account for both previous motor adaptation data and our dual-task adaptation data: a fast process that contains a single state is arranged in parallel with a slow process that contains multiple states switched via contextual cues. Our result suggests that during motor adaptation, fast and slow processes are updated simultaneously from the same motor learning errors.

Preserved motor learning after stroke is related to the degree of proprioceptive deficit

Background

Most motor learning theories posit that proprioceptive sensation serves an important role in acquiring and performing movement patterns. However, we recently demonstrated that experimental disruption of proprioception peripherally altered motor performance but not motor learning in humans. Little work has considered humans with central nervous system damage. The purpose of the present study was to specifically consider the relationship between proprioception and motor learning at the level of the central nervous system in humans.

Methods

Individuals with chronic (> 6mo) stroke and similarly aged healthy participants performed a continuous tracking task with an embedded repeating segment over two days and returned on a third day for retention testing. A limb-position matching task was used to quantify proprioception.

Results

Individuals with chronic stroke demonstrated the ability to learn to track a repeating segment; however, the magnitude of behavioral change associated with repeated segment-specific learning was directly related to the integrity of central proprioceptive processing as indexed by our limb-position matching task.

Conclusion

These results support the importance of central sensory processing for motor learning. The confirmation of central sensory processing dependent motor learning in humans is discussed in the context of our prior report of preserved motor learning when sensation is disrupted peripherally.

Optimal Integration of Gravity in Trajectory Planning of Vertical Pointing Movements

The planning and control of motor actions requires knowledge of the dynamics of the controlled limb to generate the appropriate muscular commands and achieve the desired goal. Such planning and control imply that the CNS must be able to deal with forces and constraints acting on the limb, such as the omnipresent force of gravity. The present study investigates the effect of hypergravity induced by parabolic flights on the trajectory of vertical pointing movements to test the hypothesis that motor commands are optimized with respect to the effect of gravity on the limb. Subjects performed vertical pointing movements in normal gravity and hypergravity. We use a model based on optimal control to identify the role played by gravity in the optimal arm trajectory with minimal motor costs. First, the simulations in normal gravity reproduce the asymmetry in the velocity profiles (the velocity reaches its maximum before half of the movement duration), which typically characterizes the vertical pointing movements performed on Earth, whereas the horizontal movements present symmetrical velocity profiles. Second, according to the simulations, the optimal trajectory in hypergravity should present an increase in the peak acceleration and peak velocity despite the increase in the arm weight. In agreement with these predictions, the subjects performed faster movements in hypergravity with significant increases in the peak acceleration and peak velocity, which were accompanied by a significant decrease in the movement duration. This suggests that movement kinematics change in response to an increase in gravity, which is consistent with the hypothesis that motor commands are optimized and the action of gravity on the limb is taken into account. The results provide evidence for an internal representation of gravity in the central planning process and further suggest that an adaptation to altered dynamics can be understood as a reoptimization process.

Integration of predictive feedforward and sensory feedback signals for online control of visually guided movement

Online control of movement requires complex integration of predictive central feedforward and peripheral sensory feedback signals. We studied the hand trajectories of human subjects pointing to visual targets that abruptly changed locations by different amounts and modeled the mechanism of rapid online correction using a dynamic model of a two-joint limb. Small unperceived and large detected target displacements could be attributed to different origins (motor execution errors vs. environmental changes, respectively) and compensated differently. However, the behavioral findings indicate that the rapid feedback pathway is recruited regardless of the amplitude or subjective awareness of target displacement and that the size of the earliest correction is always proportional to the amplitude of the target displacement over the tested range of perturbations. The modeling findings suggest that the rapid online corrections can be accomplished by superimposing a dynamically appropriate error correction signal onto the outgoing feedforward motor command to the original target. Furthermore, the modeling shows that the online correction mechanism must include compensation for the dynamic mechanical properties of the limb and for sensory delays in its error-correction pathway.

The separate neural control of hand movements and contact forces

To manipulate an object, we must simultaneously control the contact forces exerted on the object and the movements of our hand. Two alternative views for manipulation have been proposed: one in which motions and contact forces are represented and controlled by separate neural processes, and one in which motions and forces are controlled jointly, by a single process. To evaluate these alternatives, we designed three tasks in which subjects maintained a specified contact force while their hand was moved by a robotic manipulandum. The prescribed contact force and hand motions were selected in each task to induce the subject to attain one of three goals: (1) exerting a regulated contact force, (2) tracking the motion of the manipulandum, and (3) attaining both force and motion goals concurrently. By comparing subjects' performances in these three tasks, we found that behavior was captured by the summed actions of two independent control systems: one applying the desired force, and the other guiding the hand along the predicted path of the manipulandum. Furthermore, the application of transcranial magnetic stimulation impulses to the posterior parietal cortex selectively disrupted the control of motion but did not affect the regulation of static contact force. Together, these findings are consistent with the view that manipulation of objects is performed by independent brain control of hand motions and interaction forces.

Cortical mechanisms for online control of hand movement trajectory: the role of the posterior parietal cortex

The parietal mechanisms for the control of hand movement trajectory were studied by recording cell activity in area 5 of monkeys making direct reaches to visual targets and online corrections of movement trajectory, after change of target location in space. The activity of hand-related cells was fitted with a linear model including hand position, movement direction, and speed. The neural activity modulation mostly led, but also followed, hand movement. When a change of hand trajectory occurred, the pattern of activity associated with the movement to the first target evolved into that typical of the movement to the second one, thus following the corresponding variations of the hand kinematics. The visual signal concerning target location in space did not influence the firing activity associated with the direction of hand movement within the first 150 ms after target presentation. This might be the time necessary for the visuo-motor transformation underlying reaching. We conclude that online control of hand trajectory not only resides in the relationships between neural activity and kinematics, but, under specific circumstances, also on the coexistence of signals about ongoing and future hand movement direction.

Cortical Processing during Dynamic Motor Adaptation

In this chapter we investigate the role of motor cortex in adapting movements to novel dynamic environments. We present results from two experiments in which monkey or human subjects learned to make two-dimensional reaching movements while holding a robotic manipulandum that applied a predictable pattern of forces (a curl field) to their hand. In the first study, we analyzed electrophysiological data recorded in motor cortex while monkeys adapted or readapted to the novel forces on each day of the experiment. In the second study, we perturbed the excitability of motor cortex using repetitive transcranial magnetic stimulation (rTMS) as human participants adapted to the forces. From the first experiment, we present qualitative evidence that a network of cortical areas including the supplementary motor area, premotor cortex, and primary motor cortex (M1) not only encodes kinematic and dynamic parameters of motor execution, but also registers changes in encoding that could provide a substrate for motor memory. Based on the second experiment, we qualify the role of M1 in motor memory, by showing that its disruption by rTMS does not interfere with the process of initial motor adaptation, but rather with offline improvement as measured at retest on the following day.

Parietal encoding of action in depth

The posterior parietal cortex is a crucial node in the process of coordinates transformation for the visual control of eye and hand movements. This conviction stems from both neurophysiological studies in the behaving monkey and from the analysis of the consequences of parietal lobe lesions in humans. Despite an extensive literature concerning varying aspects of the composition and control of eye and hand movements, there is little information about the physiological processes responsible for encoding target distance and hand movement in depth or about their control and impairment in parietal patients. This review is an attempt to provide a comprehensive picture from the fragmentary material existing on this issue in the literature. This should serve as a basis for discussion of what we consider to be a prototypical function of the dorsal visuomotor stream in the primate brain, that of encoding eye and hand movement in depth.