Skilled-learning-induced potentiation in rat sensorimotor cortex: a transient form of behavioural long-term potentiation

Functional MRI of long-term potentiation: imaging network plasticity

Neurons are able to express long-lasting and activity-dependent modulations of their synapses. This plastic property supports memory and conveys an extraordinary adaptive value, because it allows an individual to learn from, and respond to, changes in the environment. Molecular and physiological changes at the cellular level as well as network interactions are required in order to encode a pattern of synaptic activity into a long-term memory. While the cellular mechanisms linking synaptic plasticity to memory have been intensively studied, those regulating network interactions have received less attention. Combining high-resolution fMRI and in vivo electrophysiology in rats, we have previously reported a functional remodelling of long-range hippocampal networks induced by long-term potentiation (LTP) of synaptic plasticity in the perforant pathway. Here, we present new results demonstrating an increased bilateral coupling in the hippocampus specifically supported by the mossy cell commissural/associational pathway in response to LTP. This fMRI-measured increase in bilateral connectivity is accompanied by potentiation of the corresponding polysynaptically evoked commissural potential in the contralateral dentate gyrus and depression of the inactive convergent commissural pathway to the ipsilateral dentate. We review these and previous findings in the broader context of memory consolidation.

Reversal of long-term potentiation-like plasticity processes after motor learning disrupts skill retention

Plasticity of synaptic connections in the primary motor cortex (M1) is thought to play an essential role in learning and memory. Human and animal studies have shown that motor learning results in long-term potentiation (LTP)-like plasticity processes, namely potentiation of M1 and a temporary occlusion of additional LTP-like plasticity. Moreover, biochemical processes essential for LTP are also crucial for certain types of motor learning and memory. Thus, it has been speculated that the occlusion of LTP-like plasticity after learning, indicative of how much LTP was used to learn, is essential for retention. Here we provide supporting evidence of it in humans. Induction of LTP-like plasticity can be abolished using a depotentiation protocol (DePo) consisting of brief continuous theta burst stimulation. We used transcranial magnetic stimulation to assess whether application of DePo over M1 after motor learning affected (1) occlusion of LTP-like plasticity and (2) retention of motor skill learning. We found that the magnitude of motor memory retention is proportional to the magnitude of occlusion of LTP-like plasticity. Moreover, DePo stimulation over M1, but not over a control site, reversed the occlusion of LTP-like plasticity induced by motor learning and disrupted skill retention relative to control subjects. Altogether, these results provide evidence of a link between occlusion of LTP-like plasticity and retention and that this measure could be used as a biomarker to predict retention. Importantly, attempts to reverse the occlusion of LTP-like plasticity after motor learning comes with the cost of reducing retention of motor learning.

Outcomes of tongue-pressure strength and accuracy training for dysphagia following acquired brain injury

The purpose of this study was to measure treatment outcomes in a group of six adults with chronic dysphagia following acquired brain injury, who each completed 24 sessions of tongue-pressure resistance training, over a total of 11-12 weeks. The treatment protocol emphasized both strength and accuracy. Biofeedback was provided using the Iowa Oral Performance Instrument. Amplitude accuracy targets were set between 20-90% of the patient's maximum isometric pressure capacity. Single subject methods were used to track changes in tongue strength (maximum isometric pressures), with functional swallowing outcomes measured using blinded ratings of a standard pre- and post-treatment videofluoroscopy protocol. Improvements were seen in post-treatment measures of tongue pressure and penetration-aspiration. No improvements were seen in pharyngeal residues, indeed worsening residue was seen in some patients.

Occlusion of low-frequency-induced, heterosynaptic long-term potentiation in the rat hippocampus in vivo following spatial training

Recent work has shown that some low-frequency stimulation (LFS) protocols can induce long-term potentiation (LTP) at hippocampal synapses. As LFS mimics certain aspects of low-frequency oscillations during slow-wave sleep, LFS-LTP may be relevant to processes of sleep-dependent consolidation. Here, alternating LFS (1 Hz) of heterosynaptic inputs arising in the medial septum and area CA3 induced LTP at hippocampal CA1 synapses of anesthetized rats. Remarkably, this LTP was absent when delivered 3 h, but not 8 or 24 h, after training in the hidden platform version of the Morris water maze, suggesting a time-specific occlusion of LFS-LTP following spatial learning. LTP assessed 3 h after training was intact in yoked swim controls and rats trained in darkness. Visible platform training resulted in heterogeneous effects, with about half of the animals showing LTP occlusion. Pharmacological experiments revealed that N-methyl-d-aspartate (NMDA)-receptor activation was required for both LFS-LTP and the retention of spatial learning. To test whether a learning-related, NMDA-dependent potentiation accounted for the occlusion effect, we blocked NMDA receptors immediately following spatial training. This manipulation reversed LTP occlusion 3 h after training. Together, these experiments indicate a mechanistic overlap between heterosynaptically induced LFS-LTP and processes mediating the consolidation of spatial information at hippocampal synapses.

Post-stroke protection from maladaptive effects of learning with the non-paretic forelimb by bimanual home cage experience in C57BL/6 mice

Behavioral experience, in the form of skilled limb use, has been found to impact the structure and function of the central nervous system, affecting post-stroke behavioral outcome in both adaptive and maladaptive ways. Learning to rely on the less-affected, or non-paretic, body side is common following stroke in both humans and rodent models. In rats, it has been observed that skilled learning with the non-paretic forelimb following ischemic insult leads to impaired or delayed functional recovery of the paretic limb. Here we used a mouse model of focal motor cortical ischemic injury to examine the effects of non-paretic limb training following unilateral stroke. In addition, we exposed some mice to increased bimanual experience in the home cage following stroke to investigate the impact of coordinated dexterous limb use on the non-paretic limb training effect. Our results confirmed that skilled learning with the non-paretic limb impaired functional recovery following stroke in C56BL/6 mice, as it does in rats. Further, this effect was avoided when the skill learning of the non-paretic limb was coupled with increased dexterous use of both forelimbs in the home cage. These findings further establish the mouse as an appropriate model in which to study the neural mechanisms of recovery following stroke and extend previous findings to suggest that the dexterous coordinated use of the paretic and non-paretic limb can promote functional outcome following injury.

Motor learning interference is proportional to occlusion of LTP-like plasticity

Learning interference occurs when learning something new causes forgetting of an older memory (retrograde interference) or when learning a new task disrupts learning of a second subsequent task (anterograde interference). This phenomenon, described in cognitive, sensory, and motor domains, limits our ability to learn multiple tasks in close succession. It has been suggested that the source of interference is competition of neural resources, although the neuronal mechanisms are unknown. Learning induces long-term potentiation (LTP), which can ultimately limit the ability to induce further LTP, a phenomenon known as occlusion. In humans we quantified the magnitude of occlusion of anodal transcranial direct current stimulation-induced increased excitability after learning a skill task as an index of the amount of LTP-like plasticity used. We found that retention of a newly acquired skill, as reflected by performance in the second day of practice, is proportional to the magnitude of occlusion. Moreover, the degree of behavioral interference was correlated with the magnitude of occlusion. Individuals with larger occlusion after learning the first skill were (1) more resilient to retrograde interference and (2) experienced larger anterograde interference when training a second task, as expressed by decreased performance of the learned skill in the second day of practice. This effect was not observed if sufficient time elapsed between training the two skills and LTP-like occlusion was not present. These findings suggest competition of LTP-like plasticity is a factor that limits the ability to remember multiple tasks trained in close succession.

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.

Imipramine counteracts corticosterone-induced enhancement of glutamatergic transmission and impairment of long-term potentiation in the rat frontal cortex

The effects of corticosterone administration lasting for 7 and 21 days were studied ex vivo in rat frontal cortex slices prepared 48 h after the last dose of the hormone. In slices originating from corticosterone-treated animals, the amplitude of extracellular field potentials recorded in cortical layer II/III was increased. Corticosterone administration also resulted in an increase of the mean frequency, but not the mean amplitude, of spontaneous excitatory postsynaptic currents (sEPSCs) in layer II/III pyramidal neurons. These effects were accompanied by a reduced magnitude of long-term potentiation (LTP) of field potentials. In a separate set of experiments, rats were treated with corticosterone for 21 days and additionally with a tricyclic antidepressant, imipramine, beginning on the eighth day of corticosterone administration. In this experimental group, the amplitude of field potentials, the mean frequency of sEPSCs and the magnitude of LTP were not different from the control, indicating that corticosterone-induced modifications of basal glutamatergic transmission and synaptic plasticity were reversed by the antidepressant.

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.

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.

High frequency stimulation alters motor maps, impairs skilled reaching performance and is accompanied by an upregulation of specific GABA, glutamate and NMDA receptor subunits

High frequency stimulation (HFS) has the potential to interfere with learning and memory. HFS and motor skill training both lead to potentiation of the stimulated network and alter motor map expression. However, the extent to which HFS can interfere with the learning and performance of a skilled motor task and the resulting effect on the representation of movement has not been examined. Moreover, the molecular mechanisms associated with HFS and skilled motor training on the motor cortex are not known. We hypothesized that HFS would impair performance on a skilled reaching task, and would be associated with alterations in motor map expression and protein levels compared to non-stimulated and untrained controls. Long Evans Hooded rats were chronically implanted with stimulating and recording electrodes in the corpus callosum and frontal neocortex, respectively. High frequency theta burst stimulation or sham stimulation was applied once daily for 20 sessions. The rats were divided into five groups: control, HFS and assessed at 1 week post stimulation, HFS and assessed 3 weeks post stimulation, reach trained, and HFS and reach trained. A subset of rats from each group was assessed with either intracortical microstimulation (ICMS) to examine motor map expression or Western blot techniques to determine protein expression of several excitatory and inhibitory receptor subunits. Firstly, we found that HFS resulted in larger and reorganized motor maps, and lower movement thresholds compared to controls. This was associated with an up-regulation of the GABA(A)α1 and NR1 receptor subunits 3 weeks after the last stimulation session only. Stimulation affected skilled reaching performance in a subset of all stimulated rats. Rats that were poor performers had larger rostral forelimb areas, higher proximal and lower distal movement thresholds compared to rats that were good performers after stimulation. Reach training alone was associated with an up-regulation of GABA(A)α1, α2, GluR2, NR1 and NR2A compared to controls. HFS and reach-trained rats showed an up-regulation of GABA(A)α2 compared to stimulated rats that were not reach-trained. Therefore, we have shown that HFS induces significant plasticity in the motor cortex, and has the potential to disrupt performance on a skilled motor task.

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.

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.

NR2B-subunit dependent facilitation of long-term potentiation in primary visual cortex following visual discrimination training of adult rats

Long-term potentiation (LTP) is an important mechanism thought to mediate changes in synaptic connectivity following various types of experience. We examined the effects of visual discrimination training on LTP in the mature, rodent thalamocortical visual system. Adult rats underwent visual discrimination training in a modified Morris Water Maze containing a Y-maze insert, requiring rats to associate visual cues with the location of a hidden escape platform placed in one of the two goal arms of the Y-maze insert. On the day following successful task acquisition (average of nine training days), rats were anesthetized (urethane), and LTP in the thalamocortical system was characterized. In task-naïve rats, theta-burst stimulation of the lateral geniculate nucleus resulted in modest (∼40%) potentiation of field postsynaptic potentials recorded in the primary visual cortex (V1). Rats trained on the visual discrimination task showed significantly greater levels of LTP (∼60%), an effect that was not seen in rats trained to swim in the maze without a predictive association between visual cues and platform location. An antagonist of the N-methyl-d-aspartate (NMDA) receptor NR2B subunit ([R-(R *,S *)]-α-(4-hydroxyphenyl)-β-methyl-4-(phenylmethyl)-1-piperidinepropanol hydrochloride (Ro 25-6981); 2 mm, applied locally at the recording site in V1) reversed the training-induced LTP enhancement without affecting LTP in task-naïve rats. An antagonist of metabotropic glutamate receptors [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY 341495); 2 mm] was ineffective in reversing the training-induced LTP facilitation. These data suggest that behavioral (visual) training can result in changes in plasticity exhibited by the mature, thalamocortical visual system that require activation of NMDA receptors containing the NR2B subunit.

Neural plasticity and neurorehabilitation: teaching the new brain old tricks

Following brain injury or disease there are widespread biochemical, anatomical and physiological changes that result in what might be considered a new, very different brain. This adapted brain is forced to reacquire behaviors lost as a result of the injury or disease and relies on neural plasticity within the residual neural circuits. The same fundamental neural and behavioral signals driving plasticity during learning in the intact brain are engaged during relearning in the damaged/diseased brain. The field of neurorehabilitation is now beginning to capitalize on this body of work to develop neurobiologically informed therapies focused on key behavioral and neural signals driving neural plasticity. Further, how neural plasticity may act to drive different neural strategies underlying functional improvement after brain injury is being revealed. The understanding of the relationship between these different neural strategies, mechanisms of neural plasticity, and changes in behavior may facilitate the development of novel, more effective rehabilitation interventions for treating brain injury and disease.

LEARNING OUTCOMES

Readers will be able to: (a) define neural plasticity, (b) understand how learning in the intact and damaged brain can drive neural plasticity, (c) identify the three basic neural strategies mediating functional improvement, and (d) understand how adjuvant therapies have the potential to upregulate plasticity and enhance functional recovery.

Larger cortical motor maps after seizures

Expansion of motor maps occurs in both clinical populations with epilepsy and in experimental models of epilepsy when the frontal lobes are involved. We have previously shown that the forelimb area of the motor cortex undergoes extensive enlargement after seizures, although the extent to which many movement representation areas are altered is not clear. Here we hypothesize that movement representations in addition to the forelimb area will be enlarged after cortical seizures. To test our hypotheses, Long Evans Hooded rats received 20 sessions of callosal (or sham) kindling, and then were subjected to intracortical microstimulation to map several movement representations including the jaw, neck, forelimb, hindlimb, trunk and tail. We found significantly larger total map areas of several movement representations, including movements that could be evoked more posterior than they are in control rats. We also show the presence of more multiple movement sites and lower movement thresholds in kindled rats, suggesting that movements not only overlap and share cortical territory after seizures, but become present in formerly non-responsive sites as they become detectable with our intracortical microstimulation methodology. In summary, several motor map areas become larger after seizures, which may contribute to the interictal motor disturbances that have been documented in patients with epilepsy.

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.

Breeder and batch-dependent variability in the acquisition and performance of a motor skill in adult Long-Evans rats

Reaching tasks are popular tools for investigating the neural mechanisms of motor skill learning and recovery from brain damage in rodents, but there is considerable unexplained variability across studies using these tasks. We investigated whether breeder, batch effects, experimenter, time of year, weight and other factors contribute to differences in the acquisition and performance of a skilled reaching task, the single pellet retrieval task, in adult male Long-Evans hooded rats. First, we retrospectively analyzed task acquisition and performance in rats from different breeding colonies that were used in several studies spanning a 3 year period in our laboratory. Second, we compared reaching variables in age-matched rats from different breeders that were trained together as a batch by the same experimenters. All rats had received daily training on the reaching task until they reached a criterion of successful reaches per attempt. We found significant breeder-dependent differences in learning rate and final performance level. This was found even when age-matched rats from different breeders were trained together by the same experimenters. There was also significant batch-to-batch variability within rats from the same breeder trained by the same experimenter. Other factors, including weight, paw preference and the experimenter, were not as strong or consistent in their contributions to differences across studies. The breeder and batch effects found within the same rat strain may reflect genetic and environmental influences on the neural substrates of motor skill learning. This is an important consideration when comparing baseline performance across studies and for controlling variability within studies.

Neural plasticity: the biological substrate for neurorehabilitation

Decades of basic science have clearly demonstrated the capacity of the central nervous system (CNS) to structurally and functionally adapt in response to experience. The field of neurorehabilitation has begun to use this body of work to develop neurobiologically informed therapies that harness the key behavioral and neural signals that drive neural plasticity. The present review describes how neural plasticity supports both learning in the intact CNS and functional improvement in the damaged or diseased CNS. A pragmatic, interdisciplinary definition of neural plasticity is presented that may be used by both clinical and basic scientists studying neurorehabilitation. Furthermore, a description of how neural plasticity may act to drive different neural strategies underlying functional improvement after CNS injury or disease is provided. The understanding of the relationship between these different neural strategies, mechanisms of neural plasticity, and changes in behavior may facilitate the development of novel, more effective rehabilitation interventions.

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.

The cortical topography of local sleep

In a recent series of experiments, we demonstrated that a visuomotor adaptation task, 12 hours of left arm immobilization, and rapid transcranial magnetic stimulation (rTMS) during waking can each induce local changes in the topography of electroencephalographic (EEG) slow wave activity (SWA) during subsequent non-rapid eye movement (NREM) sleep. However, the poor spatial resolution of EEG and the difficulty of relating scalp potentials to the activity of the underlying cortex limited the interpretation of these results. In order to better understand local cortical regulation of sleep, we used source modeling to show that plastic changes in specific cortical areas during waking produce correlated changes in SWA during sleep in those same areas. We found that implicit learning of a visuomotor adaptation task induced an increase in SWA in right premotor and sensorimotor cortices when compared to a motor control. These same areas have previously been shown to be selectively involved in the performance of this task. We also found that arm immobilization resulted in a decrease in SWA in sensorimotor cortex. Inducing cortical potentiation with repetitive transcranial magnetic stimulation (rTMS) caused an increase in SWA in the targeted area and a decrease in SWA in the contralateral cortex. Finally, we report the first evidence that these modulations in SWA may be related to the dynamics of individual slow waves. We conclude that there is a local, plasticity dependent component to sleep regulation and confirm previous inferences made from the scalp data.

Physiological effects of enriched environment exposure and LTP induction in the hippocampus in vivo do not transfer faithfully to in vitro slices

A number of experimental paradigms use in vitro brain slices to test for changes in synaptic transmission and plasticity following a behavioral manipulation. For example, a number of previous studies have reported a variety of effects of environmental enrichment (EE) exposure on field potential responses in hippocampal slices, but in no study was is it known what changes had been elicited in vivo. In the present study, we recorded from the hippocampus in vivo while rats underwent a brief period of EE. There was no detectable EE-induced change in synaptic efficacy in the dentate gyrus in vivo, but there was an increase in cellular excitability. In slices prepared from the same animals, we failed to observe any evidence of the excitability increase. We next tested whether LTP induction in vivo was better preserved in vitro. However, when slices from these rats were examined, there was no observable change in perforant path synaptic strength, although there was a modest increase in excitability that correlated with the increased excitability observed in vivo. These findings suggest that synaptic changes induced in vivo either are not preserved faithfully or are difficult to detect in hippocampal slices, while changes in cellular excitability are better preserved.

Motor cortex reorganization across the lifespan

The brain is a highly dynamic structure with the capacity for profound structural and functional change. Such neural plasticity has been well characterized within motor cortex and is believed to represent one of the neural mechanisms for acquiring and modifying motor behaviors. A number of behavioral and neural signals have been identified that modulate motor cortex plasticity throughout the lifespan in both the intact and damaged brain. Specific signals discussed in this review include: motor learning in the intact brain, motor relearning in the damaged brain, cortical stimulation, stage of development and genotype. Clinicians are encouraged to harness these signals in the development and implementation of treatment so as to maximally drive neural plasticity and functional improvements in speech, language and swallowing.

LEARNING OUTCOMES

Readers will be able to: (1) describe a set of behavioral and neural signals that modulate motor cortex plasticity in the intact and damaged brain; (2) describe the influence of stage of development on plasticity and functional outcomes; and (3) identify a known genotype that alters the capacity for motor learning and brain plasticity.

The "good" limb makes the "bad" limb worse: experience-dependent interhemispheric disruption of functional outcome after cortical infarcts in rats

Following stroke-like lesions to the sensorimotor cortex in rats, experience with the ipsi-to-lesion (ipsilesional), "nonparetic", forelimb worsens deficits in the contralesional, "paretic", forelimb. We tested whether the maladaptive effects of experience with the nonparetic limb are mediated through callosal connections and the contralesional sensorimotor cortex. Adult male rats with proficiency in skilled reaching with their dominant (for reaching) forelimb received ischemic bilateral sensorimotor cortex lesions, or unilateral lesions, with or without callosal transections. After assessing dominant forelimb function (the paretic forelimb in rats with unilateral lesions), animals were trained with their nonparetic/nondominant forelimb or underwent control procedures for 15 days. Animals were then tested with their paretic/dominant forelimb. In animals with unilateral lesions only, nonparetic forelimb training worsened subsequent performance with the paretic forelimb, as found previously. This effect was not found in animals with both callosal transections and unilateral lesions. After bilateral lesions, training the nondominant limb did not worsen function of the dominant limb compared with controls. Thus, the maladaptive effects of training the nonparetic limb on paretic forelimb function depend upon the contralesional cortex and transcallosal projections. This suggests that this experience-dependent disruption of functional recovery is mediated through interhemispheric connections of the sensorimotor cortex.

Neural cytoskeleton capabilities for learning and memory

This paper proposes a physical model involving the key structures within the neural cytoskeleton as major players in molecular-level processing of information required for learning and memory storage. In particular, actin filaments and microtubules are macromolecules having highly charged surfaces that enable them to conduct electric signals. The biophysical properties of these filaments relevant to the conduction of ionic current include a condensation of counterions on the filament surface and a nonlinear complex physical structure conducive to the generation of modulated waves. Cytoskeletal filaments are often directly connected with both ionotropic and metabotropic types of membrane-embedded receptors, thereby linking synaptic inputs to intracellular functions. Possible roles for cable-like, conductive filaments in neurons include intracellular information processing, regulating developmental plasticity, and mediating transport. The cytoskeletal proteins form a complex network capable of emergent information processing, and they stand to intervene between inputs to and outputs from neurons. In this manner, the cytoskeletal matrix is proposed to work with neuronal membrane and its intrinsic components (e.g., ion channels, scaffolding proteins, and adaptor proteins), especially at sites of synaptic contacts and spines. An information processing model based on cytoskeletal networks is proposed that may underlie certain types of learning and memory.

Neocortical movement representations are reduced and reorganized following bilateral intrastriatal 6-hydroxydopamine infusion and dopamine type-2 receptor antagonism

The neurophysiologic model of Parkinson's disease predicts nigrostriatal dopamine depletion leads to increased inhibitory basal ganglia output resulting in frontal neocortical hypoactivity. The nature of this hypoactivation is not well understood and modeled predominantly by a unilateral representation. Intracortical microstimulation (ICMS) was used to probe topographic movement representations of the left forelimb motor area 2 weeks following sham, unilateral left hemisphere or bilateral intrastriatal 6-hydroxydopamine (6-OHDA) infusion and under acute dopamine receptor antagonism with haloperidol in non-lesioned rats. 6-OHDA infusions induced a significant loss of substantia nigra pars compacta (SNc) dopamine neurons. Bilateral SNc lesions and haloperidol significantly reduced map area which was preserved in unilateral lesions. All lesion conditions and haloperidol induced significant map reorganization, characterized by increased representation of distal forelimb movements. Results suggest basal ganglia dopamine deficiency can affect the topographic organization of sensorimotor neocortex and lead to significant reduction in the size of motor representations. We conclude that the neurophysiologic model is supported but that bilateral loss of dopamine is required to see a reduction in the size of motor maps.

Effects of skilled training on sleep slow wave activity and cortical gene expression in the rat

STUDY OBJECTIVE

The best characterized marker of sleep homeostasis is the amount of slow wave activity (SWA, 0.5-4 Hz) during NREM sleep. SWA increases as a function of previous waking time and declines during sleep, but the underlying mechanisms remain unclear. We have suggested that SWA homeostasis is linked to synaptic potentiation associated with learning during wakefulness. Indeed, studies in rodents and humans found that SWA increases after manipulations that presumably enhance synaptic strength, but the evidence remains indirect. Here we trained rats in skilled reaching, a task known to elicit long-term potentiation in the trained motor cortex, and immediately after learning measured SWA and cortical protein levels of c-fos and Arc, 2 activity-dependent genes involved in motor learning.

DESIGN

Intracortical local field potential recordings and training on reaching task.

SETTING

Basic sleep research laboratory.

PATIENTS OR PARTICIPANTS

Long Evans adult male rats.

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

SWA increased post-training in the trained cortex (the frontal cortex contralateral to the limb used to learn the task), with smaller or no increase in other cortical areas. This increase was reversible within 1 hour, specific to NREM sleep, and positively correlated with changes in performance during the prior training session, suggesting that it reflects plasticity and not just motor activity. Fos and Arc levels were higher in the trained relative to untrained motor cortex immediately after training, but this asymmetry was no longer present after 1 hour of sleep.

CONCLUSION

Learning to reach specifically affects gene expression in the trained motor cortex and, in the same area, increases sleep need as measured by a local change in SWA.

Homeostatic and nonhomeostatic modulation of learning in human motor cortex

Motor learning is important throughout life for acquisition and adjustment of motor skill. The extent of motor learning may be modulated by the history of motor cortex activity, but little is known which metaplasticity rule (homeostatic vs nonhomeostatic) governs this interaction. Here, we explored in nine healthy adults the effects of three different paired associative stimulation (PAS) protocols on subsequent learning of rapid thumb flexion movements. PAS resulted in either a long-term potentiation (LTP)-like increase in excitability of the stimulated motor cortex (PAS(LTP)), or a long-term depression (LTD)-like decrease (PAS(LTD)), or no change (control condition, PAS(CON)). Learning was indexed by the increase in peak acceleration of the trained movement. Delays of 0 and 90 min between PAS and motor practice were tested. At the 0 min delay, PAS(LTD) strongly facilitated motor learning (homeostatic interaction), and PAS(LTP) also facilitated learning, although to a lesser extent (nonhomeostatic interaction). At the 90 min delay, PAS(LTD) facilitated learning, whereas PAS(LTP) depressed learning (interactions both homeostatic). Therefore, facilitation of learning by previous brain stimulation occurs primarily and most effectively through homeostatic interactions, but at the 0 min delay, nonhomeostatic mechanisms such as LTP-induced blockade of LTD and nonsaturated LTP-induced facilitation of learning might also play a role. The present findings demonstrate that motor learning in humans can be modulated by noninvasive brain stimulation and suggest the possibility of enhancing motor relearning in defined neurological patients.

Virtual enriched environments in paediatric neuropsychological rehabilitation following traumatic brain injury: Feasibility, benefits and challenges

A frequent consequence of traumatic brain injury (TBI) is a significant reduction in patients' cerebral activation/arousal, which clinicians agree is not conducive to optimal rehabilitation outcomes. In the context of paediatric rehabilitation, sustained periods of inactivity are particularly undesirable, as contemporary research has increasingly called into question the Kennard principle that youth inherently promotes greater neural plasticity and functional recovery following TBI. Therefore, the onus to create rehabilitation conditions most conducive to harnessing plasticity falls squarely on the shoulders of clinicians. Having noted the efficacy of environmental enrichment in promoting neural plasticity and positive functional outcomes in the animal literature, some researchers have suggested that the emerging technology of Virtual Reality (VR) could provide the means to increase patients' cerebral activation levels via the use of enriched Virtual Environments (VEs). However, 10 years on, this intuitively appealing concept has received almost no attention from researchers and clinicians alike. This paper overviews recent research on the benefits of enriched environments in the injured brain and identifies the potential and challenges associated with implementing VR-based enrichment in paediatric neuropsychological rehabilitation.

Phasic voluntary movements reverse the aftereffects of subsequent theta-burst stimulation in humans

Theta-burst stimulation (TBS) is a technique that elicits long-lasting changes in the excitability of human primary motor cortex (M1). Tonic contraction of the target muscle modifies the aftereffects of TBS, whereas interactions between phasic muscle contraction and the aftereffects of TBS are unknown. In this paper, we investigated whether phasic voluntary movements influence TBS-induced changes in M1 excitability. We examined whether a brief sequence of phasic finger movements performed by healthy humans before both intermittent TBS (iTBS) and continuous TBS (cTBS) influences TBS-induced aftereffects. Ten healthy subjects underwent iTBS and cTBS. To evaluate the TBS-induced aftereffects on M1 excitability, single TMS pulses were given over the FDI motor area before (T0) and 5 (T1), 15 (T2), and 30 min (T3) after TBS. To find out whether finger movements influenced the TBS-induced aftereffects, we tested motor-evoked potentials (MEPs) size by single TMS pulses at T0, immediately after movements, and at T1-T3. We also measured the kinematic variables mean amplitude and mean peak velocity of the movements. When no phasic voluntary movements preceded TBS, iTBS elicited facilitatory and cTBS elicited inhibitory aftereffects on MEP size. Conversely, movements performed before TBS elicited significant changes in the direction of the TBS-induced aftereffects. iTBS produced inhibitory instead of facilitatory aftereffects and cTBS produced facilitatory instead of inhibitory aftereffects. Finger movements alone had no effects on MEPs size tested with single-pulse TMS. Peripheral electrical stimulation had no effect on iTBS-induced aftereffects. Repeated phasic finger movements interfere with TBS-induced aftereffects probably by modulating mechanisms of brain metaplasticity.

Sequential neural changes during motor learning in schizophrenia

Positron emission tomography (PET) was used to investigate differences in neural plasticity associated with learning a unique motor task in patients with schizophrenia and healthy volunteers. Working with a robotic manipulandum, subjects learned reaching movements in a force field. Visual cues were provided to guide the reaching movements. PET rCBF measures were acquired while participants learned the motor skill over successive runs. The groups did not differ in behavioral performance but did differ in their rCBF activity patterns. Healthy volunteers displayed blood flow increases in primary motor cortex and supplementary motor area with motor learning. The patients with schizophrenia displayed an increase in the primary visual cortex with motor learning. Changes in these regions were positively correlated with changes in each group's motor accuracy, respectively. This is the first study to employ a unique arm-reaching motor learning test to assess neural plasticity during multiple phases of motor learning in patients with schizophrenia. The patients may have an inability to rapidly tune motor cortical neural populations to a preferred direction. The visual system, however, appears to be highly compensated in schizophrenia and the inability to rapidly modulate the motor cortex may be substantially corrected by the schizophrenic group's visuomotor adaptations.

Cineradiographic (video X-ray) analysis of skilled reaching in a single pellet reaching task provides insight into relative contribution of body, head, oral, and forelimb movement in rats

The forelimb movements (skilled reaching) used by rats to reach for a single food pellet to place into the mouth have been used to model many neurological conditions. They have been described as a sequence of oppositions of head-pellet, paw-pellet and pellet-mouth that can be described as movements of the distal portion of body segments in relation to their fixed proximal joints. Movement scoring is difficult, however, because the location and movement of body segments is estimated through the overlying fur and skin, which is pliable and partially obscures movement. Using moderately high-speed cineradiographic filming from lateral, dorsal, and frontal perspectives, the present study describes how forelimb and skeletal bones move during the skilled reaching act. The analysis indicates that: (i) head movements for orienting to food, enabled by the vertical orientation of the rostral spinal cord, are mainly independent of trunk movement, (ii) skilled reaching consists of a sequence of upper arm and extremity movements each involving a number of concurrent limb segment and joint movements and (iii) food pellets are retrieved from the paw using either the incisors and/or tongue. The findings are discussed in relation to the idea that X-ray cinematography is valuable tool for assisting descriptive analysis and can contribute to understanding general principles of the relations between whole body, head, oral, and upper extremity movement.

Principles of experience-dependent neural plasticity: implications for rehabilitation after brain damage

PURPOSE

This paper reviews 10 principles of experience-dependent neural plasticity and considerations in applying them to the damaged brain.

METHOD

Neuroscience research using a variety of models of learning, neurological disease, and trauma are reviewed from the perspective of basic neuroscientists but in a manner intended to be useful for the development of more effective clinical rehabilitation interventions.

RESULTS

Neural plasticity is believed to be the basis for both learning in the intact brain and relearning in the damaged brain that occurs through physical rehabilitation. Neuroscience research has made significant advances in understanding experience-dependent neural plasticity, and these findings are beginning to be integrated with research on the degenerative and regenerative effects of brain damage. The qualities and constraints of experience-dependent neural plasticity are likely to be of major relevance to rehabilitation efforts in humans with brain damage. However, some research topics need much more attention in order to enhance the translation of this area of neuroscience to clinical research and practice.

CONCLUSION

The growing understanding of the nature of brain plasticity raises optimism that this knowledge can be capitalized upon to improve rehabilitation efforts and to optimize functional outcome.

The effects of intra-oral pain on motor cortex neuroplasticity associated with short-term novel tongue-protrusion training in humans

To determine if short-term (15 min) training in a novel tongue-task is associated with rapid neuroplasticity of the tongue primary motor area (MI) in the human cerebral cortex, and if intra-oral tonic pain affects the tongue MI neuroplasticity and tongue-task training performance. Nine healthy volunteers (7 men, 2 women, mean age 24+/-1.1 years) participated in two cross-over training sessions in which the application to the tongue of the algesic chemical capsaicin (1%) or vehicle cream was randomized. Prior to and again immediately after 15 min of training in a tongue-protrusion task, transcranial magnetic stimulation (TMS) was applied to the MI in each session and motor evoked potentials (MEPs) were recorded in the tongue musculature and the first dorsal interosseous (FDI) muscle (as control). Neuroplasticity of the tongue MI, as reflected in a significantly enhanced TMS-MEP stimulus-response curve and reduced MEP threshold, was observed after the vehicle session but not after the capsaicin session. Subjects' overall mean performance scores were significantly higher in the vehicle session than in the capsaicin session. MI neuroplasticity may rapidly occur in association with successful performance in novel tongue-task training, but intra-oral tonic pain interferes with these effects. These findings suggest that nociceptive input modulates MI neuroplasticity associated with novel motor training and may impair the ability to learn a new motor task.

Plasticity of the synaptic modification range

Activity-dependent synaptic plasticity is likely to provide a mechanism for learning and memory. Cortical synaptic responses that are strengthened within a fixed synaptic modification range after 5 days of motor skill learning are driven near the top of their range, leaving only limited room for additional synaptic strengthening. If synaptic strengthening is a requisite step for acquiring new skills, near saturation of long-term potentiation (LTP) should impede further learning or the LTP mechanism should recover after single-task learning. Here we show that the initial learning-induced synaptic enhancement is sustained even long after training has been discontinued and that the synaptic modification range shifts upward. This range shift places increased baseline synaptic efficacy back within the middle of its operating range, allowing prelearning levels of LTP and long-term depression. Persistent synaptic strengthening might be a substrate for long-term retention in motor cortex, whereas the shift in synaptic modification range ensures the availability for new synaptic strengthening.

Reduction of seizure thresholds following electrical stimulation of sensorimotor cortex is dependent on stimulation intensity and is not related to synaptic potentiation

Epilepsy is characterized as a chronic brain state with a very low seizure threshold, and the occurrence of repeated seizure activity. Currently, there is no animal model of induced epilepsy that allows for the exploration of the brain mechanisms underlying a low seizure threshold without the elicitation of seizures. In this study, we employed repeated application of different intensities of electrical stimulation in an attempt to reduce afterdischarge (seizure) thresholds without eliciting seizures. We utilized an in vivo model of neocortical activation via stimulation of the corpus callosum of the adult rat. The intensities were chosen to be subthreshold (20, 30, 40, 50 microA), near threshold (150 microA), and suprathreshold (250, 500 microA) relative to the mean initial afterdischarge threshold (ADT). We also examined changes in the evoked field responses of the transcallosal pathway to the sensorimotor cortex as a measure of synaptic efficacy. Our results indicated that stimulation at 50 microA was effective at reducing the ADT, while minimizing the number of seizures elicited. Stimulation at 150 microA resulted in the concomitant reduction of ADT and repeated seizures typical of most electrical kindling studies. Finally, the 500 microA group showed repeated seizures, but no reduction of afterdischarge threshold. These stimulation intensities (50 microA, 150 microA, 500 microA and 0 microA-control) can be used to independently determine the brain mechanisms responsible for 1) the acquisition of a low afterdischarge threshold independent of the reorganizing effect of repeated seizures, and 2) the elicitation of repeated seizures independent of stimulation induced reduction of afterdischarge threshold.

Skill representation in the primary motor cortex after long-term practice

The acquisition of motor skills can lead to profound changes in the functional organization of the primary motor cortex (M1). For example, performance of movement sequences after prolonged practice is associated with an expansion of the effector representation in M1. Paradoxically, there is little evidence that the activity of M1 neurons reflects acquired skills, especially sequences of movements. We examined the activity of M1 neurons during skilled movement sequences in macaques trained to successively hit targets on a monitor. The targets appeared either pseudorandomly (Random mode) or in one of two repeating sequences (Repeating mode). With practice, response times for repeating sequences substantially declined and the monkeys performed the task predictively. Highly trained animals retained the acquired skill after long gaps in practice. After >2 yr of training, 40% of M1 neurons were differentially active during the two task modes. Variations in movement kinematics did not fully explain the task-dependent modulation of neuron activity. Differentially active neurons were more strongly influenced by task mode than by kinematics. Our results suggest that practice sculpts the response properties of M1 neurons. M1 may be a site of storage for the internal representation of skilled sequential movements.

Long-term depression and associativity in rat primary motor cortex following thalamic stimulation

Associativity is an attractive property of LTP in terms of its possible mechanism as a model for memory storage. In this study, we compare the effects of homosynaptic vs. associative stimulation on the induction of LTP and LTD in the neocortex of freely behaving rats. Using a callosal input to the motor cortex as a 'strong' input (one that potentiates reliably following homosynaptic stimulation), we paired activity of this pathway with a 'weak' thalamocortical pathway (one that does not potentiate when stimulated homosynaptically). Surprisingly, homosynaptic HFS caused a lasting depression of the field EPSP in the thalamocortical pathway. Analysis of this effect revealed that it was largely polysynaptic. Associative HFS (HFS applied to both pathways) not only failed to induce an LTP effect in the thalamocortical pathway, it increased the magnitude of the depression. Associative HFS did, however, facilitate LTP induction in the 'strong' callosal pathway. When comparing the effects of homosynaptic and associative LTD induction (HFS on one pathway anticorrelated with LFS on the other), we found that both protocols induced a similar magnitude of depression. These results show that HFS applied to the thalamocortical pathway causes a depression and this depression is enhanced, not reversed, by associative pairing with a strong input.

Parallel stages of learning and recovery of skilled reaching after motor cortex stroke: “Oppositions” organize normal and compensatory movements

Forelimb/hand motor cortex injury in rodents and primates causes impairments in skilled paw/hand movements that includes a period of movement absence followed by functional recovery/compensation. Although the postsurgical period of movement absence has been attributed to "shock" or "diaschisis", the behavior of animals during this period has not been fully described. Here, rats were trained to reach for single food pellets from a shelf and then the vasculature of the forelimb region of the sensorimotor cortex contralateral to the reaching limb was removed. A control group received a posterior parietal cortex devasularization. Frame-by-frame video analysis of reaching behavior showed that the stages of the acquisition of skilled reaching and the stages of recovery after motor cortex stroke were similar. The animals sequentially learn three relationships or "oppositions" between a body part and the food target. The oppositions are invariant relationships but each can be achieved with movements that can vary from reach to reach and between rats. A snout-pellet opposition organizes the movements of orienting, a paw-pellet opposition organizes limb transport and grasping the pellet in the digits, and a mouth-pellet opposition organizes limb withdrawal and the release of the food into the mouth. The three oppositions and the movements that they recruit were disrupted after motor cortex damage, but not parietal cortex damage. The oppositions were reestablished after stroke in the order in which they were acquired prior to stroke. Enduring impairments were more noticeable in transport and withdrawal oppositions. That the stages of recovery from motor cortex stroke parallel those of initial acquisition are discussed in relation to contemporary explanations of diaschisis and the contribution of motor cortex to motor learning.

Motor training induces experience-specific patterns of plasticity across motor cortex and spinal cord

The motor cortex and spinal cord possess the remarkable ability to alter structure and function in response to differential motor training. Here we review the evidence that the corticospinal system is not only plastic but that the nature and locus of this plasticity is dictated by the specifics of the motor experience. Skill training induces synaptogenesis, synaptic potentiation, and reorganization of movement representations within motor cortex. Endurance training induces angiogenesis in motor cortex, but it does not alter motor map organization or synapse number. Strength training alters spinal motoneuron excitability and induces synaptogenesis within spinal cord, but it does not alter motor map organization. All three training experiences induce changes in spinal reflexes that are dependent on the specific behavioral demands of the task. These results demonstrate that the acquisition of skilled movement induces a reorganization of neural circuitry within motor cortex that supports the production and refinement of skilled movement sequences. We present data that suggest increases in strength may be mediated by an increased capacity for activation and/or recruitment of spinal motoneurons while the increased metabolic demands associated with endurance training induce cortical angiogenesis. Together these results show the robust pattern of anatomic and physiological plasticity that occurs within the corticospinal system in response to differential motor experience. The consequences of such distributed, experience-specific plasticity for the encoding of motor experience by the motor system are discussed.

Hippocampal Kindling Leads to Motor Map Expansion

PURPOSE

To determine whether seizure activity, repeatedly elicited in the hippocampus, could alter the functional organization of neocortical movement representations (motor maps) and whether a relation exists between the number of afterdischarges recorded in the sensorimotor neocortex and the size of the motor maps.

METHODS

We electrically kindled the right ventral hippocampus of Long-Evans hooded rats, twice daily, for 40 sessions and recorded the afterdischarges in the stimulated hippocampus and right sensorimotor neocortex. Between 3 and 7 days after the last seizure, we used high-resolution intracortical microstimulation to derive the forelimb-movement representations in the left (un-implanted) sensorimotor neocortex.

RESULTS

In the hippocampal kindled rats, we observed a dramatic expansion of the area of neocortex that would elicit forelimb movements compared with sham-kindled controls. The number of afterdischarges recorded in the neocortex was significantly and positively correlated with the size of the motor maps.

CONCLUSIONS

Seizures propagating from the hippocampus have long-distance effects on the functional organization of motor maps.