Cortical stimulation improves skilled forelimb use following a focal ischemic infarct in the rat

Changes in somatosensory evoked potentials elicited by lateral cerebellar nucleus deep brain stimulation in the naïve rodent

Deep brain stimulation (DBS) of the deep cerebellar nuclei has been shown to enhance perilesional cortical excitability and promote motor rehabilitation in preclinical models of cortical ischemia and is currently being evaluated in patients with chronic, post-stroke deficits. Understanding the effects of cerebellar DBS on contralateral sensorimotor cortex may be key to developing approaches to optimize stimulation delivery and treatment outcomes. Using the naïve rat model, we characterized the effects of DBS of the lateral cerebellar nucleus (LCN) on somatosensory evoked potentials (SSEPs) and evaluated their potential use as a surrogate index of cortical excitability. SSEPs were recorded concurrently with continuous 30 Hz or 100 Hz LCN DBS and compared to the DBS OFF condition. Ratios of SSEP peak to peak amplitude during 100 Hz LCN DBS to DBS OFF at longer latency peaks were significantly>1, suggesting that cortical excitability was enhanced as a result of LCN DBS. Although changes in SSEP peak to peak amplitudes were observed, they were modest in relation to previously reported effects on motor cortical excitability. Overall, our findings suggest that LCN output influences thalamocortical somatosensory pathways, however further work is need to better understand the potential role of SSEPs in optimizing therapy.

Chronic muscle recordings reveal recovery of forelimb function in spinal injured female rats after cortical epidural stimulation combined with rehabilitation and chondroitinase ABC

Cervical level spinal cord injury (SCI) can severely impact upper limb muscle function, which is typically assessed in the clinic using electromyography (EMG). Here, we established novel preclinical methodology for EMG assessments of muscle function after SCI in awake freely moving animals. Adult female rats were implanted with EMG recording electrodes in bicep muscles and received bilateral cervical (C7) contusion injuries. Forelimb muscle activity was assessed by recording maximum voluntary contractions during a grip strength task and cortical motor evoked potentials in the biceps. We demonstrate that longitudinal recordings of muscle activity in the same animal are feasible over a chronic post-injury time course and provide a sensitive method for revealing post-injury changes in muscle activity. This methodology was utilized to investigate recovery of muscle function after a novel combination therapy. Cervical contused animals received intraspinal injections of a neuroplasticity-promoting agent (lentiviral-chondroitinase ABC) plus 11 weeks of cortical epidural electrical stimulation (3 h daily, 5 days/week) and behavioral rehabilitation (15 min daily, 5 days/week). Longitudinal monitoring of voluntary and evoked muscle activity revealed significantly increased muscle activity and upper limb dexterity with the combination treatment, compared to a single treatment or no treatment. Retrograde mapping of motor neurons innervating the biceps showed a predominant distribution across spinal segments C5-C8, indicating that treatment effects were likely due to neuroplastic changes in a mixture of intact and injured motor neurons. Thus, longitudinal assessments of muscle function after SCI correlate with skilled reach and grasp performance and reveal functional benefits of a novel combination therapy.

Neuroprotective Effects of Electrical Stimulation Following Ischemic Stroke in Non-Human Primates

Brain stimulation has emerged as a novel therapy for ischemic stroke, a major cause of brain injury that often results in lifelong disability. Although past works in rodents have demonstrated protective effects of stimulation following stroke, few of these results have been replicated in humans due to the anatomical differences between rodent and human brains and a limited understanding of stimulation-induced network changes. Therefore, we combined electrophysiology and histology to study the neuroprotective mechanisms of electrical stimulation following cortical ischemic stroke in non-human primates. To produce controlled focal lesions, we used the photothrombotic method to induce targeted vasculature damage in the sensorimotor cortices of two macaques while collecting electrocorticography (ECoG) signals bilaterally. In another two monkeys, we followed the same lesioning procedures and applied repeated electrical stimulation via an ECoG electrode adjacent to the lesion. We studied the protective effects of stimulation on neural dynamics using ECoG signal power and coherence. In addition, we performed histological analysis to evaluate the differences in lesion volume. In comparison to controls, the ECoG signals showed decreased gamma power across the sensorimotor cortices in stimulated animals. Meanwhile, Nissl staining revealed smaller lesion volumes for the stimulated group, suggesting that electrical stimulation may exert neuroprotection by suppressing post-ischemic neural activity. With the similarity between NHP and human brains, this study paves the path for developing effective stimulation-based therapy for acute stroke in clinical studies.

A multicenter, randomized, double-blind, placebo-controlled trial to test efficacy and safety of transcranial direct current stimulation to the motor cortex after stroke (NETS): study protocol

Introduction

The WHO estimates that each year 5 million people are left permanently disabled after stroke. Adjuvant treatments to promote the effects of rehabilitation are urgently needed. Cortical excitability and neuroplasticity can be enhanced by non-invasive brain stimulation but evidence from sufficiently powered, randomized controlled multi-center clinical trials is absent.

Methods

Neuroregeneration enhanced by transcranial direct current stimulation (tDCS) in stroke (NETS) tested efficacy and safety of anodal tDCS to the primary motor cortex of the lesioned hemisphere in the subacute phase (day 5–45) after cerebral ischemia. Stimulation was combined with standardized rehabilitative training and repeatedly applied in 10 sessions over a period of 2 weeks in a planned sample of 120 patients. Primary outcome parameter was upper-extremity function at the end of the 2-weeks intervention period of active treatment or placebo (1:1 randomization), measured by the upper-extremity Fugl-Meyer assessment. Sustainability of the treatment effect was evaluated by additional follow-up visits after 30 and 90 days. Further secondary endpoints included metrics of arm and hand function, stroke impact scale, and the depression module of the patient health questionnaire.

Perspective

NETS was aimed at providing evidence for an effective and safe adjuvant treatment for patients after stroke.

Trial registration: ClinicalTrials.gov Identifier NCT00909714. Registered May 28, 2009.

Role of the nucleus accumbens in functional recovery from spinal cord injury

Post brain damage depression impedes functional recovery. On the other hand, higher motivation facilitates functional recovery after damage to the central nervous system, but the neural mechanism of psychological effects on functional recovery is unclear. The nucleus accumbens (NAcc), a motivation center, has not been considered directly involved in motor function. Recently, it was demonstrated that the NAcc makes a direct contribution to motor performance after spinal cord injury by facilitating motor cortex activity. In this perspective, we first summarize our investigation of role of NAcc in motor control during the recovery course after spinal cord injury, followed by a discussion of the current knowledge regarding the relationship between the recovery and NAcc after neuronal damage.

Comparative Enhancement of Motor Function and BDNF Expression Following Different Brain Stimulation Approaches in an Animal Model of Ischemic Stroke

BACKGROUND

Combinatory intervention such as high-frequency (50-100 Hz) excitatory cortical stimulation (ECS) given concurrently with motor rehabilitative training (RT) improves forelimb function, except in severely impaired animals after stroke. Clinical studies suggest that low-frequency (≤1 Hz) inhibitory cortical stimulation (ICS) may provide an alternative approach to enhance recovery. Currently, the molecular mediators of CS-induced behavioral effects are unknown. Brain-derived neurotrophic factor (BDNF) has been associated with improved recovery and neural remodeling after stroke and thus may be involved in CS-induced behavioral recovery.

OBJECTIVE

To investigate whether inhibitory stimulation during RT improves functional recovery of severely impaired rats, following focal cortical ischemia and if this recovery alters BDNF expression (study 1) and depends on BDNF binding to TrkB receptors (study 2).

METHODS

Rats underwent ECS + RT, ICS + RT, or noCS + RT treatment daily for 3 weeks following a unilateral ischemic lesion to the motor cortex. Electrode placement for stimulation was either placed ipsilateral (ECS) or contralateral (ICS) to the lesion. After treatment, BDNF expression was measured in cortical tissue samples (study 1). In study 2, the TrkB inhibitor, ANA-12, was injected prior to treatment daily for 21 days.

RESULTS

ICS + RT treatment significantly improved impaired forelimb recovery compared with ECS + RT and noCS + RT treatment.

CONCLUSION

ICS given concurrently with rehabilitation improves motor recovery in severely impaired animals, and alters cortical BDNF expression; nevertheless, ICS-mediated improvements are not dependent on BDNF binding to TrkB. Conversely, inhibition of TrkB receptors does disrupt motor recovery in ECS + RT treated animals.

Imaging Developmental and Interventional Plasticity Following Perinatal Stroke

Perinatal stroke occurs around the time of birth and leads to lifelong neurological disabilities including hemiparetic cerebral palsy. Magnetic resonance imaging (MRI) has revolutionized our understanding of developmental neuroplasticity following early injury, quantifying volumetric, structural, functional, and metabolic compensatory changes after perinatal stroke. Such techniques can also be used to investigate how the brain responds to treatment (interventional neuroplasticity). Here, we review the current state of knowledge of how established and emerging neuroimaging modalities are informing neuroplasticity models in children with perinatal stroke. Specifically, we review structural imaging characterizing lesion characteristics and volumetrics, diffusion tensor imaging investigating white matter tracts and networks, task-based functional MRI for localizing function, resting state functional imaging for characterizing functional connectomes, and spectroscopy examining neurometabolic changes. Key challenges and exciting avenues for future investigations are also considered.

Regenerative Rehabilitation for Stroke Recovery by Inducing Synergistic Effects of Cell Therapy and Neurorehabilitation on Motor Function: A Narrative Review of Pre-Clinical Studies

Neurological diseases severely affect the quality of life of patients. Although existing treatments including rehabilitative therapy aim to facilitate the recovery of motor function, achieving complete recovery remains a challenge. In recent years, regenerative therapy has been considered as a potential candidate that could yield complete functional recovery. However, to achieve desirable results, integration of transplanted cells into neural networks and generation of appropriate microenvironments are essential. Furthermore, considering the nascent state of research in this area, we must understand certain aspects about regenerative therapy, including specific effects, nature of interaction when administered in combination with rehabilitative therapy (regenerative rehabilitation), and optimal conditions. Herein, we review the current status of research in the field of regenerative therapy, discuss the findings that could hold the key to resolving the challenges associated with regenerative rehabilitation, and outline the challenges to be addressed with future studies. The current state of research emphasizes the importance of determining the independent effect of regenerative and rehabilitative therapies before exploring their combined effects. Furthermore, the current review highlights the progression in the treatment perspective from a state of compensation of lost function to that of a possibility of complete functional recovery.

[Brain stimulation for treating stroke-related motor deficits]

Functional recovery of stroke-related deficits is mainly achieved through neural reorganization. Neurorehabilitative approaches, therefore, aim at supporting positive processes while suppressing maladaptive neuronal processes. This review summarizes the main findings of studies using non-invasive and invasive brain stimulation with respect to the benefits of the treatment for motor deficits after stroke. In addition, the article discusses possible approaches to enhance the effectiveness of neuromodulatory approaches and thus improve the outcome of patients.

Optogenetic Stimulation Enhanced Neuronal Plasticities in Motor Recovery after Ischemic Stroke

Motor capability recovery after ischemic stroke involves dynamic remodeling processes of neural connectomes in the nervous system. Various neuromodulatory strategies combining direct stimulating interventions with behavioral trainings for motor recovery after ischemic stroke have been developed. However, the effectiveness of these interventions varies widely due to unspecific activation or inhibition of undefined neuronal subtypes. Optogenetics is a functional and structural connection-based approach that can selectively activate or inhibit specific subtype neurons with a higher precision, and it has been widely applied to build up neuronal plasticities of the nervous system, which shows a great potential in restoring motor functions in stroke animal models. Here, we reviewed neurobiological mechanisms of enhanced brain plasticities underlying motor recovery through the optogenetic stimulation after ischemic stroke. Several brain sites and neural circuits that have been previously proven effective for motor function rehabilitation were identified, which would be helpful for a more schematic understanding of effective neuronal connectomes in the motor function recovery after ischemic stroke.

Progress in the Field of Micro-Electrocorticography

Since the 1940s electrocorticography (ECoG) devices and, more recently, in the last decade, micro-electrocorticography (µECoG) cortical electrode arrays were used for a wide set of experimental and clinical applications, such as epilepsy localization and brain⁻computer interface (BCI) technologies. Miniaturized implantable µECoG devices have the advantage of providing greater-density neural signal acquisition and stimulation capabilities in a minimally invasive fashion. An increased spatial resolution of the µECoG array will be useful for greater specificity diagnosis and treatment of neuronal diseases and the advancement of basic neuroscience and BCI research. In this review, recent achievements of ECoG and µECoG are discussed. The electrode configurations and varying material choices used to design µECoG arrays are discussed, including advantages and disadvantages of µECoG technology compared to electroencephalography (EEG), ECoG, and intracortical electrode arrays. Electrode materials that are the primary focus include platinum, iridium oxide, poly(3,4-ethylenedioxythiophene) (PEDOT), indium tin oxide (ITO), and graphene. We discuss the biological immune response to µECoG devices compared to other electrode array types, the role of µECoG in clinical pathology, and brain⁻computer interface technology. The information presented in this review will be helpful to understand the current status, organize available knowledge, and guide future clinical and research applications of µECoG technologies.

Effects of Electric Cortical Stimulation (ECS) and Transcranial Direct Current Stimulation (tDCS) on Rats With a Traumatic Brain Injury

Objective

To evaluate the effects of electric cortical stimulation (ECS) and transcranial direct current stimulation (tDCS) on motor and cognitive function recovery and brain plasticity in focal traumatic brain injury (TBI) of rats model.

Methods

Forty rats were pre-trained to perform a single pellet reaching task (SPRT), rotarod test (RRT), and Y-maze test for 14 days, then a focal TBI was induced by a weight drop model on the motor cortex. All rats were randomly assigned to one of the three groups: anodal ECS (50 Hz and 194 μs) (ECS group), tDCS (0.1 mA, 50 Hz and 200 μs) (tDCS group), and no stimulation as a control group. Four-week stimulation, including rehabilitation, was started 3 days after the operation. SPRT, RRT, and Y-maze were measured from day 1 to day 28 after the TBI was induced. Histopathological and immunohistochemistry staining evaluations were performed at 4 weeks.

Results

SPRT was improved from day 7 to day 26 in ECS, and from day 8 to day 26 in tDCS compared to the control group (p<0.05). SPRT of ECS group was significantly improved on days 3, 8, 9, and 17 compared to the tDCS group. Y-maze was improved from day 8 to day 16 in ECS, and on days 6, 12, and 16 in the tDCS group compared to the control group (p<0.05). Y-maze of the ECS group was significantly improved on day 9 to day 15 compared to the tDCS group. The c-Fos protein expression was better in the ECS group and the tDCS group compared to the control group.

Conclusion

Electric stimulation in rats modified with a focal TBI is effective for motor recovery and brain plasticity. ECS induced faster behavioral and cognitive improvements compared to tDCS during the recovery period of rats with a focal TBI.

Deep brain stimulation of the cerebellum for poststroke motor rehabilitation: from laboratory to clinical trial

Ischemic stroke is a leading cause of disability worldwide, with profound economic costs. Poststroke motor impairment is the most commonly encountered deficit resulting in significant disability and is the primary driver of stroke-associated healthcare expenditures. Although many patients derive some degree of benefit from physical rehabilitation, a significant proportion continue to suffer from persistent motor impairment. Noninvasive brain stimulation, vagal nerve stimulation, epidural cortical stimulation, and deep brain stimulation (DBS) have all been studied as potential modalities to improve upon the benefits derived from physical therapy alone. These neuromodulatory therapies aim primarily to augment neuroplasticity and drive functional reorganization of the surviving perilesional cortex. The authors have proposed a novel and emerging therapeutic approach based on cerebellar DBS targeted at the dentate nucleus. Their rationale is based on the extensive reciprocal connectivity between the dentate nucleus and wide swaths of cerebral cortex via the dentatothalamocortical and corticopontocerebellar tracts, as well as the known limitations to motor rehabilitation imposed by crossed cerebellar diaschisis. Preclinical studies in rodent models of ischemic stroke have shown that cerebellar DBS promotes functional recovery in a frequency-dependent manner, with the most substantial benefits of the therapy noted at 30-Hz stimulation. The improvements in motor function are paralleled by increased expression of markers of synaptic plasticity, synaptogenesis, and neurogenesis in the perilesional cortex. Given the findings of preclinical studies, a first-in-human trial, Electrical Stimulation of the Dentate Nucleus Area (EDEN) for Improvement of Upper Extremity Hemiparesis Due to Ischemic Stroke: A Safety and Feasibility Study, commenced in 2016. Although the existing preclinical evidence is promising, the results of this Phase I trial and subsequent clinical trials will be necessary to determine the future applicability of this therapy.

Motor cortex stimulation does not lead to functional recovery after experimental cortical injury in rats

BACKGROUND

Motor impairments are among the major complications that develop after cortical damage caused by either stroke or traumatic brain injury. Motor cortex stimulation (MCS) can improve motor functions in animal models of stroke by inducing neuroplasticity.

OBJECTIVE

In the current study, the therapeutic effect of chronic MCS was assessed in a rat model of severe cortical damage.

METHODS

A controlled cortical impact (CCI) was applied to the forelimb area of the motor cortex followed by implantation of a flat electrode covering the lesioned area. Forelimb function was assessed using the Montoya staircase test and the cylinder test before and after a period of chronic MCS. Furthermore, the effect of MCS on tissue metabolism and lesion size was measured using [18F]-fluorodesoxyglucose (FDG) μPET scanning.

RESULTS

CCI caused a considerable lesion at the level of the motor cortex and dorsal striatum together with a long-lasting behavioral phenotype of forelimb impairment. However, MCS applied to the CCI lesion did not lead to any improvement in limb functioning when compared to non-stimulated control rats. Also, MCS neither changed lesion size nor distribution of FDG.

CONCLUSION

The use of MCS as a standalone treatment did not improve motor impairments in a rat model of severe cortical damage using our specific treatment modalities.

A foldable electrode array for 3D recording of deep-seated abnormal brain cavities

OBJECTIVE

This study describes the design and microfabrication of a foldable thin-film neural implant and investigates its suitability for electrical recording of deep-lying brain cavity walls.

APPROACH

A new type of foldable neural electrode array is presented, which can be inserted through a cannula. The microfabricated electrode is specifically designed for electrical recording of the cavity wall of thalamic lesions resulting from stroke. The proof-of-concept is demonstrated by measurements in rat brain cavities. On implantation, the electrode array unfolds in the brain cavity, contacting the cavity walls and allowing recording at multiple anatomical locations. A three-layer microfabrication process based on UV-lithography and Reactive Ion Etching is described. Electrochemical characterization of the electrode is performed in addition to an in vivo experiment in which the implantation procedure and the unfolding of the electrode are tested and visualized.

MAIN RESULTS

Electrochemical characterization validated the suitability of the electrode for in vivo use. CT imaging confirmed the unfolding of the electrode in the brain cavity and analysis of recorded local field potentials showed the ability to record neural signals of biological origin.

SIGNIFICANCE

The conducted research confirms that it is possible to record neural activity from the inside wall of brain cavities at various anatomical locations after a single implantation procedure. This opens up possibilities towards research of abnormal brain cavities and the clinical conditions associated with them, such as central post-stroke pain.

Cerebral ischemia and neuroregeneration

Cerebral ischemia is one of the leading causes of morbidity and mortality worldwide. Although stroke (a form of cerebral ischemia)-related costs are expected to reach 240.67 billion dollars by 2030, options for treatment against cerebral ischemia/stroke are limited. All therapies except anti-thrombolytics (i.e., tissue plasminogen activator) and hypothermia have failed to reduce neuronal injury, neurological deficits, and mortality rates following cerebral ischemia, which suggests that development of novel therapies against stroke/cerebral ischemia are urgently needed. Here, we discuss the possible mechanism(s) underlying cerebral ischemia-induced brain injury, as well as current and future novel therapies (i.e., growth factors, nicotinamide adenine dinucleotide, melatonin, resveratrol, protein kinase C isozymes, pifithrin, hypothermia, fatty acids, sympathoplegic drugs, and stem cells) as it relates to cerebral ischemia.

Regenerative Rehabilitation: Combining Stem Cell Therapies and Activity-Dependent Stimulation

The number of clinical trials in regenerative medicine is burgeoning, and stem cell/tissue engineering technologies hold the possibility of becoming the standard of care for a multitude of diseases and injuries. Advances in regenerative biology reveal novel molecular and cellular targets, with potential to optimize tissue healing and functional recovery, thereby refining rehabilitation clinical practice. The purpose of this review is to (1) highlight the potential for synergy between the fields of regenerative medicine and rehabilitation, a convergence of disciplines known as regenerative rehabilitation; (2) provide translational examples of regenerative rehabilitation within the context of neuromuscular injuries and diseases; and (3) offer recommendations for ways to leverage activity dependence via combined therapy and technology, with the goal of enhancing long-term recovery. The potential clinical benefits of regenerative rehabilitation will likely become a critical aspect in the standard of care for many neurological and musculoskeletal disorders.

Advancing non-invasive neuromodulation clinical trials in children: Lessons from perinatal stroke

Applications of non-invasive brain stimulation including therapeutic neuromodulation are expanding at an alarming rate. Increasingly established scientific principles, including directional modulation of well-informed cortical targets, are advancing clinical trial development. However, high levels of disease burden coupled with zealous enthusiasm may be getting ahead of rational research and evidence. Experience is limited in the developing brain where additional issues must be considered. Properly designed and meticulously executed clinical trials are essential and required to advance and optimize the potential of non-invasive neuromodulation without risking the well-being of children and families. Perinatal stroke causes most hemiplegic cerebral palsy and, as a focal injury of defined timing in an otherwise healthy brain, is an ideal human model of developmental plasticity. Advanced models of how the motor systems of young brains develop following early stroke are affording novel windows of opportunity for neuromodulation clinical trials, possibly directing neuroplasticity toward better outcomes. Reviewing the principles of clinical trial design relevant to neuromodulation and using perinatal stroke as a model, this article reviews the current and future issues of advancing such trials in children.

Sensory-parietal cortical stimulation improves motor recovery in severe capsular infarct

The prevalence of subcortical white matter strokes in elderly patients is on the rise, but these patients show mixed responses to conventional rehabilitative interventions. To examine whether cortical electrical stimulation can promote motor recovery after white matter stroke, we delivered stimulation to a small or wide region of sensory-parietal cortex for two weeks in a rodent model of circumscribed subcortical capsular infarct. The sham-operated group (SOG) showed persistent and severe motor impairments together with decreased activation in bilateral sensorimotor cortices and striatum. In contrast, sensory-parietal cortex stimulation significantly improved motor recovery: final recovery levels were 72.9% of prelesion levels in the wide stimulation group (WSG) and 37% of prelesion levels in the small stimulation group (SSG). The microPET imaging showed reversal of cortical diaschisis in both groups: in both hemispheres for the WSG, and in the hemisphere ipsilateral to stimulation in the SSG. In addition, we observed activation of the corpus callosum and subcortical corticostriatal structures after stimulation. The results from the c-Fos mapping study were grossly consistent with the microPET imaging. Sensory-parietal cortex stimulation may therefore be a useful strategy for overcoming the limits of rehabilitative training in patients with severe forms of subcortical capsular infarct.

Optogenetic Approaches to Target Specific Neural Circuits in Post-stroke Recovery

Stroke is a leading cause of death and disability in the USA, yet treatment options are very limited. Functional recovery can occur after stroke and is attributed, in part, to rewiring of neural connections in areas adjacent to or remotely connected to the infarct. A better understanding of neural circuit rewiring is thus an important step toward developing future therapeutic strategies for stroke recovery. Because stroke disrupts functional connections in peri-infarct and remotely connected regions, it is important to investigate brain-wide network dynamics during post-stroke recovery. Optogenetics is a revolutionary neuroscience tool that uses bioengineered light-sensitive proteins to selectively activate or inhibit specific cell types and neural circuits within milliseconds, allowing greater specificity and temporal precision for dissecting neural circuit mechanisms in diseases. In this review, we discuss the current view of post-stroke remapping and recovery, including recent studies that use optogenetics to investigate neural circuit remapping after stroke, as well as optogenetic stimulation to enhance stroke recovery. Multimodal approaches employing optogenetics in conjunction with other readouts (e.g., in vivo neuroimaging techniques, behavior assays, and next-generation sequencing) will advance our understanding of neural circuit reorganization during post-stroke recovery, as well as provide important insights into which brain circuits to target when designing brain stimulation strategies for future clinical studies.

Brain stimulation: Neuromodulation as a potential treatment for motor recovery following traumatic brain injury

There is growing evidence that electrical and magnetic brain stimulation can improve motor function and motor learning following brain damage. Rodent and primate studies have strongly demonstrated that combining cortical stimulation (CS) with skilled motor rehabilitative training enhances functional motor recovery following stroke. Brain stimulation following traumatic brain injury (TBI) is less well studied, but early pre-clinical and human pilot studies suggest that it is a promising treatment for TBI-induced motor impairments as well. This review will first discuss the evidence supporting brain stimulation efficacy derived from the stroke research field as proof of principle and then will review the few studies exploring neuromodulation in experimental TBI studies. This article is part of a Special Issue entitled SI:Brain injury and recovery.

Enduring Poststroke Motor Functional Improvements by a Well-Timed Combination of Motor Rehabilitative Training and Cortical Stimulation in Rats

BACKGROUND

In animal stroke models, peri-infarct cortical stimulation (CS) combined with rehabilitative reach training (RT) enhances motor functional outcome and cortical reorganization, compared with RT alone. It was unknown whether the effects of CS + RT (a) persist long after treatment, (b) can be enhanced by forcing greater use of the paretic limb, and (C) vary with treatment onset time.

OBJECTIVE

To test the endurance, time sensitivity, and the potential for augmentation by forced forelimb use of CS + RT treatment effects following ischemic stroke.

METHODS

Adult rats that were proficient in skilled reaching received unilateral ischemic motor cortical lesions. RT was delivered for 3 weeks alone or concurrently with 100-Hz cathodal epidural CS, delivered at 50% of movement thresholds. In study 1, this treatment was initiated at 14 days postinfarct, with some subgroups receiving an overlapping period of continuous constraint of the nonparetic forelimb to force use of the paretic limb. The function of the paretic limb was assessed weekly for 9 to 10 months posttreatment. In study 2, rats underwent CS, RT, and the combination during the chronic postinfarct period.

RESULTS

Early onset CS + RT resulted in greater functional improvements than RT alone. The CS-related gains persisted for 9 to 10 months posttreatment and were not significantly influenced by forced use of the paretic limb. When treatment onset was delayed until 3 months post-infarct, RT alone improved function, but CS + RT was no more effective than RT alone.

CONCLUSION

CS can enhance the persistence, as well as the magnitude of RT-driven functional improvements, but its effectiveness in doing so may vary with time postinfarct.

Enhanced Motor Recovery After Stroke With Combined Cortical Stimulation and Rehabilitative Training Is Dependent on Infarct Location

BACKGROUND

Cortical electrical stimulation of the motor cortex in combination with rehabilitative training (CS/RT) has been shown to enhance motor recovery in animal models of focal cortical stroke, yet in clinical trials, the effects are much less robust. The variability of stroke location in human patient populations that include both cortical and subcortical brain regions may contribute to the failure to find consistent effects clinically.

OBJECTIVE

This study sought to determine whether infarct location influences the enhanced motor recovery previously observed in response to CS/RT. The efficacy of CS/RT to promote improvements in motor function was examined in 2 different rat models of stroke that varied the amount and location of cortical and subcortical damage.

METHODS

Ischemic infarctions were induced by injecting the vasoconstricting peptide endothelin-1 either (1) onto the middle cerebral artery (MCA) producing damage to the frontal cortex and lateral striatum or (2) into a subcortical region producing damage to the posterior thalamus and internal capsule (subcortical capsular ischemic injury [SCII]). Daily CS/RT or RT alone was then given for 20 days, during which time performance on a skilled reaching task was assessed.

RESULTS

Animals with MCA occlusion infarctions exhibited enhanced improvements on a skilled reaching task in response to CS/RT relative to RT alone. No such enhancement was observed in animals with SCII infarctions across the 20 days of treatment.

CONCLUSIONS

The efficacy of CS for enhancing motor recovery after stroke may depend in part on the extent and location of the ischemic infarct.

Effects of Subdural Monopolar Cortical Stimulation Paired With Rehabilitative Training on Behavioral and Neurophysiological Recovery After Cortical Ischemic Stroke in Adult Squirrel Monkeys

BACKGROUND

Cortical stimulation (CS) combined with rehabilitative training (RT) has proven effective for enhancing poststroke functional recovery in rats, but human clinical trials have had mixed outcomes.

OBJECTIVE

To assess the efficacy of CS/RT versus RT in a nonhuman primate model of cortical ischemic stroke.

METHODS

Squirrel monkeys learned a pellet retrieval task, then received an infarct to the distal forelimb (DFL) representation of primary motor cortex. A subdural monopolar electrode was implanted over the spared DFL representation in dorsal premotor cortex (PMD). Seven weeks postinfarct, monkeys underwent 4 to 6 weeks of RT (n = 8) or CS/RT (n = 7; 100 Hz, cathodal current) therapy. Behavioral performance was assessed before and after infarct, prior to therapy, and 1 and 12 weeks posttherapy (follow-up). The primary outcome measure was motor performance at 1 week posttherapy. Secondary outcomes included follow-up performance at 12 weeks and treatment-related changes in neurophysiological maps of spared DFL representations.

RESULTS

While postinfarct performance deficits were found in all monkeys, both groups demonstrated similar recovery profiles, with no difference in motor recovery between the RT and CS/RT groups. Posttherapy, PMD DFL area was significantly expanded in the RT group but not the CS/RT group. A significant relationship was found between motor recovery and DFL expansion in premotor cortex.

CONCLUSIONS

Results suggest that the specific parameters utilized here were not optimal for promoting behavioral recovery in nonhuman primates. Though CS/RT has consistently shown efficacy in rat stroke models, the present finding has cautionary implications for translation of CS/RT therapy to clinical populations.

Motor System Reorganization After Stroke: Stimulating and Training Toward Perfection

Stroke instigates regenerative responses that reorganize connectivity patterns among surviving neurons. The new connectivity patterns can be suboptimal for behavioral function. This review summarizes current knowledge on post-stroke motor system reorganization and emerging strategies for shaping it with manipulations of behavior and cortical activity to improve functional outcome.

Cortical Stimulation Concurrent With Skilled Motor Training Improves Forelimb Function and Enhances Motor Cortical Reorganization Following Controlled Cortical Impact

BACKGROUND

Electrical and magnetic brain stimulation can improve motor function following stroke in humans, rats, and nonhuman primates, especially when paired with rehabilitative training (RT). Previously, we found in rodent stroke models that epidural electrical cortical stimulation (CS) of the ipsilesional motor cortex (MC) combined with motor RT enhances motor function and motor cortical plasticity. It was unknown whether CS following experimental traumatic brain injury (TBI) would have similar effects.

OBJECTIVE

To test the effects of CS combined with motor training after moderate/severe TBI on behavioral outcome and motor cortical organization.

METHODS

Following unilateral controlled cortical impact (CCI) over the caudal forelimb area of the MC in adult male rats, forelimb reach training was administered daily for 9 weeks concurrently with subthreshold, 100-Hz monopolar CS or no-stimulation control procedures. The rate and magnitude of behavioral improvements and changes in forelimb movement representations in the injured MC as revealed by intracortical microstimulation were measured.

RESULTS

CCI resulted in severe motor impairments persisting throughout the 9 weeks of training in both groups, but CS-treated animals had significantly greater behavioral improvements. CS also increased wrist motor cortical representation, one of the main movements used in the training task, when compared with RT alone. However, the overall recovery level was modest, leaving animals still extremely impaired.

CONCLUSIONS

These data suggest that CS may be useful for improving rehabilitation efficacy after TBI but also raise the possibility that the CS parameters that are highly effective following stroke are suboptimal after moderate/severe TBI.

Modulation of Cortical Motor Evoked Potential After Stroke During Electrical Stimulation of the Lateral Cerebellar Nucleus

BACKGROUND

Deep brain stimulation (DBS) targeting the dentato-thalamo-cortical (DTC) pathway at its origin in the lateral cerebellar nucleus (LCN) has been shown to enhance motor recovery in a rodent model of cortical ischemia. LCN DBS also yielded frequency-specific changes in motor cortex excitability in the normal brain, indexed by motor evoked potential (MEP) amplitude.

OBJECTIVE

To investigate the effect of cortical stroke on cortical motor excitability in a rodent ischemia model and to measure the effects of LCN DBS on post-ischemia excitability as a function of stimulation parameters.

METHODS

Adult Sprague-Dawley rats were divided into two groups: naïve and stroke, with cortical ischemia induced through multiple, unilateral endothelin-1 injections. All animals were implanted with a bipolar electrode in the LCN opposite the affected hemisphere. MEPs were elicited from the affected hemisphere using intracortical microstimulation (ICMS) techniques. Multiple LCN DBS parameters were examined, including isochronal stimulation at 20, 30, 50, and 100 Hz as well as a novel burst stimulation pattern.

RESULTS

ICMS-evoked MEPs were reduced in stroke (n = 10) relative to naïve (n = 12) animals. However, both groups showed frequency-dependent augmentation of cortical excitability in response to LCN DBS. In the naïve group, LCN DBS increased MEPs by 22-58%, while in the stroke group, MEPs were enhanced by 9-41% compared to OFF-DBS conditions.

CONCLUSIONS

Activation of the DTC pathway increases cortical excitability in both naïve and post-stroke animals. These effects may underlie, at least partially, functional reorganization and therapeutic benefits associated with chronic LCN DBS in post-stroke animals.

Effect of Epidural Electrical Stimulation and Repetitive Transcranial Magnetic Stimulation in Rats With Diffuse Traumatic Brain Injury

OBJECTIVE

To evaluate the effects of epidural electrical stimulation (EES) and repetitive transcranial magnetic stimulation (rTMS) on motor recovery and brain activity in a rat model of diffuse traumatic brain injury (TBI) compared to the control group.

METHODS

Thirty rats weighing 270-285 g with diffuse TBI with 45 kg/cm(2) using a weight-drop model were assigned to one of three groups: the EES group (ES) (anodal electrical stimulation at 50 Hz), the rTMS group (MS) (magnetic stimulation at 10 Hz, 3-second stimulation with 6-second intervals, 4,000 total stimulations per day), and the sham-treated control group (sham) (no stimulation). They were pre-trained to perform a single-pellet reaching task (SPRT) and a rotarod test (RRT) for 14 days. Diffuse TBI was then induced and an electrode was implanted over the dominant motor cortex. The changes in SPRT success rate, RRT performance time rate and the expression of c-Fos after two weeks of EES or rTMS were tracked.

RESULTS

SPRT improved significantly from day 8 to day 12 in the ES group and from day 4 to day 14 in the MS group (p<0.05) compared to the sham group. RRT improved significantly from day 6 to day 11 in ES and from day 4 to day 9 in MS compared to the sham group. The ES and MS groups showed increased expression of c-Fos in the cerebral cortex compared to the sham group.

CONCLUSION

ES or MS in a rat model of diffuse TBI can be used to enhance motor recovery and brain activity.

Translating the science into practice: shaping rehabilitation practice to enhance recovery after brain damage

The revolution in neuroscience provided strong evidence for learning-dependent neuroplasticity and presaged the role of motor learning as essential for restorative therapies after stroke and other disabling neurological conditions. The scientific basis of motor learning has continued to evolve from a dominance of cognitive or information processing perspectives to a blend with neural science and contemporary social-cognitive-psychological science, which includes the neural and psychological underpinnings of motivation. This transformation and integration across traditionally separate domains is timely now that clinician scientists are developing novel, evidence-based therapies to maximize motor recovery in the place of suboptimal solutions. We will review recent evidence pertaining to therapeutic approaches that spring from an integrated framework of learning-dependent neuroplasticity along with the growing awareness of protocols that directly address the patient's fundamental psychological needs. Of importance, there is mounting evidence that when the individual's needs are considered in the context of instructions or expectations, the learning/rehabilitation process is accelerated.

Dose of arm activity training during acute and subacute rehabilitation post stroke: a systematic review of the literature

AIM

To determine the dose of activity-related arm training undertaken by stroke survivors during acute and subacute rehabilitation.

METHODS

A systematic review of PubMed, CINAHL and EMBASE up to December 2014 was completed. Studies were eligible if they defined the dose (time or repetitions) of activity-related arm training using observational methods for a cohort of adult stroke survivors receiving acute or subacute rehabilitation. All studies were quality appraised using an evidence-based learning critical appraisal checklist. Data was analysed by method of documented dose per session (minutes, repetitions), environment (acute or subacute rehabilitation) and therapy discipline (physiotherapy, occupational therapy).

RESULTS

Ten studies were included: two observed stroke survivors during acute rehabilitation and eight during subacute rehabilitation. During acute rehabilitation, one study reported 4.1 minutes per session during physiotherapy and 11.2 minutes during occupational therapy, while another study reported 5.7 minutes per session during physiotherapy only. During inpatient rehabilitation, activity-related arm training was on average undertaken for 4 minutes per session (range 0.9 to 7.9, n = 4 studies) during physiotherapy and 17 minutes per session (range 9.3 to 28.9, n = 3 studies) during occupational therapy. Repetitions per session were reported by two studies only during subacute rehabilitation. One study reported 23 repetitions per session during physiotherapy and occupational therapy, while another reported 32 repetitions per session across both disciplines.

CONCLUSION

The dose of activity-related arm training during acute and subacute rehabilitation after stroke is limited.

An animal study to examine the effects of the bilateral, epidural cortical stimulation on the progression of amyotrophic lateral sclerosis

Background

We examined the effects of the unilateral cortical stimulation on the survival of neurons showing degenerative changes and compared those in delaying the progression of amyotrophic lateral sclerosis (ALS) between the unilateral cortical stimulation and the bilateral one in an animal experimental model using mice.

Methods

We used 19 G93A transgenic mice and randomly divided into three groups: the control group (n = 6) (the implantation of electrodes in the bilateral motor cortex without electrical stimulation), the unilateral stimulation group (n = 7) (the implantation of electrodes in the unilateral motor cortex with a 24-hour cortical stimulation) and the bilateral stimulation group (n = 6) (the implantation of electrodes in the bilateral motor cortex with a 24-hour cortical stimulation).

Results

The mean survival period was significantly longer in the bilateral stimulation group as compared with the control group (124.33 ± 11.00 days vs. 109.50 ± 10.41 days) (P < 0.05). In addition, on postoperative weeks 11, 12, 13, 14 and 15, the mean Rota-rod score was significantly higher in the unilateral stimulation group as compared with the control group (P < 0.05). Furthermore, despite a lack of statistical significance, it was the lowest in the bilateral stimulation group on postoperative weeks 13, 14, 15 and 17. On postoperative weeks 11, 12, 13, 14 and 16, the mean score of paw-grip endurance was significantly higher in the unilateral stimulation group as compared with the control group (P < 0.05). Furthermore, despite a lack of statistical significance, it was the lowest in the bilateral stimulation group on postoperative weeks 13, 14, 15 and 17.

Conclusions

In conclusion, our results indicate that the bilateral epidural cortical stimulation might have a treatment effect in a murine model of ALS. But it is the limitation that we examined a small number of experimental animals. Further studies are therefore warranted to establish our results and to identify the optimal parameters of the epidural cortical stimulation in a larger number of experimental animals.

Electronic supplementary material

The online version of this article (doi:10.1186/1743-0003-11-139) contains supplementary material, which is available to authorized users.

Neurostimulation for traumatic brain injury

Traumatic brain injury (TBI) remains a significant public health problem and is a leading cause of death and disability in many countries. Durable treatments for neurological function deficits following TBI have been elusive, as there are currently no FDA-approved therapeutic modalities for mitigating the consequences of TBI. Neurostimulation strategies using various forms of electrical stimulation have recently been applied to treat functional deficits in animal models and clinical stroke trials. The results from these studies suggest that neurostimulation may augment improvements in both motor and cognitive deficits after brain injury. Several studies have taken this approach in animal models of TBI, showing both behavioral enhancement and biological evidence of recovery. There have been only a few studies using deep brain stimulation (DBS) in human TBI patients, and future studies are warranted to validate the feasibility of this technique in the clinical treatment of TBI. In this review, the authors summarize insights from studies employing neurostimulation techniques in the setting of brain injury. Moreover, they relate these findings to the future prospect of using DBS to ameliorate motor and cognitive deficits following TBI.

Invasive neurostimulation in stroke rehabilitation

The last decade has seen a growing interest in adjuvant treatments that synergistically influence mechanisms underlying rehabilitation of paretic upper limb in stroke. One such approach is invasive neurostimulation of spared cortices at the periphery of a lesion. Studies in animals have shown that during training of paretic limb, adjuvant stimulation targeting the peri-infarct circuitry enhances mechanisms of its reorganization, generating functional advantage. Success of early animal studies and clinical reports, however, failed to translate to a phase III clinical trial. As lesions in humans are diffuse, unlike many animal models, peri-infarct circuitry may not be a feasible, or consistent target across most. Instead, alternate mechanisms, such as changing transcallosal inhibition between hemispheres, or reorganization of other viable regions in motor control, may hold greater potential. Here, we review comprehensive mechanisms of clinical recovery and factors that govern which mechanism(s) become operative when. We suggest novel approaches that take into account a patient's initial clinical-functional state, and findings from neuroimaging and neurophysiology to guide to their most suitable mechanism for ideal targeting. Further, we suggest new localization schemes, and bypass strategies that indirectly target peri-lesional circuitry, and methods that serve to counter technical and theoretical challenge in identifying and stimulating such targets at the periphery of infarcts in humans. Last, we describe how stimulation may modulate mechanisms differentially across varying phases of recovery- a temporal effect that may explain missed advantage in clinical trials and help plan for the next stage. With information presented here, future trials would effectively be able to target patient's specific mechanism(s) with invasive (or noninvasive) neurostimulation for the greatest, most consistent benefit.

Chronic 30-Hz deep cerebellar stimulation coupled with training enhances post-ischemia motor recovery and peri-infarct synaptophysin expression in rodents

BACKGROUND

Over 500,000 Americans have strokes every year, making stroke the leading cause for disability in the United States and in the industrialized world. New treatments to improve poststroke motor recovery are needed.

OBJECTIVE

To investigate a novel approach for enhancing motor recovery that involves chronic, electrical stimulation of ascending cerebellar output combined with motor training.

METHODS

Adult Sprague-Dawley rats underwent unilateral endothelin-1 injections in the dominant cerebral cortex and placement of a chronic stimulating electrode in the contralateral lateral cerebellar nucleus. After 1 week, the animals were separated into 2 groups (STIM+ and STIM-), matched for poststroke motor performance in the pasta matrix task. At 2 weeks post-ischemia, the treatment phase was initiated, with animals in the STIM+ group receiving pulsed, 30-Hz stimulation for 12 hours/day. Motor training continued for both groups over 3 to 5 weeks.

RESULTS

A total of 23 animals were examined after 3 weeks of treatment. STIM+ animals showed a significant improvement in motor function compared with post-ischemia baseline performance as well as in comparison with the STIM- group. Immunohistochemistry revealed a significant increase in the perilesional expression of synaptophysin for the STIM+ vs the STIM- animals.

CONCLUSION

These results indicate that chronic activation of ascending cerebellofugal pathways enhances motor recovery after focal cortical ischemia. The recovery was associated with an increase in perilesional cortical plasticity relative to nontreated controls.

Adaptive changes in the motor cortex during and after longterm forelimb immobilization in adult rats

Experimental and clinical studies have attempted to evaluate the changes in cortical activity seen after immobilization-induced longterm sensorimotor restriction, although results remain controversial. We used intracortical microstimulation (ICMS), which provides topographic movement representations of the motor areas in both hemispheres with optimal spatial characterization, combined with behavioural testing to unravel the effects of limb immobilization on movement representations in the rat primary motor cortex (M1). Unilateral forelimb immobilization in rats was achieved by casting the entire limb and leaving the cast in place for 15 or 30 days. Changes in M1 were bilateral and specific for the forelimb area, but were stronger in the contralateral-to-cast hemisphere. The threshold current required to evoke forelimb movement increased progressively over the period in cast, whereas the forelimb area size decreased and the non-excitable area size increased. Casting resulted in a redistribution of proximal/distal movement representations: proximal forelimb representation increased, whereas distal representation decreased in size. ICMS after cast removal showed a reversal of changes, which remained partial at 15 days. Local application of the GABAA-antagonist bicuculline revealed the impairment of cortical synaptic connectivity in the forelimb area during the period of cast and for up to 15 days after cast removal. Six days of rehabilitation using a rotarod performance protocol after cast removal did not advance map size normalization in the contralateral-to-cast M1 and enabled the cortical output towards the distal forelimb only in sites that had maintained their excitability. These results are relevant to our understanding of adult M1 plasticity during and after sensorimotor deprivation, and to new approaches to conditions that require longterm limb immobilization.

Harnessing plasticity for the treatment of neurosurgical disorders: an overview

Plasticity is fundamental to normal central nervous system function and its response to injury. Understanding this adaptive capacity is central to the development of novel surgical approaches to neurologic disease. These innovative interventions offer the promise of maximizing functional recovery for patients by harnessing targeted plasticity. Developing novel therapies will require the unprecedented integration of neuroscience, bioengineering, molecular biology, and physiology. Such synergistic approaches will create therapeutic options for patients previously outside of the scope of neurosurgery, such as those with permanent disability after traumatic brain injury or stroke. In this review, we synthesize the rapidly evolving field of plasticity and explore ways that neurosurgeons may enhance functional recovery in the future. We conclude that understanding plasticity is fundamental to modern neurosurgical education and practice.

Ministrokes in channelrhodopsin-2 transgenic mice reveal widespread deficits in motor output despite maintenance of cortical neuronal excitability

We evaluated the effects of ministrokes targeted to individual pial arterioles on motor function in Thy-1 line 18 channelrhodopsin-2 (ChR2) transgenic mice within the first hours after ischemia. Using optogenetics, we directly assessed both the excitability and motor output of cortical neurons in a manner independent of behavioral state or training. Occlusion of individual arterioles within the motor cortex led to a ministroke that was verified using laser speckle contrast imaging. Surprisingly, ministrokes targeted to a relatively small region of the forelimb motor map, with an ischemic core of 0.07 ± 0.03 mm(2), impaired motor responses evoked from points across widespread areas of motor cortex even 1.5 mm away. Contrasting averaged ChR2-evoked electroencephalographic, spinal (ChR2 evoked potential), and electromyographic responses revealed a mismatch between measures of cortical excitability and motor output within 60 min after stroke. This mismatch suggests that apparently excitable cortical neurons (even >1 mm into peri-infarct areas, away from the infarct core) were impaired in their capacity to generate spinal potentials leading to even more severe deficits in motor output at muscles. We suggest that ischemia, targeted to a subset of motor cortex, leads to relatively small reductions in excitability within motor cortex, and cumulative depression of both descending spinal circuits and motor output in response to the activation of widespread cortical territories even outside of the area directly affected by the ischemia.

Use of transcranial magnetic stimulation of the brain in stroke rehabilitation

Preliminary studies suggest that stimulation of the motor cortex enhances motor recovery after stroke. Most of these studies employed transcranial magnetic stimulation of the brain and two different approaches have been evaluated. The first approach is based on the use of protocols of stimulation that increase cortical excitability, targeting the hemisphere in which the stroke occurred in order to enhance the output of the motor cortex and the response to physiotherapy. The second approach is based on the use of protocols of stimulation that suppress cortical excitability, targeting the intact hemisphere in order to counteract the imbalance due to the increased interhemispheric inhibition onto the lesioned cortex, and reducing the potential negative interference of the intact hemisphere with the function of the affected one. Cumulatively, preliminary studies suggest that transcranial magnetic stimulation might be a suitable method to combine with physiotherapy and improve recovery of useful limb function in stroke patients. However, further studies are needed to determine the best stimulation parameters and how to select patients who are likely to respond to this treatment.

Searching for the principles of brain plasticity and behavior

An important development in behavioral neuroscience in the past 25 years has been the demonstration that the brain is far more flexible in structure and function than was previously believed. Studies of laboratory animals have provided an important tool for understanding the nature of brain plasticity and behavior at many levels ranging from detailed behavioral paradigms, electrophysiology, neuronal morphology, protein chemistry, and epigenetics. Here we seek a synthesis of the multidisciplinary work on brain plasticity and behavior to identify some general principles on how the brain changes in response to a wide range of experiences over the lifetime.

In vitro and in vivo neuronal electrotaxis: a potential mechanism for restoration?

Electrical brain stimulation used to treat a variety of neurological and psychiatric diseases is entering a new period. The technique is well established and the potential complications are well known and generally manageable. Recent studies demonstrated that electrical fields (EFs) can enhance neuroplasticity-related processes. EFs applied in the physiological range induce migration of different neural cell types from different species in vitro. There are some evidences that also the speed and directedness of cell migration are enhanced by EFs. However, it is still unclear how electrical signals from the extracellular space are translated into intracellular actions resulting in the so-called electrotaxis phenomenon. Here, we aim to provide a comprehensive review of the data on responses of cells to electrical stimulation and the relation to functional recovery.

High γ power in ECoG reflects cortical electrical stimulation effects on unit activity in layers V/VI

OBJECTIVE

Cortical electrical stimulation (CES) has been used extensively in experimental neuroscience to modulate neuronal or behavioral activity, which has led this technique to be considered in neurorehabilitation. Because the cortex and the surrounding anatomy have irregular geometries as well as inhomogeneous and anisotropic electrical properties, the mechanism by which CES has therapeutic effects is poorly understood. Therapeutic effects of CES can be improved by optimizing the stimulation parameters based on the effects of various stimulation parameters on target brain regions.

APPROACH

In this study we have compared the effects of CES pulse polarity, frequency, and amplitude on unit activity recorded from rat primary motor cortex with the effects on the corresponding local field potentials (LFP), and electrocorticograms (ECoG). CES was applied at the surface of the cortex and the unit activity and LFPs were recorded using a penetrating electrode array, which was implanted below the stimulation site. ECoGs were recorded from the vicinity of the stimulation site.

MAIN RESULTS

Time-frequency analysis of LFPs following CES showed correlation of gamma frequencies with unit activity response in all layers. More importantly, high gamma power of ECoG signals only correlated with the unit activity in lower layers (V-VI) following CES. Time-frequency correlations, which were found between LFPs, ECoGs and unit activity, were frequency- and amplitude-dependent.

SIGNIFICANCE

The signature of the neural activity observed in LFP and ECoG signals provides a better understanding of the effects of stimulation on network activity, representative of large numbers of neurons responding to stimulation. These results demonstrate that the neurorehabilitation and neuroprosthetic applications of CES targeting layered cortex can be further improved by using field potential recordings as surrogates to unit activity aimed at optimizing stimulation efficacy. Likewise, the signatures of unit activity observed as changes in high gamma power in ECoGs suggest that future cortical stimulation studies could rely on less invasive feedback schemes that incorporate surface stimulation with ECoG reporting of stimulation efficacy.