BACKGROUND:Loss of hand control is a major source of poststroke disability, particularly when infarcts involve subcortical white matter. Yet, most preclinical models target the cortex and infer recovery from task performance without quantifying whether improvements reflect restoration or adaptation. To address this, we developed a novel nonhuman primate model of focal internal capsule infarct. This study is exploratory and hypothesis-testing, including prespecified hypotheses regarding motor impairment and adaptation. METHODS:Three adult male rhesus macaques (Macaca mulatta, 5-10 years) received stereotactically guided ET-1 (endothelin-1) injections (0.5 µg/µL; 9-10 µL total) into the left posterior limb of the internal capsule in a within-subject pre-infarct versus post-infarct design. Infarcts were characterized by magnetic resonance imaging and postmortem histology. Motor function was tested with a center-out joystick task and a Klüver board task using standard behavioral metrics; hand posture was additionally scored by blinded raters to quantify adaptation. Pre-infarct versus post-infarct comparisons used generalized linear mixed models with per-session random effects; adaptation covariates were evaluated using likelihood-ratio tests. RESULTS:Magnetic resonance imaging and histology confirmed focal infarcts centered on the posterior limb of the internal capsule, with variable volumes. All animals significantly reduced contralesional hand use post-infarct (Fisher exact test; all P<0.05). Generalized linear mixed models showed reduced center-out success rates in 2 animals (P<0.01) and increased path length in one (P=0.0015). Pooled generalized linear mixed models showed significant post-infarct increases in retrieval time (estimate=0.73 s; P<0.0001) and digit flexions (estimate=0.727; P<0.0001). Two animals developed a compensatory wrist-extended, digit-flexed posture by 4 weeks; augmenting generalized linear mixed models with these postural features significantly improved fit (likelihood-ratio tests; P<0.0001). CONCLUSIONS:This model recapitulates key features of human subcortical stroke, including persistent distal motor deficits, shifts in hand preference, systematic motor adaptation, and interindividual variability, establishing a translationally oriented platform for studying stroke mechanisms and evaluating recovery-promoting interventions.

Stroke is a leading cause of disability, causing chronic motor impairments in many survivors. Although recovery is correlated with cortical reorganization, the impact of lesion location on post-stroke reorganization is uncertain. We compared movement-related neural activity following experimental infarcts to the motor cortex (M1) or internal capsule in rats. Neural activity was recorded from motor and somatosensory regions during a skilled pellet retrieval task longitudinally during the course of recovery. Cortical lesions resulted in early behavioral recovery accompanied by widespread reductions in neural activity across ipsilesional regions, indicative of large-scale reorganization. In contrast, internal capsule lesions produced delayed recovery with no evidence of long-term ipsilesional cortical reorganization. These findings challenge the relevance of cortical reorganization for recovery from subcortical lesions and indicate that post-stroke recovery mechanisms are lesion-specific and that models targeting subcortical white matter are essential for maximizing translational relevance.

Loss of distal hand and finger control is among the most disabling consequences of stroke. Functional outcomes are typically worse when infarcts involve subcortical white matter tracts, particularly the internal capsule, yet most preclinical stroke models target cortical regions. To address this gap, we developed a non-human primate model of internal capsule infarct using stereotactically guided endothelin-1 injections to disrupt descending fibers from the primary motor cortex hand area. Serial structural and diffusion MRI, along with histology, confirmed subcortical infarcts centered on the targeted white matter region with no apparent cortical involvement. Motor function was assessed pre- and post-infarct using a joystick-based center-out task (proximal forelimb control) and a Klüver board task (distal forelimb control). Animals exhibited variable impairments in proximal function and consistent post-infarct deficits in distal function, including reduced contralesional hand use, longer retrieval time, and increased in-well digit flexions. One animal showed mild post-infarct impairment and the smallest lesion, highlighting that this model reflects inter-individual differences in infarct size and functional outcome as seen in human subcortical stroke. In contrast, the other two animals developed a compensatory wrist-extended posture on the Klüver board task by 4 weeks post-infarct, which stabilized the hand and enabled improved digit flexion. Incorporating this behavioral adaptation into statistical models improved prediction of motor performance. The observed adaptation may have drawn on spared corticospinal output pathways, allowing animals to re-engage pre-existing motor routines to perform the retrieval. While future studies may benefit from ethologically relevant tasks to further elucidate such adaptations, findings from this study recapitulate key features of human subcortical stroke, including persistent distal motor deficits and emergence of adaptive motor strategies. By combining precise lesioning, longitudinal imaging, and detailed behavioral analysis, this model provides a translationally oriented platform for studying white matter stroke mechanisms and evaluating interventions that promote functional recovery.

Non-human primates (NHPs) are crucial models for studies of neuronal activity. Emerging photoacoustic imaging modalities offer excellent tools for studying NHP brains with high sensitivity and high spatial resolution. In this research, a photoacoustic microscopy (PAM) device was used to provide a label-free quantitative characterization of cerebral hemodynamic changes due to peripheral mechanical stimulation. A 5 × 5 mm area within the somatosensory cortex region of an adult squirrel monkey was imaged. A deep, fully connected neural network was characterized and applied to the PAM images of the cortex to enhance the vessel structures after mechanical stimulation on the forelimb digits. The quality of the PAM images was improved significantly with a neural network while preserving the hemodynamic responses. The functional responses to the mechanical stimulation were characterized based on the improved PAM images. This study demonstrates capability of PAM combined with machine learning for functional imaging of the NHP brain.

Background Cortical electrical stimulation has been a versatile technique for examining the structure and function of cortical regions as well as for implementing novel therapies. While electrical stimulation has been used to examine the local spread of neural activity, it may also enable longitudinal examination of mesoscale interregional connectivity. Recent studies have used focal intracortical microstimulation (ICMS) with optical imaging to show cross-region spread of neural activity, but techniques are limited to utilizing hemodynamic responses within anesthetized preparations. Objective Here, we sought to use ICMS in conjunction with recordings of multi-unit action potentials to assess the mesoscale effective connectivity within sensorimotor cortex. Methods Neural recordings were made from multielectrode arrays placed into sensory, motor, and premotor regions during surgical experiments in three squirrel monkeys. During each recording, single-pulse ICMS was repeatably delivered to a single region. Mesoscale effective connectivity was calculated from ICMS-evoked changes in multi-unit firing. Results Multi-unit action potentials were able to be detected on the order of 1 ms after each ICMS pulse. Across sensorimotor regions, short-latency (< 2.5 ms) ICMS-evoked neural activity strongly correlated with known anatomic connections. Additionally, ICMS-evoked responses remained stable across the experimental period, despite small changes in electrode locations and anesthetic state. Conclusions These results show that monitoring ICMS-evoked neural activity, in a technique we refer to as Stimulation-Evoked Effective Connectivity (SEEC), is a viable way to longitudinally assess effective connectivity enabling studies comparing the time course of connectivity changes with the time course of changes in behavioral function. Highlights Short-latency neural responses to ICMS were evaluated in multiple cortical regions. Neural responses strongly correlated with known anatomical connections. Stimulation-evoked neural responses were maintained across repeated tests. ICMS-evoked activity can show longitudinal changes in effective connectivity.

Background: Cortical stimulation has been a versatile technique for examining the structure and function of cortical regions as well as for implementing novel therapies. While stimulation has been used to examine the local spread of neural activity, it may also enable longitudinal examination of mesoscale interregional connectivity. Recent studies have used focal intracortical microstimulation with optical imaging to show cross-region spread of neural activity, but the exact neural mechanisms elucidated by these modalities is uncertain. Objective: Here, we sought to use intracortical microstimulation (ICMS) in conjunction with recordings of multi-unit action potentials to assess the mesoscale effective connectivity within sensorimotor cortex. Methods: Neural recordings were made from multielectrode arrays placed into sensory, motor, and premotor regions during surgical experiments in three squirrel monkeys. During each recording, single-pulse ICMS was repeatably delivered to a single region. Mesoscale effective connectivity was calculated from ICMS-evoked changes in multi-unit firing. Results: Multi-unit action potentials were able to be detected on the order of 1 ms after each ICMS pulse. Across sensorimotor regions, short-latency (<2.5 ms) ICMS-evoked neural activity strongly correlated with known anatomic connections. Additionally, ICMS-evoked responses remained stable across the experimental period, in spite of small changes in electrode locations and anesthetic state. Conclusions: These results show that monitoring ICMS-evoked neural activity is a viable way to longitudinally assess effective connectivity, enabling studies comparing the time course of connectivity changes with the time course of changes in behavioral function.

BackgroundTechnological advances in developing experimentally controlled models of traumatic brain injury (TBI) are prevalent in rodent models and these models have proven invaluable in characterizing temporal changes in brain and behavior after trauma. To date no long-term studies in non-human primates (NHPs) have been published using an experimentally controlled impact device to follow behavioral performance over time.New methodWe have employed a controlled cortical impact (CCI) device to create a focal contusion to the hand area in primary motor cortex (M1) of three New World monkeys to characterize changes in reach and grasp function assessed for 3 months after the injury.ResultsThe CCI destroyed most of M1 hand representation reducing grey matter by 9.6 mm3, 12.9 mm3, and 15.5 mm3 and underlying corona radiata by 7.4 mm3, 6.9 mm3, and 5.6 mm3 respectively. Impaired motor function was confined to the hand contralateral to the injury. Gross hand-use was only mildly affected during the first few days of observation after injury while activity requiring skilled use of the hand was impaired over three months.Comparison with existing method(s)This study is unique in establishing a CCI model of TBI in an NHP resulting in persistent impairments in motor function evident in volitional use of the hand.ConclusionsEstablishing an NHP model of TBI is essential to extend current rodent models to the complex neural architecture of the primate brain. Moving forward this model can be used to investigate novel therapeutic interventions to improve or restore impaired motor function after trauma.

Background: The neurophysiological effects of transcranial direct current stimulation (tDCS) are typically described with respect to changes in cortical excitability, defined by using transcranial magnetic stimulation pulses to determine changes in motor evoked potentials. However, how individual cortical neurons change firing patterns under the influence of tDCS is largely unknown. While the relatively weak currents produced in the brain by tDCS may not be adequate to directly depolarize neuronal membranes, ongoing neuronal activity, combined with subthreshold changes in membrane polarization might be sufficient to alter the threshold for neural firing. Objectives: The purpose of this study was to determine the effects of tDCS on neurophysiological activity in motor cortex of freely moving, healthy rats. Methods: In nine healthy, ambulatory rats, each studied under six different stimulation conditions varying in current intensity (maximum current density = 39.8 A/m2 at 0.4 mA) and polarity (anodal or cathodal), neural activity was analyzed in response to 20 min of tDCS applied through bone screws insulated from the overlying scalp. Results: After analysis of 480 multi-unit channels that satisfied a rigid set of neurophysiological criteria, we found no systematic effect of tDCS stimulation condition on firing rate or firing pattern. Restricting the analysis to the most responsive units, subtle, but statistically significant changes occurred only in the highest intensity anodal condition. Conclusions: These results confirm that at current densities typically used in human or animal tDCS studies, observed effects of tDCS are likely to occur via mechanisms other than direct neuronal depolarization.

Unilateral limb movements are generally understood to be driven by the cortical hemisphere opposite to the moving limb, but there is increasing evidence that the hemisphere on the same side as the limb is also active during movements. These same-sided motor activations are robustly observed across numerous recording modalities and species and complex movement information can be decoded from the neural population activity. Ipsilateral motor activity has been proposed to represent a number of functions, including interhemispheric inhibition, maintaining an efferent copy of the state of the ipsilateral limb, and contributing to the planning and execution of voluntary movements. Growing evidence supports a complex role of the ipsilateral hemisphere in motor control, with the specific contributions varying according to task demands and timing. Whereas voluntary movements have long been understood to derive primarily from the cortical hemisphere contralateral to a moving limb, substantial cortical activations also occur in the same-sided, or ipsilateral, cortical hemisphere. These ipsilateral motor activations have recently been shown to be useful to decode specific movement features. Furthermore, in contrast to the classical understanding that unilateral limb movements are solely driven by the contralateral hemisphere, it appears that the ipsilateral hemisphere plays an active and specific role in the planning and execution of voluntary movements. Here we review the movement-related activations observed in the ipsilateral cortical hemisphere, interpret this evidence in light of the potential roles of the ipsilateral hemisphere in the planning and execution of movements, and describe the implications for clinical populations. Whereas voluntary movements have long been understood to derive primarily from the cortical hemisphere contralateral to a moving limb, substantial cortical activations also occur in the same-sided, or ipsilateral, cortical hemisphere. These ipsilateral motor activations have recently been shown to be useful to decode specific movement features. Furthermore, in contrast to the classical understanding that unilateral limb movements are solely driven by the contralateral hemisphere, it appears that the ipsilateral hemisphere plays an active and specific role in the planning and execution of voluntary movements. Here we review the movement-related activations observed in the ipsilateral cortical hemisphere, interpret this evidence in light of the potential roles of the ipsilateral hemisphere in the planning and execution of movements, and describe the implications for clinical populations. refers to on object on the opposite side of the body to the reference object. The cortical hemisphere on the opposite side of the body to a moving limb has the strongest neuroanatomical connections to the moving muscles and generates the majority of the outflow. on the opposite side of the body to a unilateral cortical lesion, such as a stroke or traumatic brain injury. The contralesional cortical hemisphere is the uninjured hemisphere and the contralesional limbs have more significant functional impairments. an internal cortical representation of the outflowing (efferent) motor command being executed by the limbs. electrodes used to record neural activity directly from the surface of the brain. ECoG recordings represent the summation of all ionic processes (i.e., fast action potentials, synaptic potentials, dendritic potentials, etc.) within a local area surrounding an electrode. Clinical electrodes are typically a few millimeters in diameter with an interelectrode spacing on the order of 1 cm. Because of their location directly on the cortical surface, ECoG signals record more focal representations of neural activity than electrodes placed on the surface of the scalp. electrodes used to record neural activity at the scalp. While noninvasive, EEG recordings suffer from poor spatial and spectral resolution because neural signals must past through the skull and scalp. recordings of the electrical activity associated with muscle contractions. a measurement of neural activity made using an MRI scanner. The MRI scanner is used to detect changes in local blood flow that are associated with increases and decreases in neural activity. refers to on object on the same side of the body as the reference object. The cortical hemisphere on the same side of the body as a moving limb has fewer direct neuroanatomical connections to the moving muscles than the contralateral hemisphere. on the same side of the body as a unilateral cortical lesion such as a stroke or traumatic brain injury. The ipsilesional cortical hemisphere is the injured hemisphere and the ipsilesional limbs are less impaired by the cortical lesion. the specific organization of different body parts. Within the motor cortex, representations of body parts are organized with lower-body representations in the most medial portions of the motor cortex, face and mouth representations in the most lateral portions of the motor cortex, and arm and hand representations located in between.

Whereas voluntary movements have long been understood to derive primarily from the cortical hemisphere contralateral to a moving limb, substantial cortical activations also occur in the same-sided, or ipsilateral, cortical hemisphere. These ipsilateral motor activations have recently been shown to be useful to decode specific movement features. Furthermore, in contrast to the classical understanding that unilateral limb movements are solely driven by the contralateral hemisphere, it appears that the ipsilateral hemisphere plays an active and specific role in the planning and execution of voluntary movements. Here we review the movement-related activations observed in the ipsilateral cortical hemisphere, interpret this evidence in light of the potential roles of the ipsilateral hemisphere in the planning and execution of movements, and describe the implications for clinical populations.

HomeStrokeVol. 50, No. 9Preclinical Studies of Neuroplasticity Following Experimental Brain Injury Free AccessReview ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessReview ArticlePDF/EPUBPreclinical Studies of Neuroplasticity Following Experimental Brain InjuryAn Update David T. Bundy, PhD and Randolph J. Nudo, PhD David T. BundyDavid T. Bundy From the Department of Rehabilitation Medicine (D.T.B., R.J.N.), University of Kansas Medical Center, Kansas City, KS. and Randolph J. NudoRandolph J. Nudo Correspondence to Randoph J. Nudo, PhD, Department of Rehabilitation Medicine, University of Kansas Medical Center, Kansas City, KS. Email E-mail Address: [email protected] From the Department of Rehabilitation Medicine (D.T.B., R.J.N.), University of Kansas Medical Center, Kansas City, KS. Landon Center on Aging (R.J.N.), University of Kansas Medical Center, Kansas City, KS. Originally published8 Aug 2019https://doi.org/10.1161/STROKEAHA.119.023550Stroke. 2019;50:2626–2633Other version(s) of this articleYou are viewing the most recent version of this article. Previous versions: August 8, 2019: Ahead of Print A significant challenge in the care of stroke survivors is chronic functional deficits that limit activities of daily living. Despite substantial functional recovery that can occur in the initial weeks following a stroke, over 50% of stroke survivors exhibit hemiparesis 6 months after stroke, which is particularly significant because motor recovery plateaus after 3–6 months.1 Because of the compelling need to reduce the burden of these chronic deficits, there is great interest in understanding the specific relationships between neuroplasticity and functional recovery. Although fully understanding these mechanisms will necessarily require clinical research in human cohorts, studies utilizing experimental animal models of brain injury are valuable because preclinical studies provide the ability to induce repeatable lesions, to control the specific environmental exposures that animals receive including the amount of rehabilitative training, to test novel therapies or medications, and to examine the structural and functional changes in cortical organization at different time points. Here, we review recent advances that have informed our understanding of neuroplasticity following experimental models of brain injury. Initially, we will review the impact of rehabilitation on functional recovery. Next, we will describe the functional and structural reorganization that is associated with rehabilitation and discuss the functional relevance of specific patterns of reorganization. Finally, we will conclude by describing preclinical evidence for several novel therapies designed to enhance neuroplasticity that span various stages of development towards clinical trials.Clinical Relevance of Experimental Models of Brain InjuryOne important caveat about preclinical models of stroke is that, with few exceptions, these models do not replicate the conditions of human stroke as a clinical entity and thus suffer to some extent from a lack of face validity. Except in unusual models that are infrequently used, spontaneous strokes simply do not occur in the cohort of otherwise healthy, relatively young laboratory animals utilized in preclinical stroke research. Anecdotally, in 3 decades of utilizing nonhuman primates to study the effects of ischemic lesions on sensorimotor function, performing behavioral assessment and postmortem lesion documentation on over 270 squirrel monkeys, we have not documented a single incident of a spontaneously occurring stroke. We contend that the value of preclinical models of focal ischemia does not stem from face validity per se. Rather, these studies are valuable for informing human stroke recovery research as they (1) allow hypothesis testing on basic mechanisms of neuroplasticity and the relationship of plasticity to the capacity for functional recovery, and (2) often demonstrate predictive validity, with motor impairments and recovery profiles that parallel the human recovery trajectory. Nonetheless, extending the predictive ability of such models to the translation of the effects of therapeutic interventions (drugs, devices, behavioral interventions) after ischemic injury remains a significant challenge to the field.To interpret neuroplasticity following experimental models of brain injury, it is important to consider the implications of the differences between the common animal models used and human stroke survivors. The majority of preclinical studies have used either rodent or nonhuman primate models. Importantly, both rodents and primates possess primary and secondary cortical motor regions, and substantial efforts have been made to understand functional and structural reorganization in secondary motor areas resulting from damage to primary motor areas. Assuming that secondary reorganization is functionally adaptive, such studies of postinjury cortical plasticity may yield important insights into the capacity of the remaining motor system to respond to therapeutic interventions and support functional recovery. Although a growing body of evidence supports neuroplasticity in subcortical brain and spinal cord structures after cortical lesions, the focus of this review is on reorganization in ipsilesional (same side as injury) and contralesional (intact hemisphere) cerebral cortex after focal ischemic lesions. Furthermore, advances in methods for modeling subcortical ischemic lesions (capsular, thalamic, striatal, and their combinations) have improved our ability to model more forms of clinical stroke strictly from the standpoint of stroke anatomy. These studies are largely beyond the scope of the present review. Finally, as the vast majority of preclinical studies have focused on the effects of focal ischemic lesions on forelimb function, we limit this review to neuroplasticity in spared cortical areas involved in the control of skilled forelimb function.Rodents possess 2 cortical areas associated with the control of skilled forelimb movements. The caudal forelimb area (CFA) is the equivalent of the primate primary motor cortex (M1) and abuts the primary somatosensory forelimb area on its caudal border. A second motor area, the rostral forelimb area (RFA), is located more rostrally and has been regarded as a premotor area based on similarities in connectivity and neuronal responses associated with movement and premovement planning.2 Although the presence of a secondary premotor region plays a role in the ability of rodents to perform complex grasping movements with the distal forepaw, nonhuman primates provide a better model of the human motor system with the ability to produce even more complex behaviors. This ability to perform complex movements is facilitated in part by the presence of multiple segregated secondary motor regions including the dorsal premotor cortex and ventral premotor cortex (PMv), the supplemental motor area, and the cingulate motor areas (Figure 1).3 Along with the structural and functional organization of cortical motor areas, it is important to consider the roles of the various descending pathways that facilitate motor control. For example, there are interspecies differences among nonhuman primates in the presence of monosynaptic connections within the corticospinal tract projecting to the ventral horn of the cervical spinal cord, where motoneurons innervating the hand originate.4 These structural traits are thought to underlie species-specific differences in distal forelimb dexterity.4 Additionally, the rubrospinal and reticulospinal pathways may play a greater role in motor control in nonhuman primates and rodents compared with humans, and these pathways may provide alternative substrates for recovery of function following experimental brain injuries to a much larger degree than in humans.5,6 Although corticospinal neurons originate from multiple cortical areas, a high concentration are found in M1 and PMv of primates as well as CFA and RFA of rodents.7 Taken together, although the clinical implications of experiments using animal models must be considered with care, particularly with regards to distal forelimb function, the presence of multiple distinct cortical areas makes both rodents and nonhuman primates excellent model species to examine the interaction and reorganization of these regions after brain injury, as might occur after stroke.Download figureDownload PowerPointFigure 1. Sensorimotor system organization. Both rodents (left) and nonhuman primates (right) have multiple differentiated sensory and motor regions. The rodent cortical motor system includes the caudal forelimb area (CFA, rodent M1) and the rostral forelimb area (RFA, rodent premotor cortex). Nonhuman primates possess multiple premotor areas including the dorsal and ventral premotor cortices (PMd and PMv) the supplementary motor area (SMA) and the cingulate motor areas (Cing). The rodent and nonhuman primate cortical sensory regions include the primary somatosensory cortex (S1) and secondary somatosensory cortex (S2) and parietal ventral area (PV). Although corticospinal neurons originate in multiple regions, a high concentration of corticospinal neurons is found in M1 and PMv of nonhuman primates and CFA and RFA of rodents.Rehabilitation Can Improve Functional RecoveryAlthough a significant number of human stroke survivors are left with chronic motor deficits, rehabilitation has the capacity to improve motor recovery, even in chronic stroke survivors.8 Rehabilitative training can also improve motor function following experimental lesions in animal models. For example, after lesion of the internal capsule in rodents, forced use of the impaired forelimb improved motor function with the impaired limb.9 Similarly, rehabilitative training, typically consisting of extensive use of the impaired limb for performance of a skilled forelimb task, can improve performance on the trained task in mice and rats.10–13In spite of the fact that rehabilitative training can improve function after some experimental ischemic injuries, such training alone is insufficient to restore the ability to perform skilled reaching tasks following more extensive injuries.14,15 Recent studies have shown that environmental factors, such as being housed in an enriched environment, can improve the effectiveness of rehabilitative training. For example, following large lesions to the CFA in rats (either by an induced ischemic lesion or via a controlled cortical impact), whereas neither rehabilitative training or housing in an enriched environment alone improved functional recovery relative to control animals, the combination of an enriched environment with rehabilitation did lead to significant improvements in functional recovery.14,15 In these studies, the enriched environment often includes housing in a larger cage and with multiple animals, as well as access to additional physical items including running wheels or climbing structures.16 Therefore, the additional improvement associated with housing in an enriched environment could be attributed to several adjuvant factors, including additional socialization, physical exercise, or cognitive improvements.Although therapies, such as rehabilitation, can lead to improvements in motor function, different types and time-courses of activity are not necessarily equally beneficial for motor recovery. First, the functional benefits of rehabilitative training are time-dependent. Immediately after the injury, the brain is in a susceptible state, and rehabilitative training can lead to decreases in motor function.17 Furthermore, greatly exaggerated use of the impaired limb within the first week can result in expansion of the injury.18 Following this acute period, the brain enters a subacute period of enhanced plasticity in which rehabilitation is more effective.19 Finally, after this critical period of enhanced plasticity ends, there is a more stable, chronic period in which functional recovery is possible but diminished. Interestingly, after this critical period of enhanced rehabilitation has closed, a second ischemic injury distant to the initial injury can reopen this critical window of enhanced plasticity, suggesting that the critical window is specifically tied to the molecular correlates of ischemia.19 Along with changes in the efficacy of rehabilitation over time, specific behaviors impact the effectiveness of rehabilitative training. For example, increased practice with the nonparetic limb can lead to decreases in the function of the paretic limb and can impede the functional benefits of a later period of training using the paretic limb.20Cortical Reorganization Is Associated With Functional RecoveryStudies demonstrating functional outcomes resulting from behavioral training and environmental enrichment can inform the general mechanisms of cortical plasticity. But it is also important to understand the specific patterns of functional and structural reorganization that occur concurrently with functional recovery and are thought to play a role in mediating this recovery. Although the most commonly used experimental stroke model in rodents is the intraluminal suture model of middle cerebral artery occlusion, because of the large and variable size of lesions and functional deficits associated with this model, the majority of studies examining cortical reorganization have utilized focal lesion models which are the focus of our discussion here.21Functional Reorganization of Local and Distant Motor AreasSeveral studies have examined neuroplasticity following experimental lesions by tracking the reorganization of motor regions using intracortical microstimulation. Early studies in a nonhuman primate model of small, focal ischemic lesions showed that although the spared distal forelimb representation in M1 was decreased in size without rehabilitative training, rehabilitation prevented this postinjury loss of distal representation.22 Functional recovery was also associated with enlargements of distal forelimb representations in PMv following lesions made within the distal forelimb region of M1.23 Larger cortical injuries that included both M1 and PMv resulted in expansion of the supplemental motor area distal forelimb representation.24 Along with changes in intracortical microstimulation maps, increases in activity in local and distant regions have been shown in functional imaging studies. Specifically, following lesions to M1 of macaques made with ibotenic acid, activity in the ipsilesional PMv is increased beginning within 1 to 2 months after the lesion with increased functional connectivity within the ipsilesional M1 detected at 3 to 4 months postlesion.25In addition to nonhuman primates, changes in the functional organization of the motor system have also been observed in rodent models. Optogenetic mapping of the motor cortex of the mouse shows displacement of motor representations following small lesions to the primary motor cortex.26 Following larger lesions to CFA (the rodent M1) in the rat, rehabilitation leads to increases in the size of the distal forelimb representation in the ipsilesional RFA (the rodent premotor cortex).10,11,27 Although the majority of studies have focused on examining reorganization within the ipsilesional hemisphere, several studies have also examined changes in the motor representations in the contralesional hemisphere. After recovery from a cortical lesion, the 2 hemispheres appear to interact as rehabilitative training with the impaired limb decreases the size of the contralesional CFA relative to healthy controls or lesioned animals that did not receive training.28 However, maladaptive training with the unimpaired forepaw decreases the size of the perilesional forepaw representation.20 Reorganization within the contralesional hemisphere after recovery is also impacted by the size of the lesion. When large and small ischemic lesions were induced in the motor cortex of rodents, increased lesion size correlates with the postrecovery size of RFA in both hemispheres.27 Although larger lesions are correlated with larger increases in the size of RFA bilaterally, only the size of the contralesional RFA correlates with the level of functional recovery.27 Taken together, whereas the optimal motor recovery may be associated with a normalization of motor representations in the ipsilesional hemisphere, the contralesional hemisphere may play an increased role in recovery following larger lesions that cause more substantial motor deficits.Structural Reorganization of the Motor SystemAlong with alterations in the functional organization of motor areas, recovery of function has also been associated with structural changes in the same motor regions that likely drive the observed changes in functional organization described above. Following a lesion, axonal sprouting forms new intracortical connections within perilesional areas of the ipsilesional hemisphere as well as new corticospinal and corticorubral connections stemming from both the ipsilesional and contralesional hemispheres.9,11,29–31 Recent studies have also found that rehabilitation can lead to increased dendritic density within perilesional areas of M1.13,20,32 When examined using transmission electron microscopy to determine the specific synaptic architecture, it has been found that although both maladaptive and adaptive training can increase the synaptic density in perilesional regions of M1, there were specific differences between the 2 conditions. After maladaptive training with the nonparetic limb, increased synaptic density was driven by increases in axodendritic synapses with an increased number of multisynaptic boutons.20 After adaptive training, however, the density of perforated synapses which were often associated with perisynaptic astrocytes was increased.32 Therefore, rehabilitation may lead to improved function by encouraging the maturation of appropriate synapses. When considering the changes in the structure of the cortex after a lesion, it is important to recognize that ischemic and mechanical lesions are associated with different types of structural changes.33 Therefore, the specific molecular mechanisms associated with an experimental lesion likely play important roles in the patterns of reorganization that are associated with recovery.Functional and Structural Reorganization Plays a Causal Role in Recovered Motor SkillsThe observation of structural and functional reorganization in animals that experience postinjury motor recovery after rehabilitative training suggests a causal role of both the perilesional M1 and the premotor areas in the recovered motor function. However, the observation of reorganization alone is not sufficient to determine whether these regions play a direct causal role in the recovered motor function. Several studies have used inactivation of the reorganized motor areas to test whether the M1 and ipsilesional premotor cortices are necessary for recovered motor function. First, anisomycin, a protein inhibitor that disrupts synapses, disrupts recovered function when injected into the CFA border region, but not when injected into a nonmotor region of rats that had recovered from an ischemic infarct in CFA.32 In contrast, in intact rats, disruption of the central core of CFA with anisomycin impaired performance on a skilled reaching task, but disruption of the CFA border region that was important for motor function in rehabilitated animals had no effect on function.32 Similarly, when an initial lesion is made within CFA followed by a period of rehabilitation, a second lesion to RFA leads to a reinstatement of the initial motor deficits.11 Furthermore, inactivation of PMv in macaques 1 to 2 months after a lesion to M1 also causes forelimb motor impairments to be reinstated.25 Although these studies suggest that the reorganized motor areas play a causal role in the recovered motor movements after recovery has taken place, it is important to note that the observed increases in the size of premotor cortex motor maps are delayed relative to the timing of functional recovery (Figure 2).10,24 Therefore, although secondary motor areas clearly play a causal role in recovered motor function after the recovery has been completed, the time course of the causal role of reorganized motor regions is uncertain.Download figureDownload PowerPointFigure 2. Motor map reorganization is delayed relative to functional recovery. Following an ischemic lesion to the caudal forelimb area, the area of the forelimb representation in the rostral forelimb area (RFA) initially decreases in size with a delayed expansion of RFA (A, red trace).10 Similarly, following a large lesion to M1, the dorsal premotor cortex, and the ventral premotor cortex of squirrel monkeys, the distal forelimb representation in the supplementary motor area (SMA) initially decreases in size with a delayed expansion of the representation (B, red trace).24 Although the expansion of motor maps in premotor regions is thought to contribute to motor recovery, the reorganization is delayed relative to the observed improvements in behavioral function (blue traces), raising questions about the specific roles of these secondary motor regions over the course of functional recovery.Emerging Therapies to Augment NeuroplasticityBecause reorganization of perilesional and distant motor areas plays a causal role in recovered motor function, there has been increasing interest in designing novel therapies to enhance neuroplasticity. Although these therapies will ultimately need to be translated to human stroke survivors, tests using experimental models have led to significant advancements of several potential therapies designed to enhance neuroplasticity globally or to strengthen specific intracortical or descending pathways within the motor system.Open-Loop Brain Stimulation to Enhance Global PlasticitySeveral intervention studies in experimental animal models have sought to increase the potential for neuroplasticity in a nonspecific way, which, when paired with rehabilitation, could lead to improved motor function (Figure 3A). Studies have found that 100 Hz epidural cortical stimulation to perilesional areas of rats, when paired with rehabilitation, improves motor recovery relative to rehabilitation alone.34 Importantly, although the functional benefits of epidural stimulation persisted for several months after the therapy, the benefits of cortical stimulation were only observed when started within a few weeks after the initial ischemic lesion.34 Several mechanisms have been proposed for the improvements associated with cortical stimulation, including increased neuronal survival and increased neuronal excitability from stimulation-induced depolarization of neurons.35 However, as both cathodal and anodal stimulation have led to functional improvements,35 the specific mechanism of action is uncertain. Because cortical stimulation was effective following cortical but not capsular lesions in rodents, the lesion location and surviving neural structures appear to play important roles in the effectiveness of cortical stimulation.36 Furthermore, negative results with certain stimulation parameters in nonhuman primate models of stroke and in human clinical trials show that the specific stimulation parameters and lesion characteristics play important roles in the functional benefits of cortical stimulation and that these mechanisms should be better characterized before clinical use.37,38Download figureDownload PowerPointFigure 3. Emerging technologies to enhance neuroplasticity. A number of novel technologies have been proposed to enhance and direct neuroplasticity. A, Open-loop interventions seek to nonspecifically enhance neuroplasticity to improve the efficacy of rehabilitative training through stimulation of the vagal nerve (left) or direct cortical stimulation (right). Closed-loop strategies seek to drive plasticity in targeted pathways either to strengthen intrinsic cortico-cortical connections (B) or to strengthen the descending motor output from a targeted motor region (C).Along with direct cortical stimulation, noninvasive stimulation with repetitive transcranial magnetic stimulation or transcranial direct current stimulation have also been proposed as noninvasive methods that may be able to modulate cortical excitability and increase neural plasticity after a cortical injury.39 However, the variety of possible settings tested means that future studies in animal models of brain injury will be necessary to fully characterize the specific mechanisms underlying each of these potential therapies.Behaviorally Triggered Stimulation AlgorithmsA slightly more targeted approach is to tie stimulation to behavioral events which theoretically would enhance neural plasticity during behaviorally relevant periods. For example, cortical stimulation applied immediately before a pellet retrieval task both normalized low-frequency local-field potentials in the perilesional motor cortex of rodents and improved reaching accuracies in a skilled pellet retrieval task.40 In addition to cortical stimulation, vagus nerve stimulation (Figure 3A) has also been proposed to increase cortical plasticity with potential benefits following stroke. When applied after an experimental ischemic lesion, vagus nerve stimulation temporally tied to performance of rehabilitative training leads to improvements in forelimb recovery that generalizes to nontrained tasks,41 even when delivered in the chronic state.42Closed-Loop Strategies to Target Specific PathwaysAn alternative strategy is to use neuroprosthetic systems to improve recovery by strengthening specific neural pathways. One neuroprosthetic strategy is to strengthen intrinsic connections within the central nervous system by pairing neural activity detected at one location with stimulation applied to a second location (Figure 3B). This strategy has been demonstrated in a rodent model of traumatic brain injury. Specifically, following a cortical impact lesion to the CFA, triggering stimulation in the primary sensory cortex based on the timing of action potentials that were recorded from RFA improves the performance of a skilled pellet retrieval task when compared with either an open-loop sham stimulation protocol or to a control group receiving no stimulation.43 An alternative strategy is to strengthen the extrinsic connections descending from a specific cortical region that facilitate motor output (Figure 3C). This approach has primarily been tested in human subjects using noninvasive systems that detect motor intention with EEG signals to drive an external orthosis or functional electric stimulator.44 Although rats have demonstrated the ability to control a neuroprosthetic system using neural activity recorded from their perilesional cortex,45 few studies have examined the specific mechanisms of neuroplasticity for this type of extrinsic neuroprosthetic system in animal models of brain injury.46 However, future studies utilizing animal models of brain injury will be valuable to better understand the specific mechanisms and optimal implementation of these neuroprosthetic systems for neurorehabilitation.Summary and Future DirectionsExperimental models of brain injury have provided valuable tools that have been used to investigate the mechanisms of plasticity that facilitate recovery of lost motor function. Across species and lesion models, the recovery of motor function has been associated with maintenance or increase in the size of motor representations in both the perilesional portions of M1 and secondary motor areas in the ipsilesional hemisphere. Additionally, an increase and maturation of the synaptic connections in these same regions accompany the observed functional reorganization. Importantly, the observed reorganization in experimental models corresponds with studies in human subjects that have found that optimal motor recovery is associated with a return to more normal patterns of motor activity.47Although preclinical studies allow us to perform well-controlled examinations using repeatable lesions, it is important to note that human stroke survivors demonstrate a more diverse range of stroke locations that do not always correspond well with the most commonly used lesion models.48 In particular, the lesion location and extent may be particularly relevant with regard to the role of distant motor areas, including the contralesional hemisphere in motor recovery.27 Furthermore, because this variation in lesion location will impact the success of the translation of therapies into human patients, it will be important to utilize a variety of methods for experimental animals including large and small lesions,27 subcortical lesion locations,49 cohorts that include aged animals,12 and animals with comorbidities similar to those observed in human patients to fully examine the mechanisms of recovery associated with potential therapies. Experimental models of brain injury and ischemia will also continue to be valuable to the future development of novel therapies designed to enhance neuroplasticity. Although recent studies have demonstrated the potential of these methods to improve motor function, future studies will be vital to improve our understanding of the specific mechanisms targeted by each intervention, allowing for determination of the optimal implementation for each method and thereby improving the likelihood of a successful translation into clinical populations.AcknowledgmentsThe ongoing work in the Nudo lab has been supported by National Institutes of Health (NIH) Grant R01NS030853 and Dr Bundy was supported by NIH Grant F32NS100339.DisclosuresDr Bundy holds stock equity in the start-up company Neurolut

While substantial task-related neural activity has been observed during motor tasks in rodent primary motor cortex and premotor cortex, the long-term stability of these responses in healthy rats is uncertain, limiting the interpretability of longitudinal changes in the specific patterns of neural activity associated with learning or motor recovery following injury. This study examined the stability of task-related neural activity associated with execution of two distinct reaching tasks in healthy rodents. A novel automated rodent behavioral apparatus was constructed and rats were trained to perform a reaching task combining a 'gross' lever press and a 'fine' pellet retrieval. In each animal, two chronic microelectrode arrays were implanted in motor cortex spanning the caudal forelimb area (rodent primary motor cortex) and the rostral forelimb area (rodent premotor cortex). We recorded multiunit spiking and local field potential activity from 10 days to 7-10 weeks post-implantation to characterize the patterns of neural activity observed during each task component and analyzed the consistency of channel-specific task-related neural activity. Task-related changes in neural activity were observed on the majority of channels. While the task-related changes in multi-unit spiking and local field potential spectral power were consistent over several weeks, spectral power changes were more stable, despite the trade-off of decreased spatial and temporal resolution. These results show that neural activity in rodent primary and premotor cortex is associated with specific phases of reaching movements with stable patterns of task-related activity across time, establishing the relevance of the rodent for future studies designed to examine changes in task-related neural activity during recovery from focal cortical lesions.

Objective. Activity-dependent stimulation (ADS) is designed to strengthen the connections between neuronal circuits and therefore may be a promising tool for promoting neurophysiological reorganization following a brain injury. To successfully perform this technique, two criteria must be met: (1) spikes in the extracellular electrical field potential must be detected accurately at one site of interest, and (2) stimulation pulses generated at fixed (<1 ms jitter), low-latency (<10 ms) intervals relative to each detected spike must be delivered reliably to a second site of interest. Here, we aimed to improve noise rejection in a low-cost commercial system to reliably perform ADS in awake, behaving rats, while maintaining latency requirements. Approach. We implemented a spike detection state machine on a field-programmable gate array (FPGA). Because the accuracy of spike detection can be heavily reduced in awake and behaving animals due to biological artifacts such as movement and chewing, the state machine tracks candidate spike waveforms, checking them against multiple programmable thresholds and rejecting any spikes that fail to meet a programmed threshold criterion. Main Results. A series of offline analyses showed that our implementation was able to appropriately trigger stimulation during epochs of biological artifacts with an overall accuracy between 72% and 97%, fixed computational latency of 167 µs, and an algorithmic latency of 300 µs to 800 µs. Significance. Our improvements have been made open-source and are freely available to all scientists working on closed-loop neuroprosthetic devices. Importantly, the improvements are easily incorporated into existing workflows that utilize the Intan Stimulation and Recording Controller.

OBJECTIVES/SPECIFIC AIMS: The objective of this study is to determine the degree to which the use of a contralesionally-controlled brain-computer interface for stroke rehabilitation drives change in interhemispheric motor cortical activity. METHODS/STUDY POPULATION: Ten chronic stroke patients were trained in the use of a brain-computer interface device for stroke recovery. Patients perform motor imagery to control the opening and closing of a motorized hand orthosis. This device was sent home with patients for 12 weeks, and patients were asked to use the device 1 hour per day, 5 days per week. The Action Research Arm Test (ARAT) was performed at 2-week intervals to assess motor function improvement. Before the active motor imagery task, patients were asked to quietly rest for 90 seconds before the task to calibrate recording equipment. EEG signals were acquired from 2 electrodes—one each centered over left and right primary motor cortex. Signals were preprocessed with a 60 Hz notch filter for environmental noise and referenced to the common average. Power envelopes for 1 Hz frequency bands (1–30 Hz) were calculated through Gabor wavelet convolution. Correlations between electrodes were then calculated for each frequency envelope on the first and last 5 runs, thus generating one correlation value per subject, per run. The chosen runs approximately correspond to the first and last week of device usage. These correlations were Fisher Z-transformed for comparison. The first and last 5 run correlations were averaged separately to estimate baseline and final correlation values. A difference was then calculated between these averages to determine correlation change for each frequency. The relationship between beta-band correlation changes (13–30 Hz) and the change in ARAT score was determined by calculating a Pearson correlation. RESULTS/ANTICIPATED RESULTS: Beta-band inter-electrode correlations tended to decrease more in patients achieving greater motor recovery (Pearson’s r=−0.68, p=0.031). A similar but less dramatic effect was observed with alpha-band (8–12 Hz) correlation changes (Pearson’s r=−0.42, p=0.22). DISCUSSION/SIGNIFICANCE OF IMPACT: The negative correlation between inter-electrode power envelope correlations in the beta frequency band and motor recovery indicates that activity in the motor cortex on each hemisphere may become more independent during recovery. The role of the unaffected hemisphere in stroke recovery is currently under debate; there is conflicting evidence regarding whether it supports or inhibits the lesioned hemisphere. These findings may support the notion of interhemispheric inhibition, as we observe less in common between activity in the 2 hemispheres in patients successfully achieving recovery. Future neuroimaging studies with greater spatial resolution than available with EEG will shed further light on changes in interhemispheric communication that occur during stroke rehabilitation.

There is increasing evidence that the hemisphere ipsilateral to a moving limb plays a role in planning and executing movements. However, the exact relationship between cortical activity and ipsilateral limb movements is uncertain. We sought to determine whether 3D arm movement kinematics (speed, velocity, and position) could be decoded from cortical signals recorded from the hemisphere ipsilateral to the moving limb. By having invasively monitored patients perform unilateral reaches with each arm, we also compared the encoding of contralateral and ipsilateral limb kinematics from a single cortical hemisphere. In four motor-intact human patients (three male, one female) implanted with electrocorticography electrodes for localization of their epileptic foci, we decoded 3D movement kinematics of both arms with accuracies above chance. Surprisingly, the spatial and spectral encoding of contralateral and ipsilateral limb kinematics was similar, enabling cross-prediction of kinematics between arms. These results clarify our understanding that the ipsilateral hemisphere robustly contributes to motor execution and supports that the information of complex movements is more bihemispherically represented in humans than has been previously understood. SIGNIFICANCE STATEMENT Although limb movements are traditionally understood to be driven by the cortical hemisphere contralateral to a moving limb, movement-related neural activity has also been found in the ipsilateral hemisphere. This study provides the first demonstration that 3D arm movement kinematics can be decoded from human electrocorticographic signals ipsilateral to the moving limb. Surprisingly, the spatial and spectral encoding of contralateral and ipsilateral limb kinematics was similar. The finding that specific kinematics are encoded in the ipsilateral hemisphere demonstrates that the ipsilateral hemisphere contributes to the execution of unilateral limb movements, improving our understanding of motor control. Additionally, the bihemisheric representation of voluntary movements has implications for the development of neuroprosthetic systems for reaching and for neurorehabilitation strategies following cortical injuries.

Background and Purpose— There are few effective therapies to achieve functional recovery from motor-related disabilities affecting the upper limb after stroke. This feasibility study tested whether a powered exoskeleton driven by a brain–computer interface (BCI), using neural activity from the unaffected cortical hemisphere, could affect motor recovery in chronic hemiparetic stroke survivors. This novel system was designed and configured for a home-based setting to test the feasibility of BCI-driven neurorehabilitation in outpatient environments. Methods— Ten chronic hemiparetic stroke survivors with moderate-to-severe upper-limb motor impairment (mean Action Research Arm Test=13.4) used a powered exoskeleton that opened and closed the affected hand using spectral power from electroencephalographic signals from the unaffected hemisphere associated with imagined hand movements of the paretic limb. Patients used the system at home for 12 weeks. Motor function was evaluated before, during, and after the treatment. Results— Across patients, our BCI-driven approach resulted in a statistically significant average increase of 6.2 points in the Action Research Arm Test. This behavioral improvement significantly correlated with improvements in BCI control. Secondary outcomes of grasp strength, Motricity Index, and the Canadian Occupational Performance Measure also significantly improved. Conclusions— The findings demonstrate the therapeutic potential of a BCI-driven neurorehabilitation approach using the unaffected hemisphere in this uncontrolled sample of chronic stroke survivors. They also demonstrate that BCI-driven neurorehabilitation can be effectively delivered in the home environment, thus increasing the probability of future clinical translation. Clinical Trial Registration— URL: http://www.clinicaltrials.gov . Unique identifier: NCT02552368.

Objectives: Hemispheric disconnection has been used as a treatment of medically refractory epilepsy and evolved from anatomic hemispherectomy to functional hemispherectomies to hemispherotomies. The hemispherotomy procedure involves disconnection of an entire hemisphere with limited tissue resection and is reserved for medically-refractory epilepsy due to diffuse hemispheric disease. Although it is thought to be effective by preventing seizures from spreading to the contralateral hemisphere, the electrophysiological effects of a hemispherotomy on the ipsilateral hemisphere remain poorly defined. The objective of this study was to evaluate the effects of hemispherotomy on the electrophysiologic dynamics in peri-stroke and dysplastic cortex. Methods: Intraoperative electrocorticography (ECoG) was recorded from ipsilateral cortex in 5 human subjects with refractory epilepsy before and after hemispherotomy. Power spectral density, mutual information, and phase-amplitude coupling were measured from the ECoG signals. Results: Epilepsy was a result of remote perinatal stroke in three of the subjects. In two of the subjects, seizures were a consequence of dysplastic tissue: one with hemimegalencephaly and the second with Rasmussen's encephalitis. Hemispherotomy reduced broad-band power spectral density in peri-stroke cortex. Meanwhile, hemispherotomy increased power in the low and high frequency bands for dysplastic cortex. Functional connectivity was increased in lower frequency bands in peri-stroke tissue but not affected in dysplastic tissue after hemispherotomy. Finally, hemispherotomy reduced band-specific phase-amplitude coupling in peristroke cortex but not dysplastic cortex. Significance: Disconnecting deep subcortical connections to peri-stroke cortex via a hemispherotomy attenuates power of oscillations and impairs the transfer of information from large-scale distributed brain networks to the local cortex. Hence, hemispherotomy reduces heterogeneity between neighboring cortex while impairing phase-amplitude coupling. In contrast, dysfunctional networks in dysplastic cortex lack the normal connectivity with distant networks. Therefore hemispherotomy does not produce the same effects.

Objective. Electrocorticography (ECoG) signals have emerged as a potential control signal for brain computer interface (BCI) applications due to balancing signal quality and implant invasiveness. While there have been numerous demonstrations in which ECoG signals were used to decode motor movements and to develop BCI systems, the extent of information that can be decoded has been uncertain. Therefore, we sought to determine if ECoG signals could be used to decode kinematics (speed, velocity, and position) of arm movements in 3D space. Approach. To investigate this, we designed a 3D center -out reaching task that was performed by five epileptic patients undergoing temporary placement of ECoG arrays. We used the ECoG signals within a hierarchical partial-least squares (PLS) regression model to perform offline prediction of hand speed, velocity, and position. Main Results. The hierarchical PLS regression model enabled us to predict hand speed, velocity, and position during 3D reaching movements from held -out test sets with accuracies above chance in each patient with mean correlation coefficients between 0.31 and 0.80 for speed, 0.27 and 0.54 for velocity, and 0.22 and 0.57 for position. While beta band power changes were the most significant features within the model used to classify movement and rest, the local motor potential and high gamma band power changes, were the most important features in the prediction of kinematic parameters. Significance. We believe that this study represents the first demonstration that truly three-dimensional movements can be predicted from ECoG recordings in human patients. Furthermore, this prediction underscores the potential to develop BCI systems with multiple degrees of freedom in human patients using ECoG.