Rapid functional plasticity in the primary somatomotor cortex and perceptual changes after nerve block

Representational maps in the brain: concepts, approaches, and applications

Neural systems have evolved to process sensory stimuli in a way that allows for efficient and adaptive behavior in a complex environment. Recent technological advances enable us to investigate sensory processing in animal models by simultaneously recording the activity of large populations of neurons with single-cell resolution, yielding high-dimensional datasets. In this review, we discuss concepts and approaches for assessing the population-level representation of sensory stimuli in the form of a representational map. In such a map, not only are the identities of stimuli distinctly represented, but their relational similarity is also mapped onto the space of neuronal activity. We highlight example studies in which the structure of representational maps in the brain are estimated from recordings in humans as well as animals and compare their methodological approaches. Finally, we integrate these aspects and provide an outlook for how the concept of representational maps could be applied to various fields in basic and clinical neuroscience.

Plasticity of face-hand sensorimotor circuits after a traumatic brachial plexus injury

Background

Interactions between the somatosensory and motor cortices are of fundamental importance for motor control. Although physically distant, face and hand representations are side by side in the sensorimotor cortex and interact functionally. Traumatic brachial plexus injury (TBPI) interferes with upper limb sensorimotor function, causes bilateral cortical reorganization, and is associated with chronic pain. Thus, TBPI may affect sensorimotor interactions between face and hand representations.

Objective

The aim of this study was to investigate changes in hand-hand and face-hand sensorimotor integration in TBPI patients using an afferent inhibition (AI) paradigm.

Method

The experimental design consisted of electrical stimulation (ES) applied to the hand or face followed by transcranial magnetic stimulation (TMS) to the primary motor cortex to activate a hand muscle representation. In the AI paradigm, the motor evoked potential (MEP) in a target muscle is significantly reduced when preceded by an ES at short-latency (SAI) or long-latency (LAI) interstimulus intervals. We tested 18 healthy adults (control group, CG), evaluated on the dominant upper limb, and nine TBPI patients, evaluated on the injured or the uninjured limb. A detailed clinical evaluation complemented the physiological investigation.

Results

Although hand-hand SAI was present in both the CG and the TBPI groups, hand-hand LAI was present in the CG only. Moreover, less AI was observed in TBPI patients than the CG both for face-hand SAI and LAI.

Conclusion

Our results indicate that sensorimotor integration involving both hand and face sensorimotor representations is affected by TBPI.

Neuron-specific ablation of the Krabbe disease gene galactosylceramidase in mice results in neurodegeneration

Krabbe disease is caused by a deficiency of the lysosomal galactosylceramidase (GALC) enzyme, which results in the accumulation of galactosylceramide (GalCer) and psychosine. In Krabbe disease, the brunt of demyelination and neurodegeneration is believed to result from the dysfunction of myelinating glia. Recent studies have shown that neuronal axons are both structurally and functionally compromised in Krabbe disease, even before demyelination, suggesting a possible neuron-autonomous role of GALC. Using a novel neuron-specific Galc knockout (CKO) model, we show that neuronal Galc deletion is sufficient to cause growth and motor coordination defects and inflammatory gliosis in mice. Furthermore, psychosine accumulates significantly in the nervous system of neuron-specific Galc-CKO. Confocal and electron microscopic analyses show profound neuro-axonal degeneration with a mild effect on myelin structure. Thus, we prove for the first time that neuronal GALC is essential to maintain and protect neuronal function independently of myelin and may directly contribute to the pathogenesis of Krabbe disease.

Malleability of the cortical hand map following a finger nerve block

Electrophysiological studies in monkeys show that finger amputation triggers local remapping within the deprived primary somatosensory cortex (S1). Human neuroimaging research, however, shows persistent S1 representation of the missing hand's fingers, even decades after amputation. Here, we explore whether this apparent contradiction stems from underestimating the distributed peripheral and central representation of fingers in the hand map. Using pharmacological single-finger nerve block and 7-tesla neuroimaging, we first replicated previous accounts (electrophysiological and other) of local S1 remapping. Local blocking also triggered activity changes to nonblocked fingers across the entire hand area. Using methods exploiting interfinger representational overlap, however, we also show that the blocked finger representation remained persistent despite input loss. Computational modeling suggests that both local stability and global reorganization are driven by distributed processing underlying the topographic map, combined with homeostatic mechanisms. Our findings reveal complex interfinger representational features that play a key role in brain (re)organization, beyond (re)mapping.

Plasticity of the Central Nervous System Involving Peripheral Nerve Transfer

Peripheral nerve injury can lead to partial or complete loss of limb function, and nerve transfer is an effective surgical salvage for patients with these injuries. The inability of deprived cortical regions representing damaged nerves to overcome corresponding maladaptive plasticity after the reinnervation of muscle fibers and sensory receptors is thought to be correlated with lasting and unfavorable functional recovery. However, the concept of central nervous system plasticity is rarely elucidated in classical textbooks involving peripheral nerve injury, let alone peripheral nerve transfer. This article is aimed at providing a comprehensive understanding of central nervous system plasticity involving peripheral nerve injury by reviewing studies mainly in human or nonhuman primate and by highlighting the functional and structural modifications in the central nervous system after peripheral nerve transfer. Hopefully, it will help surgeons perform successful nerve transfer under the guidance of modern concepts in neuroplasticity.

Face-hand sensorimotor interactions revealed by afferent inhibition

Reorganization of the sensorimotor cortex following permanent (e.g., amputation) or temporary (e.g., local anaesthesia) deafferentation of the hand has revealed large-scale plastic changes between the hand and face representations that are accompanied by perceptual correlates. The physiological mechanisms underlying this reorganization remain poorly understood. The aim of this study was to investigate sensorimotor interactions between the face and hand using an afferent inhibition transcranial magnetic stimulation protocol in which the motor evoked potential elicited by the magnetic pulse is inhibited when it is preceded by an afferent stimulus. We hypothesized that if face and hand representations in the sensorimotor cortex are functionally coupled, then electrocutaneous stimulation of the face would inhibit hand muscle motor responses. In two separate experiments, we delivered an electrocutaneous stimulus to either the skin over the right upper lip (Experiment 1) or right cheek (Experiment 2) and recorded muscular activity from the right first dorsal interosseous. Both lip and cheek stimulation inhibited right first dorsal interosseous motor evoked potentials. To investigate the specificity of this effect, we conducted two additional experiments in which electrocutaneous stimulation was applied to either the right forearm (Experiment 3) or right upper arm (Experiment 4). Forearm and upper arm stimulation also significantly inhibited the right first dorsal interosseous motor evoked potentials, but this inhibition was less robust than the inhibition associated with face stimulation. These findings provide the first evidence for face-to-hand afferent inhibition.

Cortical plasticity and nerve regeneration after peripheral nerve injury

With the development of neuroscience, substantial advances have been achieved in peripheral nerve regeneration over the past decades. However, peripheral nerve injury remains a critical public health problem because of the subsequent impairment or absence of sensorimotor function. Uncomfortable complications of peripheral nerve injury, such as chronic pain, can also cause problems for families and society. A number of studies have demonstrated that the proper functioning of the nervous system depends not only on a complete connection from the central nervous system to the surrounding targets at an anatomical level, but also on the continuous bilateral communication between the two. After peripheral nerve injury, the interruption of afferent and efferent signals can cause complex pathophysiological changes, including neurochemical alterations, modifications in the adaptability of excitatory and inhibitory neurons, and the reorganization of somatosensory and motor regions. This review discusses the close relationship between the cerebral cortex and peripheral nerves. We also focus on common therapies for peripheral nerve injury and summarize their potential mechanisms in relation to cortical plasticity. It has been suggested that cortical plasticity may be important for improving functional recovery after peripheral nerve damage. Further understanding of the potential common mechanisms between cortical reorganization and nerve injury will help to elucidate the pathophysiological processes of nerve injury, and may allow for the reduction of adverse consequences during peripheral nerve injury recovery. We also review the role that regulating reorganization mechanisms plays in functional recovery, and conclude with a suggestion to target cortical plasticity along with therapeutic interventions to promote peripheral nerve injury recovery.

Cortical plasticity elicited by acoustically cued monetary losses: an ERP study

Both human and animal studies have demonstrated remarkable findings of experience-induced plasticity in the cortex. Here, we investigated whether the widely used monetary incentive delay (MID) task changes the neural processing of incentive cues that code expected monetary outcomes. We used a novel auditory version of the MID task, where participants responded to acoustic cues that coded expected monetary losses. To investigate task-induced brain plasticity, we presented incentive cues as deviants during passive oddball tasks before and after two sessions of the MID task. During the oddball task, we recorded the mismatch-related negativity (MMN) as an index of cortical plasticity. We found that two sessions of the MID task evoked a significant enhancement of MMN for incentive cues that predicted large monetary losses, specifically when monetary cue discrimination was essential for maximising monetary outcomes. The task-induced plasticity correlated with the learning-related neural activity recorded during the MID task. Thus, our results confirm that the processing of (loss)incentive auditory cues is dynamically modulated by previously learned monetary outcomes.

Hyperalgesia and Reduced Offset Analgesia During Spinal Anesthesia

Introduction

Spinal anesthesia induces short-term deafferentation and causes connectivity changes in brain areas involved in endogenous pain modulation. We determined whether spinal anesthesia alters pain sensitivity and offset analgesia. Offset analgesia is a manifestation of endogenous pain modulation and characterized by profound analgesia upon a small decrease in noxious stimulation.

Methods

In this randomized controlled crossover trial, static thermal pain responses and offset analgesia were obtained in 22 healthy male volunteers during spinal anesthesia and control conditions (absence of spinal anesthesia). Pain responses and offset analgesia were measured on a remote skin area above the upper level of anesthesia (C8/Th1).

Results

Following spinal injection of the local anesthetic, the average maximum anesthesia level was Th6. Static pain scores at C8/Th1 were higher during spinal anesthesia compared to control: 59.1 ± 15.0 mm (spinal anesthesia) versus 51.7 ± 19.7 mm (control; p = 0.03). Offset analgesia responses were decreased during spinal analgesia: pain score decrease 79 ± 27% (spinal anesthesia) versus 90 ± 17% (control; p = 0.016).

Discussion

We confirmed that spinal anesthesia-induced deafferentation causes hyperalgesic responses to noxious thermal stimulation and reduced offset analgesia at dermatomes remote and above the level of deafferentation. While these data suggest that the reduction of offset analgesia has a central origin, related to alterations in brain areas involved in inhibitory pain control, we cannot exclude alternative (peripheral) mechanisms.

Trial Registration

Dutch Cochrane Center under identifier (www.trialregister.nl) NL3874.

Altered cerebrovascular response to acute exercise in patients with Huntington's disease

The objective of this study was to determine whether a single session of exercise was sufficient to induce cerebral adaptations in individuals with Huntington’s disease and to explore the time dynamics of any acute cerebrovascular response. In this case–control study, we employed arterial-spin labelling MRI in 19 Huntington’s disease gene-positive participants (32–65 years, 13 males) and 19 controls (29–63 years, 10 males) matched for age, gender, body mass index and self-reported activity levels, to measure global and regional perfusion in response to 20 min of moderate-intensity cycling. Cerebral perfusion was measured at baseline and 15, 40 and 60 min after exercise cessation. Relative to baseline, we found that cerebral perfusion increased in patients with Huntington’s disease yet was unchanged in control participants in the precentral gyrus (P = 0.016), middle frontal gyrus (P = 0.046) and hippocampus (P = 0.048) 40 min after exercise cessation (+15 to +32.5% change in Huntington’s disease participants, −7.7 to 0.8% change in controls). The length of the disease‐causing trinucleotide repeat expansion in the huntingtin gene predicted the change in the precentral gyrus (P = 0.03) and the intensity of the exercise intervention predicted hippocampal perfusion change in Huntington’s disease participants (P < 0.001). In both groups, exercise increased hippocampal blood flow 60 min after exercise cessation (P = 0.039). These findings demonstrate the utility of acute exercise as a clinically sensitive experimental paradigm to modulate the cerebrovasculature. Twenty minutes of aerobic exercise induced transient cerebrovascular adaptations in the hippocampus and cortex selectively in Huntington’s disease participants and likely represents latent neuropathology not evident at rest.

Fractal properties in sensorimotor variability unveil internal adaptations of the organism before symptomatic functional decline

If health can be defined as adaptability, then measures of adaptability are crucial. Convergent findings across clinical areas established the notion that fractal properties in bio-behavioural variability characterize the healthy condition of the organism, and its adaptive capacities in general. However, ambiguities remain as to the significance of fractal properties: the literature mainly discriminated between healthy vs. pathological states, thereby loosing perspective on the progression in between, and overlooking the distinction between adaptability and effective adaptations of the organism. Here, we design an experimental tapping paradigm involving gradual feedback deprivation in groups of healthy subjects and one deafferented man as a pathological-limit case. We show that distinct types of fractal properties in sensorimotor behaviour characterize, on the one hand impaired functional ability, and on the other hand internal adaptations for maintaining performance despite the imposed constraints. Findings may prove promising for early detection of internal adaptations preceding symptomatic functional decline.

Learning of Artificial Sensation Through Long-Term Home Use of a Sensory-Enabled Prosthesis

Upper limb prostheses are specialized tools, and skilled operation is learned by amputees over time. Recently, neural prostheses using implanted peripheral nerve interfaces have enabled advances in artificial somatosensory feedback that can improve prosthesis outcomes. However, the effect of sensory learning on artificial somatosensation has not been studied, despite its known influence on intact somatosensation and analogous neuroprostheses. Sensory learning involves changes in the perception and interpretation of sensory feedback and may further influence functional and psychosocial outcomes. In this mixed methods case study, we examined how passive learning over 115 days of home use of a neural-connected, sensory-enabled prosthetic hand influenced perception of artificial sensory feedback in a participant with transradial amputation. We examined perceptual changes both within individual days of use and across the duration of the study. At both time scales, the reported percept locations became significantly more aligned with prosthesis sensor locations, and the phantom limb became significantly more extended toward the prosthesis position. Similarly, the participant’s ratings of intensity, naturalness, and contact touch significantly increased, while his ratings of vibration and movement significantly decreased across-days for tactile channels. These sensory changes likely resulted from engagement of cortical plasticity mechanisms as the participant learned to use the artificial sensory feedback. We also assessed psychosocial and functional outcomes through surveys and interviews, and found that self-efficacy, perceived function, prosthesis embodiment, social touch, body image, and prosthesis efficiency improved significantly. These outcomes typically improved within the first month of home use, demonstrating rapid benefits of artificial sensation. Participant interviews indicated that the naturalness of the experience and engagement with the prosthesis increased throughout the study, suggesting that artificial somatosensation may decrease prosthesis abandonment. Our data showed that prosthesis embodiment was intricately related to naturalness and phantom limb perception, and that learning the artificial sensation may have modified the body schema. As another indicator of successfully learning to use artificial sensation, the participant reported the emergence of stereognosis later in the study. This study provides the first evidence that artificial somatosensation can undergo similar learning processes as intact sensation and highlights the importance of sensory restoration in prostheses.

Blocking tactile input to one finger using anaesthetic enhances touch perception and learning in other fingers

Brain plasticity is a key mechanism for learning and recovery. A striking example of plasticity in the adult brain occurs following input loss, for example, following amputation, whereby the deprived zone is “invaded” by new representations. Although it has long been assumed that such reorganization leads to functional benefits for the invading representation, the behavioral evidence is controversial. Here, we investigate whether a temporary period of somatosensory input loss to one finger, induced by anesthetic block, is sufficient to cause improvements in touch perception (“direct” effects of deafferentation). Further, we determine whether this deprivation can improve touch perception by enhancing sensory learning processes, for example, by training (“interactive” effects). Importantly, we explore whether direct and interactive effects of deprivation are dissociable by directly comparing their effects on touch perception. Using psychophysical thresholds, we found brief deprivation alone caused improvements in tactile perception of a finger adjacent to the blocked finger but not to non-neighboring fingers. Two additional groups underwent minimal tactile training to one finger either during anesthetic block of the neighboring finger or a sham block with saline. Deprivation significantly enhanced the effects of tactile perceptual training, causing greater learning transfer compared with sham block. That is, following deafferentation and training, learning gains were seen in fingers normally outside the boundaries of topographic transfer of tactile perceptual learning. Our results demonstrate that sensory deprivation can improve perceptual abilities, both directly and interactively, when combined with sensory learning. This dissociation provides novel opportunities for future clinical interventions to improve sensation.

The effect of LTP- and LTD-like visual stimulation on modulation of human orientation discrimination

Studies showing that repetitive visual stimulation protocols alter perception and induce cortical reorganization, as well-reported for the tactile domain, have been sparse. In this study, we investigated how “long-term potentiation [LTP]-like” and “long-term depression [LTD]-like” repetitive visual stimulation affects orientation discrimination ability in human observers. LTP-like stimulation with features most closely resembling the stimuli used during behavioral assessment evoked the largest improvement, while the effects were smaller in protocols that differed in shape or orientation features. This gradient suggests lower learning specificity than classical perceptual learning experiments, possibly because of an interplay of task- and feature-based factors. All modulatory effects of repetitive stimulation were superimposed on top of spontaneous task learning. Moreover, blockwise analysis revealed that LTP-like stimulation, in contrast to LTD-like or sham stimulation, prevented a loss of practice-related gain of orientation discrimination thresholds. This observation highlights a critical role of LTP-like stimulation for consolidation, typically observed during sleep.

Effects of Temporary Functional Deafferentation in Chronic Stroke Patients: Who Profits More?

Temporary functional deafferentation (TFD) by an anesthetic cream on the stroke-affected forearm was shown to improve sensorimotor abilities of stroke patients. The present study investigated different predictors for sensorimotor improvements during TFD and indicated outcome differences between patients grouped in subcortical lesions only and lesions with any cortical involvement. Thirty-four chronic stroke patients were temporarily deafferented on the more affected forearm by an anesthetic cream. Somatosensory performance was assessed using von Frey Hair and grating orientation task; motor performance was assessed by a shape-sorter-drum task. Seven potential predictors were entered into three linear multiple regression models. Furthermore, effects of TFD on outcome variables for the two groups (cortical versus subcortical lesion) were compared. Sex and sensory deficit were significant predictors for changes in motor function while age accounted for changes in grating orienting task. Males, patients with a stronger sensory deficit, and older patients profited more. None of the potential predictors made significant contributions to changes in threshold for touch. Furthermore, there were no differences in sensorimotor improvement between lesion site groups. The effects of TFD together with the low predictability of the investigated parameters suggest that characteristics of patients alone are not suitable to exclude some patients from TFD.

Tactile learning transfer from the hand to the face but not to the forearm implies a special hand-face relationship

In the primary somatosensory cortex, large-scale cortical and perceptual changes have been demonstrated following input deprivation. Recently, we found that the cortical and perceptual changes induced by repetitive somatosensory stimulation (RSS) at a finger transfer to the face. However, whether such cross-border changes are specific to the face remains elusive. Here, we investigated whether RSS-induced acuity changes at the finger can also transfer to the forearm, which is the body part represented on the other side of the hand representation. Our results confirmed the transfer of tactile learning from the stimulated finger to the lip, but no significant changes were observed at the forearm. A second experiment revealed that the same regions on the forearm exhibited improved tactile acuity when RSS was applied there, excluding the possibility of low plastic ability at the arm representation. This provides also the first evidence that RSS can be efficient on body parts other than the hand. These results suggest that RSS-induced tactile learning transfers preferentially from the hand to the face rather than to the forearm. This specificity could arise from a stronger functional connectivity between the cortical hand and face representations, reflecting a fundamental coupling between these body parts.

Therapist-assisted vision therapy improves outcome for stroke patients with homonymous hemianopia alone or combined with oculomotor dysfunction

OBJECTIVE

To improve visual performance and perception in stroke patients suffering from visual impairments by the use of therapist-assisted vision therapy.

METHODS

This study was an interventional efficacy open-label investigation. The vision therapy was designed to enhance binocular vision, and saccadic ability, and vergence ranges maximally, and for patients with hemianopia also to improve peripheral awareness. The vision training consisted of one lesson a week for 12 weeks carried out by an optometrist and a vision therapist. Between lessons, patients were to train at home for a minimum of 15-20 min daily.

RESULTS

Twenty-four patients completed the course. Significant improvements in visual performance were measured for all test parameters from the baseline to the evaluation after the last lesson of vision training. The COPM results improved both in terms of satisfaction with the completion of a task and with the way the task was carried out (p = 0.001). Groffman tracing test results improved from median 7.5 to 16 (p = 0.002), reading speed in words increased (p = 0.0004), and peripheral awareness of visual markers improved (p = 0.002).

CONCLUSION

Therapist-assisted vision therapy increased peripheral visual awareness. Furthermore patients felt safer in the traffic and in outdoor activities. Reading speed significantly increased, and the ability to keep a moving object in focus improved.

Referred cramping phantom hand pain elicited in the face and eliminated by peripheral nerve block

Phantom limb pain is a restricting condition for a substantial number of amputees with quite different characteristics of pain. Here, we report on a forearm amputee with constant phantom pain in the hand, in whom we could regularly elicit the rare phenomenon of referred cramping phantom pain by touching the face. To clarify the underlying mechanisms, we followed the cramp during the course of an axillary blockade of the brachial plexus. During the blockade, both phantom pain and the referred cramp were abolished, while a referred sensation of "being touched at the phantom" persisted. Furthermore, to identify the cortical substrate, we elicited the cramp during functional magnetic imaging. Imaging revealed that referred cramping phantom limb pain was associated with brain activation of the hand representation in the primary sensorimotor cortex. The results support the hypothesis that referred cramping phantom limb pain in this case is associated with a substantial brain activation in the hand area of the deafferented sensorimotor cortex. However, this alone is not sufficient to elicit referred cramping phantom limb pain. Peripheral inputs, both, from the arm nerves affected by the amputation and from the skin in the face at which the referred cramp is evoked, are a precondition for referred cramping phantom limb pain to occur, at least in this case.

The recalibration of tactile perception during tool use is body-part specific

Two decades of research have demonstrated that using a tool modulates spatial representations of the body. Whether this embodiment is specific to representations of the tool-using limb or extends to representations of other body parts has received little attention. Several studies of other perceptual phenomena have found that modulations to the primary somatosensory representation of the hand transfers to the face, due in part to their close proximity in primary somatosensory cortex. In the present study, we investigated whether tool-induced recalibration of tactile perception on the hand transfers to the cheek. Participants verbally estimated the distance between two tactile points applied to either their hand or face, before and after using a hand-shaped tool. Tool use recalibrated tactile distance perception on the hand-in line with previous findings-but left perception on the cheek unchanged. This finding provides support for the idea that embodiment is body-part specific. Furthermore, it suggests that tool-induced perceptual recalibration occurs at a level of somatosensory processing, where representations of the hand and face have become functionally disentangled.

Changes in Brain Resting-state Functional Connectivity Associated with Peripheral Nerve Block: A Pilot Study

BACKGROUND

Limited information exists on the effects of temporary functional deafferentation (TFD) on brain activity after peripheral nerve block (PNB) in healthy humans. Increasingly, resting-state functional connectivity (RSFC) is being used to study brain activity and organization. The purpose of this study was to test the hypothesis that TFD through PNB will influence changes in RSFC plasticity in central sensorimotor functional brain networks in healthy human participants.

METHODS

The authors achieved TFD using a supraclavicular PNB model with 10 healthy human participants undergoing functional connectivity magnetic resonance imaging before PNB, during active PNB, and during PNB recovery. RSFC differences among study conditions were determined by multiple-comparison-corrected (false discovery rate-corrected P value less than 0.05) random-effects, between-condition, and seed-to-voxel analyses using the left and right manual motor regions.

RESULTS

The results of this pilot study demonstrated disruption of interhemispheric left-to-right manual motor region RSFC (e.g., mean Fisher-transformed z [effect size] at pre-PNB 1.05 vs. 0.55 during PNB) but preservation of intrahemispheric RSFC of these regions during PNB. Additionally, there was increased RSFC between the left motor region of interest (PNB-affected area) and bilateral higher order visual cortex regions after clinical PNB resolution (e.g., Fisher z between left motor region of interest and right and left lingual gyrus regions during PNB, -0.1 and -0.6 vs. 0.22 and 0.18 after PNB resolution, respectively).

CONCLUSIONS

This pilot study provides evidence that PNB has features consistent with other models of deafferentation, making it a potentially useful approach to investigate brain plasticity. The findings provide insight into RSFC of sensorimotor functional brain networks during PNB and PNB recovery and support modulation of the sensory-motor integration feedback loop as a mechanism for explaining the behavioral correlates of peripherally induced TFD through PNB.

Effectiveness of temporary deafferentation of the arm on somatosensory and motor functions following stroke: a systematic review

BACKGROUND

After stroke, regaining functional use of the upper limb can be challenging. Temporary deafferentation (TD) is a novel approach used in neurorehabilitation to voluntarily reduce the somatosensory input in a body part by temporary anesthesia; which has been shown to improve sensorimotor functions in the affected limb.

OBJECTIVES

The primary objective of this systematic review was to present the best available evidence related to the effects of TD of the affected arm on the recovery of motor function and activity of the upper limb (arm and hand) following stroke. Further, this review aimed to assess the effects of TD on sensory function, activities of daily living (ADL) and quality of life following stroke, the acceptability and safety of the intervention as well as adverse events.

INCLUSION CRITERIA TYPES OF PARTICIPANTS

Adult patients (18 years and older) with a clinical diagnosis of stroke, either hemorrhagic or ischemic.

TYPES OF INTERVENTIONS

Reports of rehabilitation that included the use of a pneumatic tourniquet, regional anesthesia or nerve block to achieve TD of an arm, or the use of TD as a stand-alone intervention.

OUTCOMES

Primary outcomes were motor function and activity of the upper limb using assessment scales, motor tests and global motor functions.Secondary outcomes included measures of sensory function, ADL, impact of stroke and quality of life and pain.Additional outcomes were neurophysiological changes as studied with functional magnetic resonance imaging, magnetoencephalography and/or transcranial magnetic stimulation.Acceptability and safety of the intervention as well as adverse events were also included.

TYPES OF STUDIES

We included any experimental and epidemiological studies. There were no randomized controlled trials. We included non-randomized controlled trials, quasi-experimental, before and after studies and case-control studies.

SEARCH STRATEGY

We searched for both published and unpublished studies in major databases and all reference lists of relevant articles in English, German or French languages. We included studies published from January 1980 to October 2015.

DATA EXTRACTION

Data were extracted from included studies using a standardized data extraction tool from the Joanna Briggs Institute.

DATA SYNTHESIS

There was heterogeneity in the types of intervention and outcome measures, therefore statistical pooling of the findings was not appropriate. As such, the studies were grouped according to type of outcome where possible. Findings are presented in a narrative form.

RESULTS

Eight studies met the eligibility criteria. All outcome parameters related to the primary outcome (motor function and activity of the more affected upper extremity) showed an improvement during or after TD. The sensory functions significantly improved during or after TD when measured either by the grating orienting task or the grating orientation accuracy, and slightly improved when measured using the von Frey hair testing during TD.

CONCLUSION

There is evidence supporting the use of TD of the upper extremity in adults after stroke. Temporary deafferentation can be recommended (Grade B).

Effects of Sensory Deficit on Phalanx Force Deviation During Power Grip Post Stroke

The effect of sensory deficits on power grip force from individual phalanges was examined. The authors found that stroke survivors with sensory deficits (determined by the Semmes-Weinstein monofilament test) gripped with phalanx force directed more tangential to the object surface, than those without, although both groups had similar motor deficits (Chedoke-McMaster and Fugl-Meyer), grip strength, and skin friction. Altered grip force direction elevates risk of finger slippage against the object thus grip loss/object dropping, hindering activities of daily living. Altered grip force direction was associated with altered muscle activation patterns. In summary, the motor impairment level alone may not describe hand motor control in detail. Information about sensory deficits helps elucidate patients' hand motor control with functional relevance.

Bilateral sensory deprivation of trigeminal afferent fibres on corticomotor control of human tongue musculature: a preliminary study

Transcranial magnetic stimulation (TMS) has demonstrated changes in motor evoked potentials (MEPs) in human limb muscles following modulation of sensory afferent inputs. The aim of this study was to determine whether bilateral local anaesthesia (LA) of the lingual nerve affects the excitability of the tongue motor cortex (MI) as measured by TMS. The effect on MEPs after bilateral LA of the lingual nerve was studied, while the first dorsal interosseous (FDI) muscle served as a control in ten healthy participants. MEPs were measured on the right side of the tongue dorsum in four different conditions: (i) immediately prior to anaesthesia (baseline), (ii) during bilateral LA block of the lingual nerve, (iii) after anaesthesia had subjectively subsided (recovery) and (iv) 3 h after bilateral lingual block injection. MEPs were assessed using stimulus-response curves in steps of 10% of motor threshold (T). Eight stimuli were given at each stimulus level. The amplitudes of the tongue MEPs were significantly influenced by the stimulus intensity (P < 0·001) but not by condition (P = 0·186). However, post hoc tests showed that MEPS were statistically significantly higher during bilateral LA block condition compared with baseline at T + 40%, T + 50% and T + 60% (P < 0·028) and also compared with recovery at T + 60% (P = 0·010) as well as at 3 h after injection at T + 50% and T + 60% (P < 0·029). Bilateral LA block of the lingual nerve seems to be associated with a facilitation of the corticomotor pathways related to the tongue musculature.

Plasticity and Cortical Reorganization Associated With Pain

Abstract. This review focuses on plasticity and reorganization associated with pain. It is well established that noxious stimulation activates a large network of neural structures in the human brain, which is often denominated as the neuromatrix of pain. Repeated stimulation is able to induce plasticity in nearly all structures of this neuromatrix. While the plasticity to short-term stimulation is usually transient, long-term stimulation might induce persistent changes within the neuromatrix network and reorganize its functions and structures. Interestingly, a large longitudinal study on patients with subacute back pain found predictors for the persistence of pain versus remission in mesolimbic structures not usually included in the neuromatrix of pain. From these results, new concepts of nociception, pain, and transition from acute to chronic pain emerged. Overall, this review outlines a number of plastic changes in response to pain. However, the role of plasticity for chronic pain has still to be established.

Temporary deafferentation evoked by cutaneous anesthesia: behavioral and electrophysiological findings in healthy subjects

Motor function and motor excitability can be modulated by changes of somatosensory input. Here, we performed a randomized single-blind trial to investigate behavioral and neurophysiological changes during temporary deafferentation of left upper arm and forearm in 31 right-handed healthy adults. Lidocaine cream was used to anesthetize the skin from wrist to shoulder, sparing the hand. As control condition, on a different day, a neutral cream was applied to the same skin area. The sequence (first Lidocaine, then placebo or vice versa) was randomized. Behavioral measures included the Grating Orientation Task, the Von Frey hair testing and the Nine-hole-peg-test. Transcranial magnetic stimulation was used to investigate short-interval intracortical inhibition, stimulus response curves, motor evoked potential amplitudes during pre-innervation and the cortical silent period (CSP). Recordings were obtained from left first dorsal interosseous muscle and from left flexor carpi radialis muscle. During deafferentation, the threshold of touch measured at the forearm was significantly worse. Other behavioral treatment-related changes were not found. The CSP showed a significant interaction between treatment and time in first dorsal interosseous muscle. CSP duration was longer during Lidocaine application and shorter during placebo exposure. We conclude that, in healthy subjects, temporary cutaneous deafferentation of upper and lower arm may have minor effects on motor inhibition, but not on sensory or motor function for the adjacent non-anesthetized hand.

Neuromagnetic correlates of adaptive plasticity across the hand-face border in human primary somatosensory cortex

It is well established that permanent or transient reduction of somatosensory inputs, following hand deafferentation or anesthesia, induces plastic changes across the hand-face border, supposedly responsible for some altered perceptual phenomena such as tactile sensations being referred from the face to the phantom hand. It is also known that transient increase of hand somatosensory inputs, via repetitive somatosensory stimulation (RSS) at a fingertip, induces local somatosensory discriminative improvement accompanied by cortical representational changes in the primary somatosensory cortex (SI). We recently demonstrated that RSS at the tip of the right index finger induces similar training-independent perceptual learning across the hand-face border, improving somatosensory perception at the lips (Muret D, Dinse HR, Macchione S, Urquizar C, Farnè A, Reilly KT.Curr Biol24: R736-R737, 2014). Whether neural plastic changes across the hand-face border accompany such remote and adaptive perceptual plasticity remains unknown. Here we used magnetoencephalography to investigate the electrophysiological correlates underlying RSS-induced behavioral changes across the hand-face border. The results highlight significant changes in dipole location after RSS both for the stimulated finger and for the lips. These findings reveal plastic changes that cross the hand-face border after an increase, instead of a decrease, in somatosensory inputs.

Effect of imperceptible vibratory noise applied to wrist skin on fingertip touch evoked potentials - an EEG study

Random vibration applied to skin can change the sense of touch. Specifically, low amplitude white-noise vibration can improve fingertip touch perception. In fact, fingertip touch sensation can improve even when imperceptible random vibration is applied to other remote upper extremity areas such as wrist, dorsum of the hand, or forearm. As such, vibration can be used to manipulate sensory feedback and improve dexterity, particularly during neurological rehabilitation. Nonetheless, the neurological bases for remote vibration enhanced sensory feedback are yet poorly understood. This study examined how imperceptible random vibration applied to the wrist changes cortical activity for fingertip sensation. We measured somatosensory evoked potentials to assess peak-to-peak response to light touch of the index fingertip with applied wrist vibration versus without. We observed increased peak-to-peak somatosensory evoked potentials with wrist vibration, especially with increased amplitude of the later component for the somatosensory, motor, and premotor cortex with wrist vibration. These findings corroborate an enhanced cortical-level sensory response motivated by vibration. It is possible that the cortical modulation observed here is the result of the establishment of transient networks for improved perception.

Motor priming in neurorehabilitation

Priming is a type of implicit learning wherein a stimulus prompts a change in behavior. Priming has been long studied in the field of psychology. More recently, rehabilitation researchers have studied motor priming as a possible way to facilitate motor learning. For example, priming of the motor cortex is associated with changes in neuroplasticity that are associated with improvements in motor performance. Of the numerous motor priming paradigms under investigation, only a few are practical for the current clinical environment, and the optimal priming modalities for specific clinical presentations are not known. Accordingly, developing an understanding of the various types of motor priming paradigms and their underlying neural mechanisms is an important step for therapists in neurorehabilitation. Most importantly, an understanding of the methods and their underlying mechanisms is essential for optimizing rehabilitation outcomes. The future of neurorehabilitation is likely to include these priming methods, which are delivered prior to or in conjunction with primary neurorehabilitation therapies. In this Special Interest article, we discuss those priming paradigms that are supported by the greatest amount of evidence, including (i) stimulation-based priming, (ii) motor imagery and action observation, (iii) sensory priming, (iv) movement-based priming, and (v) pharmacological priming.Video Abstract available. (see Supplemental Digital Content 1, http://links.lww.com/JNPT/A86) for more insights from the authors.

Effect of deafferentation from spinal anesthesia on pain sensitivity and resting-state functional brain connectivity in healthy male volunteers

Patients may perceive paradoxical heat sensation during spinal anesthesia. This could be due to deafferentation-related functional changes at cortical, subcortical, or spinal levels. In the current study, the effect of spinal deafferentation on sensory (pain) sensitivity was studied and linked to whole-brain functional connectivity as assessed by resting-state functional magnetic resonance imaging (RS-fMRI) imaging. Deafferentation was induced by sham or spinal anesthesia (15 mg bupivacaine injected at L3-4) in 12 male volunteers. RS-fMRI brain connectivity was determined in relation to eight predefined and seven thalamic resting-state networks (RSNs) and measured before, and 1 and 2 h after spinal/sham injection. To measure the effect of deafferentation on pain sensitivity, responses to heat pain were measured at 15-min intervals on nondeafferented skin and correlated to RS-fMRI connectivity data. Spinal anesthesia altered functional brain connectivity within brain regions involved in the sensory discriminative (i.e., pain intensity related) and affective dimensions of pain perception in relation to somatosensory and thalamic RSNs. A significant enhancement of pain sensitivity on nondeafferented skin was observed after spinal anesthesia compared to sham (area-under-the-curve [mean (SEM)]: 190.4 [33.8] versus 13.7 [7.2]; p<0.001), which significantly correlated to functional connectivity changes observed within the thalamus in relation to the thalamo-prefrontal network, and in the anterior cingulate cortex and insula in relation to the thalamo-parietal network. Enhanced pain sensitivity from spinal deafferentation correlated with functional connectivity changes within brain regions involved in affective and sensory pain processing and areas involved in descending control of pain.

Effect of remote sensory noise on hand function post stroke

Hand motor impairment persists after stroke. Sensory inputs may facilitate recovery of motor function. This pilot study tested the effectiveness of tactile sensory noise in improving hand motor function in chronic stroke survivors with tactile sensory deficits, using a repeated measures design. Sensory noise in the form of subthreshold, white noise, mechanical vibration was applied to the wrist skin during motor tasks. Hand dexterity assessed by the Nine Hole Peg Test and the Box and Block Test and pinch strength significantly improved when the sensory noise was turned on compared with when it was turned off in chronic stroke survivors. The subthreshold sensory noise to the wrist appears to induce improvements in hand motor function possibly via neuronal connections in the sensoriomotor cortex. The approach of applying concomitant, unperceivable mechanical vibration to the wrist during hand motor tasks is easily adoptable for clinic use as well as unsupervised home use. This pilot study suggests a potential for a wristband-type assistive device to complement hand rehabilitation for stroke survivors with sensorimotor deficit.

Sensorimotor deprivation induces interdependent changes in excitability and plasticity of the human hand motor cortex

Prolonged limb immobilization deprives sensorimotor cortical areas of an important source of excitatory input, as well as of motor output. Previous work has described effects on motor excitability but it is unclear whether motor plasticity is also influenced. In two groups of eight healthy human subjects, the left hand was immobilized for 8 h to induce sensorimotor deprivation of the cortical representation of the abductor pollicis brevis muscle. We used transcranial magnetic stimulation protocols to evaluate motor excitability with motor-evoked potentials, input-output (IOcurve) and short-latency intracortical inhibition (SICI) recruitment curves, as well as long-term potentiation (LTP)/long-term depression (LTD)-like plasticity with paired-associative stimulation (PAS) of the median nerve and motor cortex using an interstimulus interval of 25 ms (PAS25) or 10 ms (PAS10), respectively, in two sessions at least 7 d apart (baseline and after immobilization). After immobilization, the slope of the IOcurve decreased, and SICI at lower conditioning pulse intensities was reduced. The LTP-like effects of PAS25 and the LTD-like effect of PAS10 were both significantly enhanced. The effects differed among individuals: the more IOslope decreased after immobilization, the greater the increase of PAS25 and the smaller the increase of PAS10 effects. We suggest that sensorimotor deprivation has two effects. It increases the sensitivity to remaining sensory inputs and therefore increases the effectiveness of both PAS protocols. In addition, it reduces neuronal excitability to an individually different level, as reflected in the reduced IOslope and leads to an interdependent modulation of synaptic plasticity as such as it shifts the threshold of LTP/LTD-like plasticity induction.

The functional significance of cortical reorganization and the parallel development of CI therapy

For the nineteenth and the better part of the twentieth centuries two correlative beliefs were strongly held by almost all neuroscientists and practitioners in the field of neurorehabilitation. The first was that after maturity the adult CNS was hardwired and fixed, and second that in the chronic phase after CNS injury no substantial recovery of function could take place no matter what intervention was employed. However, in the last part of the twentieth century evidence began to accumulate that neither belief was correct. First, in the 1960s and 1970s, in research with primates given a surgical abolition of somatic sensation from a single forelimb, which rendered the extremity useless, it was found that behavioral techniques could convert the limb into an extremity that could be used extensively. Beginning in the late 1980s, the techniques employed with deafferented monkeys were translated into a rehabilitation treatment, termed Constraint Induced Movement therapy or CI therapy, for substantially improving the motor deficit in humans of the upper and lower extremities in the chronic phase after stroke. CI therapy has been applied successfully to other types of damage to the CNS such as traumatic brain injury, cerebral palsy, multiple sclerosis, and spinal cord injury, and it has also been used to improve function in focal hand dystonia and for aphasia after stroke. As this work was proceeding, it was being shown during the 1980s and 1990s that sustained modulation of afferent input could alter the structure of the CNS and that this topographic reorganization could have relevance to the function of the individual. The alteration in these once fundamental beliefs has given rise to important recent developments in neuroscience and neurorehabilitation and holds promise for further increasing our understanding of CNS function and extending the boundaries of what is possible in neurorehabilitation.

Cortical reorganization after macroreplantation at the upper extremity: a magnetoencephalographic study

With the development of microsurgical techniques, replantation has become a feasible alternative to stump treatment after the amputation of an extremity. It is known that amputation often induces phantom limb pain and cortical reorganization within the corresponding somatosensory areas. However, whether replantation reduces the risk of comparable persisting pain phenomena as well as reorganization of the primary somatosensory cortex is still widely unknown. Therefore, the present study aimed to investigate the potential development of persistent pain and cortical reorganization of the hand and lip areas within the sensory cortex by means of magnetoencephalographic dipole analyses after replantation of a traumatically amputated upper limb proximal to the radiocarpal joint. Cortical reorganization was investigated in 13 patients with limb replantation using air puff stimulation of the phalanges of both thumbs and both corners of the lower lip. Displacement of the centre of gravity of lip and thumb representations and increased cortical activity were found in the limb and face areas of the primary somatosensory cortex contralateral to the replanted arm when compared to the ipsilateral hemisphere. Thus, cortical reorganization in the primary somatosensory cortex also occurs after replantation of the upper extremity. Patients' reports of pain in the replanted body part were negatively correlated with the amount of cortical reorganization, i.e. the more pain the patients reported, the less reorganization of the subjects' hand representation within the primary somatosensory cortex was observed. Longitudinal studies in patients after macroreplantation are necessary to assess whether the observed reorganization in the primary somatosensory cortex is a result of changes within the representation of the replanted arm and/or neighbouring representations and to assess the relationship between the development of persistent pain and reorganization.

Differences in somatosensory and motor improvement during temporary functional deafferentation in stroke patients and healthy subjects

Temporary functional deafferentation is of interest to become an additional tool in neurorehabilitative treatments. Temporary functional deafferentation is known to improve sensory and motor outcomes in chronic stroke patients and healthy subjects. The present study soughts to indicate differences in the efficiency of pharmacologically induced temporary functional deafferentation between chronic stroke patients and matched healthy subjects. 46 chronic stroke patients and 20 age- and gender-matched healthy subjects were deafferented on one forearm by an anesthetic cream. Somatosensory performance was assessed using von-Frey Hair testing and Grating orientation task; motor performance was assessed by means of a shape-sorter-drum task. Grating orientation task and shape-sorter-drum task were significantly improved during temporary functional deafferentation in stroke patients but not in healthy subjects. Von-Frey Hair testing revealed no improvement of absolute tactile thresholds during temporary functional deafferentation in both groups. Furthermore, the stroke patients showed deficits at baseline measurement in all assessments except the von-Frey Hair test. Temporary functional deafferentation of a forearm by an anesthetic cream results in improvements of motor performance and somatosensory discrimination in stroke patients but not in healthy subjects. Therefore, it is reasonable to test in a next step whether temporary functional deafferentation might become an additional tool in motor rehabilitation of post stroke patients.

Effects of temporary functional deafferentation on the brain, sensation, and behavior of stroke patients

Following stroke, many patients suffer from chronic motor impairment and reduced somatosensation in the stroke-affected body parts. Recent experimental studies suggest that temporary functional deafferentation (TFD) of parts of the stroke-affected upper limb or of the less-affected contralateral limb might improve the sensorimotor capacity of the stroke-affected hand. The present study sought evidence of cortical reorganization and related sensory and motor improvements following pharmacologically induced TFD of the stroke-affected forearm. Examination was performed during 2 d of Constraint-Induced Movement Therapy. Thirty-six human patients were deafferented on the stroke-affected forearm by an anesthetic cream (containing lidocaine and prilocaine) on one of the 2 d, and a placebo cream was applied on the other. The order of TFD and placebo treatment was counterbalanced across patients. Somatosensory and motor performance were assessed using a Grating orienting task and a Shape-sorter-drum task, and with somatosensory-evoked magnetic fields. Evoked magnetic fields showed significant pre- to postevaluation magnitude increases in response to tactile stimulation of the thumb of the stroke-affected hand during TFD but not following placebo treatment. We also observed a rapid extension of the distance between cortical representations of the stroke-affected thumb and little finger following TFD but not following placebo treatment. Moreover, somatosensory and motor performance of the stroke-affected hand was significantly enhanced during TFD but not during placebo treatment. Thus, pharmacologically induced TFD of a stroke-affected forearm might improve the somatosensory and motor functions of the stroke-affected upper limb, accompanied by cortical plasticity.

Effects of aging on properties of the local circuit in rat primary somatosensory cortex (S1) in vitro

During aging receptive field properties degrade, the ability of the circuit to process temporal information is impaired and behaviors mediated by the circuit can become impaired. These changes are mediated by changes in the properties of neural circuits, particularly the balance of excitation and inhibition, the intrinsic properties of neurons, and the anatomy of connections in the circuit. In this study, properties of thalamorecipient pyramidal neurons in layer 3 were examined in the hindpaw region of rat primary somatosensory cortex (S1) in vitro. Excitatory and inhibitory postsynaptic currents (IPSCs) resulting from trains of electrical stimulation of thalamocortical afferents were recorded. Excitatory postsynaptic currents were larger in old S1, but showed no difference in temporal dynamics; IPSCs showed significantly less suppression across the train in old S1, partly due to a decrease in GABAB signaling. Neurons in old S1 were more likely to exhibit burst firing, due to an increase in T-current. Significant differences in dendritic morphology were also observed in old S1, accompanied by a decrease in dendritic spine density. These data directly demonstrate changes in the properties of the thalamorecipient circuit in old S1 and help to explain the changes observed in responses during aging.

Changes in corticomotor excitability and inhibition after exercise are influenced by hand dominance and motor demand

Previous studies on handedness have often reported functional asymmetries in corticomotor excitability (CME) associated with voluntary movement. Recently, we have shown that the degree of post-exercise corticomotor depression (PED) and increase in short-interval cortical inhibition (SICI) after a repetitive finger movement task was less when the task was performed at a maximal voluntary rate (MVR) than when it was performed at a submaximal sustainable rate (SR). In the current study, we have compared the time course of PED and SICI in the dominant (DOM) and nondominant (NDOM) hands after an MVR and SR finger movement task to determine the influence of hand dominance and task demand. We tracked motor-evoked potential (MEP) amplitude from the first dorsal interosseous muscle of the DOM and NDOM hand for 20 min after a 10-s index finger flexion-extension task at MVR and SR. For all hand-task combinations, we report a period of PED and increased SICI lasting for up to 8 min. We find that the least demanding task, one that involved index finger movement of the DOM hand at SR, was associated with the greatest change in PED and SICI from baseline (63.6±5.7% and 79±2%, P<0.001, PED and SICI, respectively), whereas the most demanding task (MVR of the NDOM hand) was associated with the least change from baseline (PED: 88.1±3.6%, SICI: 103±2%; P<0.001). Our findings indicate that the changes in CME and inhibition associated with repetitive finger movement are influenced both by handedness and the degree of demand of the motor task and are inversely related to task demand, being smallest for an MVR task of the NDOM hand and greatest for an SR task of the DOM hand. The findings provide additional evidence for differences in neuronal processing between the dominant and nondominant hemispheres in motor control.

Diverse excitatory and inhibitory synaptic plasticity outcomes in complex horizontal circuits near a functional border of adult neocortex

The primary somatosensory cortex (SI) is topographically organized into a map of the body. This organization is dynamic, undergoing experience-dependent modifications throughout life. It has been hypothesized that excitatory and inhibitory synaptic plasticity of horizontal intracortical connections contributes to functional reorganization. However, very little is known about synaptic plasticity of these connections; particularly the characteristics of inhibitory synaptic plasticity, its relationship to excitatory synaptic plasticity, and their relationship to the functional organization of the cortex. To investigate this, we located the border between the forepaw and lower jaw representation of SI in vivo, and used whole cell-patch electrophysiology to record post-synaptic excitatory and inhibitory currents in complex horizontal connections in vitro. Connections that remained within the representation (continuous) and those that crossed from one representation to another (discontinuous) were stimulated differentially, allowing us to examine differences associated with the border. To induce synaptic plasticity, tetanic stimulation was applied to either continuous or discontinuous pathways. Tetanic stimulation induced diverse forms of excitatory and inhibitory synaptic plasticity, with LTP dominating for excitation and LTD dominating for inhibition. The border did not restrict plasticity in either case. In contrast, tetanization elicited LTP of monosynaptic inhibitory responses in continuous, but not discontinuous connections. These results demonstrate that continuous and discontinuous pathways are capable of diverse synaptic plasticity responses that are differentially inducible. Furthermore, continuous connections can undergo monosynaptic inhibitory LTP, independent of excitatory drive onto interneurons. Thus, coordinated excitatory and inhibitory synaptic plasticity of horizontal connections are capable of contributing to functional reorganization.

Deafferentation of the affected arm: a method to improve rehabilitation?

BACKGROUND AND PURPOSE

Reduced somatosensation is a common impairment after stroke. This somatosensory deficit is known to be a reliable predictor of poor rehabilitation outcome. Several methods of physical therapy have addressed this problem, but with only moderate success. Here, we used a new neural plasticity-based approach, ie, a simple, inexpensive, pharmacologically induced temporary functional deafferentation (TFD) of the forearm to investigate whether TFD might result in beneficial effects on the somatosensory sensibility and motor capacity of the stroke-affected hand.

METHODS

Examination was performed over 2 consecutive days of an efficient rehabilitation program for stroke patients referred to as constraint-induced movement therapy. Patients were deafferented on one of these days but not on the other (placebo session). The order of deafferentation and nondeafferentation was counterbalanced across patients. TFD of the stroke-affected forearm was realized using an anesthetic cream. Somatosensory abilities were assessed by a Grating orienting task, and a shape-sorter drum task was used to test motor performance. Both tests were performed each day before and after the constraint-induced movement therapy training session.

RESULTS

We found significantly better outcomes for Grating orienting task and shape-sorter drum task after TFD on the forearm as compared to placebo, indicating increased somatosensory abilities and motor performance in stroke patients using the simple TFD procedure.

CONCLUSIONS

The improvement was achieved during the course of one of the best established poststroke rehabilitation programs, suggesting that TFD on the more affected forearm might become an efficient additional tool in stroke rehabilitation.

The packet switching brain

The computer metaphor has served brain science well as a tool for comprehending neural systems. Nevertheless, we propose here that this metaphor be replaced or supplemented by a new metaphor, the "Internet metaphor," to reflect dramatic new network theoretic understandings of brain structure and function. We offer a "weak" form and a "strong" form of this metaphor: The former suggests that structures and processes unique to Internet-like architectures (e.g., domains and protocols) can profitably guide our thinking about brains, whereas the latter suggests that one particular feature of the Internet-packet switching-may be instantiated in the structure of certain brain networks, particularly mammalian neocortex.

Transcranial imaging of somatotopic map plasticity after tail cut in mice

Peripheral afferent denervation induces reorganization of somatotopic maps in the primary somatosensory cortex (S1). In the present study, we investigated somatotopic map plasticity after tail cut. Neonatal mice at postnatal days (P) 2-3 and adult mice at eight weeks of age were anesthetized with ether, and approximately two thirds of the tail was cut from the tip. Both groups of mice were anesthetized with urethane (1.7g/kg, i.p.) at 10weeks of age, and transcranial flavoprotein fluorescence imaging was performed in the S1. Neural activities in the S1 were elicited by vibratory stimulation applied to the contralateral hindpaw or the tail in control mice. The cortical areas activated by hindpaw, tail base, and tail tip stimuli were placed in this order according to the medial and posterior direction. In mice with tail cut, the tail base area moved to the more medial and posterior area corresponding to the tail tip in control mice. The shift of the tail base area was observed in both neonatal and adult tail cut mice, indicating the absence of a critical period before eight weeks. Medial and posterior shift of the tail base area with regard to the bregma was confirmed in tail cut mice. These data suggest that transcranial flavoprotein fluorescence imaging is a useful technique for investigating somatosensory map plasticity in mice.

Profiles of precentral and postcentral cortical mean thicknesses in individual subjects over acute and subacute time-scales

Human precentral and postcentral cortical areas interact to generate sensorimotor functions. Recent imaging work suggests that pre- and postcentral cortical thicknesses of an individual vary over time-scales of years and decades due to aging, disease, and other factors. In contrast, there is little understanding of how thicknesses of these areas vary in an individual over time-scales of minutes and weeks. This study used longitudinal magnetic resonance imaging (MRI) and computational morphometry approaches in 5 healthy subjects to assess how mean thicknesses, and intra- and interhemispheric relationships in mean thicknesses, of these areas vary in an individual subject over minutes and weeks. Within each individual, absolute differences in thicknesses over these times were small and similar in the precentral (mean = 0.02-0.04 mm) and postcentral (mean = 0.03-0.05 mm) areas. Each individual also had a consistent intrahemispheric disparity and interhemispheric asymmetrical or symmetrical relationship in thicknesses of these areas over these times. The results provide new understanding of within-individual cortical thickness variability in these areas and raise the possibility that longitudinal thickness profiling can provide a baseline definition of short time-scale thickness variability that can be used to detect acute and subacute changes in pre- and postcentral thicknesses at an individual subject level.

From maps to form to space: touch and the body schema

Evidence from patients has shown that primary somatosensory representations are plastic, dynamically changing in response to central or peripheral alterations, as well as experience. Furthermore, recent research has also demonstrated that altering body posture results in changes in the perceived sensation and localization of tactile stimuli. Using evidence from behavioral studies with brain-damaged and healthy subjects, as well as functional imaging, we propose that the traditional concept of the body schema should be divided into three components. First are primary somatosensory representations, which are representations of the skin surface that are typically somatotopically organized, and have been shown to change dynamically due to peripheral (usage, amputation, deafferentation) or central (lesion) modifications. Second, we argue for a mapping from a primary somatosensory representation to a secondary representation of body size and shape (body form representation). Finally, we review evidence for a third set of representations that encodes limb position and is used to represent the location of tactile stimuli relative to the subject using external, non-somatotopic reference frames (postural representations).