The motor cortex and its role in phantom limb phenomena

A preliminary study exploring the effects of transcutaneous spinal cord stimulation on spinal excitability and phantom limb pain in people with a transtibial amputation

Objective. Phantom limb pain (PLP) is debilitating and affects over 70% of people with lower-limb amputation. Other neuropathic pain conditions correspond with increased spinal excitability, which can be measured using reflexes andF-waves. Spinal cord neuromodulation can be used to reduce neuropathic pain in a variety of conditions and may affect spinal excitability, but has not been extensively used for treating PLP. Here, we propose using a non-invasive neuromodulation method, transcutaneous spinal cord stimulation (tSCS), to reduce PLP and modulate spinal excitability after transtibial amputation.Approach. We recruited three participants, two males (5- and 9-years post-amputation, traumatic and alcohol-induced neuropathy) and one female (3 months post-amputation, diabetic neuropathy) for this 5 d study. We measured pain using the McGill Pain Questionnaire (MPQ), visual analog scale (VAS), and pain pressure threshold (PPT) test. We measured spinal reflex and motoneuron excitability using posterior root-muscle (PRM) reflexes andF-waves, respectively. We delivered tSCS for 30 min d-1for 5 d.Main Results. After 5 d of tSCS, MPQ scores decreased by clinically-meaningful amounts for all participants from 34.0 ± 7.0-18.3 ± 6.8; however, there were no clinically-significant decreases in VAS scores. Two participants had increased PPTs across the residual limb (Day 1: 5.4 ± 1.6 lbf; Day 5: 11.4 ± 1.0 lbf).F-waves had normal latencies but small amplitudes. PRM reflexes had high thresholds (59.5 ± 6.1μC) and low amplitudes, suggesting that in PLP, the spinal cord is hypoexcitable. After 5 d of tSCS, reflex thresholds decreased significantly (38.6 ± 12.2μC;p< 0.001).Significance. These preliminary results in this non-placebo-controlled study suggest that, overall, limb amputation and PLP may be associated with reduced spinal excitability and tSCS can increase spinal excitability and reduce PLP.

Smart ArM: a customizable and versatile robotic arm prosthesis platform for Cybathlon and research

BACKGROUND

In the last decade, notable progress in mechatronics paved the way for a new generation of arm prostheses, expanding motor capabilities thanks to their multiple active joints. Yet, the design of control schemes for these advanced devices still poses a challenge, especially with the limited availability of command signals for higher levels of arm impairment. When addressing this challenge, current commercial devices lack versatility and customizing options to be employed as test-beds for developing novel control schemes. As a consequence, researchers resort to using lab-specific experimental apparatuses on which to deploy their innovations, such as virtual reality setups or mock prosthetic devices worn by unimpaired participants.

METHODS

To meet this need for a test-bed, we developed the Smart Arm platform, a human-like, multi-articulated robotic arm that can be worn as a trans-humeral arm prosthesis. The design process followed three principles: provide a reprogrammable embedded system allowing in-depth customization of control schemes, favor easy-to-buy parts rather than custom-made components, and guarantee compatibility with industrial standards in prosthetics.

RESULTS

The Smart ArM platform includes motorized elbow and wrist joints while being compatible with commercial prosthetic hands. Its software and electronic architecture can be easily adapted to build devices with a wide variety of sensors and actuators. This platform was employed in several experiments studying arm prosthesis control and sensory feedback. We also report our participation in Cybathlon, where our pilot with forearm agenesia successfully drives the Smart Arm prosthesis to perform activities of daily living requiring both strength and dexterity.

CONCLUSION

These application scenarios illustrate the versatility and adaptability of the proposed platform, for research purposes as well as outside the lab. The Smart Arm platform offers a test-bed for experimenting with prosthetic control laws and command signals, suitable for running tests in lifelike settings where impaired participants wear it as a prosthetic device. In this way, we aim at bridging a critical gap in the field of upper limb prosthetics: the need for realistic, ecological test conditions to assess the actual benefit of a technological innovation for the end-users.

Changes in Primary Somatosensory Cortex Following Allogeneic Hand Transplantation or Autogenic Hand Replantation

Former amputees who undergo allogeneic hand transplantation or autogenic hand replantation (jointly, "hand restoration") present a unique opportunity to measure the range of post-deafferentation plastic changes in the nervous system, especially primary somatosensory cortex (S1). However, few such patients exist, and previous studies compared single cases to small groups of typical adults. Here, we studied 5 individuals (n = 8 sessions: a transplant with 2 sessions, a transplant with 3 sessions, and three replants with 1 session each). We used functional magnetic resonance imaging (fMRI) to measure S1 responsiveness to controlled pneumatic tactile stimulation delivered to each patient's left and right fingertips and lower face. These data were compared with responses acquired from typical adults (n = 29) and current unilateral amputees (n = 19). During stimulation of the affected hand, patients' affected S1 (contralateral to affected hand) responded to stimulation in a manner similar both to amputees and to typical adults. The presence of contralateral responses indicated grossly typical S1 function, but responses were universally at the low end of the range of typical variability. Patients' affected S1 showed substantial individual variability in responses to stimulation of the intact hand: while all patients fell within the range of typical adults, some patient sessions (4/8) had substantial ipsilateral responses similar to those exhibited by current amputees. Unlike hand restoration patients, current amputees exhibited substantial S1 reorganization compared to typical adults, including bilateral S1 responses to stimulation of the intact hand. In all three participant groups, we assessed tactile localization by measuring individuals' ability to identify the location of touch on the palm and fingers. Curiously, while transplant patients improved their tactile sensory localization over time, this was uncorrelated with changes in S1 responses to tactile stimuli. Overall, our results provide the first description of cortical responses to well-controlled tactile stimulation after hand restoration. Our case studies indicate that hand restoration patients show S1 function within the range of both typical adults and amputees, but with low-amplitude and individual-specific responses that indicate a wide range of potential cortical neurological changes following de-afferentation and re-afferentation.

The various forms of sensorimotor plasticity following limb amputation and their link with rehabilitation strategies

Limb amputation is characterized by complex and intermingled brain reorganization processes combining sensorimotor deprivation induced by the loss of the limb per se, and compensatory behaviors, such as the over-use of the intact or remaining limb. While a large body of evidence documents sensorimotor representation plasticity following arm amputation, less investigations have been performed to fully understand the use-dependent plasticity phenomenon and the role of behavioral compensation in brain reorganization. In this article, I will review the findings on sensorimotor plasticity after limb amputation, focusing on these two aspects: sensorimotor deprivation and adaptive patterns of limb usage, and describe the models that attempt to link these reorganizational processes with phantom limb pain. Two main models have been proposed: the maladaptive plasticity model which states that the reorganization of the adjacent cortical territories into the representation of the missing limb is proportional to phantom pain intensity, and the persistent representation model, which rather suggests that the intensity of residual brain activity associated with phantom hand movements scales with phantom limb pain intensity. I will finally illustrate how this fundamental research helps designing new therapeutic strategies for phantom plain relief.

Surgical prevention of terminal neuroma and phantom limb pain: a literature review

The incidence of extremity amputation is estimated at about 200,000 cases annually. Over 25% of patients suffer from terminal neuroma or phantom limb pain (TNPLP), resulting in pain, inability to wear a prosthetic device, and lost work. Once TNPLP develops, there is no definitive cure. Therefore, there has been an emerging focus on TNPLP prevention. We examined the current literature on TNPLP prevention in patients undergoing extremity amputation. A literature review was performed using Ovid Medline, Cochrane Collaboration Library, and Google Scholar to identify all original studies that addressed surgical prophylaxis against TNPLP. The search was conducted using both Medical Subject Headings and free-text using the terms “phantom limb pain,” “amputation neuroma,” and “surgical prevention of amputation neuroma.” Fifteen studies met the inclusion criteria, including six prospective trials, two comprehensive literature reviews, four retrospective chart reviews, and three case series/technique reviews. Five techniques were identified, and each was incorporated into a target-based classification system. A small but growing body of literature exists regarding the surgical prevention of TNPLP. Targeted muscle reinnervation (TMR), a form of physiologic target reassignment, has the greatest momentum in the academic surgical community, with multiple recent prospective studies demonstrating superior prevention of TNPLP. Neurorrhaphy and transposition with implantation are supported by less robust evidence, but merit future study as alternatives to TMR.

Aberrant activity in an intact residual muscle is associated with phantom limb pain in above-knee amputees

Many individuals who undergo limb amputation experience persistent phantom limb pain (PLP), but the underlying mechanisms of PLP are unknown. The traditional hypothesis was that PLP resulted from maladaptive plasticity in sensorimotor cortex that degrades the neural representation of the missing limb. However, a recent study of individuals with upper limb amputations has shown that PLP is correlated with aberrant electromyographic (EMG) activity in residual muscles, posited to reflect a retargeting of efferent projections from a preserved representation of a missing limb. Here, we assessed EMG activity in a residual thigh muscle (vastus lateralis, VL) in patients with transfemoral amputations during cyclical movements of a phantom foot. VL activity on the amputated side was compared to that recorded on patients' intact side while they moved both the phantom and intact feet synchronously. VL activity in the patient group was also compared to a sample of control participants with no amputation. We show that phantom foot movement is associated with greater VL activity in the amputated leg than that seen in the intact leg as well as that exhibited by controls. The magnitude of residual VL activity was also positively related to ratings of PLP. These results show that phantom limb movement is associated with aberrant activity in a residual muscle after lower-limb amputation and provide evidence of a positive relationship between this activity and phantom limb pain.NEW & NOTEWORTHY This study is the first to assess residual muscle activity during movement of a phantom limb in individuals with lower limb amputations. We find that phantom foot movement is associated with aberrant recruitment of a residual thigh muscle and that this aberrant activity is related to phantom limb pain.

Short-Term Suppression of Somatosensory Evoked Potentials and Perceived Sensations in Healthy Subjects Following TENS

Transcutaneous electrical nerve stimulation (TENS) has been reported to alleviate pain in chronic pain patients. Currently, there is limited knowledge how TENS affects can cause cortical neuromodulation and lead to modulation of non-painful and painful sensations. Our aim was therefore to investigate the effect of conventional, high-frequency TENS on cortical activation and perceived sensations in healthy subjects. We recorded somatosensory evoked potentials (SEPs) and perceived sensations following high-frequency TENS (100 Hz) in 40 healthy subjects (sham and intervention group). The effect of TENS was examined up to an hour after the intervention phase, and results revealed significant cortical inhibition. We found that the magnitude of N100, P200 waves, and theta and alpha band power was significantly suppressed following the TENS intervention. These changes were associated with a simultaneous reduction in the perceived intensity and the size of the area where the sensation was felt. Although phantom limb pain relief previously has been associated with an inhibition of cortical activity, the efficacy of the present TENS intervention to induce such cortical inhibition and cause pain relief should be verified in a future clinical trial.

Sensory stimulation enhances phantom limb perception and movement decoding

OBJECTIVE

A major challenge for controlling a prosthetic arm is communication between the device and the user's phantom limb. We show the ability to enhance phantom limb perception and improve movement decoding through targeted transcutaneous electrical nerve stimulation in individuals with an arm amputation.

APPROACH

Transcutaneous nerve stimulation experiments were performed with four participants with arm amputation to map phantom limb perception. We measured myoelectric signals during phantom hand movements before and after participants received sensory stimulation. Using electroencephalogram (EEG) monitoring, we measured the neural activity in sensorimotor regions during phantom movements and stimulation. In one participant, we also tracked sensory mapping over 2 years and movement decoding performance over 1 year.

MAIN RESULTS

Results show improvements in the participants' ability to perceive and move the phantom hand as a result of sensory stimulation, which leads to improved movement decoding. In the extended study with one participant, we found that sensory mapping remains stable over 2 years. Sensory stimulation improves within-day movement decoding while performance remains stable over 1 year. From the EEG, we observed cortical correlates of sensorimotor integration and increased motor-related neural activity as a result of enhanced phantom limb perception.

SIGNIFICANCE

This work demonstrates that phantom limb perception influences prosthesis control and can benefit from targeted nerve stimulation. These findings have implications for improving prosthesis usability and function due to a heightened sense of the phantom hand.

Sensory- and Action-Oriented Embodiment of Neurally-Interfaced Robotic Hand Prostheses

Embodiment is the percept that something not originally belonging to the self becomes part of the body. Feeling embodiment for a prosthesis may counteract amputees' altered image of the body and increase prosthesis acceptability. Prosthesis embodiment has been studied longitudinally in an amputee receiving feedback through intraneural and perineural multichannel electrodes implanted in her stump. Three factors-invasive (vs non-invasive) stimulation, training, and anthropomorphism-have been tested through two multisensory integration tasks: visuo-tactile integration (VTI) and crossing-hand effect in temporal order judgment (TOJ), the former more sensible to an extension of a safe margin around the body and the latter to action-oriented remapping. Results from the amputee participant were compared with the ones from healthy controls. Testing the participant with intraneural stimulation produced an extension of peripersonal space, a sign of prosthesis embodiment. One-month training extended the peripersonal space selectively on the side wearing the prostheses. More and less-anthropomorphic prostheses benefited of intraneural feedback and extended the peripersonal space. However, the worsening of TOJ performance following arm crossing was present only wearing the more trained, despite less anthropomorphic, prosthesis, suggesting that training was critical for our participant to achieve operative tool-like embodiment.

Peripheral input and phantom limb pain: A somatosensory event-related potential study

BACKGROUND

Following amputation, nearly all amputees report nonpainful phantom phenomena and many of them suffer from chronic phantom limb pain (PLP) and residual limb pain (RLP). The aetiology of PLP remains elusive and there is an ongoing debate on the role of peripheral and central mechanisms. Few studies have examined the entire somatosensory pathway from the truncated nerves to the cortex in amputees with PLP compared to those without PLP. The relationship among afferent input, somatosensory responses and the change in PLP remains unclear.

METHODS

Transcutaneous electrical nerve stimulation was applied on the truncated median nerve, the skin of the residual limb and the contralateral homologous nerve in 22 traumatic upper-limb amputees (12 with and 10 without PLP). Using somatosensory event-related potentials, the ascending volley was monitored from the brachial plexus, the spinal cord, the brainstem and the thalamus to the primary somatosensory cortex.

RESULTS

Peripheral input could evoke PLP in amputees with chronic PLP (7/12), but not in amputees without a history of PLP (0/10). The amplitudes of the somatosensory components were comparable between amputees with and without PLP. In addition, evoked potentials from the periphery through the spinal, subcortical and cortical segments were not significantly associated with PLP.

CONCLUSIONS

Peripheral input can modulate PLP but seems insufficient to cause PLP. These findings suggest the multifactorial complexity of PLP and different mechanisms for PLP and RLP.

SIGNIFICANCE

Peripheral afferent input plays a role in PLP and has been assumed to be sufficient to generate PLP. In this study we found no significant differences in the electrical potentials generated by peripheral stimulation from the truncated nerve and the skin of the residual limb in amputees with and without PLP. Peripheral input could enhance existing PLP but could not cause it. These findings indicate the multifactorial complexity of PLP and an important role of central processes in PLP.

Examination of Regional Interdependence Theory in Chronic Neck Pain: Interpretations from Correlation of Strength Measures in Cervical and Pain-Free Regions

OBJECTIVE

Impairments present in chronic pain conditions have been reported not to be limited to the painful region. Pain-free regions have also been proposed to be adversely affected as a cause or consequence of the painful condition. The aim of this study was to investigate the association between muscle strength in painful and pain-free regions and chronic neck pain.

DESIGN

A cross-sectional study.

SETTING

Rehabilitation hospital laboratory.

SUBJECTS

One hundred twenty-two patients with chronic neck pain (87 female) and 98 asymptomatic volunteers (52 female) were included in the study.

METHODS

Maximal isometric strength measures of the neck, scapulothoracic, shoulder, trunk, and hip muscles were assessed using a hand-held dynamometer in all participants. Pain intensity and pain-related disability were also assessed in patients through visual analog scale and Neck Disability Index scores, respectively.

RESULTS

Principal component analysis revealed one component for each of the studied regions. Multivariate analysis of variance found neck (d = 0.46), scapulothoracic (d = 0.46), shoulder (d = 0.60), trunk flexor (d = 0.38), extensor (d = 0.36), and hip (d = 0.51) strength components to be lower in the neck pain patients compared with asymptomatic participants (P < 0.01). Logistic and linear regression analyses found the shoulder strength component both to be a significant predictor for neck pain occurrence (β = 0.53, P < 0.01) and to have a considerable effect on pain intensity score (β=-0.20, P = 0.02), respectively.

CONCLUSIONS

The results found that some pain-free regions in addition to the cervical spine to exhibit lower levels of muscular strength in neck pain patients. These findings support the regional interdependence theory, which proposes that impairments are not limited to the painful area and are possibly mediated by central mechanisms.

Augmented manipulation ability in humans with six-fingered hands

Neurotechnology attempts to develop supernumerary limbs, but can the human brain deal with the complexity to control an extra limb and yield advantages from it? Here, we analyzed the neuromechanics and manipulation abilities of two polydactyly subjects who each possess six fingers on their hands. Anatomical MRI of the supernumerary finger (SF) revealed that it is actuated by extra muscles and nerves, and fMRI identified a distinct cortical representation of the SF. In both subjects, the SF was able to move independently from the other fingers. Polydactyly subjects were able to coordinate the SF with their other fingers for more complex movements than five fingered subjects, and so carry out with only one hand tasks normally requiring two hands. These results demonstrate that a body with significantly more degrees-of-freedom can be controlled by the human nervous system without causing motor deficits or impairments and can instead provide superior manipulation abilities.

A functional Magnetic Resonance Imaging study of patients with Polar Type II/III complex shoulder instability

The pathophysiology of Stanmore Classification Polar type II/III shoulder instability is not well understood. Functional Magnetic Resonance Imaging was used to measure brain activity in response to forward flexion and abduction in 16 patients with Polar Type II/III shoulder instability and 16 age-matched controls. When a cluster level correction was applied patients showed significantly greater brain activity than controls in primary motor cortex (BA4), supramarginal gyrus (BA40), inferior frontal gyrus (BA44), precentral gyrus (BA6) and middle frontal gyrus (BA6): the latter region is considered premotor cortex. Using voxel level correction within these five regions a unique activation was found in the primary motor cortex (BA4) at MNI coordinates -38 -26 56. Activation was greater in controls compared to patients in the parahippocampal gyrus (BA27) and perirhinal cortex (BA36). These findings show, for the first time, neural differences in patients with complex shoulder instability, and suggest that patients are in some sense working harder or differently to maintain shoulder stability, with brain activity similar to early stage motor sequence learning. It will help to understand the condition, design better therapies and improve treatment of this group; avoiding the common clinical misconception that their recurrent shoulder dislocations are a form of attention-seeking.

Upper limb cortical maps in amputees with targeted muscle and sensory reinnervation

Neuroprosthetics research in amputee patients aims at developing new prostheses that move and feel like real limbs. Targeted muscle and sensory reinnervation (TMSR) is such an approach and consists of rerouting motor and sensory nerves from the residual limb towards intact muscles and skin regions. Movement of the myoelectric prosthesis is enabled via decoded electromyography activity from reinnervated muscles and touch sensation on the missing limb is enabled by stimulation of the reinnervated skin areas. Here we ask whether and how motor control and redirected somatosensory stimulation provided via TMSR affected the maps of the upper limb in primary motor (M1) and primary somatosensory (S1) cortex, as well as their functional connections. To this aim, we tested three TMSR patients and investigated the extent, strength, and topographical organization of the missing limb and several control body regions in M1 and S1 at ultra high-field (7 T) functional magnetic resonance imaging. Additionally, we analysed the functional connectivity between M1 and S1 and of both these regions with fronto-parietal regions, known to be important for multisensory upper limb processing. These data were compared with those of control amputee patients (n = 6) and healthy controls (n = 12). We found that M1 maps of the amputated limb in TMSR patients were similar in terms of extent, strength, and topography to healthy controls and different from non-TMSR patients. S1 maps of TMSR patients were also more similar to normal conditions in terms of topographical organization and extent, as compared to non-targeted muscle and sensory reinnervation patients, but weaker in activation strength compared to healthy controls. Functional connectivity in TMSR patients between upper limb maps in M1 and S1 was comparable with healthy controls, while being reduced in non-TMSR patients. However, connectivity was reduced between S1 and fronto-parietal regions, in both the TMSR and non-TMSR patients with respect to healthy controls. This was associated with the absence of a well-established multisensory effect (visual enhancement of touch) in TMSR patients. Collectively, these results show how M1 and S1 process signals related to movement and touch are enabled by targeted muscle and sensory reinnervation. Moreover, they suggest that TMSR may counteract maladaptive cortical plasticity typically found after limb loss, in M1, partially in S1, and in their mutual connectivity. The lack of multisensory interaction in the present data suggests that further engineering advances are necessary (e.g. the integration of somatosensory feedback into current prostheses) to enable prostheses that move and feel as real limbs.

Motor correlates of phantom limb pain

Following amputation, individuals ubiquitously report experiencing lingering sensations of their missing limb. While phantom sensations can be innocuous, they are often manifested as painful. Phantom limb pain (PLP) is notorious for being difficult to monitor and treat. A major challenge in PLP management is the difficulty in assessing PLP symptoms, given the physical absence of the affected body part. Here, we offer a means of quantifying chronic PLP by harnessing the known ability of amputees to voluntarily move their phantom limbs. Upper-limb amputees suffering from chronic PLP performed a simple finger-tapping task with their phantom hand. We confirm that amputees suffering from worse chronic PLP had worse motor control over their phantom hand. We further demonstrate that task performance was consistent over weeks and did not relate to transient PLP or non-painful phantom sensations. Finally, we explore the neural basis of these behavioural correlates of PLP. Using neuroimaging, we reveal that slower phantom hand movements were coupled with stronger activity in the primary sensorimotor phantom hand cortex, previously shown to associate with chronic PLP. By demonstrating a specific link between phantom hand motor control and chronic PLP, our findings open up new avenues for PLP management and improvement of existing PLP treatments.

Sensorimotor Cortical Neuroplasticity in the Early Stage of Bell's Palsy

Neuroplasticity is a common phenomenon in the human brain following nerve injury. It is defined as the brain's ability to reorganize by creating new neural pathways in order to adapt to change. Here, we use task-related and resting-state fMRI to investigate neuroplasticity in the primary sensory (S1) and motor cortex (M1) in patients with acute Bell's palsy (BP). We found that the period directly following the onset of BP (less than 14 days) is associated with significant decreases in regional homogeneity (ReHo), fractional amplitude of low frequency fluctuations (fALFF), and intrinsic connectivity contrast (ICC) values in the contralateral S1/M1 and in ReHo and ICC values in the ipsilateral S1/M1, compared to healthy controls. The regions with decreased ReHo, fALFF, and ICC values were in both the face and hand region of S1/M1 as indicated by resting-state fMRI but not task-related fMRI. Our results suggest that the early stages of BP are associated with functional neuroplasticity in both the face and hand regions of S1/M1 and that resting-state functional fMRI may be a sensitive tool to detect these early stages of plasticity in patient populations.

Revealing the neural fingerprints of a missing hand

The hand area of the primary somatosensory cortex contains detailed finger topography, thought to be shaped and maintained by daily life experience. Here we utilise phantom sensations and ultra high-field neuroimaging to uncover preserved, though latent, representation of amputees’ missing hand. We show that representation of the missing hand’s individual fingers persists in the primary somatosensory cortex even decades after arm amputation. By demonstrating stable topography despite amputation, our finding questions the extent to which continued sensory input is necessary to maintain organisation in sensory cortex, thereby reopening the question what happens to a cortical territory once its main input is lost. The discovery of persistent digit topography of amputees’ missing hand could be exploited for the development of intuitive and fine-grained control of neuroprosthetics, requiring neural signals of individual digits.

DOI:http://dx.doi.org/10.7554/eLife.15292.001

The brain has a remarkable ability to adapt to changes in circumstances. But what happens to the brain when it loses a key source of input, for example, following the amputation of a limb? A region of the brain known as primary somatosensory cortex processes sensory inputs from all over the body. The more sensitive an area of the body is, the more fine-grained its representation is in the cortex. For example, the hand is represented with a highly detailed map, with each finger represented seperately.

The brain is thought to require ongoing sensory signals from the body to maintain these detailed representations in the cortex. Indeed, textbooks typically state that the brain will ‘overwrite’ its representation of a body part if input from that area no longer arrives. According to this view, people who have lost a hand should show little or no activity in the area of primary somatosensory cortex that used to represent it.

However, many people who have had a limb amputated continue to experience vivid sensations of the missing limb long after its loss. When asked to move their so-called ‘phantom’ limb, these individuals report being able to feel the movement. Kikkert, Kolasinski et al. now show, using advanced imaging techniques, that the brains of individuals with phantom hands continue to represent the missing hand several decades after its loss. Indeed, asking the subjects to move individual fingers of their phantom hand activates fine-grained representations of those fingers, similar to those seen in two-handed controls.

By showing that the brain ‘remembers’ an amputated hand, Kikkert, Kolasinski et al. demonstrate that ongoing sensory input is not required to maintain representations of the body in somatosensory cortex. This, in turn, offers new hope for developing prosthetic limbs that are under direct brain control. If the brain continues to represent individual fingers many years after their loss, it should be possible to exploit those pathways to achieve intuitive fine-grained control of artificial fingers.

DOI:http://dx.doi.org/10.7554/eLife.15292.002

Repetitive Transcranial Magnetic Stimulation for Phantom Limb Pain in Land Mine Victims: A Double-Blinded, Randomized, Sham-Controlled Trial

We evaluated the effects of repetitive transcranial magnetic stimulation (rTMS) in the treatment of phantom limb pain (PLP) in land mine victims. Fifty-four patients with PLP were enrolled in a randomized, double-blinded, placebo-controlled, parallel group single-center trial. The intervention consisted of real or sham rTMS of M1 contralateral to the amputated leg. rTMS was given in series of 20 trains of 6-second duration (54-second intertrain, intensity 90% of motor threshold) at a stimulation rate of 10 Hz (1,200 pulses), 20 minutes per day, during 10 days. For the control group, a sham coil was used. The administration of active rTMS induced a significantly greater reduction in pain intensity (visual analogue scale scores) 15 days after treatment compared with sham stimulation (-53.38 ± 53.12% vs -22.93 ± 57.16%; mean between-group difference = 30.44%, 95% confidence interval, .30-60.58; P = .03). This effect was not significant 30 days after treatment. In addition, 19 subjects (70.3%) attained a clinically significant pain reduction (>30%) in the active group compared with 11 in the sham group (40.7%) 15 days after treatment (P = .03). The administration of 10 Hz rTMS on the contralateral primary motor cortex for 2 weeks in traumatic amputees with PLP induced significant clinical improvement in pain.

PERSPECTIVE

High-frequency rTMS on the contralateral primary motor cortex of traumatic amputees induced a clinically significant pain reduction up to 15 days after treatment without any major secondary effect. These results indicate that rTMS is a safe and effective therapy in patients with PLP caused by land mine explosions.

Revealing humans' sensorimotor functions with electrical cortical stimulation

Direct electrical stimulation (DES) of the human brain has been used by neurosurgeons for almost a century. Although this procedure serves only clinical purposes, it generates data that have a great scientific interest. Had DES not been employed, our comprehension of the organization of the sensorimotor systems involved in movement execution, language production, the emergence of action intentionality or the subjective feeling of movement awareness would have been greatly undermined. This does not mean, of course, that DES is a gold standard devoid of limitations and that other approaches are not of primary importance, including electrophysiology, modelling, neuroimaging or psychophysics in patients and healthy subjects. Rather, this indicates that the contribution of DES cannot be restricted, in humans, to the ubiquitous concepts of homunculus and somatotopy. DES is a fundamental tool in our attempt to understand the human brain because it represents a unique method for mapping sensorimotor pathways and interfering with the functioning of localized neural populations during the performance of well-defined behavioural tasks.

Immediate and Sustained Effects of 5-Day Transcranial Direct Current Stimulation of the Motor Cortex in Phantom Limb Pain

The study explored the analgesic effects of transcranial direct current stimulation (tDCS) over the motor cortex on postamputation phantom limb pain (PLP). Eight subjects with unilateral lower or upper limb amputation and chronic PLP were enrolled in a crossover, double-blind, sham-controlled treatment program. For 5 consecutive days, anodal (active or sham) tDCS was applied over the motor cortex for 15 minutes at an intensity of 1.5 mA. The 5-day treatment with active, but not sham, tDCS induced a sustained decrease in background PLP and in the frequency of PLP paroxysms, which lasted for 1 week after the end of treatment. Moreover, on each day of active tDCS, patients reported an immediate PLP relief, along with an increased ability to move their phantom limb. Patients' immediate responses to sham tDCS, on the contrary, were variable, marked by an increase or decrease of PLP levels from baseline. These results show that a 5-day treatment of motor cortex stimulation with tDCS can induce stable relief from PLP in amputees. Neuromodulation targeting the motor cortex appears to be a promising option for the management of this debilitating neuropathic pain condition, which is often refractory to classic pharmacologic and surgical treatments.

PERSPECTIVE

The study describes sustained and immediate effects of motor cortex stimulation by tDCS on postamputation PLP, whose analgesic action seems linked to the motor reactivation of the phantom limb. These results are helpful for the exploitation of tDCS as a therapeutic tool for the management of neuropathic pain.

Observation of limb movements reduces phantom limb pain in bilateral amputees

Background

Mirror therapy has been demonstrated to reduce phantom limb pain (PLP) experienced by unilateral limb amputees. Research suggests that the visual feedback of observing a limb moving in the mirror is critical for therapeutic efficacy.

Objective

Since mirror therapy is not an option for bilateral lower limb amputees, the purpose of this study was to determine if direct observation of another person’s limbs could be used to relieve PLP.

Methods

We randomly assigned 20 bilateral lower limb amputees with PLP to visual observation (n = 11) or mental visualization (n = 9) treatment. Treatment consisted of seven discrete movements which were mimicked by the amputee’s phantom limbs moving while visually observing the experimenter’s limbs moving, or closing the eyes while visualizing and attempting the movements with their phantom limbs, respectively. Participants performed movements for 20 min daily for 1 month. Response to therapy was measured using a 100-mm visual analog scale (VAS) and the McGill Short-Form Pain Questionnaire (SF-MPQ).

Results

Direct visual observation significantly reduced PLP in both legs (P < 0.05). Amputees assigned to the mental visualization condition did not show a significant reduction in PLP.

Interpretation

Direct visual observation therapy is an inexpensive and effective treatment for PLP that is accessible to bilateral lower limb amputees.

Neural representations of ethologically relevant hand/mouth synergies in the human precentral gyrus

Complex motor responses are often thought to result from the combination of elemental movements represented at different neural sites. However, in monkeys, evidence indicates that some behaviors with critical ethological value, such as self-feeding, are represented as motor primitives in the precentral gyrus (PrG). In humans, such primitives have not yet been described. This could reflect well-known interspecies differences in the organization of sensorimotor regions (including PrG) or the difficulty of identifying complex neural representations in peroperative settings. To settle this alternative, we focused on the neural bases of hand/mouth synergies, a prominent example of human behavior with high ethological value. By recording motor- and somatosensory-evoked potentials in the PrG of patients undergoing brain surgery (2-60 y), we show that two complex nested neural representations can mediate hand/mouth actions within this structure: (i) a motor representation, resembling self-feeding, where electrical stimulation causes the closing hand to approach the opening mouth, and (ii) a motor-sensory representation, likely associated with perioral exploration, where cross-signal integration is accomplished at a cortical site that generates hand/arm actions while receiving mouth sensory inputs. The first finding extends to humans' previous observations in monkeys. The second provides evidence that complex neural representations also exist for perioral exploration, a finely tuned skill requiring the combination of motor and sensory signals within a common control loop. These representations likely underlie the ability of human children and newborns to accurately produce coordinated hand/mouth movements, in an otherwise general context of motor immaturity.

Cortical motor activity and reorganization following upper-limb amputation and subsequent targeted reinnervation

Previous studies have postulated that the amount of brain reorganization following peripheral injuries may be correlated with negative symptoms or consequences. However, it is unknown whether restoring effective limb function may then be associated with further changes in the expression of this reorganization. Recently, targeted reinnervation (TR), a surgical technique that restores a direct neural connection from amputated sensorimotor nerves to new peripheral targets such as muscle, has been successfully applied to upper-limb amputees. It has been shown to be effective in restoring both peripheral motor and sensory functions via the reinnervated nerves as soon as a few months after the surgery. However, it was unclear whether TR could also restore normal cortical motor representations for control of the missing limb. To answer this question, we used high-density electroencephalography (EEG) to localize cortical activity related to cued motor tasks generated by the intact and missing limb. Using a case study of 3 upper-limb amputees, 2 of whom went through pre and post-TR experiments, we present unique quantitative evidence for the re-mapping of motor representations for the missing limb closer to their original locations following TR. This provides evidence that an effective restoration of peripheral function from TR can be linked to the return of more normal cortical expression for the missing limb. Therefore, cortical mapping may be used as a potential guide for monitoring rehabilitation following peripheral injuries.

Amputation and prosthesis implantation shape body and peripersonal space representations

Little is known about whether and how multimodal representations of the body (BRs) and of the space around the body (Peripersonal Space, PPS) adapt to amputation and prosthesis implantation. In order to investigate this issue, we tested BR in a group of upper limb amputees by means of a tactile distance perception task and PPS by means of an audio-tactile interaction task. Subjects performed the tasks with stimulation either on the healthy limb or the stump of the amputated limb, while wearing or not wearing their prosthesis. When patients performed the tasks on the amputated limb, without the prosthesis, the perception of arm length shrank, with a concurrent shift of PPS boundaries towards the stump. Conversely, wearing the prosthesis increased the perceived length of the stump and extended the PPS boundaries so as to include the prosthetic hand, such that the prosthesis partially replaced the missing limb.

Re-establishing the merits of electrical brain stimulation

During the past decades, direct electrical stimulation (DES) has been a key method not only in determining the organization of brain networks mediating movement, language, and cognition but also in establishing many central concepts of modern neuroscience, such as the electrical nature of neural transmission, the localization of brain functions, and the homuncular arrangement of sensorimotor areas. However, recent criticisms have questioned the utility of DES and argued that data collected with this technique may be flawed and unreliable. As with every other neuroscientific method, DES does have limitations. However, existing evidence argues strongly for its validity and usefulness by demonstrating that DES produces highly specific outcomes at well-defined anatomical sites and significantly minimizes postoperative deficits in brain-damaged patients.

[Functional magnetic resonance imaging. What are the benefits expected in hand surgery?]

Functional MRI (fMRI) allowed considerable advances upon understanding of cerebral functioning. Cortical plasticity, which allows the voluntary command of a restored function by a transferred muscle remains to be investigated in its intimacy. The authors present here the round table held at the 48th annual meeting of the French Society for Surgery of the Hand on December 22nd, 2012. It tries to review the analysis of the phenomenon observed during multiple tendinous transfers for restoration of proximal radial nerve palsy. Were successively approached: 1) Methods of acquisition and analysis of the signals (C. D-M.); 2) Movement reorganization (O.M.); 3) Motor plasticity after hand allograft (A. S.); 4) The potential interest of the fMRI in hand rehabilitation (F. D.); 5) The analysis of cerebral plasticity in general (H. B.). A rather philosophical conclusion opens other fields to f MRI (A.M.).

Motor and parietal cortex stimulation for phantom limb pain and sensations

Limb amputation may lead to chronic painful sensations referred to the absent limb, ie phantom limb pain (PLP), which is likely subtended by maladaptive plasticity. The present study investigated whether transcranial direct current stimulation (tDCS), a noninvasive technique of brain stimulation that can modulate neuroplasticity, can reduce PLP. In 2 double-blind, sham-controlled experiments in subjects with unilateral lower or upper limb amputation, we measured the effects of a single session of tDCS (2 mA, 15 min) of the primary motor cortex (M1) and of the posterior parietal cortex (PPC) on PLP, stump pain, nonpainful phantom limb sensations and telescoping. Anodal tDCS of M1 induced a selective short-lasting decrease of PLP, whereas cathodal tDCS of PPC induced a selective short-lasting decrease of nonpainful phantom sensations; stump pain and telescoping were not affected by parietal or by motor tDCS. These findings demonstrate that painful and nonpainful phantom limb sensations are dissociable phenomena. PLP is associated primarily with cortical excitability shifts in the sensorimotor network; increasing excitability in this system by anodal tDCS has an antalgic effect on PLP. Conversely, nonpainful phantom sensations are associated to a hyperexcitation of PPC that can be normalized by cathodal tDCS. This evidence highlights the relationship between the level of excitability of different cortical areas, which underpins maladaptive plasticity following limb amputation and the phenomenology of phantom limb, and it opens up new opportunities for the use of tDCS in the treatment of PLP.

The proprioceptive senses: their roles in signaling body shape, body position and movement, and muscle force

This is a review of the proprioceptive senses generated as a result of our own actions. They include the senses of position and movement of our limbs and trunk, the sense of effort, the sense of force, and the sense of heaviness. Receptors involved in proprioception are located in skin, muscles, and joints. Information about limb position and movement is not generated by individual receptors, but by populations of afferents. Afferent signals generated during a movement are processed to code for endpoint position of a limb. The afferent input is referred to a central body map to determine the location of the limbs in space. Experimental phantom limbs, produced by blocking peripheral nerves, have shown that motor areas in the brain are able to generate conscious sensations of limb displacement and movement in the absence of any sensory input. In the normal limb tendon organs and possibly also muscle spindles contribute to the senses of force and heaviness. Exercise can disturb proprioception, and this has implications for musculoskeletal injuries. Proprioceptive senses, particularly of limb position and movement, deteriorate with age and are associated with an increased risk of falls in the elderly. The more recent information available on proprioception has given a better understanding of the mechanisms underlying these senses as well as providing new insight into a range of clinical conditions.

Cortical depression and potentiation: basic mechanisms for phantom pain

People experience the feeling of the missing body part long after it has been removed after amputation are known as phantom limb sensations. These sensations can be painful, sometimes becoming chronic and lasting for several years (or called phantom pain). Medical treatment for these individuals is limited. Recent neurobiological investigations of brain plasticity after amputation have revealed new insights into the changes in the brain that may cause phantom limb sensations and phantom pain. In this article, I review recent progresses of the cortical plasticity in the anterior cingulate cortex (ACC), a critical cortical area for pain sensation, and explore how they are related to abnormal sensory sensations such as phantom pain. An understanding of these alterations may guide future research into medical treatment for these disorders.

Functional expansion of sensorimotor representation and structural reorganization of callosal connections in lower limb amputees

Previous studies have indicated that amputation or deafferentation of a limb induces functional changes in sensory (S1) and motor (M1) cortices, related to phantom limb pain. However, the extent of cortical reorganization after lower limb amputation in patients with nonpainful phantom phenomena remains uncertain. In this study, we combined functional magnetic resonance (fMRI) and diffusion tensor imaging (DTI) to investigate the existence and extent of cortical and callosal plasticity in these subjects. Nine "painless" patients with lower limb amputation and nine control subjects (sex- and age-matched) underwent a 3-T MRI protocol, including fMRI with somatosensory stimulation. In amputees, we observed an expansion of activation maps of the stump in S1 and M1 of the deafferented hemisphere, spreading to neighboring regions that represent the trunk and upper limbs. We also observed that tactile stimulation of the intact foot in amputees induced a greater activation of ipsilateral S1, when compared with controls. These results demonstrate a functional remapping of S1 in lower limb amputees. However, in contrast to previous studies, these neuroplastic changes do not appear to be dependent on phantom pain but do also occur in those who reported only the presence of phantom sensation without pain. In addition, our findings indicate that amputation of a limb also induces changes in the cortical representation of the intact limb. Finally, DTI analysis showed structural changes in the corpus callosum of amputees, compatible with the hypothesis that phantom sensations may depend on inhibitory release in the sensorimotor cortex.

The map is not the territory: motor system reorganization in upper limb amputees

It is generally considered that hand amputation changes primary motor cortex (M1) stump muscle representations. Transcranial magnetic stimulation (TMS) studies show that the corticospinal excitability of a stump muscle and its homologous muscle on the intact side is not equivalent, and that the resting level of excitability is higher in the stump muscle. Since changes in M1 stump muscle map characteristics (e.g., size and location) are identified by comparing stump and intact muscle maps, such changes might reflect between-side differences in corticospinal excitability rather than a true reorganization of the stump muscle's map. In eight above-elbow amputees we used TMS to map the M1 representation of a stump muscle and its homologous muscle on the intact side during rest and contraction. Importantly, the same relative stimulation intensity was used to construct each map; stimulation was performed at 120% of the motor threshold of each muscle (intact/amputated limb) measured in each condition (rest/active contraction). Resting motor threshold was lower in the stump muscle, but active motor thresholds did not differ. Motor-evoked potential amplitudes increased between the rest and muscle contraction conditions, but this increase was smaller for the stump muscle because its at-rest corticospinal excitability was higher than that of the intact muscle. When the between-side difference in excitability was considered no interhemispheric difference was found for map areas or for their medio-lateral locations. The present results challenge the view that after an upper limb amputation the stump representation moves laterally and occupies a larger M1 territory.

Prevention of chronic pain after surgical nerve injury: amputation and thoracotomy

Although techniques for acute pain management have improved in recent years, a dramatic reduction in the incidence and severity of chronic pain following surgery has not occurred. Amputation and thoracotomy, although technically different, share the commonalities of unavoidable nerve injury and the frequent presence of persistent postsurgical neuropathic pain. The authors review the risk factors for the development of chronic pain following these surgeries and the current evidence that supports analgesic interventions. The inconclusive results from many preemptive analgesic studies may require us to reconceptualize the perioperative treatment period as a time of gradual neurologic remodeling.

Former hand territory activity increases after amputation during intact hand movements, but is unaffected by illusory visual feedback

BACKGROUND

In healthy adults, hand movements are controlled largely by the contralateral primary motor cortex. Following amputation, however, movements of the intact hand are accompanied by increased activity in the sensorimotor cortices of both cerebral hemispheres.

OBJECTIVE

The authors tested whether use of the intact hand reactivates the cortical territory formerly devoted to the now missing hand and whether these effects can be augmented by motor imagery (MI) and/or exposure to illusory visual "feedback" (VF) of the absent hand created with a mirror.

METHODS

Functional magnetic resonance imaging (fMRI) was used to delineate the boundaries of normative sensorimotor hand representations in healthy controls. Brain activity from 11 unilateral hand amputees was recorded while they performed aurally paced thumb-finger sequencing movements with their intact hands under 4 conditions: (1) motor execution of the intact hand alone (ME), (2) ME with corresponding MI of the amputated hand, (3) ME with VF of the amputated hand, and (4) ME with MI and VF.

RESULTS

Intact hand movements increased activity specifically within the former sensorimotor hand territory during all conditions, an effect that may be attributable to decreased levels of interhemispheric inhibition and/or use-dependent functional reorganization following amputation. This effect was not significantly increased by the addition of VF and/or MI of the amputated hand. However, in amputees, MI was associated with an expansion of this ipsilateral response into parietal, premotor, and presupplementary motor areas.

CONCLUSION

Active engagement of the intact hand may be critical for therapies seeking to stimulate the former hand territory.

Mirror-sensory synaesthesia: exploring 'shared' sensory experiences as synaesthesia

Recent research suggests the observation or imagination of somatosensory stimulation in another (e.g., touch or pain) can induce a similar somatosensory experience in oneself. Some researchers have presented this experience as a type of synaesthesia, whereas others consider it an extreme experience of an otherwise normal perception. Here, we present an argument that these descriptions are not mutually exclusive. They may describe the extreme version of the normal process of understanding somatosensation in others. It becomes synaesthesia, however, when this process results in a conscious experience comparable to the observed person's state. We describe these experiences as 'mirror-sensory synaesthesia'; a type of synaesthesia identified by its distinct social component where the induced synaesthetic experience is a similar sensory experience to that perceived in another person. Through the operationalisation of this intriguing experience as synaesthesia, existing neurobiological models of synaesthesia can be used as a framework to explore how mechanisms may act upon social cognitive processes to produce conscious experiences similar to another person's observed state.

Interactions between Pain and the Motor Cortex: Insights from Research on Phantom Limb Pain and Complex Regional Pain Syndrome

PURPOSE

Pain is a significantly disabling problem that often interacts with other deficits during the rehabilitation process. The aim of this paper is to review evidence of interactions between pain and the motor cortex in order to attempt to answer the following questions: (1) Does acute pain interfere with motor-cortex activity? (2) Does chronic pain interfere with motor-cortex activity, and, conversely, does motor-cortex plasticity contribute to chronic pain? (3) Can the induction of motor plasticity by means of motor-cortex stimulation decrease pain? (4) Can motor training result in both motor-cortex reorganization and pain relief?

SUMMARY OF KEY POINTS

Acute experimental pain has been clearly shown to exert an inhibitory influence over the motor cortex, which can interfere with motor learning capacities. Current evidence also suggests a relationship between chronic pain and motor-cortex reorganization, but it is still unclear whether one causes the other. However, there is growing evidence that interventions aimed at normalizing motor-cortex organization can lead to pain relief.

CONCLUSIONS

Interactions between pain and the motor cortex are complex, and more studies are needed to understand these interactions in our patients, as well as to develop optimal rehabilitative strategies.

Motor cortex representation of the upper-limb in individuals born without a hand

The body schema is an action-related representation of the body that arises from activity in a network of multiple brain areas. While it was initially thought that the body schema developed with experience, the existence of phantom limbs in individuals born without a limb (amelics) led to the suggestion that it was innate. The problem with this idea, however, is that the vast majority of amelics do not report the presence of a phantom limb. Transcranial magnetic stimulation (TMS) applied over the primary motor cortex (M1) of traumatic amputees can evoke movement sensations in the phantom, suggesting that traumatic amputation does not delete movement representations of the missing hand. Given this, we asked whether the absence of a phantom limb in the majority of amelics means that the motor cortex does not contain a cortical representation of the missing limb, or whether it is present but has been deactivated by the lack of sensorimotor experience. In four upper-limb amelic subjects we directly stimulated the arm/hand region of M1 to see 1) whether we could evoke phantom sensations, and 2) whether muscle representations in the two cortices were organised asymmetrically. TMS applied over the motor cortex contralateral to the missing limb evoked contractions in stump muscles but did not evoke phantom movement sensations. The location and extent of muscle maps varied between hemispheres but did not reveal any systematic asymmetries. In contrast, forearm muscle thresholds were always higher for the missing limb side. We suggest that phantom movement sensations reported by some upper limb amelics are mostly driven by vision and not by the persistence of motor commands to the missing limb within the sensorimotor cortex. We propose that prewired movement representations of a limb need the experience of movement to be expressed within the primary motor cortex.

Large-scale expansion of the face representation in somatosensory areas of the lateral sulcus after spinal cord injuries in monkeys

Transection of dorsal columns of the spinal cord in adult monkeys results in large-scale expansion of the face inputs into the deafferented hand region in the primary somatosensory cortex (area 3b) and the ventroposterior nucleus of thalamus. Here, we determined whether the upstream cortical areas, secondary somatosensory (S2) and parietal ventral (PV) areas, also undergo reorganization after lesions of the dorsal columns. Areas S2, PV, and 3b were mapped after long-term unilateral lesions of the dorsal columns at cervical levels in adult macaque monkeys. In areas S2 and PV, we found neurons responding to touch on the face in regions in which responses to touch on the hand and other body parts are normally seen. In the reorganized parts of S2 and PV, inputs from the chin as well as other parts of the face were observed, whereas in area 3b only the chin inputs expand into the deafferented regions. The results show that deafferentations lead to a more widespread brain reorganization than previously known. The data also show that reorganization in areas S2 and PV shares a common substrate with area 3b, but there are specific features that emerge in S2 and PV.

Motor control over the phantom limb in above-elbow amputees and its relationship with phantom limb pain

Recent evidence shows that the primary motor cortex continues to send motor commands when amputees execute phantom movements. These commands are retargeted toward the remaining stump muscles as a result of motor system reorganization. As amputation-induced reorganization in the primary motor cortex has been associated with phantom limb pain we hypothesized that the motor control of the phantom limb would differ between amputees with and without phantom limb pain. Eight above-elbow amputees with or without pain were included in the study. They were asked to produce cyclic movements with their phantom limb (hand, wrist, and elbow movements) while simultaneously reproducing the same movement with the intact limb. The time needed to complete a movement cycle and its amplitude were derived from the kinematics of the intact limb. Electromyographic (EMG) activity from different stump muscles and from the homologous muscles on the intact side was recorded. Different EMG patterns were recorded in the stump muscles depending on the movement produced, showing that different phantom movements are associated with distinct motor commands. Phantom limb pain was associated with some aspects of phantom limb motor control. The time needed to complete a full cycle of a phantom movement was systematically shorter in subjects without phantom limb pain. Also, the amount of EMG modulation recorded in a stump muscle during a phantom hand movement was positively correlated with the intensity of phantom limb pain. Since phantom hand movement-related EMG patterns in above-elbow stump muscles can be considered as a marker of motor system reorganization, this result indirectly supports the hypothesis that amputation-induced plasticity is associated with phantom limb pain severity. The discordance between the (amputated) hand motor command and the feedback from above-elbow muscles might partially explain why subjects exhibiting large EMG modulation during phantom hand movement have more phantom limb pain.

Re-emergence of hand-muscle representations in human motor cortex after hand allograft

The human primary motor cortex (M1) undergoes considerable reorganization in response to traumatic upper limb amputation. The representations of the preserved arm muscles expand, invading portions of M1 previously dedicated to the hand, suggesting that former hand neurons are reassigned to the control of remaining proximal upper limb muscles. Hand allograft offers a unique opportunity to study the reversibility of such long-term cortical changes. We used transcranial magnetic stimulation in patient LB, who underwent bilateral hand transplantation 3 years after a traumatic amputation, to longitudinally track both the emergence of intrinsic (from the donor) hand muscles in M1 as well as changes in the representation of stump (upper arm and forearm) muscles. The same muscles were also mapped in patient CD, the first bilateral hand allograft recipient. Newly transplanted intrinsic muscles acquired a cortical representation in LB's M1 at 10 months postgraft for the left hand and at 26 months for the right hand. The appearance of a cortical representation of transplanted hand muscles in M1 coincided with the shrinkage of stump muscle representations for the left but not for the right side. In patient CD, transcranial magnetic stimulation performed at 51 months postgraft revealed a complete set of intrinsic hand-muscle representations for the left but not the right hand. Our findings show that newly transplanted muscles can be recognized and integrated into the patient's motor cortex.

Training With Virtual Visual Feedback to Alleviate Phantom Limb Pain

Background. Performing phantom movements with visual virtual feedback, or mirror therapy, is a promising treatment avenue to alleviate phantom limb pain. However the effectiveness of this approach appears to vary from one patient to another. Objective. To assess the individual response to training with visual virtual feedback and to explore factors influencing the response to that approach. Methods. Eight male participants with phantom limb pain (PLP) resulting from either a traumatic upper limb amputation or a brachial plexus avulsion participated in this single case multiple baseline study. Training was performed 2 times per week for 8 weeks where a virtual image of a missing limb performing different movements was presented and the participant was asked to follow the movements with his phantom limb. Results. Patients reported an average 38% decrease in background pain on a visual analog scale (VAS), with 5 patients out of 8 reporting a reduction greater than 30%. This decrease in pain was maintained at 4 weeks postintervention in 4 of the 5 participants. No significant relationship was found between the long-term pain relief and the duration of the deafferentation or with the immediate pain relief during exposure to the feedback. Conclusions. These results support the use of training with virtual feedback to alleviate phantom limb pain. Our observations suggest that between-participant differences in the effectiveness of the treatment might be related more to a difference in the susceptibility to the virtual visual feedback, than to factors related to the lesion, such as the duration of the deafferentation.