Cortical reorganization and phantom phenomena in congenital and traumatic upper-extremity amputees

The motor cortex and its role in phantom limb phenomena

Limb amputation results in plasticity of connections between the brain and muscles; the cortical motor representation of the missing limb seemingly disappears. The disappearance of the hand's motor representation is, however, difficult to reconcile with evidence that a perceptual representation of the missing limb persists in the form of a phantom limb endowed with sensory and motor qualities. Here, we argue that despite considerable reorganization within the motor cortex of upper-limb amputees, the representation of the amputated hand does not disappear. We hypothesize that two levels of hand-movement representation coexist within the primary motor cortex; at one level, limb movements are specified in terms of arm and hand motor commands, and at another level, limb movements are specified as muscles synergies. We propose that primary motor cortex reorganization after amputation concerns primarily the upper limb's muscular map but not its motor command map and that the integrity of the motor command map underlies the existence of the phantom limb.

Allogreffe de main chez le nouveau-né agénésique: étude de faisabilité

Would a newborn with a single hand benefit from hand allograft? Transantebrachial aplasia is the chosen clinical form of agenesia in our interrogation. The feasibility study presents several aspects: 1) ethical and psychological aspects. Is this a desired surgery for agenesic population? Which are the functional, psychological and social situations of agenesic patient? Is the hand transplantation in newborn ethically acceptable? What is the parents' attitude toward agenesia? Can we envisage organ donation in neonatal period? 2) immunological aspects. The non-vital character of this condition and its' good functional tolerance cannot make accepting the risk of adverse effects of hand allotransplantation. Hence, one may consider this surgery only without immunosuppression. Can the peculiarities of the neonate "immature" immune system represent an opportunity of easier tolerance obtaining, avoiding immunosuppression? 3) anatomical and technical aspects. The proximal tissues at the level of amputation are all hypoplastic in agenesic patients. Can we efficaciously suture those structures with donor eutrophic tissues? 4) cognitive aspects. Is a neonate born with only one hand is able to use two? A feasibility study on such a subject needs to take into account all these aspects. This research is useful because, even if hand allograft in agenesic newborn will never be done, the provided information will allow to progress in the vaster domain of composite tissue allotransplantation in perinatology.

Studying the human somatosensory hand area: A new way to compare fMRI and MEG

Valid localization is a prerequisite to study plasticity of the somatosensory cortex in humans. We compared the localizations of left and right thumb and little finger in the primary somatosensory cortex obtained with fMRI and MEG. Representations were investigated in 11 healthy right-handed subjects using echoplanar fMRI and 122-channel MEG together with electric finger stimulation. Activation observed with fMRI was based on an increase in the BOLD signal. Most of the activation clusters (71.1%) were located on the lateral surface of the postcentral gyrus. Representations of thumb and little finger were 17mm apart on average and consistently showed a somatotopic arrangement with the thumb representation inferior, lateral, and anterior to the representation of the little finger. Activation observed with MEG was modelled by equivalent current dipoles. Dipole localization was compatible with an assumed origin of activation within the posterior wall of the central sulcus. The Euclidian distance between corresponding dipoles was 11.5mm on average with deviations from the expected spatial arrangement of 35, 30, and 20% in the x-, y- und z-direction, respectively. Our study demonstrates how relative localization of somatosensory activations can serve as an indicator for localization validity when comparing different methods or studying somatosensory plasticity.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

Tinnitus and Neural Plasticity of the Brain

OBJECTIVE

To describe the current ideas about the manifestations of neural plasticity in generating tinnitus.

DATA SOURCES

Recently published source articles were identified using MEDLINE, PubMed, and Cochrane Library according to the key words mentioned below.

STUDY SELECTION

Review articles and controlled trials were particularly selected.

DATA EXTRACTION

Data were selected systematically, scaled on validity and comparability.

CONCLUSION

An altered afferent input to the auditory pathway may be the initiator of a complex sequence of events, finally resulting in the generation of tinnitus at the central level of the auditory nervous system. The effects of neural plasticity can generally be divided into early modifications and modifications with a later onset. The unmasking of dormant synapses, diminishing of (surround) inhibition and initiation of generation of new connections through axonal sprouting are early manifestations of neural plasticity, resulting in lateral spread of neural activity and development of hyperexcitability regions in the central nervous system. The remodeling process of tonotopic receptive fields within auditory pathway structures (dorsal cochlear nucleus, inferior colliculus, and the auditory cortex) are late manifestations of neural plasticity. The modulation of tinnitus by stimulating somatosensory or visual systems in some people with tinnitus might be explained via the generation of tinnitus following the nonclassical pathway. The similarities between the pathophysiological processes of phantom pain sensations and tinnitus have stimulated the theory that chronic tinnitus is an auditory phantom perception.

Phantom limb pain: a case of maladaptive CNS plasticity?

Phantom pain refers to pain in a body part that has been amputated or deafferented. It has often been viewed as a type of mental disorder or has been assumed to stem from pathological alterations in the region of the amputation stump. In the past decade, evidence has accumulated that phantom pain might be a phenomenon of the CNS that is related to plastic changes at several levels of the neuraxis and especially the cortex. Here, we discuss the evidence for putative pathophysiological mechanisms with an emphasis on central, and in particular cortical, changes. We cite both animal and human studies and derive suggestions for innovative interventions aimed at alleviating phantom pain.

Decrease of thalamic gray matter following limb amputation

Modern neuroscience has elucidated general mechanisms underlying the functional plasticity of the adult mammalian brain after limb deafferentation. However, little is known about possible structural alterations following amputation and chronic loss of afferent input in humans. Using voxel-based morphometry (VBM), based on high-resolution magnetic resonance images, we investigated the brain structure of 28 volunteers with unilateral limb amputation and compared them to healthy controls. Subjects with limb amputation exhibited a decrease in gray matter of the posterolateral thalamus contralateral to the side of the amputation. The thalamic gray matter differences were positively correlated with the time span after the amputation but not with the frequency or magnitude of coexisting phantom pain. Phantom limb pain was unrelated to thalamic structural variations, but was positively correlated to a decrease in brain areas related to the processing of pain. No gray matter increase was detected. The unilateral thalamic differences may reflect a structural correlate of the loss of afferent input as a secondary change following deafferentation.

A sham-controlled, phase II trial of transcranial direct current stimulation for the treatment of central pain in traumatic spinal cord injury

Past evidence has shown that motor cortical stimulation with invasive and non-invasive brain stimulation is effective to relieve central pain. Here we aimed to study the effects of another, very safe technique of non-invasive brain stimulation--transcranial direct current stimulation (tDCS)--on pain control in patients with central pain due to traumatic spinal cord injury. Patients were randomized to receive sham or active motor tDCS (2mA, 20 min for 5 consecutive days). A blinded evaluator rated the pain using the visual analogue scale for pain, Clinician Global Impression and Patient Global Assessment. Safety was assessed with a neuropsychological battery and confounders with the evaluation of depression and anxiety changes. There was a significant pain improvement after active anodal stimulation of the motor cortex, but not after sham stimulation. These results were not confounded by depression or anxiety changes. Furthermore, cognitive performance was not significantly changed throughout the trial in both treatment groups. The results of our study suggest that this new approach of cortical stimulation can be effective to control pain in patients with spinal cord lesion. We discuss potential mechanisms for pain amelioration after tDCS, such as a secondary modulation of thalamic nuclei activity.

Motor cortex stimulation for long-term relief of chronic neuropathic pain: A 10 year experience

Chronic subthreshold stimulation of the contralateral precentral gyrus is used in patients with intractable neuropathic pain for more than 15 years. The aim of this study was to analyse retrospectively our own patient group with long term follow-up of 10 years. Seventeen patients with chronic neuropathic pain were treated with contralateral epidural stimulation electrodes. In 10 cases, trigeminal neuropathic pain (TNP) and in seven cases post-stroke pain (PSP) were diagnosed. The placement of the electrodes was performed in local anaesthesia using neuronavigation and intraoperative neuromonitoring. A test trial of minimum one week including double-blind testing was conducted and pain intensity was measured using a visual analogue scale (VAS). Correct placement of the electrode was achieved in all patients using intraoperative neurophysiological monitoring. Double-blind testing was able to identify 6 (35%) non-responders. In 5 of 10 (50%) with TNP and 3 of 7 (43%) with PSP a positive effect with pain reduction > or = 50% was observed. The mean follow-up period was 3.6 years (range 1-10 years) and includes a patient with 10 years of positive stimulation effect. Stimulation of the motor cortex is a treatment option for patients with chronic neuropathic pain localized in the face or upper extremity. Double-blind testing can identify non-responders. Patients with TNP profit more than patients with PSP. The positive effect can last for ten years in long-term follow-up.

A PET activation study of brush-evoked allodynia in patientswith nerve injury pain

Acute experimental brush-evoked allodynia induces a cortical activation pattern that differs from that typically seen during experimental nociceptive pain. In this study, we used positron emission tomography to measure changes in regional cerebral blood flow (rCBF) in patients with clinical allodynia. Nine patients with peripheral nerve injury were scanned during rest, brush-evoked allodynia, and brushing of normal contralateral skin. PET data were analyzed for the whole group and for single subjects. Allodynic stimulation activated the contralateral orbitofrontal cortex (BA 11) in every patient. Whereas normal brushing activated most strongly the contralateral insular cortex, allodynic brushing produced an ipsilateral activation in this area. Another important difference between normal and allodynic brushing was the absence of a contralateral primary somatosensory cortex (SI) activation during allodynic brushing. No thalamic activation was observed during allodynic or control brushing. Although no anterior cingulate cortex (ACC) activation could be demonstrated in the group analysis, single subject analysis revealed that four patients activated this region during brush-evoked allodynia. A direct post hoc comparison of brush -and allodynia-induced rCBF changes showed that allodynia was associated with significantly stronger activations in orbitofrontal cortex and ipsilateral insula whereas non-painful brushing more strongly activated SI and BA 5/7. These findings indicate that activity in the cortical network involved in the sensory-discriminative processing of nociceptive pain is downregulated in neuropathic pain. Instead, there is an upregulation of activity in the orbitofrontal and insular cortices, which is probably due to the stronger emotional load of neuropathic pain and higher computational demands of processing a mixed sensation of brush and pain.

Direct neural sensory feedback and control of a prosthetic arm

Evidence indicates that user acceptance of modern artificial limbs by amputees would be significantly enhanced by a system that provides appropriate, graded, distally referred sensations of touch and joint movement, and that the functionality of limb prostheses would be improved by a more natural control mechanism. We have recently demonstrated that it is possible to implant electrodes within individual fascicles of peripheral nerve stumps in amputees, that stimulation through these electrodes can produce graded, discrete sensations of touch or movement referred to the amputee's phantom hand, and that recordings of motor neuron activity associated with attempted movements of the phantom limb through these electrodes can be used as graded control signals. We report here that this approach allows amputees to both judge and set grip force and joint position in an artificial arm, in the absence of visual input, thus providing a substrate for better integration of the artificial limb into the amputee's body image. We believe this to be the first demonstration of direct neural feedback from and direct neural control of an artificial arm in amputees.

Phantom Limb Pain Treatment with Mirtazapine: A Case Series

OBJECTIVES

To pilot the efficacy of mirtazapine for relief of phantom limb pain (PLP); to correlate the putative drug mechanism with theoretical PLP mechanisms; and to develop a rationale for further study of mirtazapine in this population.

DESIGN

Open-label case series.

SUBJECTS/PATIENTS

Four individuals with PLP for at least 3 months after amputation.

METHODS

All subjects received oral mirtazapine between 7.5 and 30 mg/day. An 11-point numeric rating scale (0 to 10) measured pain intensity and relief during monitored outpatient follow-up visits.

RESULTS/DISCUSSION

Mirtazapine use improved the PLP experienced by these subjects by at least 50%. Subjects with PLP-related sleeping difficulties reported the greatest pain relief concomitant with improved sleep quality. One subject was able to eliminate the use of a selective serotonin reuptake inhibitor antidepressant while using mirtazapine for PLP and depression without change in mood or affect. Mirtazapine enhances noradrenergic and serotonergic activity and may modulate PLP by central mechanisms. Current concepts of the proposed pathophysiology of PLP and the hypothetical impact of mirtazapine are discussed.

CONCLUSION

Mirtazapine may be an effective treatment for PLP that can also potentially enhance sleep and mood. This information provides preliminary reinforcement for more formal, controlled studies concerning mirtazapine use in PLP.

The saltation illusion demonstrates integrative processing of spatiotemporal information in thermoceptive and nociceptive networks

In sensory saltation, first reported by Geldard and Sherrick (Science 178:178-179, 1972), a stimulus is displaced towards a second one following closely in time and space as a function of the delay between the stimuli. The distance between stimulus locations is restricted by the extension of sensory fields in the primary somatosensory cortex. Saltation is assumed to reflect dynamic changes in these cortical representations. The present study demonstrates for the first time saltation in thermoceptive and nociceptive pathways with CO(2) laser stimulation. Stimuli were presented to the dorsal forearms of 18 healthy subjects at two intensities. Saltation patterns consisted of a reference stimulus S0 near the wrist, the first test stimulus S1 at the reference location after a fixed onset delay of 1,000 ms, and a second test stimulus S2 at a location 105 mm distant from reference after a variable onset delay of 60-516 ms. Perceived positions were indicated by the subjects without skin contact with a 3D tracker. As expected, subjects mislocalized S1 towards S2. Mean S1 displacement was 51+/-36 mm. Decreasing delays between S1 and S2 resulted in increasing displacements, independent of intensity. However, since no clear-cut discrimination of thermal versus nociceptive activation could be achieved definite conclusions about differences between the two modalities cannot be drawn. In addition, effects of body site on the saltation characteristics were observed. The saltation paradigm constitutes a promising approach to the functional analysis of spatiotemporal dynamics in thermoceptive and nociceptive networks to supplement brain-mapping approaches to cortical sensory fields.

Vers une théorie unifiée des symptômes positifs

Defined more than one century ago, the concept of positive symptoms has become obsolete, except in the psychiatric domain. However, its relevance remains intact today when considering such pathophysiologies as neuropathic (phantom) pain, movement disorders, tinnitus, epilepsy, and psychiatric disorders. Beside their very different clinical characteristics, all these symptoms arise from a lesion in the nervous system. Furthermore, they are paradoxical in the sense that they correspond to a spontaneous hyperactivity of the injured functional system, concomitant to the usual deficits resulting from the lesion. Could these similarities reflect the existence of some common pathophysiological process? A peculiar electrophysiological property of thalamic cells is likely to be compatible with this hypothesis. A thalamic cell produces action potentials when depolarised by excitatory inputs. Conversely, its ability to produce action potentials is decreased or even completely suppressed when the same cell is hyperpolarized by inhibitory influences. However, depending on its level of hyperpolarization, this cell can also produce rhythmic paradoxical bursts of activity at low frequency (3-4 Hz). In this context, a lesion involving, for example, the somatosensory excitatory fibres gives rise to hyperpolarization of the corresponding thalamic cells, which may produce such rhythmic bursting activity. This causes an increase of low frequency thalamo-cortical activity, which, through reduction of collateral cortico-cortical inhibition, induces high frequency activity in neighbouring thalamo-cortical loops ("edge effect"). This leads to the appearance of the clinical symptoms, in this case, pain. Electrophysiological recordings performed in patients suffering from sensory or motor positive symptoms have shown the presence of such deleterious sequence of events. Furthermore, the efficiency of neurosurgical treatments that are used against some positive symptoms can be explained on the basis of such a dynamic process. Both considerations support the validity of the proposed hypothesis and open avenues for the control of other positive symptoms.

Neuromagnetic indicators of auditory cortical reorganization of tinnitus

Animal studies show that following damage to inner-ear receptors, central representations of intact lesion-edge (LE) frequencies become enlarged (map reorganization). One theory of tinnitus holds that this process could be related to the tinnitus sensation. To test this hypothesis, neuromagnetic evoked fields of tinnitus participants with high-frequency hearing loss and normal hearing controls were measured, while subjects listened to monaurally presented LE or control (CO; an octave below LE) tones. The predictions made based on the map reorganization hypothesis of tinnitus were that neuronal responses to LE frequencies would be enhanced, and that source localizations for LE would be distorted. N1m equivalent dipole moments for LE were not supranormal in the tinnitus group, whereas responses to CO of tinnitus patients compared to controls were enlarged in the right hemisphere. This effect was positively associated with tinnitus-related distress. Abnormal source locations were found for generators activated by LE tones in the right hemisphere of the tinnitus group. This right-hemispheric map distortion was not associated with subjective variables of tinnitus. A positive correlation with tinnitus distress was found for the left hemisphere with more anterior sources being associated with enhanced distress. However, this result was independent of the frequency of the stimulus. Overall, the present study suggests that mechanisms of map reorganization, although present in the data, cannot satisfactorily explain the emergence of tinnitus and that differential hemispheric involvement must be considered.

Recovery of impaired songs following unilateral but not bilateral lesions of nucleus uvaeformis of adult zebra finches

Zebra finches utilize neural circuits in both cerebral hemispheres to produce their learned songs. Although direct reciprocal connections do not exist between song control nuclei across hemispheres, premotor activity in these nuclei during singing is precisely and continuously coordinated between the hemispheres. We hypothesized that this interhemispheric coordination is mediated by bilateral feedback projections from medullary and midbrain song control nuclei to the thalamic song control nucleus uvaeformis (Uva). Consistent with our hypothesis, bilateral lesions of Uva severely impaired singing. This impairment was long-lasting, as it persisted for at least 35 days after the lesions. Unilateral lesions of Uva on either side also resulted in an immediate singing impairment. However, song recovered substantially after less than 15 days, suggesting a possible compensation by the unlesioned side. Although the acoustic structure of individual syllables recovered fully after unilateral lesioning, subtle changes in the sequencing of syllables were observed after song recovery, suggesting that the lesion led to an alteration in the functioning of the remaining song control network. These results demonstrate that the adult songbird brain can adjust to damage to certain parts of the song control network and recover from their associated singing deficits. The well-identified and localized central neural pathways mediating birdsong production provide an advantageous model system to analyze systematically the sensorimotor contexts and the specific sites and mechanisms for behavioral recovery following partial damage to a behavior-producing neural circuit.

Reduced somatosensory hand representation in thalidomide-induced dysmelia as revealed by fMRI

The concept of cerebral plasticity suggests that the hand representation in somatosensory cortex is abnormal in congenital malformation disorders. To investigate this issue we studied 11 subjects with different degrees of upper extremity dysmelia due to thalidomide embryopathy in comparison to 10 control subjects. In the affected subjects fingers are typically missing in radio-ulnar order beginning with the thumb. Haemodynamic responses to electrical stimulation of the radial-most and ulnar-most fingers were measured in each subject using functional magnetic resonance tomography. The size of the hand area in the primary somatosensory cortex was estimated by calculating the Euclidian distance between corresponding activation peaks on the lateral postcentral gyrus. The cortical somatosensory hand representation was found to be significantly smaller in dysmelic subjects as compared with the control subjects (P <0.001). The shrinkage of the hand area was not proportional to the number of missing fingers. Furthermore, the cortical representation of the ulnar fingers in the dysmelic subjects was shifted towards the cortical thumb representation of the control group. We suggest that the unproportional reduction of the hand area together with the observed shift may reflect use-dependent rather than malformation-induced reorganization of the somatosensory hand area.

Shifting of cortical somatosensory areas in a man with amelia

The human brain represents the body in a homuncular fashion. Although these cortical representations are to some degree plastic, it is unclear how extensive this plasticity can be. Examinations of amputated or amelic subjects can contribute to answering this question. We examined a man who lacked both arms since birth and was free of phantom pain. Magnetic source imaging revealed that not only had the somatotopically adjacent representation of the lip moved into the cortical space usually representing the hand; but the representation of the foot had similarly moved towards the cortical hand area. These data illustrate a case of extensive cortical reorganization and suggest that the potential overlap of different body parts is much greater than previously anticipated.

Hand movement observation by individuals born without hands: phantom limb experience constrains visual limb perception

Increasing evidence suggests that the visual analysis of other people's actions depends upon the observer's own body representation or schema. This raises the question of how differences in observers' body structure and schema impact their perception of human movement. We investigated the visual experiences of two persons born without arms, one with and the other without phantom sensations. These participants, plus six normally-limbed control observers, viewed depictions of upper limb movement under conditions of apparent motion. Consistent with previous results (Shiffrar M, Freyd JJ (1990) Psychol Sci 1:257), normally-limbed observers perceived rate-dependent paths of apparent human movement. Specifically, biologically impossible motion trajectories were reported at rapid display rates while biologically possible trajectories were reported at slow display rates. The aplasic individual with phantom experiences showed the same perceptual pattern as control participants, while the aplasic individual without phantom sensations did not. These preliminary results suggest that phantom experiences may constrain the visual analysis of the human body. These results further suggest that it may be time to move beyond the question of whether aplasic phantoms exist and instead focus on the question of why some people with limb aplasia experience phantom sensations while others do not. In this light, the current results suggest that somesthetic representations are not sufficient to define body schema. Instead, neural systems matching action observation, action execution and motor imagery likely contribute to the definition of body schema in profound ways. Additional research with aplasic individuals, having and lacking phantom sensations, is needed to resolve this issue.

Prospective demonstration of short-term motor plasticity following acquired central pareses

The effect of newly acquired central pareses on functional MRI (fMRI) signal pattern is not known, since up to now all investigated patients were examined while they already experienced the motor weakness. We describe the first prospective and controlled study demonstrating the impact of new, acquired central pareses on fMRI motor activation pattern. Six patients suffering from a new central paresis after resection of a brain tumor infiltrating the central region were prospectively compared with a control group of five patients without postoperative paresis and a group of six healthy, age-matched controls who were investigated at two time points. fMRI signal was postoperatively analyzed during the performance of hand motor tasks and compared to the preoperative fMRI results. The relative signal change between rest and activation was evaluated for five cortical regions: the primary motor cortex (M1) and the ipsilateral primary motor cortex (M1i), the supplementary motor area (SMA), the premotor area (PMA), and the superior parietal lobule (SPL). In the patients with new postoperative central pareses, significant (P = 0.0313) decreases in fMRI activation were found in M1, whereas significant (P = 0.0313) increases were found in SMA and PMA. For M1i and SPL, there was a signal increase on average as well, but it failed to reach significance (P = 0.1250). In both control groups, no significant changes between both examinations were seen. Even though the number of investigated patients is too small to draw definite conclusions, our results support the concept of short-term motor plasticity being mediated by redundant systems that may take over function after damage of the primary motor cortex. The findings potentially also reflect increased functional demands imposed upon the motor network subsequent to a loss of dexterity.

Shrinkage of Somatosensory Hand Area in Subjects With Upper Extremity Dysmelia Revealed by Magnetoencephalography

The effect of peripheral lesions on cerebral somatosensory representations is well studied for experimentally induced amputations and deafferentations acquired later in life. However, few studies have investigated the brain's capacity for plastic changes in congenital malformations. We studied somatosensory-evoked fields to electrical stimulation of the bordering fingers in 10 subjects with upper extremity dysmelia in comparison with 10 control subjects using a 122-channel whole-head magnetometer. The number of developed fingers varied between two and four in the affected subjects. We localized finger representations in the primary somatosensory cortex and calculated Euclidian distances to estimate the size of the somatosensory hand area. Euclidian distances were significantly smaller in dysmelic subjects (5.7 mm) than in control subjects (11.6 mm) and were related to the number of the developed fingers on the contralateral hand. In contrast, individual finger representations were not found to be reduced. We suggest that the shrinkage of the somatosensory hand area might be related to the congenital nature of the malformation, to the smaller anatomical hand size in the affected subjects, and/or to use-dependent effects due to impaired hand function.

[Visualisation of phantom- and backpain using imaging techniques. Implication for treatment]

If patients with chronic low back pain are stimulated in the painful region, an expanded representation of the back in the primary somatosensory cortex becomes visible that increases with chronicity. This "pain memory" might play an important role in the chronicity process. In patients with phantom limb pain, e.g. subsequent to the amputation of an arm or leg, a shift in the representation of neighboring areas into the deafferented area in primary somatosensory cortex has been observed. This reorganization of functional brain maps is not present in congenital amputees or amputees without phantom limb pain. The magnitude of such pain is positively correlated with this reorganization. We present a model of phantom limb pain that assigns an important role to pre-existing chronic pain. The modulation of plasticity and phantom limb pain by anesthesiological manipulation, the use of NMDA receptor antagonists and opioids is presented. Behaviorally relevant stimulation, e.g. by the use of a myoelectric prosthesis or sensory discrimination training can also influence the cortical somatosensory pain memory. More recent studies focus also on brain areas such as the cingulate gyrus believed to be involved in the affective processing of pain.

P300-amplitudes in upper limb amputees with and without phantom limb pain in a visual oddball paradigm

The aim of the study was to investigate to what extent cortical hyper-reactivity to visual stimuli is present in upper limb amputees. Five amputees with phantom limb pain (PLP), five amputees without PLP (Non-PLP) and 10 healthy controls (HC) were investigated using a visual oddball paradigm. Two hundred visual stimuli were presented with target stimuli occurring at a probability of 25% and standard stimuli at a probability of 75%. Event-related potentials were recorded from nine scalp positions (F3, F4, Fz, C3, C4, Cz, P3, P4, Pz). The PLP-patients had significantly higher P300-amplitudes to both types of stimuli compared to the non-PLP-patients. The HC were not significantly different from both amputee groups. P300-amplitude to targets at frontal sites in the hemisphere contralateral to the amputation was higher in the PLP patients. P300-latencies to target stimuli differed only at frontal sites with PLP-patients showing significantly longer latencies than non-PLP-patients. To standard stimuli, however, they showed significantly shorter latencies at central and parietal scalp positions. The HC had significantly shorter latencies than both amputee groups. The size of the P300-amplitude was positively correlated with the intensity of PLP. These findings suggest a higher magnitude of non-specific cortical excitability in amputees with PLP and a reduced excitability in amputees without PLP. This extends previous findings of differences in cortical excitability in PLP and non-PLP patients in the sensorimotor domain.

Neuroelectric source imaging of steady-state movement-related cortical potentials in human upper extremity amputees with and without phantom limb pain

Whereas several studies reported a close relationship between changes in the somatotopic organization of primary somatosensory cortex and phantom limb pain, the relationship between alterations in the motor cortex and amputation-related phenomena has not yet been explored in detail. This study used steady-state movement-related cortical potentials (MRCPs) combined with neuroelectric source imaging to assess the relationship of changes in motor cortex and amputation-related phenomena such as painful and non-painful phantom and residual limb sensations, telescoping, and prosthesis use. Eight upper limb amputees were investigated. A significant positive relationship between reorganization of the motor cortex (distance of the MRCP source location from the mirrored source for hand movement) and phantom limb pain was found. Non-painful phantom sensations as well as painful and non-painful residual limb sensations were unrelated to motor cortical reorganization. A higher amount of motor reorganization was associated with less daily prosthesis use, which also tended to be related to more severe phantom limb pain. These results extend previous findings of a positive relationship between somatosensory reorganization and phantom limb pain to the motor domain and suggest a potential positive effect of prosthesis use on phantom limb pain and cortical reorganization.

Prospective Diary Study of Nonpainful and Painful Phantom Sensations in a Preselected Sample of Child and Adolescent Amputees Reporting Phantom Limbs

OBJECTIVE

To prospectively study factors associated with the occurrence of phantom sensations and pains in a pre-selected sample of child and adolescent amputees reporting phantom limbs.

DESIGN

Prospective diary study over 1 month.

PARTICIPANTS

Fourteen child and adolescent amputees from 10-18 years of age who were missing a limb due to trauma (n = 12) or congenital limb deficiency (n = 2), and who had previously reported having phantom sensations and pain.

MAIN OUTCOME MEASURE

Diary used to assess the occurrence of non-painful and painful phantom sensations. Items included age, sex, location and cause of amputation, past experience with stump pain and pre-amputation pain, and intensity, quality, duration, and triggers of the sensations and pains.

RESULTS

Thirteen amputees reported having 104 incidents of non-painful phantom sensations with an average intensity of 4.17 (SD = 2.14) on a 0-10 rating scale. Fifty-three incidents of phantom pain with an average intensity of 6.43 (SD = 1.76) were recorded by 8 amputees. Both amputees with a congenital limb deficiency reported phantom phenomena. Girls reported more psychosocial triggers than did boys whereas boys were more likely than girls to report that they could not identify a trigger (P = 0.0001). Boys also reported a higher proportion of physical triggers than psychosocial triggers while there were no differences for girls (P = 0.0001).

DISCUSSION

Child and adolescent amputees experience phantom sensations and pains on a regular basis over a 1-month period. Differences in triggers of phantom phenomena between boys and girls may be due to differences in activities, awareness, attribution, and willingness to report psychosocial triggers.

Postlesional vestibular reorganization improves the gain but impairs the spatial tuning of the maculo-ocular reflex in frogs

The ramus anterior (RA) of N.VIII was sectioned unilaterally. Two months later we analyzed in vivo responses of the ipsi- and of the contralesional abducens nerve during horizontal and vertical linear acceleration in darkness. The contralesional abducens nerve had become responsive again to linear acceleration either because of a synaptic reorganization in the vestibular nuclei on the operated side and/or because of a reinnervation of the utricular macula by regenerating afferent nerve fibers. Significant differences in the onset latencies and in the acceleration sensitivities allowed a separation of RA frogs in a group without and in a group with functional utricular reinnervation. Most important, the vector orientation for maximal abducens nerve responses was clearly altered: postlesional synaptic reorganization resulted in the emergence of abducens nerve responses to vertical linear acceleration, a response component that was barely detectable in RA frogs with utricular reinnervation and that was absent in controls. The ipsilesional abducens nerve, however, exhibited unaltered responses in either group of RA frogs. The altered spatial tuning properties of contralesional abducens nerve responses are a direct consequence of the postlesional expansion of signals from intact afferent nerve and excitatory commissural fibers onto disfacilitated 2nd-order vestibular neurons on the operated side. These results corroborate the notion that postlesional vestibular reorganization activates a basic neural reaction pattern with more beneficial results at the cellular than at the network level. However, given that the underlying mechanism is activity-related, rehabilitative training after vestibular nerve lesion can be expected to shape the ongoing reorganization.

Cortical Areas Involved in Virtual Movement of Phantom Limbs: Comparison with Normal Subjects

OBJECTIVE

To demonstrate that amputees performing “virtual” movements of their amputated limb activate cortical areas previously devoted to their missing limb, we studied amputees with functional magnetic resonance imaging (fMRI) and positron emission tomographic (PET) scans and compared the results with those of normal volunteers performing imaginary movements during fMRI acquisitions.

METHODS

Ten amputees (age range, 33–92 yr; average age, 49 yr; six men and four women; eight upper-limb and two lower-limb amputations) able to move their phantom limb at will were studied by fMRI (all patients) and PET scan (seven patients). The time between amputation and fMRI and PET studies ranged from 1 to 27 years (average, 13 yr). Patients were asked to perform virtual movements of the amputated limb and normal movements of the contralateral normal limb according to the functional images acquisition procedure. Movements of the stump were also used to differentiate stump cortical areas from virtual movement-activated areas. Ten right-handed volunteers, age- and sex-matched to the amputees, were also studied by fMRI. All volunteers were asked to perform four tasks during their fMRI study: imaginary movements of their right arm (1 task) and foot (1 task) and real movements of their left arm (1 task) and foot (1 task).

RESULTS

In amputees, virtual movements of the missing limbs produced contralateral primary sensorimotor cortex activation on both fMRI and PET scans. These activation areas, different from the stump activation areas, were similar in location to contralateral normal limb-activated areas. Quantitatively, in two amputees who claimed to be able to perform both slow and fast virtual movements, regional cerebral blood flow measured by PET scan in the precentral gyrus increased significantly during fast movements in comparison with slow virtual movements. In normal subjects, significant differences between real versus imaginary fMRI activations were found (for both foot and hand movements); imaginary right hand and foot tasks activated primarily the contralateral supplementary motor areas, with no significant activation detected in the contralateral precentral or postcentral gyri.

CONCLUSION

Primary sensorimotor cortical areas can be activated by phantom-limb movements and thus can be considered functional for several years or decades after amputation. In this study, we found that the location of the activation of these areas is comparable to that of activations produced by normal movements in control subjects or in amputees.

NMDA-mediated mechanisms in cortical excitability changes after limb amputation

OBJECTIVES

The aim of our study was to determine the role of N-methyl-d-aspartate (NMDA)-mediated mechanisms in cortical excitability changes after limb amputation, and their possible relationship to phantom pain.

MATERIALS AND METHODS

Sixteen upper limb amputees who were suffering from chronic phantom pain received the NMDA-antagonist memantine or placebo for 3 weeks. Intracortical inhibition (ICI) and intracortical facilitation (ICF) were determined at baseline and on day 21 using transcranial magnetic stimulation. Simultaneously, phantom pain intensity was assessed.

RESULTS

Memantine reduced ICF and enhanced ICI to roughly the same extent as seen in healthy subjects in a previous study. These changes were not correlated to the reduction of phantom pain.

CONCLUSION

We therefore conclude that NMDA-mediated mechanisms influence changes of ICI and ICF occurring after limb amputation. However, our results suggest that these cortical excitability changes and phantom pain are independent of each other.

A pain neuromatrix approach to patients with chronic pain

This paper presents an approach to rehabilitation of pain patients. The fundamental principles of the approach are (i) pain is an output of the brain that is produced whenever the brain concludes that body tissue is in danger and action is required, and (ii) pain is a multisystem output that is produced when an individual-specific cortical pain neuromatrix is activated. When pain becomes chronic, the efficacy of the pain neuromatrix is strengthened via nociceptive and non-nociceptive mechanisms, which means that less input, both nociceptive and non-nociceptive, is required to produce pain. The clinical approach focuses on decreasing all inputs that imply that body tissue is in danger and then on activating components of the pain neuromatrix without activating its output. Rehabilitation progresses to increase exposure to threatening input across sensory and non-sensory domains.

Efficacy of the NMDA-receptor antagonist memantine in patients with chronic phantom limb pain--results of a randomized double-blinded, placebo-controlled trial

Phantom limb pain (PLP) associated neuroplastic changes are partly mediated by excitatory amino acids at NMDA receptor sites. This study was undertaken to deduce if NMDA-receptor antagonists may be effective in patients with chronic PLP. Therefore a four week double-blinded, randomized placebo-controlled trial was performed to evaluate the efficacy of 30 mg memantine/day, an orally administrable NMDA receptor antagonist.Thirty-six patients, 18 per group, with a history of at least 12 months PLP and an average pain of at least 4 on the 11-point numeric rating scale (NRS) were enrolled. The patients completed a standardized questionnaire before the trial. PLP intensity and the level of eight complaints were assessed during the trial. Number needed to treat (NNT) was calculated based on the average PLP during the 3rd week (steady state). In both groups, PLP declined significantly in comparison with the baseline (verum: 5.1 (+/-2.1) to 3,8 (+/-2,3), placebo from 5.1 (+/-2.0) to 3.2 (+/-1,46) NRS) without a re-rising of the PLP during the washout period. Mean pain relief was 47% in the memantine group (10 patients reported more than 50% relief), 40% in the placebo group (6>50%): NNT were 4.5 (KI: 2.1-10.6). Analysis of covariance demonstrated a significant impact only on the prior PLP intensity, but no treatment effect. Two patients have demonstrated long-term pain relief under memantine until now (16 months). The total number of slight adverse events were comparable in both groups, but the overall number of severe events was higher in the memantine group (P<0.05). This trial failed to demonstrate a significant clinical benefit of the NMDA-receptor antagonist memantine in chronic PLP. The administration of a higher dosage is probably not tolerable.

An fMRI investigation of hand representation in paraplegic humans

OBJECTIVE

Cortical reorganization can occur after deaf-ferentation due to loss of a limb, but the nature of the cortical reorganization after spinal cord injury (SCI) is still in debate.

METHODS

Using a 1.5T MRI, we scanned paraplegic and noninjured participants during hand movement and palm stimulation, to determine whether longterm paraplegics would show different patterns of cortical activity from the noninjured participants.

RESULTS

The SCI group showed stronger activation in areas posterior, rather than superior, to the areas activated by non-SCIs. Conversely, the non-SCIs showed stronger activation in more anterior areas. The signal at each individual's maximally significant voxel had a greater modulation for the SCI group than for the non-SCIs, in response to movement.

CONCLUSIONS

In this study of sensory and motor representations within the same subjects, the authors show for the first time the increase in the BOLD fMRI signal modulation in SCI. The authors do not find evidence of expansion of the hand representation into nearby cortical areas, and they corroborate previous EEG studies indicating a posterior shift for hand motor representation after SCI, while showing that the sensory representation does not undergo a posterior shift of similar magnitude. The difference between the reorganization found here and the reorganization typically found following amputations suggests a rationale for the differences in neuropathic pain symptoms following a spinal cord injury or amputation.

Central tinnitus

Tinnitus is likely initiated by a discontinuity in the spontaneous or low-level-stimulus induced neural activity across auditory nerve fibers with different characteristic frequency (CF). This discontinuity may be caused by functional loss of outer hair cells in those regions where inner hair cells are preserved. The reduced spontaneous activity for nerve fibers with CFs in the hearing loss range may result in a reduction of lateral inhibition at more central levels. This reduced lateral inhibition of neurons with CFs close to the edge frequency of the audiogram induces hypersensitivity and hyperactivity in these neurons. Persistent changes in lateral inhibition result in increased numbers of neurons that are tuned to a limited range of frequencies at the edge of the cochlear lesion. Thus, the frequency map in auditory cortex, the tonotopic map, becomes reorganized as it reflects these changes. The spontaneous neuronal firings in the auditory cortex after insults that cause tinnitus show increased synchrony, thereby mimicking one aspect of the responses to normal sound stimulation. All long-standing tinnitus may thus be called central tinnitus, despite the fact that it is initiated by cochlear hearing loss or is localized to the hearing loss ear.

Chronic motor cortex stimulation for phantom limb pain: correlations between pain relief and functional imaging studies

Chronic motor cortex stimulation (CMCS) has provided satisfactory control of pain in patients with central or trigeminal neuropathic pain. We used this technique in 3 patients with intractable phantom limb pain after upper limb amputation. Functional magnetic resonance imaging (fMRI) correlated to anatomical MRI permitted frameless image guidance for electrode placement. Pain control was obtained for all the patients initially and the relief was stable in 2 of the 3 patients at 2 year follow-up. CMCS can be used to relieve phantom limb pain. fMRI data are useful in assisting the neurosurgeon in electrode placement for this indication.

Human brain plasticity: an emerging view of the multiple substrates and mechanisms that cause cortical changes and related sensory dysfunctions after injuries of sensory inputs from the body

Injuries of peripheral inputs from the body cause sensory dysfunctions that are thought to be attributable to functional changes in cerebral cortical maps of the body. Prevalent theories propose that these cortical changes are explained by mechanisms that preeminently operate within cortex. This paper reviews findings from humans and other primates that point to a very different explanation, i.e. that injury triggers an immediately initiated, and subsequently continuing, progression of mechanisms that alter substrates at multiple subcortical as well as cortical locations. As part of this progression, peripheral injuries cause surprisingly rapid neurochemical/molecular, functional, and structural changes in peripheral, spinal, and brainstem substrates. Moreover, recent comparisons of extents of subcortical and cortical map changes indicate that initial subcortical changes can be more extensive than cortical changes, and that over time cortical and subcortical extents of change reach new balances. Mechanisms for these changes are ubiquitous in subcortical and cortical substrates and include neurochemical/molecular changes that cause functional alterations of normal excitation and inhibition, atrophy and degeneration of normal substrates, and sprouting of new connections. The result is that injuries that begin in the body become rapidly further embodied in reorganizational make-overs of the entire core of the somatosensory brain, from peripheral sensory neurons to cortex. We suggest that sensory dysfunctions after nerve, root, dorsal column (spinal), and amputation injuries can be viewed as diseases of reorganization in this core.

Dynamic representational plasticity in sensory cortex

Studies of the effects of peripheral and central lesions, perceptual learning and neurochemical modification on the sensory representations in cortex have had a dramatic effect in alerting neuroscientists and therapists to the reorganizational capacity of the adult brain. An intriguing aspect of some of these investigations, such as partial peripheral denervation, is the short-term expression of these changes. Indeed, in visual cortex, auditory cortex and somatosensory cortex loss of input from a region of the peripheral receptor epithelium (retinal, basilar and cutaneous, respectively) induces rapid expression of ectopic, or expanded, receptive fields of affected neurons and reorganization of topographic maps to fill in the representation of the denervated area. The extent of these changes can, in some cases, match the maximal extents demonstrated with chronic manipulations. The rapidity, and reversibility, of the effects rules out many possible explanations which involve synaptic plasticity and points to a capacity for representational plasticity being inherent in the circuitry of a topographic pathway. Consequently, topographic representations must be considered as manifestations of physiological interaction rather than as anatomical constructs. Interference with this interaction can produce an unmasking of previously inhibited responsiveness. Consideration of the nature of masking inhibition which is consistent with the precision and order of a topographic representation and which has a capacity for rapid plasticity requires, in addition to stimulus-driven inhibition, a source of tonic input from the periphery. Such input, acting locally to provide tonic inhibition, has been directly demonstrated in the somatosensory system and is consistent with results obtained in auditory and visual systems.

Carpal tunnel syndrome modifies sensory hand cortical somatotopy: a MEG study

The adult somatosensory system has shown reorganizational abilities at cortical and subcortical levels after peripheral nerve lesions. In the present study the effects of carpal tunnel syndrome (CTS) are investigated as reflected on the somatotopy of the primary cortical hand representation. Position and intensity of cortical sources activated by the separate electrical stimulation of median nerve and Digits 1, 3, and 5 of both affected and non-affected hands are evaluated by magnetoencephalographic (MEG) technique. Correlation of MEG results with patient-, physician- and neurophysiological-oriented evaluations of CTS was carried out. Patients showed changes in cortical hand somatotopy in strict relationship to self-referred assessment of symptoms and hand disability in daily activities, including: 1) a more extended representation of the affected hand when paresthesias prevailed; and 2) a more restricted representation due to lateral shift of the little finger was observed when pain symptoms dominated the clinical picture. Contralateral to the side of CTS, the cortical sources activated by Digit 5-stimulation appeared significantly enhanced with respect to contralateral ones from non-affected hand. When comparing the amplitude of peripheral sensory nerve action potentials of median and ulnar nerves to that of cortical responses (i.e., ECD strengths of M20 and M30 components after stimulation of Digits 3 and 5), a significant selective amplification of M30 with respect to M20 and sensory nerve action potential (SNAP) appeared during Digit 3 stimulation compared to that observed for Digit 5. This has been interpreted as a central magnification mechanism in brain responsiveness, possibly revealing a safety factor enabling sensory perception despite the small peripheral signal due to nerve trunk dysfunction. Hum.

Phantom-limb pain: characteristics, causes, and treatment

Phantom-limb pain is a common sequela of amputation, occurring in up to 80% of people who undergo the procedure. It must be differentiated from non-painful phantom phenomena, residual-limb pain, and non-painful residual-limb phenomena. Central changes seem to be a major determinant of phantom-limb pain; however, peripheral and psychological factors may contribute to it. A comprehensive model of phantom-limb pain is presented that assigns major roles to pain occurring before the amputation and to central as well as peripheral changes related to it. So far, few mechanism-based treatments for phantom-limb pain have been proposed. Most published reports are based on anecdotal evidence. Interventions targeting central changes seem promising. The prevention of phantom-limb pain by peripheral analgesia has not yielded consistent results. Additional measures that reverse or prevent the formation of central memory processes might be more effective.

Dynamic changes in area 1 somatosensory cortex during transient sensory deprivation: a preliminary study

To investigate the neural plasticity in the somatosensory cortex, changes in somatosensory evoked potentials (SSEPs) during finger ischemia were evaluated and compared with those affected by touch or movement interference. Somatosensory evoked potentials were recorded in the vicinity of the central sulcus in four patients with intractable epilepsy. During electrical stimulation to a selected finger, ischemic anesthesia was induced in another finger. Effects of tactile or movement interference were examined during electrical stimulation to the selected finger by applying tactile stimulation to or inducing voluntary movement of the other finger. Dynamic SSEPs were recorded during varying levels of sensory deprivation and different types of interference, and the dynamic nature of the SSEP changes within an individual was studied in detail. Somatosensory evoked potential changes appeared during finger ischemia and tended to persist during the postischemic stage, which is indicative of sensory plasticity and the maintenance of new conditioning. Amplitudes of the early and late cortical components increased when complete finger anesthesia was induced-a sign of the unmasking phenomenon. Amplitudes of early cortical SSEPs decreased when ischemic anesthesia was incomplete, similar to the findings when tactile or movement interference was applied. Surrounding inhibition, therefore, may become dominant before the unmasking phenomenon appears in early cortical SSEPs.

Phantom smelling

A case of phantom smelling (phantosmia) is described in a 28-yr.-old man who developed permanent bilateral anosmia after a serious injury to olfaction-related brain structures at the age of 25 years. The findings indicate that, even years after loss of input from olfactory receptors, the neural representation of olfactory perception can still recreate olfactory sensations without any conscious recall of them. This indicates that the neural representation of olfactory sensations remains functional and implies that neuronal activity in the olfactory organ or in other brain structures gives rise to olfactory experiences perceived as originating from the perception of original odor substances. The report suggests the intriguing possibility that the olfactory perception is not a passive process that merely reflects its normal input from the olfactory system but is continuously generated by a neural representation in the olfactory organ or in other olfaction-related brain structures, based on both genetic and sensory determinants. To the author's knowledge this is the first reported case of its kind.

Spatial acuity after digit amputation

Digit amputation in human and non-human primates results in reorganization of somatosensory cortex in which the representations of adjacent, intact digits expand to fill the cortical region previously devoted to the amputated digits. Whether this expanded representation results in improved sensory performance has not been determined. Consequently, we measured the ability to recognize small objects (raised letters) with a digit adjacent to the amputation and the same digit on the normal, contralateral hand in 15 amputees. The same digits were also tested in 15 age-matched, amputation-free subjects. There was no significant difference in recognition scores between digits in the amputees or between amputees and control subjects. More detailed analyses of specific confusion patterns and of the improvement with practice showed no significant differences. As far as we could determine, the cortical expansion that is presumed to accompany digit amputation had no effect on tactile pattern recognition performance.

Somatotopic organization of the ventral and dorsal finger surface representations in human primary sensory cortex evaluated by magnetoencephalography

Cortical reorganization of the subtly differentiated hand map after peripheral nerve injury might be better understood if there was a topographic conception of the homuncular representation of the dorsal finger surfaces in humans, in addition to the well-established sequential rostrocaudal array of the ventral finger aspects in cortical area 3b. In the present magnetoencephalographic study, tactile pneumatic stimulation was delivered to the fingertip and to the ventral and dorsal proximal phalanx of each digit of the dominant hand in 20 right-handed volunteers. Source localization of equivalent current dipoles underlying the recorded somatosensory evoked magnetic field was performed using a Cartesian coordinate system established by the anatomical landmarks nasion and preauricular points. Of the first major peak of each somatosensory evoked field, the region with the maximum field power (root-mean-square across channels) was selected for source reconstruction. Analysis of variance for repeated measures yielded significant results with respect to the arrangement of digits along the vertical coordinate axis, demonstrating a sequential array from the most inferiorly located D1 to the most superiorly located D5 for all different stimulus positions. This is the first study providing evidence for a sequential topographical arrangement of not only the ventral but also the dorsal surface representations of the individual digits in the human somatosensory cortex. The study contributes to a better understanding of the somatosensory hand representation in human primary cortex and provides useful information with regard to cortical plasticity studies in patients with peripheral nerve injuries at the upper extremity.