Modulation of plasticity in human motor cortex after forearm ischemic nerve block

Interhemispheric asymmetry of corticomotor excitability after chronic cerebellar infarcts

Early after stroke, there is loss of intracortical facilitation (ICF) and increase in short-interval intracortical inhibition (SICI) in the primary motor cortex (M1) contralateral to a cerebellar infarct. Our goal was to investigate intracortical M1 function in the chronic stage following cerebellar infarcts (>4 months). We measured resting motor threshold (rMT), SICI, ICF, and ratios between motor-evoked potential amplitudes (MEP) and supramaximal M response amplitudes (MEP/M; %), after transcranial magnetic stimulation was applied to the M1 contralateral (M1(contralesional)) and ipsilateral (M1(ipsilesional)) to the cerebellar infarct in patients and to both M1s of healthy age-matched volunteers. SICI was decreased in M1(contralesional) compared to M1(ipsilesional) in the patient group in the absence of side-to-side differences in controls. There were no significant interhemispheric or between-group differences in rMT, ICF, or MEP/M (%). Our results document disinhibition of M1(contralesional) in the chronic phase after cerebellar stroke.

Exploring the after-effects of theta burst magnetic stimulation on the human motor cortex: a functional imaging study

Theta burst stimulation (TBS) is a protocol of subthreshold repetitive transcranial magnetic stimulation (rTMS) inducing changes in cortical excitability. From functional imaging studies with conventional subthreshold rTMS protocols, it remains unclear what type of modulation occurs (direction and dependency to neural activity) and whether putative effects are bound to unspecific changes in cerebral perfusion or require a functional challenge. In a within-subjects (n = 17) repeated measurement design including real TBS and a control session without stimulation, we examined neural activation in a choice-reaction task after application of intermittent TBS, a protocol, which enhances cortical excitability over the left motor cortex (M1). Brain activity was monitored by blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging interleaved with measuring regional cerebral blood flow (rCBF) at rest using MR-based perfusion imaging. On a separate day, TMS-induced compound muscle action potentials (cMAPs) amplitude of the right hand was measured after excitatory TBS. Compared to control, a significant decrease in BOLD signal due to right hand motor activity during the choice-reaction task was observed mainly in the stimulated M1 and motor-related remote areas after stimulation. This decrease might represent a facilitating effect, because cMAPs amplitude increased upon TBS compared to control. No changes in rCBF at rest were observed. The data demonstrate that subthreshold intermittent TBS targets both the stimulated cortical area as well as remote areas. The facilitation changing the efficacy of neural signal transmission seems to be reflected by a BOLD signal decrease, whereas the network at rest does not appear to be affected.

Dissecting the mechanisms underlying short-interval intracortical inhibition using exercise

Recently, 2 physiologically distinct phases of short-interval intracortical inhibition (SICI) have been identified, a larger phase at interstimulus interval (ISI) 3 ms and a smaller phase at ISI 1 ms. While the former is mediated by synaptic processes, the mechanisms underlying the first phase of SICI remain a matter of debate. Separately, it is known that fatiguing hand exercise reduces SICI, a measure of cortical excitability. Consequently, the present study assessed effects of fatiguing hand exercise on the 2 SICI phases, using threshold tracking transcranial magnetic stimulation techniques, to yield further information on underlying mechanisms. Studies were undertaken on 22 subjects, with SICI assessed at baseline, after each voluntary contraction (VC) period of 120 s and 5, 10, and 20 min after last VC, with responses recorded over abductor pollicis brevis. Exercise resulted in significant reduction of SICI at ISI 1 ms (SICI_baseline 9.5 ± 2.7%; SICI_{MAXIMUM REDUCTION} 2.5 ± 2.5%, P < 0.05) and 3 ms (SICI_baseline 16.8 ± 1.7%; SICI_{MAXIMUM REDUCTION} 11.6 ± 2.1%, P < 0.05), with the time course of reduction being different for the 2 phases. Taken together, findings from the present study suggest that synaptic processes were the predominant mechanism underlying the different phases of SICI.

A preliminary transcranial magnetic stimulation study of cortical inhibition and excitability in high-functioning autism and Asperger disorder

AIM

Controversy surrounds the distinction between high-functioning autism (HFA) and Asperger disorder, but motor abnormalities are associated features of both conditions. This study examined motor cortical inhibition and excitability in HFA and Asperger disorder using transcranial magnetic stimulation (TMS).

METHOD

Participants were diagnosed by experienced clinicians strictly according to DSM-IV criteria. Participants with HFA (nine males, two females; mean age 16y 8mo, SD 4y 5mo) or Asperger disorder (11 males, three females; mean age 19y 1mo, SD 4y 2mo) and neurotypical participants (eight males, three females; mean age 19y 0mo, SD 3y 1mo) were administered a paired-pulse TMS paradigm intended to assess motor cortical inhibition and excitability. Responses to TMS were recorded by electromyography.

RESULTS

Cortical inhibition was significantly reduced in the HFA group compared with both the Asperger disorder (p<0.001) and neurotypical (p<0.001) groups, suggesting disruption of activity at gamma-aminobutyric acid A (GABA(A)) receptors. There was no group difference in cortical excitability.

INTERPRETATION

Cortical inhibition deficits may underlie motor dysfunction in autism, and perhaps even relate to specific clinical symptoms (e.g. repetitive behaviours). These findings provide novel evidence for a possible neurobiological dissociation between HFA and Asperger disorder based on GABAergic function.

Enhanced human brain associative plasticity in Costello syndrome

Costello syndrome (CS) is a rare multiple congenital anomaly disorder which is caused by germline mutations in the v-Ha-ras Harvey rat sarcoma viral oncogene homologue (HRAS) proto-oncogene. Experimental data suggest perturbing effects of the mutated protein on the functional and structural organization of networks of cerebral cortex and on the activity-dependent strengthening of synaptic transmission known as long term potentiation (LTP). In five patients with molecularly proven diagnosis of CS and in a group of 13 age-matched control subjects we investigated activity-dependent synaptic plasticity. To this end, we used a paired associative stimulation (PAS) protocol, in which left ulnar nerve stimuli were followed by transcranial magnetic stimulation (TMS) pulses to right cortical hand area, and recorded motor evoked potentials (MEPs) by single pulse TMS from left first dorsal interosseus (FDI) muscle before and after PAS. In 4 out of 5 CS patients and in a subgroup of nine control subjects we also evaluated the time course and the topographical specificity of PAS after-effects. In these two subgroups, MEPs were measured before, immediately after and 30 min after PAS in the left FDI and left abductor pollicis brevis (APB). While the PAS protocol led to a 65% increase of the FDI MEP amplitude in controls, the LTP-like phenomenon was significantly more pronounced in CS patients, with motor responses increased by 230%. In addition, CS patients showed a similar MEP increase in both muscles while control subjects showed a slight increase in APB and only immediately after PAS. We hypothesize that the extremely enhanced PAS after-effects could be due to the influence of HRAS activity on the susceptibility of synapses to undergo LTP.

Determinants of the induction of cortical plasticity by non-invasive brain stimulation in healthy subjects

The ability to induce cortical plasticity with non-invasive brain stimulation (NBS) techniques has provided novel and exciting opportunities for examining the role of the human cortex during a variety of behaviours. Additionally, and importantly, the induction of lasting changes in cortical excitability can, under some conditions, reversibly modify behaviour and interact with normal learning. Such findings have driven a large number of recent studies examining whether by using such approaches it might be possible to induce functionally significant changes in patients with a large variety of neurological and psychiatric conditions including stroke, Parkinson's disease and depression. However, even in neurologically normal subjects the variability in the neurophysiological and behavioural response to such brain stimulation techniques is high. This variability at present limits the therapeutic usefulness of these techniques. The cause of this variability is multifactorial and to some degree still unknown. However, a number of factors that can influence the induction of plasticity have been identified. This review will summarise what is known about the causes of variability in healthy subjects and propose additional factors that are likely to be important determinants. A greater understanding of these determinants is critical for optimising the therapeutic applications of non-invasive brain stimulation techniques.

Afferent Regulation of Leg Motor Cortex Excitability After Incomplete Spinal Cord Injury

An incomplete spinal cord injury (SCI) impairs neural conduction along spared ascending sensory pathways to disrupt the control of residual motor movements. To characterize how SCI affects the activation of the motor cortex by spared ascending sensory pathways, we examined how stimulation of leg afferents facilitates the excitability of the motor cortex in subjects with incomplete SCI. Homo- and heteronymous afferents to the tibialis anterior (TA) representation in the motor cortex were electrically stimulated, and the responses were compared with uninjured controls. In addition, we examined if cortical excitability could be transiently increased by repetitively pairing stimulation of spared ascending sensory pathways with transcranial magnetic stimulation (TMS), an intervention termed paired associative stimulation (PAS). In uninjured subjects, activating the tibial nerve at the ankle 45–50 ms before a TMS pulse in a conditioning-test paradigm facilitated the motor-evoked potential (MEP) in the heteronymous TA muscle by twofold on average. In contrast, prior tibial nerve stimulation did not facilitate the TA MEP in individuals with incomplete SCI ( n = 8 SCI subjects), even in subjects with less severe injuries. However, we provide evidence that ascending sensory inputs from the homonymous common peroneal nerve (CPN) can, unlike the heteronymous pathways, facilitate the motor cortex to modulate the TA MEP ( n = 16 SCI subjects) but only in subjects with less severe injuries. Finally, by repetitively coupling CPN stimulation with coincident TA motor cortex activation during PAS, we show that 7 of 13 SCI subjects produced appreciable (>20%) facilitation of the MEP following the intervention. The increase in corticospinal tract excitability by PAS was transient (<20 min) and tended to be more prevalent in SCI subjects with stronger functional ascending sensory pathways.

Differential Effects of Neuromuscular Electrical Stimulation Parameters on Submental Motor-Evoked Potentials

Background. Neuromuscular electrical stimulation (NMES) of the muscles underlying the pharynx and faucial pillars affects the excitability of corticobulbar projections in a frequency- and duration-specific manner. The anterior hyomandibular (submental) muscles are primary targets for the clinical application of NMES to improve disordered swallowing, but the optimal NMES parameters for this application are unknown. Objective. To determine the influence of NMES parameters on the excitability of corticobulbar projections to the submental musculature. Methods. Transcranial magnetic stimulation (TMS) was used in event-related protocols, triggered by either volitional contraction of the submental muscles or pharyngeal swallowing, to assess corticobulbar excitability prior to, immediately following, and 30, 60, and 90 minutes post-NMES in 25 healthy volunteers. In the first 2 experiments, 4 stimulus frequencies (5, 20, 40, and 80 Hz) and 3 NMES dosages, manipulated through stimulus train durations or number of repetitions, were evaluated. The optimal excitatory NMES triggered by volitional swallowing (event-related NMES) was then replicated in a new sample and contrasted with non-event-related NMES (either discrete events or continuously for 1 hour). Results. It was found that 80Hz NMES increased motor-evoked potential (MEP) amplitude at 30 minutes and 60 minutes poststimulation only after 60 repetitions of 4-s event-related NMES trains. Non-event-related and continuous NMES did not affect MEP amplitudes. No changes in MEP onset latencies were observed. Conclusions. Changes in corticobulbar excitability induced by NMES of the submental muscle group are frequency and dose dependent and only occur after NMES triggered by volitional swallowing. Underlying neural mechanisms are discussed.

Reversal of cortical reorganization in human primary motor cortex following thumb reconstruction

Deafferentation such as the amputation of a body part causes cortical reorganization in the primary motor cortex (M1). We investigated whether this reorganization is reversible after reconstruction of the lost body part. We tested two patients who had long-standing thumb amputations followed by thumb reconstruction with toe-to-thumb transfer 9 to 10 mo later and one patient who underwent thumb replantation immediately following traumatic amputation. Using transcranial magnetic stimulation, we measured the motor evoked potential (MEP) threshold, latency, short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF) at different time points in the course of recovery in abductor pollicis brevis muscle. For the two patients who underwent late toe-to-thumb transfer, the rest motor threshold was lower on the injured side than that on the intact side before surgery and it increased with time after reconstruction, whereas the active motor threshold remained unchanged. The rest and active MEP latencies were similar on the injured side before and < or =15 wk after surgery and followed by restoration of expected latency differences. SICI was reduced before surgery and progressively normalized with the time after surgery. ICF did not change with time. These physiological measures correlated with the recovery of motor and sensory functions. All the measurements on the intact side of the toe-to-thumb transfer patients and in the patient with thumb replantation immediately following traumatic amputation remained stable over time. We conclude that chronic reorganization occurring in the M1 after amputation can be reversed by reconstruction of the lost body part.

Reorganization and preservation of motor control of the brain in spinal cord injury: a systematic review

Reorganization of brain function in people with CNS damage has been identified as one of the fundamental mechanisms involved in the recovery of sensorimotor function. Spinal cord injury (SCI) brain mapping studies during motor tasks aim for assessing the reorganization and preservation of brain networks involved in motor control. Revealing the activation of cortical and subcortical brain areas in people with SCI can indicate principal patterns of brain reorganization when the neurotrauma is distal to the brain. This review assessed brain activation after SCI in terms of intensity, volume, and somatotopic localization, as well as preservation of activation during attempted and/or imagined movements. Twenty-five studies meeting the inclusion criteria could be identified in Medline (1980 to January 2008). Relevant characteristics of studies (level of lesion, time after injury, motor task) and mapping techniques varied widely. Changes in brain activation were found in both cortical and subcortical areas of individuals with SCI. In addition, several studies described a shift in the region of brain activation. These patterns appeared to be dynamic and influenced by the level, completeness, and time after injury, as well as extent of clinical recovery. In addition, several aspects of reorganization of brain function following SCI resembled those reported in stroke. This review demonstrates that brain networks involved in different demands of motor control remain responsive even in chronic paralysis. These findings imply that therapeutic strategies aimed at restoring spinal cord function, even in people with chronic SCI, can build on preserved competent brain control.

Continuous theta burst stimulation of right dorsolateral prefrontal cortex induces changes in impulsivity level

There is evidence that the right dorsolateral prefrontal cortex (DLPFC) may play a certain role in decision making related to reward value and time perception and, in particular, in the inhibitory control of impulsive decision making. Using the theta burst stimulation (TBS) and a delay discounting (DD) task, we investigated the potential role of right DLPFC in impulsive decision making defined by the rate of discounting delayed reward. Healthy right-handed volunteers underwent three stimulation sessions, intermittent TBS (iTBS), continuous TBS (cTBS), and sham. The steepness of the discount function (k-value), reaction time for choice and consistency were measured for each subjects. cTBS of the DLPFC reduced by 36.88 % the k-value of the DD task compared to sham condition. In contrast, iTBS did not affect impulsivity level. There were no changes neither in reaction time for choice nor consistency after either the iTBS or cTBS compared with the sham stimulation. These results demonstrate that cTBS-induced modulation of cortical excitability of the right DLPFC may affect and reduce impulsive decision making. These observations may provide some insights into the role of the right DLPFC in modulating impulsivity level and calculating reward value at different time scales under less ambiguous circumstances.

Contribution of transcranial magnetic stimulation to the understanding of functional recovery mechanisms after stroke

Motor impairments are a major cause of morbidity and disability after stroke. This article reviews evidence obtained using transcranial magnetic stimulation (TMS) that provides new insight into mechanisms of impaired motor control and disability. They briefly discuss the use of TMS in the diagnosis, prognosis, and therapy of poststroke motor disability. Particular emphasis is placed on TMS as a tool to explore mechanisms of neuroplasticity during spontaneous and treatment-induced recovery of motor function to develop more rational and clinically useful interventions for stroke rehabilitation.

Central neuromuscular dysfunction of the deltoid muscle in patients with chronic rotator cuff tears

Background

Previous surface electromyogram (EMG) studies have shown that chronic rotator cuff tears (RCT) may be associated with a altered activation of adjacent shoulder muscles. The effect of RCT on central neuromuscular control mechanisms of the shoulder girdle muscles such as the deltoideus muscle (DM), a key muscle of shoulder function, has as not yet been studied in detail.

Materials and methods

This study investigated the corticospinal excitability of the DM to assess the effects of RCT on the central neuromuscular function of proximal upper limb muscles. The motor-evoked potentials (MEP) in response to transcranial magnetic stimulation of DM on both sides were obtained from patients with unilateral RCT and compared with healthy control subjects.

Results

In patients, stimulus response curves of DM demonstrated a bilateral hyperexcitability at rest and a hypoexcitability during voluntary activation (F = 3.82, P = 0.007).

Conclusions

The DM hyperexcitability may be related to alterations in the sensory output from the shoulder. The insufficient facilitation of the DM points toward a bilateral central activation deficit. These findings seem to be assigned to adaptive changes in the motor cortex as a consequence of chronic RCT, and the neuromuscular alteration of the DM should be considered when treating patients with RCT.

High-Frequency Transcranial Magnetic Stimulation on Motor Cortex of Patients Affected by Migraine With Aura: A Way to Restore Normal Cortical Excitability?

We showed reduced motor intracortical inhibition (ICI) and paradoxical increase of intracortical facilitation (ICF) to 1 Hz repetitive transcranial magnetic stimulation (rTMS) in patients affected by migraine with aura (MA). In conditions of enhanced excitability due to a reduced inhibition, high-frequency rTMS was found to potentiate intracortical inhibition. Here we explored the conditioning effects of high-frequency priming stimulation of motor cortex with the aim of normalizing excitability reverting paradoxical facilitation by 1 Hz rTMS in MA. Nine patients with MA and nine healthy controls underwent a paired-pulse TMS paradigm to evaluate motor intracortical excitability (ICI and ICF) before and after the following rTMS conditions: 1 Hz alone or preceded by a real or sham conditioning high-frequency (10 Hz) rTMS. Sham was used to control for rTMS specificity. In baseline, ICI was significantly lower in migraineurs with respect to controls. One hertz stimulation reduced motor evoked potential amplitude and ICF in healthy controls, while it caused a significant paradoxical ICF increase in migraineurs. High-frequency rTMS conditioning normalized excitability in migraine, increasing short ICI and so reversing the paradoxical effects of 1 Hz rTMS. These findings raise the possibility that the interictal reduced intracortical inhibition in migraine could be normalized by high-frequency rTMS. This would open perspectives for new treatment strategies in migraine prevention.

Cortical inhibition and habituation to evoked potentials: relevance for pathophysiology of migraine

Dysfunction of neuronal cortical excitability has been supposed to play an important role in etiopathogenesis of migraine. Neurophysiological techniques like evoked potentials (EP) and in the last years non-invasive brain stimulation techniques like transcranial magnetic stimulation (TMS) and transcranial direct current stimulation gave important contribution to understanding of such issue highlighting possible mechanisms of cortical dysfunctions in migraine. EP studies showed impaired habituation to repeated sensorial stimulation and this abnormality was confirmed across all sensorial modalities, making defective habituation a neurophysiological hallmark of the disease. TMS was employed to test more directly cortical excitability in visual cortex and then also in motor cortex. Contradictory results have been reported pointing towards hyperexcitability or on the contrary to reduced preactivation of sensory cortex in migraine. Other experimental evidence speaks in favour of impairment of inhibitory circuits and analogies have been proposed between migraine and conditions of sensory deafferentation in which down-regulation of GABA circuits is considered the more relevant pathophysiological mechanism. Whatever the mechanism involved, it has been found that repeated sessions of high-frequency rTMS trains that have been shown to up-regulate inhibitory circuits could persistently normalize habituation in migraine. This could give interesting insight into pathophysiology establishing a link between cortical inhibition and habituation and opening also new treatment strategies in migraine.

Selective activation of human finger muscles after stroke or amputation

Individuated finger movements of the human hand require selective activation of particular sets of muscles. Such selective activation is controlled primarily by the motor cortex via the corticospinal tract. Is this selectivity therefore lost when lesions damage the corticospinal tract? Or when the motor cortex reorganizes after amputation? We studied finger movements in normal human subjects and in patients who had recovered substantially from pure motor hemiparesis caused by lacunar strokes, which damage the corticospinal tract without affecting other pathways. Even after substantial recovery from these strokes, individuation of finger movements remained reduced-both for flexion/extension and for adduction/ abduction motion of the fingers. Stroke subjects regained the ability to move the instructed digit through a normal range, but unintentional motion of other digits was increased. This increase did not result from a change in the passive biomechanical coupling of the fingers. Rather, voluntary contractions of muscles that move the intended digit were accompanied by inappropriate contractions in muscles acting on additional digits. These observations suggest that the normal corticospinal system produces individuated finger movements not only by selectively activating certain muscles, but also by suppressing activation of other muscles during voluntary effort to move a given digit. In a separate experiment, reversible amputation of the hand was produced in normal subjects by ischemic nerve block at the wrist. Motor output to the intrinsic muscles and sensory input both become blocked under these conditions, effectively amputating the hand from the nervous system. But the long extrinsic muscles that flex and extend the digits remain normally innervated, and thus flexion forces still can be generated at the fingertips. During reversible amputation of the hand produced by ischemic nerve block, the ability of subjects to activate subdivisions of extrinsic muscles and to exert flexion force at individual fingertips continued to show essentially normal selectivity. Voluntary activation of the remaining muscles thus continues to be selective after amputation, in spite of both the loss of sensory input from the amputated hand, and reorganization within the primary motor cortex. During cortical reorganization after amputation, then, voluntary patterns of motor output intended for finger muscles may not be lost. We therefore examined activity in the stump muscles of above-elbow amputees, who have no remaining hand muscles. Different movements of the phantom hand were accompanied by different patterns of EMG in remaining proximal muscles, distinct from the EMG patterns associated with movement of the phantom elbow. We infer that voluntary motor output patterns that normally control finger movements after amputation may become diverted to remaining proximal muscles.

Motor improvement and corticospinal modulation induced by hybrid assistive neuromuscular dynamic stimulation (HANDS) therapy in patients with chronic stroke

BACKGROUND AND OBJECTIVE

We devised a therapeutic approach to facilitate the use of the hemiparetic upper extremity (UE) in daily life by combining integrated volitional control electrical stimulation with a wrist splint, called hybrid assistive neuromuscular dynamic stimulation (HANDS).

METHODS

Twenty patients with chronic hemiparetic stroke (median 17.5 months) had moderate to severe UE weakness. Before and immediately after completing 3 weeks of training in 40-minute sessions, 5 days per week over 3 weeks and wearing the system for 8 hours each day, clinical measures of motor impairment, spasticity, and UE functional scores, as well as neurophysiological measures including electromyography activity, reciprocal inhibition, and intracortical inhibition were assessed. A follow-up clinical assessment was performed 3 months later.

RESULTS

UE motor function, spasticity, and functional scores improved after the intervention. Neurophysiologically, the intervention induced restoration of presynaptic and long loop inhibitory connections as well as disynaptic reciprocal inhibition. Paired pulse transcranial magnetic stimulation study indicated disinhibition of the short intracortical inhibition in the affected hemisphere. The follow-up assessment showed that improved UE functions were maintained at 3 months.

CONCLUSION

The combination of hand splint and volitional and electrically induced muscle contraction can induce corticospinal plasticity and may offer a promising option for the management of the paretic UE in patients with stroke. A larger sample size with randomized controls is needed to demonstrate effectiveness.

Differing effects of intracortical circuits on plasticity

Practice of a motor task leads to an increase in amplitude of motor-evoked potentials (MEP) in the exercised muscle. This is termed practice-dependent plasticity, and is abolished by the NMDA antagonist dextromethorphan and the GABA(A) agonist lorazepam. Here, we sought to determine whether specific subtypes of GABA(A) circuits are responsible for this effect by comparing the action of the non-selective agonist, lorazepam with that of the selective GABA(A)-alpha(1) receptor agonist, zolpidem. In seven healthy subjects, transcranial magnetic stimulation (TMS) was used to quantify changes in amplitude of MEP after practice of a ballistic motor task. In addition we measured how the same drugs affected MEP amplitudes and the excitability of a number of cortical inhibitory circuits [short-interval intracortical inhibition (SICI), short-interval afferent inhibition (SAI) and long-interval intracortical inhibition]. This allowed us to explore correlations between drugs effects in measures of cortical excitability and practice-dependent plasticity of MEP amplitudes. As previously reported, lorazepam increased SICI and decreased SAI, while zolpidem only decreased SAI. The new findings were that practice-dependent plasticity of MEPs was impaired by lorazepam but not zolpidem, and that this was negatively correlated with lorazepam-induced changes in SICI but not SAI. This suggests that the intracortical circuits involved in SICI (and not neurons expressing GABA(A)-alpha(1) receptor subunits that are implicated in SAI) may be involved in controlling the amount of practice-dependent MEP plasticity.

Cortical plasticity following nerve transfer in the upper extremity

With increasing clinical experience, peripheral nerve surgeons have come to appreciate the important role that cortical plasticity and motor relearning play in functional recovery following a nerve transfer. Neurostimulation (transcranial magnetic stimulation), and neuroimaging (functional MRI, structural MRI, magnetoencephalography) measure different aspects of cortical physiology and when used together are powerful tools in the study of cortical plasticity. The mechanisms of cortical plasticity, according to current and widely accepted opinions, involve the unmasking of previously ineffective connections or the sprouting of intact afferents from adjacent cortical or subcortical territories. Although significant strides have been made in our understanding of cortical plasticity following nerve transfer and during motor relearning, a great deal remains that we do not understand. Cortical plasticity and its manipulation may one day become important contributors to improve functional outcome following nerve transfer.

Neuromuscular plasticity during and following 3 wk of human forearm cast immobilization

Prolonged reductions in muscle activity results in alterations in neuromuscular properties; however, the time course of adaptations is not fully understood, and many of the specific adaptations have not been identified. This study evaluated the temporal evolution of adaptations in neuromuscular properties during and following 3 wk of immobilization. We utilized a combination of techniques involving nerve stimulation and transcranial magnetic stimulation to assess changes in central activation of muscle, along with spinal (H reflex) and corticospinal excitability [i.e., motor-evoked potential (MEP) amplitude, silent period (SP)] and contractile properties in 10 healthy humans undergoing 3 wk of forearm immobilization and 9 control subjects. Immobilization induced deficits in central activation (85 +/- 3 to 67 +/- 7% ) that returned to baseline levels 1 wk after cast removal. The flexor carpii radialis MEP amplitude increased greater than twofold after the first week of immobilization and remained elevated throughout immobilization and 1 wk after cast removal. Additionally, we observed a prolongation of the SP 1 wk after cast removal compared with baseline (78.5 +/- 7.1 to 98.2 +/- 8.7 ms). The contractile properties were also altered, since the rate of evoked force relaxation was slower following immobilization (-14.5 +/- 1.4 to -11.3 +/- 1.0% peak force/ms), and remained depressed 1 wk after cast removal (-10.5 +/- 0.8% peak force/ms). These observations detail the time course of adaptations in corticospinal and contractile properties associated with disuse and illustrate the profound effect of immobilization on the human neuromuscular system as evidenced by the alterations in corticospinal excitability persisting 1 wk following cast removal.

Consensus: Motor cortex plasticity protocols

Noninvasive transcranial stimulation is being increasingly used by clinicians and neuroscientists to alter deliberately the status of the human brain. Important applications are the induction of virtual lesions (for example, transient dysfunction) to identify the importance of the stimulated brain network for a certain sensorimotor or cognitive task, and the induction of changes in neuronal excitability, synaptic plasticity or behavioral function outlasting the stimulation, for example, for therapeutic purposes. The aim of this article is to review critically the properties of the different currently used stimulation protocols, including a focus on their particular strengths and weaknesses, to facilitate their appropriate and conscientious application.

Cortical excitability is abnormal in patients with the "fixed dystonia" syndrome

A form of fixed dystonia (FD) without evidence of basal ganglia lesions or neurodegeneration has been recently characterized (Schrag et al., Brain 2004;127:2360-2372), which may overlap the clinical spectrum of either complex regional pain syndrome or psychogenic dystonia. Transcranial magnetic stimulation studies in typically mobile dystonia revealed abnormal motor cortical excitability and sensori-motor integration. We compared 12 patients with limb FD to 10 patients with primary adult-onset typically mobile dystonia and 11 age-matched healthy volunteers. Measurements at the first digital interosseus representation area on both hemispheres included: short intracortical inhibition (SICI), contralateral silent period (cSP), and short and long afferent inhibition (SAI and LAI). Repeated measure ANOVA and post-hoc t-tests were used for statistical analysis. SICI was significantly reduced in both hemispheres of patients with "typical" and FD, compared to healthy subjects. For both hemispheres, cSP duration was shorter in both fixed and "typical" dystonia patients. SAI and LAI did not significantly differ between the three groups. The abnormal cortical excitability observed in FD might represent an underlying trait predisposing to different clinical forms of dystonia.

Selectivity of voluntary finger flexion during ischemic nerve block of the hand

During ischemic nerve block of an extremity the cortical representations of muscles proximal to the block are known to expand, increasing the overlap of different muscle representations. Such reorganization mimics that seen in actual amputees. We investigated whether such changes degrade voluntary control of muscles proximal to the block. Nine subjects produced brief, isometric flexion force selectively with each fingertip before, during, and after ischemic block at the wrist. We recorded the isometric force exerted at the distal phalanx of each digit, along with electromyographic (EMG) activity from intrinsic and extrinsic finger muscles. Despite paralysis of the intrinsic hand muscles, and associated decrements in the flexion forces exerted by the thumb, index, and little fingers, the selectivity of voluntary finger flexion forces and of EMG activity in the extrinsic finger muscles that generated these forces remained unchanged. Our observations indicate that during ischemic nerve block reorganization does not eliminate or degrade motor representations of the temporarily deafferented and paralyzed fingers.

Paired-pulse behavior of visually evoked potentials recorded in human visual cortex using patterned paired-pulse stimulation

Paired-pulse stimulation techniques are used as common tools to investigate cortical excitability and cortical plastic changes. Similar to investigations in the somatosensory and motor system here we applied a new paired-pulse paradigm to study the paired-pulse behavior of visually evoked potentials (VEPs) in 25 healthy subjects. VEPs were recorded and the responses to the first and the second P100 peak were analyzed at different SOAs [stimulus onset asynchrony (SOA) = interstimulus interval (ISI) + pulse duration (13 ms)]. Two measures describe the paired pulse interaction: the "amplitude ratio", the ratio of the second to the first amplitude, and the "latency shift", the difference of the inter-peak interval between the P100 peaks and the respective SOA. To separate alterations in the amplitude of the second VEP response due to changes in paired-pulse inhibition from those originating from superposition of the two waveforms, particularly at short SOAs, we created a waveform template from recordings made at SOAs of 1 s, where interaction can be assumed to be negligible. Superposed traces of VEP recordings were then created by adding two templates at delays corresponding to the SOAs used. The original recordings were then digitally subtracted from the traces obtained by superposition. Analysis of the subtracted traces revealed evidence that at short SOAs the second VEP response is substantially suppressed, a finding comparable to the paired-pulse inhibition described for motor and somatosensory cortex following paired-pulse stimulation. However, paired-pulse inhibition seen in V1 varied considerably from subject to subject, both in respect to amplitude, and to time of maximal inhibition. We found paired-pulse inhibition ranging from 12 to 76% (mean 34%) at SOAs between 80 (shortest discriminable SOA) and 320 ms (mean 128 ms). At intermediate SOAs between 80 and 720 ms (mean 215 ms) the amplitude ratios were between 94 and 145% (mean 116%) indicative of slight paired-pulse facilitation. Comparable to recovery studies by means of paired-pulse median nerve stimulation in somatosensory cortex, at shorter SOAs we found a delayed second VEP response. Combined together, our findings suggest that VEPs are characterized by significant paired-pulse inhibition at short SOAs, a phenomenon reminiscent of findings reported in other modalities. Possible mechanisms and pharmacological properties of the described paired-pulse behavior in visual cortex remain to be explored.

Abnormal cortical excitability with preserved brainstem and spinal reflexes in sialidosis type I

OBJECTIVE

To examine neurophysiological evidence of functional involvement of the brainstem and spinal cord and motor cortical excitability in sialidosis type I, a rare inherited neurodegenerative disorder caused by mutations in the NEU1 gene.

METHODS

We investigated particular pathways in the brainstem, spinal cord and motor cortex in 12 genetically proven cases of sialidosis type I by assessing blink reflex recovery cycle (BR), spinal reciprocal inhibition (RI), input-output curves (I/O), short interval intracortical inhibition (SICI), intracortical facilitation (ICF) and silent period (SP).

RESULTS

The BR and RI were normal. The slope of I/O was significantly increased, and SICI and the duration of SP were reduced in sialidosis patients.

CONCLUSIONS

Despite reports of pathology involving brainstem and anterior horn neurones, there were no obvious abnormalities in spinal and brainstem reflexes in the present patients, suggesting that the major clinical effects may be caused by changes at a level above the brainstem.

SIGNIFICANCE

For the first time, the integrity of certain brainstem and spinal cord reflexes in addition to motor cortical facilitatory and inhibitory circuits has been assessed in genetically proven type I sialidosis. This provides new data to aid in understanding of the pathophysiology of motor system dysfunction in this condition.

Modifying motor learning through gating and homeostatic metaplasticity

Synaptic plasticity in the motor cortex is involved at least in some forms of motor learning. Recent evidence showed that the extent of practice-dependent plasticity in the motor cortex can be purposefully enhanced by experimental manipulation. One way of improving motor learning is to transiently increase the excitability of the motor cortex during motor learning. This can be achieved by weakening the excitability of intracortical inhibitory circuits concurrently with practice. This principle is being referred to as "gating." Another strategy to boost learning is to decrease the threshold for induction of synaptic plasticity by lowering neuronal activity in the motor cortex before practice. This approach invokes homeostatic metaplasticity. Here we highlight how transcranial brain stimulation can exploit gating and homeostatic metaplasticity to enhance motor learning in healthy subjects and in patients after stroke.

Contribution of transcranial magnetic stimulation to the understanding of cortical mechanisms involved in motor control

Transcranial magnetic stimulation (TMS) was initially used to evaluate the integrity of the corticospinal tract in humans non-invasively. Since these early studies, the development of paired-pulse and repetitive TMS protocols allowed investigators to explore inhibitory and excitatory interactions of various motor and non-motor cortical regions within and across cerebral hemispheres. These applications have provided insight into the intracortical physiological processes underlying the functional role of different brain regions in various cognitive processes, motor control in health and disease and neuroplastic changes during recovery of function after brain lesions. Used in combination with neuroimaging tools, TMS provides valuable information on functional connectivity between different brain regions, and on the relationship between physiological processes and the anatomical configuration of specific brain areas and connected pathways. More recently, there has been increasing interest in the extent to which these physiological processes are modulated depending on the behavioural setting. The purpose of this paper is (a) to present an up-to-date review of the available electrophysiological data and the impact on our understanding of human motor behaviour and (b) to discuss some of the gaps in our present knowledge as well as future directions of research in a format accessible to new students and/or investigators. Finally, areas of uncertainty and limitations in the interpretation of TMS studies are discussed in some detail.

Neural observer based spasticity quantification during therapeutic muscle stimulation

Repetitive peripheral magnetic stimulation (RPMS) is an innovative approach in treatment of central paresis, e.g. after stroke. The main goals of our current research are the improvement of the therapeutic effect by inducing closed loop controlled movements on the one hand, and the objective assessment of the RPMS therapy on the other hand. One important parameter that allows the evaluation of the therapy progress and that gives insight in neurological processes is the level of spasticity. Current methods to evaluate spasticity are not completely objective and error-prone. This paper presents a novel method of spasticity quantification. The used algorithms are based on parameter estimation methods and can be executed during the therapeutic stimulation. Hence, objective spasticity parameters can be obtained without applying any extra equipment. The presented method has been tested with one patient.

Sustained increase of somatosensory cortex excitability by tactile coactivation studied by paired median nerve stimulation in humans correlates with perceptual gain

Cortical excitability can be reliably assessed by means of paired-pulse stimulation techniques. Recent studies demonstrated particularly for motor and visual cortex that cortical excitability is systematically altered following the induction of learning processes or during the development of pathological symptoms. A recent tactile coactivation protocol developed by Godde and coworkers showed that improvement of tactile performance in humans can be achieved also without training through passive stimulation on a time scale of a few hours. Tactile coactivation evokes plastic changes in somatosensory cortical areas as measured by blood oxygenation level-dependent (BOLD) activation in fMRI or SEP-dipole localization, which correlated with the individual gain in performance. To demonstrate changes in excitability of somatosensory cortex after tactile coactivation, we combined assessment of tactile performance with recordings of paired-pulse SEPs after electrical median nerve stimulation of both the right coactivated and left control hand at ISIs of 30 and 100 ms before, 3 h after and 24 h after tactile coactivation. Amplitudes and latencies of the first and second cortical N20/P25 response components were calculated. For the coactivated hand, we found significantly lowered discrimination thresholds and significantly reduced paired-pulse ratios (second N20/P25 response/first N20/P25 response) at an ISI of 30 ms after tactile coactivation indicating enhanced cortical excitability. No changes in paired-pulse behaviour were observed for ISIs of 100 ms. Both psychophysical and cortical effects recovered to baseline 24 h after tactile coactivation. The individual increase of excitability correlated with the individual gain in discrimination performance. For the left control hand we found no effects of tactile coactivation on paired-pulse behaviour and discrimination threshold. Our results indicate that changes in cortical excitability are modified by tactile coactivation and were scaled with the degree of improvement of the individual perceptual learning. Conceivably, changes of cortical excitability seem to constitute an additional important marker and mechanism underlying plastic reorganization.

Transcranial magnetic stimulation for the treatment of tinnitus: effects on cortical excitability

Background

Low frequency repetitive transcranial magnetic stimulation (rTMS) has been proposed as an innovative treatment for chronic tinnitus. The aim of the present study was to elucidate the underlying mechanism and to evaluate the relationship between clinical outcome and changes in cortical excitability. We investigated ten patients with chronic tinnitus who participated in a sham-controlled crossover treatment trial. Magnetic-resonance-imaging and positron-emission-tomography guided 1 Hz rTMS were performed over the auditory cortex on 5 consecutive days. Active and sham treatments were separated by one week. Parameters of cortical excitability (motor thresholds, intracortical inhibition, intracortical facilitation, cortical silent period) were measured serially before and after rTMS treatment by using single- and paired-pulse transcranial magnetic stimulation. Clinical improvement was assessed with a standardized tinnitus-questionnaire.

Results

We noted a significant interaction between treatment response and changes in motor cortex excitability during active rTMS. Specifically, clinical improvement was associated with an increase in intracortical inhibition, intracortical facilitation and a prolongation of the cortical silent period. These results indicate that intraindividual changes in cortical excitability may serve as a correlate of response to rTMS treatment.

Conclusion

The observed alterations of cortical excitability suggest that low frequency rTMS may evoke long-term-depression like effects resulting in an improvement of subcortical inhibitory function.

Effect of physiological activity on an NMDA-dependent form of cortical plasticity in human

Retention of motor learning can be enhanced or degraded by subsequent performance of a different task. Neurophysiologically this may reflect interference in synaptic plasticity by ongoing neural activity in the brain. Here we demonstrate that N-methyl-D-aspartate (NMDA) dependent aftereffects of repetitive transcranial magnetic stimulation (rTMS) also are subject to interference effects, suggesting that it may be possible to investigate these basic mechanisms in the intact human brain. We measured the motor-evoked potential (MEP) amplitude and short-interval intracortical inhibition (SICI) in the first dorsal interosseous (FDI) muscle after continuous or intermittent theta burst (cTBS/iTBS) forms of rTMS. In resting subjects, cTBS depressed MEPs and reduced SICI for about 20 min, whereas iTBS had the opposite effect. However, if subjects contracted the FDI during TBS, then effects on the MEP were abolished, although effects of cTBS on SICI remained. Contraction immediately after TBS enhanced the facilitatory effect of iTBS and reversed the usual inhibitory effect of cTBS into facilitation. Contraction 10 min after cTBS (iTBS not tested) had only a transient (3-4 min) effect on MEPs. These interactions with behavior may relate to mechanisms of interference between learning paradigms in human and be similar to effects on synaptic long-term potentiation/depression described in animal experiments.

Short-interval intracortical inhibition is modulated by high-frequency peripheral mixed nerve stimulation

Cortical excitability can be modulated by manipulation of afferent input. We investigated the influence of peripheral mixed nerve stimulation on the excitability of the motor cortex. Motor evoked potentials (MEPs), short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) in the right abductor pollicis brevis (APB), extensor carpi radialis (ECR) and first dorsal interosseous (FDI) muscles were evaluated using paired-pulse transcranial magnetic stimulation (TMS) before and after high-frequency peripheral mixed nerve stimulation (150 Hz, 30 min) over the right median nerve at the wrist. The MEP amplitude and SICI of the APB muscle decreased transiently 0-10 min after the intervention, whereas the ICF did not change. High-frequency peripheral mixed nerve stimulation reduced the excitability of the motor cortex. The decrement in the SICI, which reflects the function of GABA(A)ergic inhibitory interneurons, might compensate for the reduced motor cortical excitability after high-frequency peripheral mixed nerve stimulation.

Mental practice with motor imagery: evidence for motor recovery and cortical reorganization after stroke

OBJECTIVES

To measure the efficacy of a program combining mental and physical practice with the efficacy of a program composed of only constraint-induced movement therapy (CIMT) or only mental practice on stroke patients' levels of upper-extremity impairment and upper-extremity functional outcomes and to establish the relationship between changes in blood-oxygen-level dependent (BOLD) functional magnetic resonance imaging response during a specific motor or imagery task and improvement in motor function between intervention groups.

DESIGN

Case series.

SETTING

Licensed, 56-bed, freestanding, university-affiliated rehabilitation hospital.

PARTICIPANTS

Three men and 1 woman with moderate upper-limb hemiparesis after stroke were randomized.

INTERVENTIONS

Two patients received mental practice and CIMT, 1 patient received only mental practice, and 1 received only CIMT.

MAIN OUTCOME MEASURES

Wolf Motor Function Test (WMFT), Motor Activity Log (MAL), Sirigu break test, Movement Imagery Questionnaire-Revised, and Vividness of Movement Imagery Questionnaire.

RESULTS

The mental practice intervention alone led to slight improvement in certain functional and mental imagery measures (Sirigu, MAL, WMFT) but did not result in a clinically meaningful improvement with notable right cerebellar hemisphere activation that was not present before intervention. After CIMT, only the single patient showed clinically meaningful improvement of his affected hand as exhibited by decreased times on the MAL and WMFT. The patient showed increased bilateral cortical activation in both the motor and premotor areas during execution of a finger flexion and extension task. In contrast, during a second task, which was an imagined flexion and extension task, motor, occipital, and inferior parietal activation mainly in the contralateral hemisphere were observed. After 2 weeks of CIMT plus mental practice a patient with a lesion restricted to the parietal cortex showed little improvement in upper-extremity function and mental imagery in comparison with the patient with damage to nonparietal areas, who showed clinically meaningful improvement. The pattern of activation after 2 weeks of CIMT plus mental practice in the patient with nonparietal damage led to more focal contralateral activation in primary motor cortex when executing a voluntary flexion and extension task.

CONCLUSIONS

The case series indicates that for these patients with chronic, moderate upper-extremity impairment after stroke, a 2-week regimen of CIMT or CIMT plus mental practice only (in 1 case) resulted in modest changes occurring as a decrease in impairment, with functional improvement. Mental practice alone did not result in a clinically meaningful improvement in upper-limb impairment. We describe how these interventions may elicit "plastic" changes in the brain. Further investigations to determine the appropriate delivery and dosing of both physical and mental practice, as well as to determine whether mental practice-induced changes positively correlate with distinct patterns of cortical activation, should be undertaken before the efficacy of their use can be ascertained among patients with limitations comparable with these participants.

Central mechanisms in phantom limb perception: The past, present and future

Phantom limbs provide valuable insight into the mechanisms underlying bodily awareness and ownership. This paper reviews the complexity of phantom limb phenomena (proprioception, form, position, posture and telescoping), and the various contributions of internal constructs of the body, or body schema, and neuromatrix theory in explaining these phenomena. Specific systems and processes that have received little attention in phantom limb research are also reviewed and highlighted as important future directions, These include prosthesis embodiment and extended physiological proprioception (i.e., the extension of the body's "area of influence" that thereby extends one's innate sense of proprioception, mirror neurons and cross-referencing of the phantom limb with the intact limb (and the related phenomena of perceiving referred sensations and mirrored movements in the phantom form the intact limb). The likely involvements of the body schema and the body-self neuromatrix, mirror neurons, and cross-callosal and ipsilateral mechanisms in phantom limb phenomena all suggest that the perception of a "normal" phantom limb (that is, a non-painful phantom that has the sensory qualities of an intact limb) is more than likely an epiphenomenon of normal functioning, action understanding and empathy, and potentially may even be evolutionarily adaptive and perhaps necessary. Phantom pain, however, may be a maladaptive failure of the neuromatrix to maintain global bodily constructs.

Suppression of LTP-like plasticity in human motor cortex by the GABAB receptor agonist baclofen

Previous experiments in slice preparations revealed that pharmacological activation of GABAB receptors down- or up-regulates long-term potentiation (LTP), depending on whether increase of GABAB receptor mediated inhibitory postsynaptic potentials or decrease of presynaptic auto-inhibition of GABAA receptor mediated inhibition predominates. The effects of GABAB receptor activation on LTP in humans in vivo are unknown. Here we show, by using transcranial magnetic stimulation, that the GABAB receptor agonist baclofen decreases paired associative stimulation induced LTP-like plasticity in human motor cortex. This suggests that increased GABAB mediated inhibitory postsynaptic potentials drive this effect, and that baclofen may have a negative impact on LTP-dependent behavioural processes such as motor learning.

Changes in Excitability of the Cortical Projections to the Human Tibialis Anterior After Paired Associative Stimulation

Paired associative stimulation (PAS) based on Hebb's law of association can induce plastic changes in the intact human. The optimal interstimulus interval (ISI) between the peripheral nerve and transcranial magnetic stimulus is not known for muscles of the lower leg. The aims of this study were to investigate the effect of PAS for a variety of ISIs and to explore the efficacy of PAS when applied during dynamic activation of the target muscle. PAS was applied at 0.2 Hz for 30 min with the tibialis anterior (TA) at rest. The ISI was varied randomly in seven sessions ( n = 5). Subsequently, PAS was applied ( n = 14, ISI = 55 ms) with the TA relaxed or dorsi-flexing. Finally, an optimized ISI based on the subject somatosensory evoked potential (SEP) latency plus a central processing delay (6 ms) was used ( n = 13). Motor-evoked potentials (MEPs) were elicited in the TA before and after the intervention, and the size of the TA MEP was extracted. ISIs of 45, 50, and 55 ms increased and 40 ms decreased TA MEP significantly ( P = 0.01). PAS during dorsi-flexion increased TA MEP size by 92% ( P = 0.001). PAS delivered at rest resulted in a nonsignificant increase; however, when the ISI was optimized from SEP latency recordings, all subjects showed significant increases ( P = 0.002). No changes in MEP size occurred in the antagonist. Results confirm that the excitability of the corticospinal projections to the TA but not the antagonist can be increased after PAS. This is strongly dependent on the individualized ISI and on the activation state of the muscle.

Exercise-induced depression of the diaphragm motor evoked potential is not affected by non-invasive ventilation

Whole body exercise is followed by a depression of the diaphragm motor evoked potential (MEP). It is unknown whether the change is due to diaphragm activity or whole body exercise. To test the hypothesis that exercise-induced MEP depression was related to diaphragm activity, we performed two experiments. The first examined the effect of whole body exercise, performed with and without the use of non-invasive ventilation (NIV). NIV resulted in significant unloading of the diaphragm (pressure time product 101+/-68 cm H(2)O/s/min versus 278+/-95 cm H(2)O/s/min, p<0.001). Both conditions produced significant MEP depression compared to the control condition (% drop at 5 min, after exercise and exercise with NIV: 29 and 34%, p=0.77). Study 2 compared exercise with isocapnic hyperventilation. At 20 min the MEP had fallen by 29% in the exercise session versus 5% with hyperventilation (p=0.098). We conclude that the work of breathing during whole body exercise is not the primary driver of exercise-induced diaphragm MEP depression.

Cortical plasticity and motor activity studied with transcranial magnetic stimulation

For decades cortical representations of the parts of the body have been considered to be unchangeable. This view has changed radically during the past 20 years using new tools designed to study plasticity in the adult human brain. Transcranial magnetic stimulation (TMS) is a valuable non-invasive technique for exploring the ability of the motor cortex to change during motor skill acquisition. Results obtained with TMS in neurological patients as well as in normal subjects demonstrate that cortical plasticity is a necessity for correct adaptation to the continuously changing environment. Topographical reorganization of the motor cortex depends on the types of movements performed by the subjects. During simple training, the cortical representation is enlarged, and it returns to its initial size when the task is overlearned. These transient modifications characterize simple motor training. Motor skills in which coordination of distal and proximal muscles, precision of the task and spatio-temporal constraints are associated, has a different impact on cortical reorganization. We propose that years of practice of a complex motor skill induces a new cortical topography that must be interpreted as structural plasticity which provides the capacity to execute a plastic behaviour instead of a stereotypical movement. We review the neuronal mechanisms underlying plasticity in different types of movement. We stress new emerging notions, such as overlap of cortical maps, and system dynamics at single neuron and network levels, to explain the reorganization of movement representations that encode motor skill. Dendritic arborizations as functional computing elements, newly generated neurons in adult brain, and plastic architectures of cortical networks operating as distributed functional modules are new hypotheses for structural plasticity.