Rapid modulation of GABA in sensorimotor cortex induced by acute deafferentation

Somatosensory cortical plasticity in carpal tunnel syndrome--a cross-sectional fMRI evaluation

Carpal tunnel syndrome (CTS) is a common entrapment neuropathy of the median nerve characterized by paresthesias and pain in the first, second, and third digits. We hypothesize that aberrant afferent input in CTS will lead to cortical plasticity. Functional MRI (fMRI) and neurophysiological testing were performed on CTS patients and healthy adults. Median nerve innervated digit 2 (D2), and digit 3 (D3) and ulnar nerve innervated digit 5 (D5) were stimulated during fMRI. Surface-based and ROI-based analyses consistently demonstrated more extensive and stronger contralateral sensorimotor cortical representations of D2 and D3 for CTS patients as compared to healthy adults (P < 0.05). Differences were less profound for D5. Moreover, D3 fMRI activation in both the contralateral SI and motor cortex correlated positively with the D3 sensory conduction latency. Analysis of somatotopy suggested that contralateral SI representations for D2 and D3 were less separated for CTS patients (3.8 +/- 1.0 mm) than for healthy adults (7.5 +/- 1.2 mm). Furthermore, the D3/D2 separation distance correlated negatively with D2 sensory conduction latency-the greater the latency, the closer the D2/D3 cortical representations (r = -0.79, P < 0.05). Coupled with a greater extent of SI representation for these CTS affected digits, the closer cortical representations can be interpreted as a blurred somatotopic arrangement for CTS affected digits. These findings provide further evidence that CTS is not manifest in the periphery alone. Our results are consistent with Hebbian plasticity mechanisms, as our cohort of CTS patients had predominant paresthesias, which produce more temporally coherent afferent signaling from affected digits.

Exploring Motor Cortical Plasticity Using Transcranial Magnetic Stimulation in Humans

It is generally accepted that functional properties of the motor cortex in adult humans can be altered through behavioral or pharmacological manipulations, as well as in some pathological conditions. The ability and capacity of adult human cortex to undergo any adaptive or reorganizational changes is referred to as plasticity. Much of the evidence concerning motor cortical plasticity have been derived from studies using the non-invasive technique of transcranial magnetic stimulation (TMS). TMS has proven to be a suitable tool to explore representational plasticity and to interact with neuronal activity in settings of induction protocols either alone or coupled with altered sensory inputs. Furthermore, plastic changes induced by motor learning protocols have attracted particular interest because of their relevance in functional recovery. Recent studies support the view that learning in human motor cortex occurs through long-term potentiation (LTP)-like mechanisms. Purposeful modulation of motor cortical plastic changes by manipulative TMS protocols may offer useful rehabilitative strategies in patients with chronic motor deficits.

Drivers of brain plasticity

PURPOSE OF REVIEW

Neural plasticity represents a crucial mechanism of the human brain to adapt to environmental changes in the developing and adult human central nervous system. This property of the central nervous system contributes to learning and functional recovery from neurological diseases such as stroke. Novel interventional approaches have been proposed and are under investigation to modulate neural plasticity, enhance it when it plays an adaptive role and downregulate it when it is considered maladaptive.

RECENT FINDINGS

One of the purposes of research in neurorehabilitation has been to develop interventional approaches to enhance the beneficial effects of training. Procedures like cortical stimulation, administration of central nervous system active drugs and modulation of afferent input have been evaluated as drivers of neural plasticity in healthy subjects and in small groups of patients with stroke. So far, these studies have shown promising results and translation into the clinic is under investigation.

SUMMARY

Cortical stimulation and purposeful changes in afferent input that modulate neural plasticity impact on behavioral markers of performance, learning and functional recovery and represent promising tools in neurorehabilitation.

Visual deprivation effects on human motor cortex excitability

Single and paired-pulse transcranial magnetic stimulation (TMS) were applied to the motor cortex of 12 healthy volunteers, who were instructed to relax under eyes-open and eyes-closed conditions with room lights on and after 30 min of blindfolding. Compared to the eyes-open condition, significantly larger motor-evoked potentials and less intracortical inhibition were observed during blindfolding. Visual deafferentation changes resting human motor cortex excitability and might be a novel way to promote brain plasticity. These results raise the issue of how widespread the effects of temporary deafferentation may be and whether they are mediated by discrete or diffuse systems. These findings also illustrate an important potential confound in TMS studies of the motor cortex.

Rapid modulation of GABA concentration in human sensorimotor cortex during motor learning

Movement representations within the human primary motor and somatosensory cortices can be altered by motor learning. Decreases in local GABA concentration and its release may facilitate this plasticity. Here we use in vivo magnetic resonance spectroscopy (MRS) to noninvasively measure serial changes in GABA concentration in humans in a brain region including the primary sensorimotor cortex contralateral to the hand used for an isometric motor sequence learning task. Thirty minutes of motor sequence learning reduced the mean GABA concentration within a 2 x 2 x 2-cm3 voxel by almost 20%. This reduction was specific to motor learning: 30 min of similar, movements with an unlearnable, nonrepetitive sequence were not associated with changes in GABA concentration. No significant changes in GABA concentration were found in the primary sensorimotor cortex ipsilateral to the hand used for learning. These changes suggest remarkably rapid, regionally specific short-term presynaptic modulation of GABAergic input that should facilitate motor learning. Although apparently confined to the contralateral hemisphere, the magnitude of changes seen within a large spectroscopic voxel suggests that these changes occur over a wide local neocortical field.

Refined myoelectric control in below-elbow amputees using artificial neural networks and a data glove

PURPOSE

To develop a system for refined motor control of artificial hands based on multiple electromyographic (EMG) recordings, allowing multiple patterns of hand movements.

METHODS

Five subjects with traumatic below-elbow amputations and 1 subject with a congenital below- elbow failure of formation performed 10 imaginary movements with their phantom hand while surface electrodes recorded the EMG data. In a training phase a data glove with 18 degrees of freedom was used for positional recording of movements in the contralateral healthy hand. These movements were performed at the same time as the imaginary movements in the phantom hand. An artificial neural network (ANN) then could be trained to associate the specific EMG patterns recorded from the amputation stump with the analogous specific hand movements synchronously performed in the healthy hand. The ability of the ANN to predict the 10 imaginary movements offline, when they were reflected in a virtual computer hand, was assessed and calculated.

RESULTS

After the ANN was trained the subjects were able to perform and control 10 hand movements in the virtual computer hand. The subjects showed a median performance of 5 types of movement with a high correlation with the movement pattern of the data glove. The subjects seemed to relearn to execute motor commands rapidly that had been learned before the accident, independent of how old the injury was. The subject with congenital below-elbow failure of formation was able to perform and control several hand movements in the computer hand that cannot be performed in a myoelectric prosthesis (eg, opposition of the thumb).

CONCLUSIONS

With the combined use of an ANN and a data glove, acting in concert in a training phase, amputees rapidly can learn to execute several imaginary movements in a virtual computerized hand, this opens promising possibilities for motor control of future hand prostheses.

Frequency specific changes in regional cerebral blood flow and motor system connectivity following rTMS to the primary motor cortex

Repetitive transcranial magnetic stimulation (rTMS) to the human primary motor cortex (M1) causes bidirectional changes in corticospinal excitability depending on the stimulation frequency used. We used functional brain imaging to compare the effects of 5 Hz and 1 Hz-rTMS on local and inter-regional connectivity within the motor system. Regional cerebral blood flow (rCBF) was measured as a marker of synaptic activity at rest and during freely selected finger movements. We hypothesized that increased cortical excitability induced by 5 Hz-rTMS over M1 has an opposite effect on the synaptic activity and the connectivity of the motor network from the decreased cortical excitability induced by 1 Hz-rTMS. rTMS at both frequencies induced similar changes in rCBF at the site of stimulation and within areas of the motor network engaged by the task. The two frequencies showed different effects on movement-related coupling between motor areas. Connectivity analyses also indicated a differential effect of 5 and 1 Hz-rTMS on motor network connectivity, suggesting a role for an inferomedial portion of left M1 and left dorsal premotor cortex in maintaining performance. These results suggest that rapid reorganization of the motor system occurs to maintain task performance during periods of altered cortical excitability. This reorganization differs according to the modulation of excitability which is a function of rTMS frequency. This study extends the work of Lee et al. (Lee, L., Siebner, H.R., Rowe, J.B., Rizzo, V. Rothwell, J.C. Frackowiak, R.S. Friston, K.J., 2003. Acute remapping within the motor system induced by low-frequency repetitive transcranial magnetic stimulation. J. Neurosci. 23, 5308-5318.) by providing evidence that the pattern of acute reorganization in the motor network following rTMS depends on the direction of conditioning.

Neural plasticity and recovery of function

Recovery of the function after stroke is a consequence of many factors including resolution of oedema and survival of the ischaemic penumbra. In addition there is a growing interest in the role of central nervous system (CNS) reorganization. Much of the evidence supporting this comes from animal models of focal brain injury, but non-invasive techniques such as functional magnetic resonance imaging, transcranial magnetic stimulation, electroencephalography and magnetoencephalography now allow the study of the working human brain. Using these techniques it is apparent that the motor system of the brain adapts to damage in a way that attempts to preserve motor function. This has been demonstrated after stroke, as part of the ageing process, and even after disruption of normal motor cortex with repetitive transcranial magnetic stimulation. The result of this reorganization is a new functional architecture, one which will vary from patient to patient depending on the anatomy of the damage, the biological age of the patient and lastly the chronicity of the lesion. The success of any given therapeutic intervention will depend on how well it interacts with this new functional architecture. Thus it is crucial that the study of novel therapeutic strategies for treating motor impairment after stroke take account of this. This review maps out the attempts to describe functionally relevant adaptive changes in the human brain following focal damage. A greater understanding of how these changes are related to the recovery process will allow not only the development of novel therapeutic techniques that are based on neurobiological principles and designed to minimize impairment in patients suffering from stroke, but also to target these therapies at the appropriate patients.

Brain Gamma-Aminobutyric Acid Levels Are Decreased in Patients With Phantageusia and Phantosmia Demonstrated by Magnetic Resonance Spectroscopy

BACKGROUND

Olfactory and gustatory hallucinations (phantosmias and phantageusias, respectively) are sensory distortions that commonly follow losses of olfactory and gustatory acuity (hyposmia and hypogeusia, respectively). The biochemical basis of these hallucinations is unclear. Functional magnetic resonance imaging has been used previously to demonstrate widespread and robust central nervous system (CNS) activation to memories of these sensory distortions in patients with these symptoms. In this study, possible CNS mechanisms responsible for these distortions were evaluated using magnetic resonance spectroscopy, because this technique has been used to measure various CNS metabolites in patients with neurologic disorders.

METHODS

Forty-seven subjects were studied: 28 normal volunteers (13 men and 15 women) and 19 patients (8 men and 11 women) with persistent oral global phantageusia and/or birhinal phantosmia studied before any treatment. Four patients (1 man and 3 women) were studied before and after pharmacologic treatment that reduced the severity of their sensory distortions. All subjects were studied in a Signa 1.5-T magnetic resonance scanner with a quadrature head coil using a modified standard 2-dimensional J-point resolved excitation in the steady state (PRESS) sequence by which gamma-aminobutyric acid (GABA), glutamic acid, choline, N-acetylaspartate, and creatine (Cre) were measured in various CNS regions. Results were expressed using Cre as a denominator to determine ratios for each measurement. Differences were defined between normal subjects and patients before treatment and in patients before and after successful pharmacologic treatment.

RESULTS

Before treatment, GABA levels in several CNS regions were lower in patients than in normal volunteers and were the only biochemical changes found; significantly lowered GABA levels were found in the cingulate, right and left insula, and left amygdala. No differences between patients and normal volunteers were found in any of the metabolites in the posterior occipital region. After treatment that inhibited sensory distortions, CNS GABA levels increased in the cingulate, insula, and amygdala but significantly only in the left insula and in the right and left amygdala. After this successful treatment, no change in any biochemical parameter was found in the posterior occipital region.

CONCLUSIONS

These results indicate that decreased brain GABA levels can serve as biochemical markers of phantageusia and/or phantosmia in patients with these distortions and are the first biochemical changes in the CNS that reflect these sensory changes. After successful treatment of these distortions, CNS GABA levels increased to levels at or near normal, consistent with functional remission of these symptoms. These results substantiate a role for CNS GABA in the generation and inhibition of these sensory hallucinations. Although the underlying biochemical mechanism(s) for the generation of these decreased GABA levels are complex, because similar types of sensory hallucinations occur as auras or prodromata of epileptic seizure and migraine activity, these results suggest that there may be common biochemical changes among these disorders.

Short-term effects of functional electrical stimulation on motor-evoked potentials in ankle flexor and extensor muscles

Stimulating sensory afferents can increase corticospinal excitability. Intensive use of a particular part of the body can also induce reorganization of neural circuits (use-dependent plasticity) in the central nervous system (CNS). What happens in the CNS when the nerve stimulation is applied in concert with the use of particular muscle groups? The purpose of this study was to investigate short-term effects of electrical stimulation of the common peroneal (CP) nerve during walking on motor-evoked potentials (MEPs) in the ankle flexors and extensors in healthy subjects. Since the stimulation was applied during the swing phase of the step cycle when the ankle flexors are active, this is referred to as functional electrical stimulation (FES). The following questions were addressed: (1) can FES during walking increase corticospinal excitability more effectively than passively received repetitive nerve stimulation and (2) does walking itself improve the descending connection. FES was delivered using a foot drop stimulator that activates ankle dorsiflexors during the swing phase of the step cycle. MEPs in the tibialis anterior (TA) and soleus muscles were measured before, between, and after periods of walking with or without FES, using transcranial magnetic stimulation. After 30 min of walking with FES, the half-maximum peak-to-peak MEP (MEP(h)) in the TA increased in amplitude and this facilitatory effect lasted for at least 30 min. In contrast, walking had no effects on the TA MEP(h) without FES. The increase in the TA MEP(h) with FES (approximately 40%) was similar to that with repetitive CP nerve stimulation at rest. The soleus MEP(h) was also increased after walking with FES, but not without FES, which differs from the previous observation with CP nerve stimulation at rest. With FES, the TA silent period at MEP(h) was unchanged or slightly decreased, while it increased after walking without FES. Increased cortical excitability accompanied by unchanged cortical inhibition (no changes in the silent period with FES) suggests that FES did not simply increase general excitability of the cortex, but had specific effects on particular cortical neurons.

Local GABA Receptor Blockade Reveals Hindlimb Responses in the SI Forelimb-Stump Representation of Neonatally Amputated Rats

In adult rats that sustained forelimb amputation on the day of birth, there are numerous multi-unit recording sites in the forelimb-stump representation of primary somatosensory cortex (SI) that also respond to cutaneous stimulation of the hindlimb when cortical receptors for GABA are blocked. These normally suppressed hindlimb inputs originate in the SI hindlimb representation and synapse in the dysgranular cortex before exciting SI forelimb-stump neurons. In our previous studies, GABA (A + B) receptor blockade was achieved by topically applying a bicuculline methiodide/saclofen solution (BMI/SAC) to the cortical surface. This treatment blocks receptors throughout SI and does not allow determination of where along the above circuit the GABA-mediated suppression of hindlimb information occurs. In this study, focal injections of BMI/SAC were delivered to three distinct cortical regions that are involved in the hindlimb-to-forelimb-stump pathway. Blocking GABA receptors in the SI hindlimb representation and in the dysgranular cortex was largely ineffective in revealing hindlimb inputs (∼10% of hindlimb inputs were revealed in both cases). In contrast, when the blockade was targeted at forelimb-stump recording sites, >80% of hindlimb inputs were revealed. Thus GABAergic interneurons within the forelimb-stump representation suppress the expression of reorganized hindlimb inputs to the region. A circuit model incorporating these and previous observations is presented and discussed.

Plasticity in the human cerebral cortex: lessons from the normal brain and from stroke

The adult brain maintains the ability for reorganization or plasticity throughout life. Results from neurophysiological and neuroanatomical experiments in animals and noninvasive neuroimaging and electrophysiological studies in humans show considerable plasticity of motor representations with use or nonuse, skill learning, or injury to the nervous system. An important concept of reorganization in the motor cortex is that of a distributed neuronal network in which multiple overlapping motor representations are functionally connected through an extensive horizontal network. By changing the strength of horizontal connections between motor neurons, functionally different neuronal assemblies can form, thereby providing a substrate to construct dynamic motor output zones. Modulation of inhibition and synaptic efficacy are mechanisms involved. Recent evidence from animal experiments indicates that these functional changes are accompanied by anatomical changes. Because plasticity of the brain plays a major role in the recovery of function after stroke, the knowledge of the principles of plasticity may help to design strategies to enhance plasticity when it is beneficial, such as after brain infarction.

Persistence of visual–tactile enhancement in humans

We report two experiments in which non-informative vision of the finger enhanced tactile acuity on the fingertip. The right index finger was passively lifted to contact a grating. Twelve participants judged orientations of tactile gratings while viewing either the fingertip, or a neutral object presented via a mirror at the fingertip's location. In Expt. 1, tactile orientation discrimination for near-threshold gratings was improved when viewing the fingertip, compared to viewing the neutral object. Experiment 2 examined the temporal persistence of this effect, and found significant visual-tactile enhancement when a dark interval of up to 10 s intervened between viewing the finger and tactile stimulation. These results suggest that viewing the body modulates the neural circuitry of primary somatosensory cortex, outlasting visual inputs.