Inhibition of ipsilateral motor cortex during phasic generation of low force

The effect of age on task-related modulation of interhemispheric balance

Normal aging is associated with less lateralised task-related activation of the primary motor cortices. It has been hypothesized, but not tested, that this phenomenon is mediated transcallosaly. We have used Transcranial Magnetic Stimulation to look for age-related changes in interhemispheric inhibition (IHI). Thirty healthy individuals (aged 19–78 years) were studied using a paired-pulse protocol at rest and during a low-strength isometric contraction with the right hand. The IHI targeting the right motor cortex was assessed at two intervals, 10 ms (IHI10) and 40 ms (IHI40). The corticospinal excitability of the left hemisphere was assessed by means of input–output curves constructed during voluntary construction. Age was not correlated with IHI10 or IHI40 at rest. During muscle contraction IHI tended to increase at both intervals. However, this increase in IHI during the active condition (changeIHI) was less evident with advancing age for the 40 ms interval (r = 0.444, P = 0.02); in fact a degree of disinhibition was often present. There was no correlation between age and changeIHI10. Age was negatively correlated with the area under the recruitment curve (r = −0.585, P = 0.001) and the size of the maximum MEP collected (r = −0.485, P = 0.007). ChangeIHI and measures of corticospinal excitability were not intercorrelated. In conclusion, task-related increases in interhemispheric inhibition seem to diminish with advancing age. This phenomenon is specific for long-latency IHI and may underlie the age-related bihemispheric activation seen in functional imaging studies. The mechanism underlying changes in IHI with advancing age and the association with changes in corticospinal excitability need further investigation.

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.

Effect of corpus callosum damage on ipsilateral motor activation in patients with multiple sclerosis: a functional and anatomical study

Functional MRI (fMRI) studies have shown increased activation of ipsilateral motor areas during hand movement in patients with multiple sclerosis (MS). We hypothesized that these changes could be due to disruption of transcallosal inhibitory pathways. We studied 18 patients with relapsing-remitting MS. Conventional T1- and T2-weighted images were acquired and lesion load (LL) measured. Diffusion tensor imaging (DTI) was performed to estimate fractional anisotropy (FA) and mean diffusivity (MD) in the body of the corpus callosum (CC). fMRI was obtained during a right-hand motor task. Patients were studied to evaluate transcallosal inhibition (TCI, latency and duration) and central conduction time (CCT). Eighteen normal subjects were studied with the same techniques. Patients showed increased MD (P < 0.0005) and reduced FA (P < 0.0005) in the body of the CC. Mean latency and duration of TCI were altered in 12 patients and absent in the others. Between-group analysis showed greater activation in patients in bilateral premotor, primary motor (M1), and middle cingulate cortices and in the ipsilateral supplementary motor area, insula, and thalamus. A multivariate analysis between activation patterns, structural MRI, and neurophysiological findings demonstrated positive correlations between T1-LL, MD in the body of CC, and activation of the ipsilateral motor cortex (iM1) in patients. Duration of TCI was negatively correlated with activation in the iM1. Our data suggest that functional changes in iM1 in patients with MS during a motor task partially represents a consequence of loss of transcallosal inhibitory fibers.

Mirror therapy enhances lower-extremity motor recovery and motor functioning after stroke: a randomized controlled trial

OBJECTIVE

To evaluate the effects of mirror therapy, using motor imagery training, on lower-extremity motor recovery and motor functioning of patients with subacute stroke.

DESIGN

Randomized, controlled, assessor-blinded, 4-week trial, with follow-up at 6 months.

SETTING

Rehabilitation education and research hospital.

PARTICIPANTS

A total of 40 inpatients with stroke (mean age, 63.5 y), all within 12 months poststroke and without volitional ankle dorsiflexion.

INTERVENTIONS

Thirty minutes per day of the mirror therapy program, consisting of nonparetic ankle dorsiflexion movements or sham therapy, in addition to a conventional stroke rehabilitation program, 5 days a week, 2 to 5 hours a day, for 4 weeks.

MAIN OUTCOME MEASURES

The Brunnstrom stages of motor recovery, spasticity assessed by the Modified Ashworth Scale (MAS), walking ability (Functional Ambulation Categories [FAC]), and motor functioning (motor items of the FIM instrument).

RESULTS

The mean change score and 95% confidence interval (CI) of the Brunnstrom stages (mean, 1.7; 95% CI, 1.2-2.1; vs mean, 0.8; 95% CI, 0.5-1.2; P=.002), as well as the FIM motor score (mean, 21.4; 95% CI, 18.2-24.7; vs mean, 12.5; 95% CI, 9.6-14.8; P=.001) showed significantly more improvement at follow-up in the mirror group compared with the control group. Neither MAS (mean, 0.8; 95% CI, 0.4-1.2; vs mean, 0.3; 95% CI, 0.1-0.7; P=.102) nor FAC (mean, 1.7; 95% CI, 1.2-2.1; vs mean, 1.5; 95% CI, 1.1-1.9; P=.610) showed a significant difference between the groups.

CONCLUSIONS

Mirror therapy combined with a conventional stroke rehabilitation program enhances lower-extremity motor recovery and motor functioning in subacute stroke patients.

Putting the brain on the map: use of transcranial magnetic stimulation to assess and induce cortical plasticity of upper-extremity movement

The plasticity of the brain is an increasingly important topic for physical therapists interested in childhood development, learning, and repair following injury. The study of plasticity directly in the human nervous system presents numerous challenges, such as the ability to assess neuronal function in vivo because of physical impediments, such as the skull, skin, and dura. Transcranial magnetic stimulation (TMS), however, has become a suitable, noninvasive, and painless technique that can be applied to detect changes in cortical excitability as an indicator of neurological changes. Furthermore, repetitive trains of TMS themselves can induce plasticity. This article discusses the use of TMS to investigate and manipulate plasticity in the human nervous system.

Intermanual Differences in movement-related interhemispheric inhibition

Interhemispheric inhibition (IHI) between motor cortical areas is thought to play a critical role in motor control and could influence manual dexterity. The purpose of this study was to investigate IHI preceding movements of the dominant and nondominant hands of healthy volunteers. Movement-related IHI was studied by means of a double-pulse transcranial magnetic stimulation protocol in right-handed individuals in a simple reaction time paradigm. IHI targeting the motor cortex contralateral (IHI(c)) and ipsilateral (IHI(i)) to each moving finger was determined. IHI(c) was comparable after the go signal, a long time preceding movement onset, in both hands. Closer to movement onset, IHI(c) reversed into facilitation for the right dominant hand but remained inhibitory for left nondominant hand movements. IHI(i) displayed a nearly constant inhibition with a trough early in the premovement period in both hands. In conclusion, our results unveil a more important modulation of interhemispheric interactions during generation of dominant than nondominant hand movements. This modulation essentially consisted of a shift from a balanced IHI at rest to an IHI predominantly directed toward the ipsilateral primary motor cortex at movement onset. Such a mechanism might release muscles from inhibition in the contralateral primary motor cortex while preventing the occurrence of the mirror activity in ipsilateral primary motor cortex and could therefore contribute to intermanual differences in dexterity.

The effects of age and attention on motor overflow production—A review

Motor overflow refers to overt involuntary movement, or covert muscle activity, that sometimes co-occurs with voluntary movement. Various clinical populations exhibit overflow. Motor overflow is also present in healthy children and the elderly, although in young adults, overt overflow is considered abnormal unless elicited under conditions of extreme force or muscle fatigue. Current theories of overflow imply that the corpus callosum may mediate production of this phenomenon. However, given that the corpus callosum is a conduit enabling the transfer of cortical information, surprisingly few studies have considered the cortical or subcortical structures underlying overflow. This review considers the developmental trend of motor overflow production, specifically in the upper-limbs, and the mechanisms thought to underlie this age-related phenomenon. Potential neurological correlates of motor overflow will be discussed in conjunction with higher order attentional processes which also regulate motor overflow production. Future research investigating the impact of attentional processes on overflow production may be particularly valuable for designing rehabilitation strategies for patients experiencing induced pathological overflow or conversely, to develop techniques to encourage the recovery of movement function in individuals with paretic limbs.

Complex spatio-temporal dynamics of fMRI BOLD: A study of motor learning

Many studies have investigated the temporal properties of BOLD signal responses to task performance in regions of interest, often noting significant departures from the conventionally modelled response shape, and significant variation between regions. However, these investigations are rarely extended across the whole brain nor incorporated into the routine analysis of fMRI studies. As a result, little is known about the range of response shapes generated in the brain by common paradigms. The present study finds such temporal dynamics can be complex. We made a detailed investigation of BOLD signal responses across the whole brain during a two minute motor-sequence task, and tracked changes due to learning. The multi-component OSORU (Onset, Sustained, Offset, Ramp, Undershoot) linear model, developed by Harms and Melcher (J.Neurophysiology, 2003), was extended to characterise responses. In many regions, signal transients persisted for over thirty seconds, with large signal spikes at onset often followed by a dip in signal below the final sustained level of activation. Training altered certain features of the response shape, suggesting that different features of the response may reflect different aspects of neuro-vascular dynamics. Unmodelled, this may give rise to inconsistent results across paradigms of varying task durations. Few of the observed effects have been thoroughly addressed in physiological models of the BOLD response. The complex, extended dynamics generated by this simple, often employed task, suggests characterisation and modelling of temporal aspects of BOLD responses needs to be carried out routinely, informing experimental design and analysis, and physiological modelling.

Cross Education

Resistance training can be defined as the act of repeated voluntary muscle contractions against a resistance greater than those normally encountered in activities of daily living. Training of this kind is known to increase strength via adaptations in both the muscular and nervous systems. While the physiology of muscular adaptations following resistance training is well understood, the nature of neural adaptations is less clear. One piece of indirect evidence to indicate that neural adaptations accompany resistance training comes from the phenomenon of 'cross education', which describes the strength gain in the opposite, untrained limb following unilateral resistance training. Since its discovery in 1894, subsequent studies have confirmed the existence of cross education in contexts involving voluntary, imagined and electrically stimulated contractions. The cross-education effect is specific to the contralateral homologous muscle but not restricted to particular muscle groups, ages or genders. A recent meta-analysis determined that the magnitude of cross education is approximately equal to 7.8% of the initial strength of the untrained limb. While many features of cross education have been established, the underlying mechanisms are unknown. This article provides an overview of cross education and presents plausible hypotheses for its mechanisms. Two hypotheses are outlined that represent the most viable explanations for cross education. These hypotheses are distinct but not necessarily mutually exclusive. They are derived from evidence that high-force, unilateral, voluntary contractions can have an acute and potent effect on the efficacy of neural elements controlling the opposite limb. It is possible that with training, long-lasting adaptations may be induced in neural circuits mediating these crossed effects. The first hypothesis suggests that unilateral resistance training may activate neural circuits that chronically modify the efficacy of motor pathways that project to the opposite untrained limb. This may subsequently lead to an increased capacity to drive the untrained muscles and thus result in increased strength. A number of spinal and cortical circuits that exhibit the potential for this type of adaptation are considered. The second hypothesis suggests that unilateral resistance training induces adaptations in motor areas that are primarily involved in the control of movements of the trained limb. The opposite untrained limb may access these modified neural circuits during maximal voluntary contractions in ways that are analogous to motor learning. A better understanding of the mechanisms underlying cross education may potentially contribute to more effective use of resistance training protocols that exploit these cross-limb effects to improve the recovery of patients with movement disorders that predominantly affect one side of the body.

Contralateral effects of unilateral strength training: evidence and possible mechanisms

If exercises are performed to increase muscle strength on one side of the body, voluntary strength can increase on the contralateral side. This effect, termed the contralateral strength training effect, is usually measured in homologous muscles. Although known for over a century, most studies have not been designed well enough to show a definitive transfer of strength that could not be explained by factors such as familiarity with the testing. However, an updated meta-analysis of 16 properly controlled studies (range 15–48 training sessions) shows that the size of the contralateral strength training effect is ∼8% of initial strength or about half the increase in strength of the trained side. This estimate is similar to results of a large, randomized controlled study of training for the elbow flexors (contralateral effect of 7% initial strength or one-quarter of the effect on the trained side). This is likely to reflect increased motoneuron output rather than muscular adaptations, although most methods are insufficiently sensitive to detect small muscle contributions. Two classes of central mechanism are identified. One involves a “spillover” to the control system for the contralateral limb, and the other involves adaptations in the control system for the trained limb that can be accessed by the untrained limb. Cortical, subcortical and spinal levels are all likely to be involved in the “transfer,” and none can be excluded with current data. Although the size of the effect is small and may not be clinically significant, study of the phenomenon provides insight into neural mechanisms associated with exercise and training.

Ipsilateral corticospinal projections do not predict congenital mirror movements: a case report

Congenital mirror movements (CMMs) are involuntary, symmetric movements of one hand during the production of voluntary movements with the other. CMMs have been attributed to a range of physiological mechanisms, including excessive ipsilateral projections from each motor cortex to distal extremities. We examined this hypothesis with an individual showing pronounced CMMs. Mirror movements were characterized for a set of hand muscles during a simple contraction task. Transcranial magnetic stimulation (TMS) was then used to map the relative input to each muscle from both motor cortices. Contrary to our expectations, CMMs were most prominent for muscles with the strongest contralateral representation rather than in muscles that were activated by stimulation of either hemisphere. These findings support a bilateral control hypothesis whereby CMMs result from the recruitment of both motor cortices during intended unimanual movements. Consistent with this hypothesis, bilateral motor cortex activity was evident during intended unimanual movements in an fMRI study. To assess the level at which bilateral recruitment occurs, motor cortex excitability during imagined unimanual movements was assessed with TMS. Facilitory excitation was only observed in the contralateral motor cortex. Thus, the bilateral recruitment of the hemispheres for unilateral actions in individuals with CMMs appears to occur during movement execution rather than motor planning.

Motor cortex excitability during unilateral muscle activity

The effect of unilateral tonic muscle activity with and without co-activation of the antagonists on motor cortex excitability has been studied. Motor evoked potentials (MEPs) were recorded from the first dorsal interosseus muscles of both hands in response to transcranial magnetic stimulation (TMS) during relax, isometric index finger abduction and antagonistic co-activation. The intracortical inhibition (ICI) and intracortical facilitation (ICF) were investigated by paired-pulse TMS with interstimulus intervals of 3 and 13 ms. The unilateral tonic activation of the right hand facilitated contralateral and ipsilateral responses (cMEP and iMEP) recorded from both hands with an exception of iMEPs recorded from the left hand. During paired-pulse TMS ICI for cMEPs was not influenced by the unilateral tonic activity in both hands, while ICF was suppressed when MEPs were recorded from the active right hand. The effect of unilateral tonic activity on iMEP in response to paired-pulse TMS was essentially different: generally, ICI was greater for iMEPs and ICF was completely abolished with an exception of iMEPs recorded from the left hand during right finger isometric abduction when a strong ICF was evident. The decreased ICF and/or increased ICI are assumed to reflect mechanisms underlying the co-activation of antagonists.

The origin of activity in the biceps brachii muscle during voluntary contractions of the contralateral elbow flexor muscles

During strong voluntary contractions, activity is not restricted to the target muscles. Other muscles, including contralateral muscles, often contract. We used transcranial magnetic stimulation (TMS) to analyse the origin of these unintended contralateral contractions (termed "associated" contractions). Subjects (n = 9) performed maximal voluntary contractions (MVCs) with their right elbow-flexor muscles followed by submaximal contractions with their left elbow flexors. Electromyographic activity (EMG) during the submaximal contractions was matched to the associated EMG in the left biceps brachii during the right MVC. During contractions, TMS was delivered to the motor cortex of the right or left hemisphere and excitatory motor evoked potentials (MEPs) and inhibitory (silent period) responses recorded from left biceps. Changes at a spinal level were investigated using cervicomedullary stimulation to activate corticospinal paths (n = 5). Stimulation of the right hemisphere produced silent periods of comparable duration in associated and voluntary contractions (218 vs 217 ms, respectively), whereas left hemisphere stimulation caused a depression of EMG but no EMG silence in either contraction. Despite matched EMG, MEPs elicited by right hemisphere stimulation were approximately 1.5-2.5 times larger during associated compared to voluntary contractions (P < 0.005). Similar inhibition of the associated and matched voluntary activity during the silent period suggests that associated activity comes from the contralateral hemisphere and that motor areas in this (right) hemisphere are activated concomitantly with the motor areas in the left hemisphere. Comparison of the MEPs and subcortically evoked potentials implies that cortical excitability was greater in associated contractions than in the matched voluntary efforts.

Subcortical reorganization in amyotrophic lateral sclerosis

The cerebral cortex reorganizes in response to central or peripheral lesions. Although basal ganglia and cerebellum are key components of the network dedicated to movement control, their role in motor reorganization remains elusive. We therefore tested if slowly progressive neurodegenerative motor disease alters the subcortical functional anatomy of the basal ganglia-thalamo-cerebellar circuitry. Ten patients with amyotrophic lateral sclerosis (ALS) and ten healthy controls underwent functional magnetic resonance imaging (fMRI), while executing a simple finger flexion task. Cued by an acoustic trigger, they squeezed a handgrip force transducer with their right hand at 10% of their maximum voluntary contraction force. Movement frequency, amplitude, and force were controlled. Statistical parametric mapping of task-related BOLD-response revealed increased activation in ALS patients as compared to healthy controls. The main activation increases were found in the supplementary motor area, basal ganglia, brainstem, and cerebellum. These findings suggest that degeneration of cortical and spinal motor neurons in ALS leads to a recruitment of subcortical motor structures. These subcortical activation patterns strongly resemble functional activation in motor learning and might therefore represent adaptations of cortico-subcortical motor loops as a - albeit finally ineffective - mechanism to compensate for the ongoing loss of motor neurons in ALS.

Corticomotor excitability during a choice-hand reaction time task

Fourteen neurologically healthy, right-handed subjects performed a choice-hand reaction time (RT) task, which involved wrist flexion or extension of either the left or right hand to one of three fixed target locations corresponding to 45 degrees flexion, 20 degrees flexion, or 20 degrees extension from the starting position. In each trial, a pre-cue provided information regarding the forthcoming target location. The hand was specified by the imperative signal. Focal transcranial magnetic stimulation (TMS) was delivered over the hand motor area of either the right or left hemisphere at set times during the foreperiod, and at random intervals during the RT interval defined by electromyography onset. As expected, an increase in corticomotor excitability was observed in the agonist of the responding hand over the RT interval. When the cue appeared at a location that required flexion with either hand, an increase in excitability was observed following stimulation over the hemisphere ipsilateral to the responding hand, indicating activation of the homologous muscle. However, when the cue appeared at a location at which the response would require flexion with one hand and extension with the other, the modulation of excitability was also based on the direction of the response. This direction-specific effect was only observed for TMS delivered to the left hemisphere during the left-hand movement, and suggested goal-based preparation in the left hemisphere independent of whether the actual movement is made with the right or left hand. These results indicate that both the homologous-muscle and the directional-specific constraints affect the corticomotor excitability of the non-responding hand.

Change of facilitation during voluntary bilateral hand activation after stroke

BACKGROUND AND PURPOSE

The relearning of daily activities after stroke also involves performance of bimanual tasks. This raises the possibility that concurrent activation of the healthy hemisphere interferes with reorganization processes in the affected hemisphere due to inhibitory pathways between homologous motor cortex representations. This study investigated the effect of voluntary, simultaneous activation of both hands upon the non-dominant (healthy subjects) or affected (stroke patients) hemisphere.

METHODS

Eleven healthy subjects and 16 stroke patients were investigated using transcranial stimulation (TMS). TMS was applied over the non-dominant/affected hemisphere during performance of an isometric pinch grip at different force levels (10% or 50% of maximal voluntary contraction) with the contralateral hand. The ipsilateral hand had to perform the pinch grip at various force levels (10%, 50%, or 100% of maximal voluntary contraction) simultaneously. Peak-to-peak amplitudes of motor evoked potentials (MEPs) were compared to assess differences in motor cortex excitability.

RESULTS

Unilateral activity of either hand alone exerted a facilitatory effect upon the non-dominant or affected hemisphere. In healthy subjects, the activation of the ipsilateral hand simultaneously with the contralateral hand did not produce any significant change of the MEP amplitudes compared to unilateral activation of the contralateral hand. In patients, however, the additional activation of the ipsilateral hand caused an additional increase of the peak-to-peak amplitudes.

CONCLUSION

In healthy subjects voluntary activation of the ipsilateral hand does not change the excitability of the motor cortex of the non-dominant hemisphere, when the contralateral hand is simultaneously activated. The facilitation of the contralateral hand seems to gate further facilitation by the ipsilateral hand. However, in stroke patients simultaneous activation of both hands causes an additional facilitation compared to activation of the affected hand alone.

Neural pathways mediating bilateral interactions between the upper limbs

The ease with which we perform tasks such as opening the lid of a jar, in which the two hands execute quite different actions, belies the fact that there is a strong tendency for the movements of the upper limbs to be drawn systematically towards one another. Mirror movements, involuntary contractions during intended unilateral engagement of the opposite limb, are considered pathological, as they occur in association with specific disorders of the CNS. Yet they are also observed frequently in normally developing children, and motor irradiation, an increase in the excitability of the (opposite) homologous motor pathways when unimanual movements are performed, is a robust feature of the mature motor system. The systematic nature of the interactions that occur between the upper limbs has also given rise to the expectation that functional improvements in the control of a paretic limb may occur when movements are performed in a bimanual context. In spite of the ubiquitous nature of these phenomena, there is remarkably little consensus concerning the neural basis of their mediation. In the present review, consideration is given to the putative roles of uncrossed corticofugal fibers, branched bilateral corticomotoroneuronal projections, and segmental networks. The potential for bilateral interactions to occur in various brain regions including the primary motor cortex, the supplementary motor area, non-primary motor areas, the basal ganglia, and the cerebellum is also explored. This information may provide principled bases upon which to evaluate and develop task and deficit-specific programs of movement rehabilitation and therapy.

Hemodynamic and metabolic responses to activation, deactivation and epileptic discharges

To investigate the coupling between the hemodynamic and metabolic changes following functional brain activation as well as interictal epileptiform discharges (IEDs), blood oxygenation level dependent (BOLD), perfusion and oxygen consumption responses to a unilateral distal motor task and interictal epileptiform discharges (IEDs) were examined via continuous EEG-fMRI. Seven epilepsy patients performed a periodic (1 Hz) right-hand pinch grip using approximately 8% of their maximum voluntary contraction, a paradigm previously shown to produce contralateral MI neuronal excitation and ipsilateral MI neuronal inhibition. A multi-slice interleaved pulsed arterial spin labeling and T(2)*-weighted gradient echo sequence was employed to quantify cerebral blood flow (CBF) and BOLD changes. EEG was recorded throughout the imaging session and reviewed to identify the IEDs. During the motor task, BOLD, CBF and cerebral metabolic rate of oxygen consumption (CMR(O(2))) signals increased in the contra- and decreased in the ipsilateral primary motor cortex. The relative changes in CMR(O(2)) and CBF were linearly related, with a slope of 0.46 +/- 0.05. The ratio of contra- to ipsilateral CBF changes was smaller in the present group of epilepsy patients than in the healthy subjects examined previously. IEDs produced both increases and decreases in BOLD and CBF signals. In the two case studies for which the estimation criteria were met, the coupling ratio between IED-induced CMR(O(2)) and CBF changes was estimated at 0.48 +/- 0.17. These findings provide evidence for a preserved coupling between hemodynamic and metabolic changes in response to both functional activation and, for the two case studies available, in response to interictal epileptiform activity.

Bilateral Processing of Motor Commands in the Motor Cortex of the Cat During Target-Reaching

Single-unit activity of the motor cortex (area 4γ) was studied in cats performing reaching with the contra- versus ipsilateral forelimb. Reaching was initiated by a tone burst (Go cue), different limbs were used in separate blocks of trials. During reaching performed with the contralateral limb, three types of neurons were observed. The first type had biphasic pattern with an initial component locked to the Go cue followed by a component locked to the onset of reaching. The second type of neurons had monophasic discharges correlated both with the onset of the stimulus and with the movement. The third type showed responses related to the movement. Activity of the same cells investigated during reaching performed with the ipsilateral limb revealed that the cue-locked responses of the cells of the first type were effector independent, i.e., similar discharges locked to the Go cue were generated. The movement-related component of these cells was drastically reduced. The activity of some cells of the second type was suppressed during reaching with the ipsilateral limb. When performance was switched between limbs, a significant change of background discharge frequency was observed in 31% of the cells. The present results suggest that the sensory cue triggers elaboration of motor commands for reaching in both motor cortices, but further sensorimotor transformation is completed in only one hemisphere but is aborted actively in the other. It is also suggested that a certain pattern of background activity may serve a tuning function for elaboration of the command in the proper hemisphere.

Plastic changes in interhemispheric inhibition with practice of a two-hand force production task: a transcranial magnetic stimulation study

We studied the effects of practice of an unusual two-hand finger force production task on electromyographic and force responses to transcranial magnetic stimulation (TMS). Prior to practice, force production by a hand resulted in decreased TMS-induced responses in the other hand. After practice, fingers that were explicitly required to produce force during practice showed a significant drop in these inhibitory effects, while other fingers did not. We conclude that interhemispheric inhibitory projections can show plastic changes that favor the execution of a practiced task.

Mirror, mirror on the wall: viewing a mirror reflection of unilateral hand movements facilitates ipsilateral M1 excitability

Primary motor cortex (M1) excitability is modulated by both ipsilateral limb movement and passive observation of movement of the contralateral limb. An interaction of these effects within M1 may account for recent research suggesting improved functional recovery of the impaired arm following stroke by viewing a mirror reflection of movements of the unimpaired arm superimposed over the (unseen) impaired arm. This hypothesis was tested in the present study using single-pulse transcranial magnetic stimulation (TMS) in eight neurologically healthy subjects. Excitability of M1 ipsilateral to a phasic, unilateral hand movement was measured while subjects performed paced (1 Hz), unilateral index finger-thumb opposition movements. Motor evoked potentials (MEPs) were obtained from the inactive first dorsal interosseous (FDI) in each of four viewing conditions: Active (viewing the active hand), Central (viewing a mark positioned between hands), Inactive (viewing the inactive hand) and Mirror (viewing a mirror-reflection of the active hand in a mirror oriented in the mid-sagittal plane) and with both hands at rest (Rest). MEPs were significantly enhanced during ipsilateral hand movement compared with the Rest condition (P<0.05). Largest MEPs were obtained in the Mirror condition, and this was significant compared with both the Inactive and Central viewing conditions (P<0.05). There was no difference between the dominant and non-dominant hand. Excitability of M1 ipsilateral to a unilateral hand movement is facilitated by viewing a mirror reflection of the moving hand. This finding provides neurophysiological evidence supporting the application of mirror therapy in stroke rehabilitation.

Análise da distribuição de potência cortical em função do aprendizado de datilografia

O objetivo do presente estudo foi investigar alterações nos padrões eletroencefalográficos de sujeitos normais e destros durante o aprendizado motor de uma tarefa manual. Estudos recentes têm demonstrado que o córtex cerebral é suscetível a modificações em vários aspectos durante a aprendizagem e que tais alterações nos padrões eletrocorticais são resultado da aquisição de habilidades motoras e consolidação de memória. Para tal, a atividade elétrica cortical dos sujeitos foi analisada antes e depois da prática motora. Os dados foram captados pelo Braintech 3000 e analisados pelo programa Neurometrics. Para a análise estatística, variáveis comportamentais tais como tempo e erro foram observadas através de uma ANOVA one-way, blocos como efeito principal. Na variável neurofisiológica, potência absoluta, foi utilizado um teste t pareado a fim de detectar alterações entre os momentos pré e pós-aprendizagem, e diferentes eletrodos, CZ-C3/CZ-C4 em teta e alfa, e O1-P3/T3-F7 em beta. Os resultados principais demonstraram mudança na performance através das variáveis tempo e número de erros. Concomitantemente, foi verificado aumento de potência na banda alfa sobre áreas centrais (CZ-C3/CZ-C4) e diminuição em beta localizada na área temporoparietal esquerda (O1-P3/T3-F7). Alterações na banda teta, como demonstradas em outros experimentos, não ocorreram neste estudo. Tais resultados sugerem uma adaptação do córtex sensório-motor em que a alteração da atividade elétrica cortical é condizente com uma transição ao automatismo motor.

fMRI signal decreases in ipsilateral primary motor cortex during unilateral hand movements are related to duration and side of movement

Interactions between the primary motor cortices of each hemisphere during unilateral hand movements appear to be inhibitory, although there is evidence that the strengths of these interactions are asymmetrical. In the present study, functional magnetic resonance imaging (fMRI) was used to investigate the effects of motor task duration and hand used on unilateral movement-related BOLD signal increases and decreases in the hand region of primary motor cortex (M1) of each hemisphere in six right-handed volunteers. Significant task-related BOLD signal decreases were observed in ipsilateral M1 during single and brief bursts of unilateral movements for both hands. However, these negative-to-baseline responses were found to intensify with increasing movement duration in parallel with greater task-related increases in contralateral M1. Movement-related BOLD signal decreases in ipsilateral M1 were also stronger for the right, dominant hand than for the left hand in our right-handed subjects. These findings would be consistent with the existence of interhemispheric interactions between M1 of each hemisphere, whereby increased neuronal activation in M1 of one hemisphere induces reduced neuronal activity in M1 of the opposite hemisphere. The observation of a hemispheric asymmetry in inhibition between M1 of each hemisphere agrees well with previous neuroimaging and electrophysiological data. These findings are discussed in the context of current understanding of the physiological origins of negative-to-baseline BOLD responses.

Investigating the cortical origins of motor overflow

Motor overflow refers to the involuntary movements which may accompany the production of voluntary movements. While overflow is not usually seen in the normal population, it does present in children and the elderly, as well as those suffering certain neurological dysfunctions. Advancements in methodology over the last decade have allowed for more convincing conclusions regarding the cortical origins of motor overflow. However, despite significant research, the exact mechanism underlying the production of motor overflow is still unclear. This review presents a more comprehensive conceptualization of the theories of motor overflow, which have often been only vaguely defined. Further, the major findings are explored in an attempt to differentiate the competing theories of motor overflow production. This exploration is done in the context of a range of neurological and psychiatric disorders, in order to elucidate the possible underlying mechanisms of overflow.

Enhanced Motor Cortex Excitability During Ipsilateral Voluntary Hand Activation in Healthy Subjects and Stroke Patients

BACKGROUND AND PURPOSE

It is still a matter of debate whether the ipsilateral voluntary hand activation has a facilitatory or inhibitory effect on the nondominant or affected hemisphere. To give an answer to this question is of great importance for the rehabilitation of stroke patients, because they often use the unaffected hand for compensation.

METHODS

Ten healthy volunteers and 11 stroke patients were investigated using transcranial magnetic stimulation (TMS). TMS was applied to the dominant/unaffected hemisphere during performance of different tasks (simple index finger abduction, pinch grip, and power grip) at various force levels (5%, 10%, 50%, and 100% maximal voluntary contraction) with the ipsilateral hand. Peak-to-peak amplitudes of motor-evoked potentials were used as measure for motor cortex excitability.

RESULTS

Both simple and complex tasks led to a facilitation of the contralateral corticospinal system at all levels of applied force. Not only the facilitatory effect in general but also the slope of the relationship between force level and MEP amplitude were significantly lower in stroke patients indicating that both the general activation level of the impaired motor system and the bandwidth of possible activation levels are diminished.

CONCLUSIONS

Voluntary activation of the hand does not exert an inhibitory effect on the excitability of the ipsilateral hemisphere in healthy volunteers or in stroke patients.

Kinematically Specific Interhemispheric Inhibition Operating in the Process of Generation of a Voluntary Movement

Unilateral hand movements are accompanied by a transient decrease in corticospinal (CS) excitability of muscles in the opposite hand. However, the rules that govern this phenomenon are not completely understood. We measured the amplitude of motor evoked potentials (MEP) in the left first dorsal interosseus (FDI) elicited by transcranial magnetic stimulation (TMS) of the primary motor cortex in order to assess CS excitability changes that preceded eight possible combinations of unilateral and bilateral index finger movements with different right hand positions. Left FDI MEP amplitude (MEP(Left FDI)) increased when this muscle acted as an agonist and tended to decrease when it was an antagonist. Additionally, MEP(Left FDI) decreased substantially before right index finger abduction (a movement mediated by the right FDI) when both hands were lying flat (a movement mirroring left index finger abduction) but not when the right hand was turned at 90 degrees or flat with the palm up. Therefore, CS excitability of the resting FDI was differentially modulated depending on the direction of the opposite index finger movement, regardless of muscles engaged in the task. These results indicate that inhibitory interactions preceding unilateral finger movements are determined by movement kinematics possibly to counteract the default production of mirror motions.

Hemodynamic and metabolic responses to neuronal inhibition

Functional magnetic resonance imaging (fMRI) was used to investigate the changes in blood oxygenation level dependent (BOLD) signal, cerebral blood flow (CBF) and cerebral metabolic rate of oxygen consumption (CMR(O(2))) accompanying neuronal inhibition. Eight healthy volunteers performed a periodic right-hand pinch grip every second using 5% of their maximum voluntary contraction (MVC), a paradigm previously shown to produce robust ipsilateral neuronal inhibition. To simultaneously quantify CBF and BOLD signals, an interleaved multislice pulsed arterial spin labeling (PASL) and T(2)*-weighted gradient echo sequence was employed. The CMR(O(2)) was calculated using the deoxyhemoglobin dilution model, calibrated by data measured during graded hypercapnia. In all subjects, BOLD, CBF and CMR(O(2)) signals increased in the contralateral and decreased in the ipsilateral primary motor (M1) cortex. The relative changes in CMR(O(2)) and CBF were linearly related, with a slope of approximately 0.4. The coupling ratio thus established for both positive and negative CMR(O(2)) and CBF changes is in close agreement with the ones observed by earlier studies investigating M1 perfusion and oxygen consumption increases. These findings characterize the hemodynamic and metabolic downregulation accompanying neuronal inhibition and thereby establish the sustained negative BOLD response as a marker of neuronal deactivation.

Interhemispheric effects of high and low frequency rTMS in healthy humans

OBJECTIVE

We investigated whether repetitive transcranial magnetic stimulation (rTMS) applied to the right motor cortex modified the excitability of the unstimulated left motor cortex.

METHODS

Interhemispheric effects of 0.5 and 5 Hz subthreshold rTMS over the right motor cortex were examined by single pulse and paired pulse TMS and by transcranial electrical stimulation (TES) applied to the unstimulated left motor cortex. The effects of (a) 1800 pulses real and sham rTMS with 5 Hz, (b) 180 pulses real and sham rTMS with 0.5 Hz and (c) 1800 pulses real rTMS with 0.5 Hz were studied.

RESULTS

Following 5 Hz right motor rTMS motor evoked potential (MEP) amplitudes induced by single pulse TMS over the left motor cortex increased significantly. Intracortical inhibition (ICI) and facilitation (ICF) and MEP amplitudes evoked by TES were unchanged. Sham stimulation had no influence on motor cortex excitability. After 180 pulses right motor cortex rTMS with 0.5 Hz a significant decrease of left motor ICF, but no change in single pulse MEP amplitudes was found. A similar trend was observed with 1800 pulses rTMS with 0.5 Hz.

CONCLUSIONS

High frequency right motor rTMS can increase left motor cortex excitability whereas low frequency right motor rTMS can decrease it. These effects outlast the rTMS by several minutes. The underlying mechanisms mediating interhemispheric excitability changes are likely to be frequency dependent.

Event-related desynchronization and excitability of the ipsilateral motor cortex during simple self-paced finger movements

OBJECTIVE

To study the time course of oscillatory EEG activity and corticospinal excitability of the ipsilateral primary motor cortex (iM1) during self-paced phasic extension movements of fingers II-V.

METHODS

We designed an experiment in which cortical activation, measured by spectral-power analysis of 28-channel EEG, and cortical excitability, measured by transcranial magnetic stimulation (TMS), were assessed during phasic self-paced extensions of the right fingers II-V in 28 right-handed subjects. TMS was delivered to iM1 0-1500 ms after movement onset.

RESULTS

Ipsilateral event-related desynchronization (ERD) during finger movement was paralleled by increased cortical excitability of iM1 from 0-200 ms after movement onset and by increased intracortical facilitation (ICF) without changes in intracortical inhibition (ICI) or peripheral measures (F waves). TMS during periods of post-movement event-related synchronization (ERS) revealed no significant changes in cortical excitability in iM1.

CONCLUSIONS

Our findings indicate that motor cortical ERD ipsilateral to the movement is associated with increased corticospinal excitability, while ERS is coupled with its removal. These data are compatible with the concept that iM1 contributes actively to motor control. No evidence for inhibitory modulation of iM1 was detected in association with self-paced phasic finger movements.

SIGNIFICANCE

Understanding the physiological role of iM1 in motor control.

Cortical correlates of neuromotor development in healthy children

OBJECTIVE

To examine the relationship between acquisition of fine motor skills in childhood and development of the motor cortex.

METHODS

We measured finger tapping speed and mirror movements in 43 healthy right-handed subjects (6-26 years of age). While recording surface electromyographic activity from right and left first dorsal interosseus, we delivered focal transcranial magnetic stimulation (TMS) over the hand areas of each motor cortex. We measured motor evoked potential (MEP) threshold, and ipsilateral (iSP) and contralateral (CSP) silent periods.

RESULTS

As children got older, finger speeds got faster, MEP threshold decreased, iSP duration increased and latency decreased. Finger tapping speed got faster as motor thresholds and iSP latency decreased, but was unrelated to CSP duration. In all subjects right hemisphere MEP thresholds were higher than those on the left and duration of right hemisphere CSP was longer than that on the left. Children under 10 years of age had higher left hand mirror movement scores, and fewer left hemisphere iSPs which were of longer duration.

CONCLUSIONS

Maturation of finger tapping skills is closely related to developmental changes in the motor threshold and iSP latency. Studies are warranted to explore the relationship between these measures and other neuromotor skills in children with motor disorders.

SIGNIFICANCE

TMS can provide important insights into certain functional aspects of neurodevelopment in children.

Functional cortical changes in patients with multiple sclerosis and nonspecific findings on conventional magnetic resonance imaging scans of the brain

Recent functional magnetic resonance imaging (fMRI) work has suggested that cortical reorganisation might have an adaptive role in limiting the clinical impact of multiple sclerosis (MS) structural damage. In this study, we investigated whether, in patients with MS, the presence and extent of structural damage of the normal-appearing brain tissue are associated with the extent of the movement-associated pattern of cortical activations. Using fMRI and a general search method, we assessed the patterns of brain activations associated with simple motor tasks in 12 right-handed patients with clinically definite MS and nonspecific T2-weighted abnormalities on conventional MRI scans of the brain and compared them with those from 12 sex- and age-matched right- handed healthy controls. Also investigated were the extent to which the fMRI changes correlated with normal-appearing white matter and grey matter (GM) pathology, measured using diffusion tensor MRI. When performing the simple motor task with the dominant hand, MS patients had more significant activations of the ipsilateral supplementary motor area (SMA), the ipsilateral superior frontal sulcus, the contralateral superior temporal gyrus, and the thalamus than controls. On the contrary, healthy subjects showed more significant activations of the medial part of the contralateral parieto-occipital fissure and the ipsilateral primary sensorimotor cortex (SMC) than patients with MS. In patients with MS, the relative activation of the ipsilateral SMA was correlated with the peak height (r = -0.88, P < 0.001) and position (r = 0.87, P < 0.001) of the GM mean diffusivity histogram. This study shows that cortical reorganisation occurs over a rather distributed sensorimotor network even in patients with MS and nonspecific abnormalities on conventional brain MRI scans. This suggests that, in patients with MS, an increased recruitment of movement-associated cortical network can be elicited by the presence of normal-appearing tissue pathology, which is independent of macroscopic T2-weighted abnormalities.

Power grip disinhibits the ipsilateral sensorimotor cortex: a TMS and fMRI study

Electrophysiological studies have shown that forceful activation of the hand muscles (power grip) is accompanied by an increased excitability of the ipsilateral corticospinal system. This increase in excitability may be due to spinal or cortical mechanisms. Here we show with fMRI that this phenomenon is at least in part mediated at a cortical level. We used TMS to show that the increased ipsilateral excitability during a forceful maneuver leads to enhanced stimulus-response curves. fMRI was used to compare the activation during a repetitive hand movement with or without an accompanying power grip on the opposite site. The power grip reduced movement-related activation in the ipsilateral sensorimotor cortex. Peak deactivation was located in the left postcentral gyrus extending into the adjacent precentral gyrus. This finding suggests that a forceful activation of the hand muscles disinhibits a distinct functional representation in the ipsilateral sensorimotor cortex. Consequently, the excitability of the corticospinal system increases and less neuronal excitatory activity is needed to perform a given task. The results may be important for a variety of studies as they suggest that fMRI may show decreased hemodynamic response under conditions in which other neurophysiological methods have shown increased functional activity.

Contralateral activity in a homologous hand muscle during voluntary contractions is greater in old adults

This study compared the amount of contralateral activity produced in a homologous muscle by young (18-32 yr) and old (66-80 yr) adults when they performed unilateral isometric and anisometric contractions with a hand muscle. The subjects were not aware that the focus of the study was the contralateral activity. The tasks involved the performance of brief isometric contractions to six target forces, slowly lifting and lowering six inertial loads, and completing a set of 10 repetitions with a heavy load. The unintended force exerted by the contralateral muscle during the isometric contractions increased with target force, but the average force was greater for the old adults (means +/- SD; 12.6 +/- 15.3%) compared with the young adults (6.91 +/- 11.1%). The contralateral activity also increased with load during the anisometric contractions, and the average contralateral force was greater for the old subjects (5.28 +/- 6.29%) compared with the young subjects (2.10 +/- 3.19%). Furthermore, the average contralateral force for both groups of subjects was greater during the eccentric contractions (4.17 +/- 5.24%) compared with the concentric contractions (3.20 +/- 5.20%). The rate of change in contralateral activity during the fatigue task also differed between the two groups of subjects. The results indicate that old subjects have a reduced ability to suppress unintended contralateral activity during the performance of goal-directed, unilateral tasks.