The modulation of short and long-latency interhemispheric inhibition during bimanually coordinated movements.

Bimanual coordination is essential for the performance of many everyday tasks. There are several types of bimanually coordinated movements, classified according to whether the arms are acting to achieve a single goal (cooperative) or separate goals (independent), and whether the arms are moving symmetrically or asymmetrically. Symmetric bimanual movements are thought to facilitate corticomotor excitability (CME), while asymmetric bimanual movements are thought to recruit interhemispheric inhibition to reduce functional coupling between the motor cortices. The influences of movement symmetry and goal conceptualisation on interhemispheric interactions have not been studied together, and not during bimanually active dynamic tasks. The present study used transcranial magnetic stimulation (TMS) to investigate the modulation of CME and short- and long-latency interhemispheric inhibition (SIHI and LIHI, respectively) during bimanually active dynamic tasks requiring different types of bimanual coordination. Twenty healthy right-handed adults performed four bimanual tasks in which they held a dumbbell in each hand (independent) or a custom device between both hands (cooperative) while rhythmically flexing and extending their wrists symmetrically or asymmetrically. Motor-evoked potentials were recorded from the right extensor carpi ulnaris. We found CME was greater during asymmetric tasks than symmetric tasks, and movement symmetry did not modulate SIHI or LIHI. There was no effect of goal conceptualisation nor any interaction with movement symmetry for CME, SIHI or LIHI. Based on these results, movement symmetry and goal conceptualisation may not modulate interhemispheric inhibition during dynamic bimanual tasks. These findings contradict prevailing thinking about the roles of CME and interhemispheric inhibition in bimanual coordination.

Cooperative hand movements: task-dependent modulation of ipsi- and contralateral cortical control.

Cooperative hand movements are known to be controlled by a task-specific neural coupling associated with an involvement of the respective ipsilateral hemispheres. The aim of this study was to explore in how far this neural control applies to and is modulated during various, fine and gross, cooperative hand movements required during activities of daily living. Somatosensory evoked potentials and contralateral electromyographic reflex responses to unilateral ulnar nerve stimulation were simultaneously recorded in healthy participants during three different cooperative hand movement tasks and a resting condition. Amplitude ratio (ipsi-/contralateral) of the somatosensory evoked potentials, which is a measure for the involvement of the ipsilateral hemisphere in movement control, was higher in all three movement tasks compared to resting. This ratio was highest during the fine cooperative movement studied here. Contralateral reflex responses, as a measure for the functional coupling of the arms, were elicited following stimulation of both arms during gross cooperative movements. However, such a response could only be elicited in the dominant arm during fine movement. It is concluded that the neural coupling and thus enhancement of ipsilateral cortical control is preserved through different cooperative hand movement tasks, independently whether fine or gross motor tasks are performed. However, modulation of cortical control can be observed as ipsilateral cortical control is stronger during fine movements and functional coupling of the arms more focused to the dominant hand compared to gross cooperative tasks.

Cooperative hand movements: effect of a reduced afference on the neural coupling mechanism.

The aim of this study was to evaluate the influence of unilateral reduction of afferent input on the 'neural coupling' mechanism during cooperative hand movements. This 'neural coupling' is reflected in the task-specific appearance of contralateral reflex responses in forearm muscles to unilateral arm nerve stimulation. Sensory input from the right hand was reduced by ischemic nerve block at the right wrist. Ipsilateral and contralateral reflex responses elicited by stimulation of the ulnar nerve either at the left or the right wrist proximal to the nerve block were recorded in forearm extensors during the performance of cooperative hand movements. During ischemia of the right hand, a significant difference was found in the magnitude of the contralateral responses, that is, contralateral reflex responses in the right arm were significantly higher compared with the left arm (P=0.04). Ipsilateral reflex responses were not affected by ischemic nerve block. The reduced afference from the ischemic hand during cooperative hand movements is assumed to weaken the activity in ipsilateral pathways involved in the neural coupling mechanism. Consequently, a shift in the interhemispheric balance might lead to the relative increase and decrease in the contralateral responses to left and right nerve stimulation, respectively. The study provides novel information on the involvement of ipsilateral hemispheres in the performance of cooperative hand movements.

Neuroplastic Changes in Older Adults Performing Cooperative Hand Movements.

The aim of this study was to examine whether older adults use the same task-specific brain activation patterns during two different bimanual hand movement tasks as younger adults. Functional magnetic resonance brain imaging was performed in 18 younger (mean age: 30.3 ± 3.6 years) and 11 older adults (62.6 ± 6.8 years) during the execution of cooperative (mimicking opening a bottle) or non-cooperative (bimanual pro-/supination) hand movements. We expected to see a stronger task-specific involvement of the secondary somatosensory cortex (S2) during cooperative hand movements in older compared to younger adults. However, S2 activation was present in both groups during the cooperative task and was only significantly stronger compared to the non-cooperative task in younger adults. In a whole brain-analysis, the contrast between older and younger adults revealed a hyperactivation of the bilateral dorsal premotor cortex (precentral gyrus), right thalamus, right frontal operculum, anterior cingulate cortex, and supplementary motor areas in older adults (p < 0.001), with some of them being visible after correcting for age. Age was positively associated with fMRI signal changes in these regions across the whole sample. Older adults showed reduced gray matter volume but not in regions showing task-related fMRI group differences. We also found an increase in functional connectivity between SMA, M1, thalamus, and precentral gyri in older adults. In contrast, younger adults showed hyperconnectivity between S2 and S1. We conclude that older compared to younger adults show age-related functional neuroplastic changes in brain regions involved in motor control and performance.

Increasing cognitive load attenuates right arm swing in healthy human walking.

Human arm swing looks and feels highly automated, yet it is increasingly apparent that higher centres, including the cortex, are involved in many aspects of locomotor control. The addition of a cognitive task increases arm swing asymmetry during walking, but the characteristics and mechanism of this asymmetry are unclear. We hypothesized that this effect is lateralized and a Stroop word-colour naming task-primarily involving left hemisphere structures-would reduce right arm swing only. We recorded gait in 83 healthy subjects aged 18-80 walking normally on a treadmill and while performing a congruent and incongruent Stroop task. The primary measure of arm swing asymmetry-an index based on both three-dimensional wrist trajectories in which positive values indicate proportionally smaller movements on the right-increased significantly under dual-task conditions in those aged 40-59 and further still in the over-60s, driven by reduced right arm flexion. Right arm swing attenuation appears to be the norm in humans performing a locomotor-cognitive dual-task, confirming a prominent role of the brain in locomotor behaviour. Women under 60 are surprisingly resistant to this effect, revealing unexpected gender differences atop the hierarchical chain of locomotor control.

Effect of Locomotor Training on Exhaustion of Leg Muscle Activity in Chronic Complete Spinal Cord Injury.

The aim of this study was to evaluate the effect of a continuous locomotor training on leg muscle electromyographic (EMG) exhaustion during assisted stepping movements in a patient with motor complete spinal cord injury (SCI). EMG exhaustion and loss of potentials starts to develop in untrained patients at ∼6 months after injury. In the trained patient examined in this study, exhaustion was also observed but occurred with a delay of several months. In contrast to an untrained patient, no more EMG exhaustion was observed in the very chronic stage. At this time (12 years after injury) a basic locomotor pattern of leg muscle activity of reduced amplitude could still be elicited, but it was resistant to exhaustion and unchanged in amplitude after 12 min of assisted stepping. It is suggested that fatigue-resistant motor units prevail at this stage and can still be activated during stepping as a result of the training.

Neural coupling of cooperative hand movements after stroke: role of ipsilateral afference.

We investigated the role of ipsilateral ascending pathways in the neural coupling underlying cooperative hand movements of stroke subjects. Ipsi- and contralateral somatosensory evoked potentials (SSEP) were recorded following ulnar nerve stimulation during cooperative and non-cooperative hand movements. The amplitude ratio, that is, ipsilateral divided by contralateral amplitude, was highest during the cooperative task when the affected arm was stimulated, reflecting an enhanced afferent volley to the unaffected hemisphere. The presence of ipsilateral SSEP from the paretic arm was closely related with the patients' hand function. This shows for the first time a laterality in ascending pathways after unilateral stroke and implies an involvement of the unaffected hemisphere in the control of paretic hand movements.

Control of functional movements in healthy and post-stroke subjects: Role of neural interlimb coupling.

In recent years it has become evident that, in a number of functional movements, synergistically acting limbs become task-specifically linked by a soft-wired 'neural coupling' mechanism (e.g. the legs during balancing, the arms and legs during gait and both arms during cooperative hand movements). Experimentally this mechanism became evident by the analysis of reflex responses as a marker for a neural coupling. It is reflected by the task-specific appearance of reflex EMG responses to non-noxious nerve stimulation, not only in muscles of the stimulated limb, but also, with same long latency, in muscles of meaningful coupled (contralateral) limb(s). After a stroke, nerve stimulation of the unaffected limb during such cooperative tasks is followed by EMG responses in muscles of the (contralateral) coupled affected limb, i.e. unaffected motor centres support synergistically acting movements of the paretic limb. In contrast, following stimulation of the affected limb, no contralateral responses appear due to defective processing of afferent input. As a consequence, it may be therapeutically possible to strengthen the influence of unaffected motor centres on the performance of affected limb movements through training of cooperative limb movements required during activities of daily living.

Cooperative hand movements in post-stroke subjects: Neural reorganization.

OBJECTIVE: Recent research indicates a task-specific neural coupling controlling cooperative hand movements reflected in bilateral electromyographic reflex responses in arm muscles following unilateral nerve stimulation. Reorganization of this mechanism was explored in post-stroke patients in this study. METHODS: Electromyographic reflex responses in forearm muscles to unilateral electrical ulnar nerve stimulation were examined during cooperative and non-cooperative hand movements. RESULTS: Stimulation of the unaffected arm during cooperative hand movements led to electromyographic responses in bilateral forearm muscles, similar to those seen in healthy subjects, while stimulation of the affected side was followed only by ipsilateral responses. No contralateral reflex responses could be evoked in severely affected patients. The presence of contralateral responses correlated with the clinical motor impairment as assessed by the Fugl-Meyer test. CONCLUSION: The observations suggest that after stroke an impaired processing of afferent input from the affected side leads to a defective neural coupling and is associated with a greater involvement of fiber tracts from the unaffected hemisphere during cooperative hand movements. SIGNIFICANCE: The mechanism of neural coupling underlying cooperative hand movements is shown to be defective in post-stroke patients. The neural re-organizations observed have consequences for the rehabilitation of hand function.

Neural coupling of cooperative hand movements: a reflex and fMRI study.

The neural control of "cooperative" hand movements reflecting "opening a bottle" was explored in human subjects by electromyographic (EMG) and functional magnetic resonance imaging (fMRI) recordings. EMG responses to unilateral nonnoxious ulnar nerve stimulation were analyzed in the forearm muscles of both sides during dynamic movements against a torque applied by the right hand to a device which was compensated for by the left hand. For control, stimuli were applied while task was performed in a static/isometric mode and during bilateral synchronous pro-/supination movements. During the dynamic cooperative task, EMG responses to stimulations appeared in the right extensor and left flexor muscles, regardless of which side was stimulated. Under the control conditions, responses appeared only on the stimulated side. fMRI recordings showed a bilateral extra-activation and functional coupling of the secondary somatosensory cortex (S2) during the dynamic cooperative, but not during the control, tasks. This activation might reflect processing of shared cutaneous input during the cooperative task. Correspondingly, it is assumed that stimulation-induced unilateral volleys are processed in S2, leading to a release of EMG responses to both forearms. This indicates a task-specific neural coupling during cooperative hand movements, which has consequences for the rehabilitation of hand function in poststroke patients.

Task-specific role of ipsilateral pathways: somatosensory evoked potentials during cooperative hand movements.

Task-specific neural coupling during cooperative hand movements has been described in healthy volunteers, manifested by bilateral reflex electromyographic responses in forearm muscles following unilateral ulnar nerve stimulation and by task-specific activation of secondary somatosensory cortical areas (S2) in functional MRI. The aim of this study was to investigate the role of sensory input to the ipsilateral and contralateral cortex during a cooperative task. Somatosensory evoked potentials from the ulnar nerve were recorded over the ipsilateral and contralateral cortex during resting and during cooperative and noncooperative hand movements. Ipsilateral potentials with smaller amplitude were present under all conditions in almost all participants. In relation to the resting condition, the amplitudes of both the ipsilateral and the contralateral potential were reduced during the cooperative and the noncooperative tasks. Nevertheless, the reduction in amplitude was similar for the ipsilateral and the contralateral potentials in the noncooperative task, but less on the ipsilateral compared with the contralateral side during the cooperative task. The ratio of ipsilateral/contralateral somatosensory evoked potential amplitude was thus significantly larger during the cooperative task compared with the control task and the resting condition. This indicates a functional role of ipsilateral pathways connecting the cervical spinal cord with the cortex during the cooperative task. These observations favor the idea of a task-specific mediation of sensory input from both hands to the ipsilateral and contralateral hemispheres as the basis of neuronal coupling.