Gwyn N. Lewis

Phasic modulation of corticomotor excitability during passive movement of the upper limb : effects of movement frequency and muscle specificity

Modulations in the excitability of spinal reflex pathways during passive rhythmic movements of the lower limb have been demonstrated by a number of previous studies [4]. Less emphasis has been placed on the role of supraspinal pathways during passive movement, and on tasks involving the upper limb. In the present study, transcranial magnetic stimulation (TMS) was delivered to subjects while undergoing passive flexion-extension movements of the contralateral wrist. Motor evoked potentials (MEPs) of flexor carpi radialis (FCR) and abductor pollicus brevis (APB) muscles were recorded. Stimuli were delivered in eight phases of the movement cycle during three different frequencies of movement. Evidence of marked modulations in pathway excitability was found in the MEP amplitudes of the FCR muscle, with responses inhibited and facilitated from static values in the extension and flexion phases, respectively. The results indicated that at higher frequencies of movement there was greater modulation in pathway excitability. Paired-pulse TMS (sub-threshold conditioning) at short interstimulus intervals revealed modulations in the extent of inhibition in MEP amplitude at high movement frequencies. In the APB muscle, there was some evidence of phasic modulations of response amplitude, although the effects were less marked than those observed in FCR. It is speculated that these modulatory effects are mediated via Ia afferent pathways and arise as a consequence of the induced forearm muscle shortening and lengthening. Although the level at which this input influences the corticomotoneuronal pathway is difficult to discern, a contribution from cortical regions is suggested.

Bimanual coordination dynamics in poststroke hemiparetics.

Poststroke hemiparetic individuals (n = 9) and a control group (n = 9) completed a frequency-scaled circle-drawing task in unimanual and bimanual conditions. Measures of intralimb spatial and temporal task accuracy and interlimb coordination parameters were analyzed. Significant reductions in task performance were seen in both limbs of the patients and controls with the introduction of bimanual movement. Spatial performance parameters suggested that the 2 groups focused on different hands during bimanual conditions. In the controls, interlimb coordination variables indicated predictable hand dominance effects, whereas in the patient group, dominance was influenced by the side of impairment and prior handedness of the individual. Therefore, in this particular bimanual task, performance improvements in the hemiplegic side could not be elicited. Intrinsic coupling asymmetries between the hands can be altered by unilateral motor deficits.

The subdominant hand increases the efficacy of voluntary alterations in bimanual coordination

Abstract. Subjects performed a bimanual circle-tracing task in time with an auditory metronome while restricted to moving with either proximal or distal musculature of the upper limb. Patterns were made in symmetric or asymmetric directions with respect to the midline. Symmetric patterns were more stable than asymmetric patterns. In response to a visual stimulus, subjects reversed the direction of one limb. Unwanted disruptions (momentary or lasting reversals) in the limb contralateral to the reversing limb were observed in 48% of trials. Incidence of disruption was equivalent between postures, but occurred more frequently when the dominant hand reversed direction. This result is consistent with anisotropy in coupling between hands and reveals a unifying constraint between spontaneous and intentional dynamics in bimanual coordination.

Modulations in corticomotor excitability during passive upper-limb movement: Is there a cortical influence?

Modulations in the excitability of corticomotor pathways to forearm musculature have previously been demonstrated during passive wrist movement [Brain Res. 900 (2001) 282]. Investigations were conducted to determine the level of the neuroaxis at which these modulations arise, and to establish the influence of proprioceptive task constraints on pathway excitability. Forearm motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) were examined during passive wrist movement while subjects maintained a low-level muscle activation, thus stabilising the excitability of the motoneuron pool. Modulations in response amplitude during movement were evident in both forearm flexor and extensor muscles. The pattern of modulation generally mirrored that seen in quiescent musculature during movement, with responses potentiated during the phases where the muscle was in a shortened position. Variations in MEP amplitude were not detected while the wrist was constrained statically at various joint angles. This suggests a dynamic influence of movement, most likely mediated by spindle receptors, arising at a supraspinal level. We also investigated the influence of a kinesthetic tracking task on corticomotor excitability during passive movement of the wrist joint. MEPs were recorded from the target driven limb while the contralateral limb was stationary, while the contralateral limb actively tracked the movements of the target limb, and while the contralateral limb moved actively in time with a metronome. The results revealed no differences in MEP characteristics in the driven limb between the three conditions. Placing the movement elicited afferent information in an active movement context does not appear to enhance the modulations in cortical excitability.

Bimanual coordination in Parkinson's disease: Deficits in movement frequency, amplitude, and pattern switching

Six patients with idiopathic Parkinsons disease (PD) and six age‐matched controls participated in a variety of rhythmic bimanual coordination tasks. The main goal of the task was to perform inphase or antiphase patterns of pronation and supination of the forearms at a specified tempo, and to switch from one pattern to the other upon presentation of a visual cue. The availability of advance information was varied to examine whether deficits would emerge under choice versus pre‐cue constraints. In pre‐cue conditions, the subjects knew in advance which hand would be cued to initiate pattern change. In choice conditions, the cued hand was not known until the imperative stimulus was presented. Overall, the PD patients made movements with significantly lower frequencies and smaller amplitudes relative to controls. Patients exhibited spontaneous pattern switching from antiphase to inphase at significantly lower movement frequencies than controls. During intentional switching trials, the control group was significantly faster at initiating pattern change. PD and control groups differed in the time to initiate pattern switching to a greater extent under choice conditions, suggesting that patients used advance information to increase the speed of their response. The control group exhibited a preference for spontaneous switching and intentional switching through the subdominant hand. Patients exhibited a switching preference using the impaired limb (whether or not it was subdominant). The control group made more correct responses when the subdominant side was either pre‐cued or presented in choice conditions. The patients maintained the subdominant/impaired side advantage under pre‐cue conditions but not choice. In the maintenance of rhythmic movement, individuals with PD were able to use advance information in terms of both speed and accuracy.

Effector-Specific Visual Information Influences Kinesthesis and Reaction Time Performance in Parkinson's Disease

Abstract Twelve patients diagnosed with idiopathic Parkinsons disease and 11 age-matched control participants performed a continuous bimanual wrist flexion-extension tracking task while vision of their hands was manipulated. Participants were required to match the frequency and amplitude of movements of 1 limb that was driven at 0.6 Hz by a torque motor by actively moving the contralateral limb. In half the trials, the more affected limb (sub-dominant for controls) was driven, and in the other half, the less affected limb (dominant for controls) was driven. Vision of both hands, vision of the driven hand only, vision of the active hand only, or no vision of the hands was allowed. Simple and probe reaction times were assessed. Parkinsons disease patients performed the tracking task to a reasonable level of temporal and spatial accuracy as compared with control participants in terms of hand phasing and root mean square error. Patients demonstrated a marked posture deviation (toward flexion), which was exaggerated when the less affected limb was active. Amplitude deviations were smaller in both groups when the less affected (dominant) limb was active and when participants had vision of the driven hand. Overall, patients delivered slower responses in both simple and probe conditions. Reaction times of Parkinsons disease patients who were allowed vision of only the active hand were longer than were those of patients in all other visual conditions, whereas visual conditions did not affect the reaction times of control participants. The authors conclude that central demands increase when movement regulation must be based solely on kinesthetic information and when vision directs attention away from the most relevant source of kinesthetic information.

The Modulation of Excitability in Corticospinal Pathways during Rhythmic Movement

We report on several studies that have examined changes in corticospinal excitability during rhythmic movement of the upper limbs. During passive wrist flexion-extension there is a cyclic potentiation and inhibition of corticospinal excitability. The potentiation appears to be associated with a release of intracortical inhibition directed toward the same muscle, and is also associated with an overall depression of segmental excitability, as determined by H-reflex. There appears to be suppression of intracortical inhibition when homologous forearm muscles are shortened and leng thened synchronously, whereas less suppression of intracortical inhibition is evident when the movement pattern is asynchronous. H-reflex modulation does not distinguish between the two patterns and hence, differences likely reflect a cortical phenomenon. The composition ofmuscle synergies is shown to impact directly upon between-limb neural coupling and cortical input to spinal motoneurons appears to be strongly modulated by changes in the functional context of opposite limb muscles. The clinical implications of this research are described in a pilot study of patients recovering from monohemispheric stroke, and results show promise for interventions that exploit changes along the neuroaxis associated with interlimb movement.

A method to monitor corticomotor excitability during passive rhythmic movement of the upper limb.

A procedure is outlined in which the excitability of the corticomotor pathway is examined during rhythmic, passive upper limb movement. Using a custom built apparatus and software, wrist flexion-extension movements of a programmable frequency, amplitude and duration are induced while transcranial magnetic stimuli are delivered to the contralateral cortex over the area representing the flexor carpi radialis muscle. Stimuli are timed to occur during different phases of the movement cycle in order to examine the influence of ascending sensory input on the excitability of the corticospinal pathway. The protocol enables modulations in evoked responses to be analysed during movement of different frequencies and amplitudes, and permits alterations in cortical excitability to be examined by using paired pulse paradigms. The technique may also be utilised to examine hemispheric and segmental transfer if a stationary target limb is probed while the contralateral limb is passively moved. The protocol has potential use in examining corticomotor excitability in subjects with deficits in sensory and/or motor function, such as patients with Parkinsons disease or individuals recovering from stroke.