Biomedical subjects

Gwyn N Lewis

Publications and source records attributed to Gwyn N Lewis.

10 recordsLinked to original sources

PubMed2006

The effect of task instruction on the excitability of spinal and supraspinal reflex pathways projecting to the biceps muscle.

There is controversy within the literature regarding the influence of task instruction on the size of the long-latency stretch reflex (M2) elicited by a joint displacement. The aim of this study was to investigate if the previously reported task-dependent modulation of the M2 is specific to the M2 or can be explained by an early release of the intended voluntary response. We took advantage of the fact that the M2 is absent when the duration of the applied perturbation is less than a critical time period. This allowed us to examine modulation of muscle activity with and without the contribution of the M2. In addition, we applied transcranial magnetic stimulation (TMS) over the primary motor cortex to examine the modulation of corticomotor excitability with task instruction. Elbow joint extension displacements were used to elicit a stretch reflex in the biceps muscle. Subjects were instructed to "do not intervene" (DNI) with the applied perturbation, or to oppose the perturbation by activating the elbow flexors in response to the perturbation (FLEX). Electromyographic (EMG) activity in the time period corresponding to the M2 was significantly facilitated in the FLEX task instruction both with and without the presence of the M2. Motor evoked potentials (MEPs) elicited by TMS were also facilitated during the FLEX condition in the absence of the M2. EMG and MEP responses were not facilitated until immediately prior to the onset of the M2. Paired-pulse TMS revealed a significant reduction in short-interval intracortical inhibition (SICI) during the M2 response, but the level of SICI was not altered by the task instruction. We conclude that the task-dependent modulation of the biceps M2 results, at least in part, from an early release of the prepared movement and is accompanied by an increase in corticospinal excitability that is not specific to the M2 pathway. Task-dependent modulation of the response cannot be explained by an alteration in the excitability of intracortical inhibitory circuits.

Adult↗

PubMed2005

The influence of perturbation duration and velocity on the long-latency response to stretch in the biceps muscle.

Different neural pathways are proposed to mediate the long-latency stretch reflex response (M2) in muscles spanning distal and proximal joints of the upper limb. The M2 at the wrist joint is present only if the duration of the perturbation exceeds a critical time. Lee and Tatton put forward a converging input hypothesis, requiring an interaction of excitatory volleys at the spinal cord, to account for this feature. The goal of the present study was to examine the influence of the duration of perturbation on M2 responses elicited in a muscle spanning the elbow joint. Reflex responses were induced in the biceps brachii muscle by applying ramp-and-hold position displacements to the elbow. It was found that the M2 was strongly dependent on the duration of the perturbation. On average, responses were not elicited following perturbations of less than 35+/-5 ms. Using a novel double-movement paradigm, we were unable to provide support for the converging input hypothesis. The effect of the duration of perturbation on the M2 may account for the conflicting characteristics of the M2 that have been provided by previous studies applying mechanical or electrical perturbations of varying time durations.

Adult↗

PubMed2004

The effects of repetitive proprioceptive stimulation on corticomotor representation in intact and hemiplegic individuals.

OBJECTIVE: To determine the effects of a passive wrist movement intervention on cortical representation of forearm musculature. METHODS: Transcranial magnetic stimulation was used to map cortical representation of a forearm flexor muscle in healthy individuals and in individuals following stroke before and immediately after a 30 min session of passive wrist movement. RESULTS: In the healthy individuals, no changes in map area or map centre of gravity were noted after the intervention; however, map volume increased significantly across all subjects. In the stroke patient group there were no significant changes in any parameters following the intervention. CONCLUSIONS: It is speculated that the enlargement in map volume following the passive movement intervention arose through a heightened synaptic efficacy of the corticospinal pathway in response to the increase in afferent information. Short-term proprioceptive stimulation can induce alterations in corticomotor excitability in the target musculature. SIGNIFICANCE: These findings provide a potential neural substrate to account for alterations in motor and sensory function in stroke patients in response to long-term passive movement interventions.

Aged↗

PubMed2004

Neurophysiological and behavioural adaptations to a bilateral training intervention in individuals following stroke.

OBJECTIVE: To examine the neurophysiological and behavioural adaptations to a bilateral training protocol in individuals with hemiparesis due to stroke. DESIGN: Subjects performed daily practice sessions of three specific upper limb tasks over a four-week period. Performance of the tasks was recorded on video for later analysis over the duration of the study. For the first week of the training period the tasks were practised with the impaired upper limb. Only then, with a random start day between 8 and 13, the tasks were practised using a bilateral training protocol. Functional assessments of the affected upper limb and neurophysiological investigations of contra- and ipsilateral pathways from the affected and nonaffected cortical hemispheres were completed prior to the start of the intervention, at the end of the unimanual practice period, and at the conclusion of the bimanual practice period. SUBJECTS: Six individuals between one month and four years post stroke. INTERVENTIONS: Unimanual and bimanual training protocols. MAIN MEASURES: Task performance with the affected limb only; motor impairment of the affected upper limb; ipsi- and contralateral corticospinal pathway excitability. RESULTS: Baseline task performance varied markedly between subjects. Most individuals demonstrated little, if any, additional beneficial effect of bimanual practice on task performance. Results of neurophysiological investigations were inconsistent between subjects. CONCLUSIONS: Short-term bilateral training following unilateral training may have limited effectiveness in enhancing upper limb motor performance in acute and chronic individuals poststroke. The neural mechanisms associated with bilateral activation therapies remain uncertain.

Adaptation, Physiological↗

PubMed2004

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.

Adult↗

PubMed2003

Proposed cortical and sub-cortical contributions to the long-latency stretch reflex in the forearm.

The short- and long-latency (SLSR, LLSR) components of the stretch reflex response were investigated in the forearm using a paired transcranial magnetic stimulation (TMS)-stretch reflex protocol. Responses to TMS were recorded in the flexor and extensor carpi radialis muscles (FCR, ECR) after conditioning with a rapid wrist extension movement. The cortical stimuli were timed to elicit a motor-evoked potential (MEP) at either the SLSR or LLSR onset in the FCR muscle. Responses were also collected in TMS-alone and stretch reflex-alone conditions. Six intensities of magnetic stimulation were applied in all conditions. In the FCR muscle, MEP amplitude when timed to arrive at the LLSR onset was significantly greater than the sum of the MEP and stretch reflex responses when given separately. MEP amplitudes at the SLSR onset in the FCR muscle and in the ECR muscle at both SLSR and LLSR onset were not significantly different from that expected from the sum of the two stimuli given separately. This indicates heightened corticospinal excitability at a time corresponding to the passage of an afferent volley induced by the stretch, and raises the possibility of a transcortical loop of the LLSR in the forearm. The extent of MEP facilitation was generally consistent across all stimulus intensities tested. A reduced component of the LLSR was evident when the stretch response was timed to arrive during the silent period following the cortical stimulus, suggesting both cortical and sub-cortical components to the reflex response.

Adult↗

PubMed2003

Effector-specific visual information influences kinesthesis and reaction time performance in Parkinson's disease.

Twelve patients diagnosed with idiopathic Parkinson's 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 (subdominant 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. Parkinson's 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 Parkinson's 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.

Aged↗

PubMed2002

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.

Adult↗

PubMed2002

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

Six patients with idiopathic Parkinson's 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.

Aged↗

PubMed2002

Altered sensorimotor integration in Parkinson's disease.

Transcranial magnetic stimulation (TMS) was used to investigate sensorimotor integration in the upper limb of 10 patients with Parkinson's disease and 10 age-matched controls. Non-conditioned and subthreshold conditioned (2 ms interstimulus interval) responses were recorded in the flexor and extensor carpi radialis muscles (FCR and ECR) of the more impaired (non-dominant) limb. Stimuli were delivered while the wrist joint was positioned statically at various joint angles as well as during different phases of passive movement of the wrist joint (90 degrees amplitude, 0.2 Hz). The FCR and ECR muscles remained relaxed during all stimulation. In both groups, responses in the static condition were larger when the target muscle was in a shortened position. Responses were also facilitated in the muscle shortening phases of passive movement. In both static and dynamic conditions, the extent of modulations in response amplitude was significantly reduced in the patient group. The level of intracortical inhibition (ICI) was also significantly less in the Parkinson's disease patients in static conditions. During passive movement, control subjects demonstrated a clear reduction in ICI compared with the static trials; however, the level of ICI was unchanged in the Parkinson's disease group in the dynamic condition. The results suggest an abnormal influence of afference on corticomotor excitability in Parkinson's disease. This may be related to abnormal sensory input, a defective integrative unit or an inappropriate motor response.

Aged↗