Michael I. Garry

Bilateral and unilateral movement training on upper limb function in chronic stroke patients: A TMS study

The use of activity-dependent interventions has shown some success in promoting recovery of upper limb function in chronic stroke patients. This study compared the neurophysiological and behavioural changes associated with two such rehabilitation protocols: unilateral and bilateral movement training. Twelve chronic stroke patients were randomly assigned to the two training protocols involving six daily practice sessions. Each session consisted of 50 trials of a dowel placement task performed either with both impaired and unimpaired arm moving synchronously (bilateral training group) or with only the impaired arm moving (unilateral training). Kinematic measurements of upper limb movements were made in four unilateral test trials performed prior to and following each practice session. Functional assessments of the impaired upper limb and neurophysiological assessments, using transcranial magnetic stimulation (TMS), of the affected and non-affected cortical hemispheres were made prior to and following the intervention sessions. Individuals receiving bilateral training showed a reduction in movement time of the impaired limb and increased upper limb functional ability compared to individuals receiving unilateral training. In some patients changes to upper limb function were associated with changes to the cortical representation of a target muscle in the non-affected hemisphere. Overall, these findings suggest that a short-term bilateral training intervention may be effective in facilitating upper limb motor function in chronic stroke patients.

The effect of test TMS intensity on short-interval intracortical inhibition in different excitability states.

The paired-pulse transcranial magnetic stimulation (TMS) paradigm is increasingly employed to examine intracortical inhibitory processes in different motor tasks. Short-interval intracortical inhibition (SICI) has been shown to vary with the size of the MEP elicited by the test TMS pulse. This suggests that factors that alter MEP size, such as changes in cortical excitability, may confound the interpretation of SICI. However, the effect of excitability on SICI has not been systematically investigated. The present study examined SICI in 11 volunteers. The effect of test TMS intensities ranging from 90 to 150% resting motor threshold (RMT) on SICI was examined in three excitability states in the right first dorsal interosseous muscle: rest, isometric abduction of the left index finger (Contra) and isometric abduction of the right index finger (Active). For all excitability states SICI was not observed when test TMS intensity was less than 110% resting motor threshold. This was true even for the Active condition in which 90 and 100% test TMS intensities elicited large and consistent MEPs. For all conditions moderately suprathreshold test TMS intensities (110–120% RMT) yielded the greatest measure of SICI; increasing test TMS intensities resulted in a progressive reduction in the estimate of SICI. These results suggest that estimates of SICI are systematically affected by the intensity of the test TMS pulse, regardless of excitability state. The results suggest that SICI should be examined using a constant test TMS intensity regardless of changes in cortical excitability and test MEP size.

Age-related differences in inhibitory processes during interlimb coordination

The study examined the neurophysiological correlates of age-related changes in the coordination of hand and foot movements. Young and older adults (N=30) performed cyclical isodirectional and non-isodirectional hand-foot movements with contralateral and ipsilateral limb combinations. Motor evoked potentials (MEPs) and silent period durations following transcranial magnetic stimulation (TMS) were measured from the right extensor carpi radialis (ECR) muscle during the interlimb coordination tasks. Older adults demonstrated lower coordination stability than younger adults, particularly when performing non-isodirectional movements with ipsilateral limbs. For all coordination tasks, MEP amplitude was lower in older compared with young participants. Young adults showed significantly longer silent period durations when the coordination pattern involved ipsilateral limbs than during contralateral limb coordination. In contrast, silent period durations did not differ between contralateral and ipsilateral limb coordination in older adults. These results suggest that deterioration in motor performance with advancing age may be associated with a decreased ability to modulate inhibitory function.

Absence of cross-limb transfer of performance gains following ballistic motor practice in older adults

The phenomenon of cross-limb transfer, in which unilateral strength training can result in bilateral strength gains, has recently been tested for ballistic movements. Performance gains associated with repetitive motor practice, and the associated transfer, occur within a few minutes. In this study, young and older adults were trained to perform ballistic abductions of their dominant (right) index finger as quickly as possible. Performance was assessed bilaterally before, during, and after this training. Both groups exhibited large performance gains in the right hand as a result of training (P < 0.001; young 84% improvement, older 70% improvement), which were not significantly different between groups (P = 0.40). Transcranial magnetic stimulation revealed that the performance improvements were accompanied by increases in excitability, together with decreases in intracortical inhibition, of the projections to both the trained muscle and the homologous muscle in the contralateral limb (P < 0.05). The young group also exhibited performance improvements as a result of cross-limb transfer in the left (untrained) hand (P < 0.005), equivalent to 75% of the performance increase in the trained hand. In contrast, there were no significant performance gains in the left hand for the older group (P = 0.23). This was surprising given that the older group exhibited a significantly greater degree of mirror activity than the young group (P < 0.01) in the left first dorsal interosseus muscle (FDI) during right hand movements. Our findings suggest that older adults exhibit a reduced capacity for cross-limb transfer, which may have implications for motor rehabilitation programs after stroke.

Differences in motor learning success are associated with differences in M1 excitability.

Primary motor cortex (M1) plays a role in motor learning, although the exact nature of that involvement remains unclear. The present study examined the relationship between motor learning and cortical plasticity by manipulating augmented feedback during motor training. Two groups of 10 participants performed a wrist flexion-extension waveform-tracking task with either concurrent and terminal augmented feedback after every trial (100% FB) or only terminal feedback after every alternate trial (50% FB). Single- and paired-pulse transcranial magnetic stimulation (TMS) was used to assess cortical excitability short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) before, after, and 24 h following (retention) motor training. The 50% FB group performed better at retention than the 100% FB group, indicative of enhanced learning due to reduced FB scheduling. Cortical excitability did not change during acquisition for either group, however, the 50% FB group had elevated M1 excitability at retention, suggesting M1 involvement in the consolidation of learning. Reduced SICI following practice suggests a reduction of intracortical inhibition during motor skill acquisition. ICF was unchanged. It is concluded that the nature of M1 modulation associated with the acquisition and retention of a novel motor skill appears to vary with the nature and complexity of task requirements.

Dual-task interference: Attentional and neurophysiological influences

Performing two tasks simultaneously often degrades performance of one or both tasks. While this dual-task interference is classically interpreted in terms of shared attentional resources, where two motor tasks are performed simultaneously interactions within primary motor cortex (i.e., activity-dependent coupling) may also be a contributing factor. In the present study TMS (transcranial magnetic stimulation) was used to examine the contribution of activity-dependent coupling to dual-task interference during concurrent performance of a bimanual coordination task and a discrete probe reaction time (RT) task involving the foot. Experiments 1 and 2 revealed that activity-dependent coupling within the leg corticomotor pathway was greater during dual-task performance than single-task performance, and this was associated with interference on the probe RT task (i.e., increased RT). Experiment 3 revealed that dual-task interference occurred regardless of whether the dual-task involved two motor tasks or a motor and cognitive task, however activity-dependent coupling was present only when a dual motor task was performed. This suggests that activity-dependent coupling is less detrimental to performance than attentional processes operating upstream of the corticomotor system. Finally, while prioritising the RT task reduced, but did not eliminate, dual-task interference the contribution of activity-dependent coupling to dual-task interference was not affected by task prioritisation. This suggests that although activity-dependent coupling may contribute to dual motor-task interference, attentional processes appear to be more important. It also suggests that activity-dependent coupling may not be subject to modulation by attentional processes.

Age-related Differences in Corticomotor Excitability and Inhibitory Processes during a Visuomotor RT Task

This study tested the postulation that change in the ability to modulate corticospinal excitability and inhibitory processes underlie age-related differences in response preparation and generation during tasks requiring either rapid execution of a motor action or actively withholding that same action. Younger (n = 13, mean age = 26.0 years) and older adults (n = 13, mean age = 65.5 years) performed an RT task in which a warning signal (WS) was followed by an imperative signal (IS) to which participants were required to respond with a rapid flexion of the right thumb (go condition) or withhold their response (no-go condition). We explored the neural correlates of response preparation, generation, and inhibition using single- and paired-pulse TMS, which was administered at various times between WS and IS (response preparation phase) and between IS and onset of response-related muscle activity in the right thumb (response generation phase). Both groups exhibited increases in motor-evoked potential amplitudes (relative to WS onset) during response generation; however, this increase began earlier and was more pronounced for the younger adults in the go condition. Moreover, younger adults showed a general decrease in short-interval intracortical inhibition during response preparation in both the go and no-go conditions, which was not observed in older adults. Importantly, correlation analysis suggested that for older adults the task-related increases of corticospinal excitability and intracortical inhibition were associated with faster RT. We propose that the declined ability to functionally modulate corticospinal activity with advancing age may underlie response slowing in older adults.

Selective suppression of the incorrect response implementation in choice behavior assessed by transcranial magnetic stimulation

Selecting the adequate alternative in choice situations may involve an inhibition process. Here we assessed response implementation during the reaction time of a between-hand choice task with single- or paired-pulse (3 or 15 ms interstimulus intervals [ISIs]) transcranial magnetic stimulation of the motor cortex. The amplitude of the single-pulse motor evoked potential (MEP) initially increased for both hands. At around 130 ms, the single-pulse MEP kept increasing for the responding hand and decreased for the nonresponding hand. The paired-pulse MEP revealed a similar pattern for both ISIs with no effect on short intracortical inhibition and intracortical facilitation measures. The results suggest that the incorrect response implementation was selectively suppressed before execution of the correct response, preventing errors in choice context. The results favor models assuming that decision making involves an inhibition process.

Attentional influences on short-interval intracortical inhibition.

OBJECTIVE The allocation of attention to sensory stimulation and movement might influence cortical activity. Two experiments were conducted to investigate the effect of variation of intensity of attention (Experiment 1) and direction of attention (Experiment 2) on cortical excitability and short-interval intracortical inhibition (SICI) during performance of a simple index finger abduction task. METHODS Subjects responded to subtle cutaneous electrical stimulation delivered to the index finger while single and paired TMS pulses were delivered during muscle relaxation between successive responses. In Experiment 1, attentional resources allocated to the task were manipulated using a dual task paradigm involving a backward-counting task. In Experiment 2, spatial attention was varied by delivering cutaneous stimuli to the responding or the non-responding index finger. RESULTS In Experiment 1, SICI was reduced during performance, but was unaffected by variation in the intensity of attention. The results of Experiment 2, however, showed that SICI was significantly lower when attention was directed to the responding hand compared with when it was directed to the non-responding hand. CONCLUSIONS While SICI was not affected by variation of attentional resources, it was influenced by spatial attention. SIGNIFICANCE These findings may be relevant in future investigations of the underlying neurophysiology of plasticity.

Visuospatial ability and memory are associated with falls risk in older people: a population-based study

Background/Aims: Our purpose was to examine whether falls risk is associated with cognitive functions beyond executive function/attention and processing speed. Methods: Cognitive function was measured in a population-based sample (n = 300) of people aged 60–86 years. The physiological profile assessment was used to estimate the falls risk. Results: After adjusting for confounders, visual construction (p < 0.01), executive function/attention and memory (both p < 0.05) were independently associated with falls risk. The associations for visual construction (p < 0.01) and memory (p < 0.01) remained after adjusting for executive function/ attention. Conclusions: The neural basis underlying the associations of visuospatial function and memory with falls risk require further study.