Corticomotor excitability and plasticity following complex visuomotor training in young and old adults

Abstract

Previous studies with transcranial magnetic stimulation (TMS) have shown that advancing age may influence plasticity induction in human motor cortex (M1), but these changes have been assessed with TMS-induced paradigms or simple motor tasks. The aim of this study was to examine changes in corticospinal excitability and intracortical inhibition as markers of corticomotor plasticity following complex motor training in young and old adults. Electromyographic recordings were obtained from the right first dorsal interosseous (FDI) muscle of 16 young (20-35 years) and 16 older (aged 60-75 years) adults before and after motor skill training. Motor training consisted of three 6-minute blocks of a complex visuomotor task that required matching the metacarpophalangeal (MCP) joint angle of the index finger using abduction-adduction movements. Single- and paired-pulse TMS over the left M1 was used to assess changes in right FDI motor-evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) before and after each training block. Visuomotor tracking performance was diminished in old compared with young adults throughout training. However, improvement in tracking error was similar for young and old adults (7-24% increase in each training block). For young and old adults, motor training increased FDI MEP amplitude (≥ 20%) and reduced the magnitude of SICI (≥ 19%) after each visuomotor training block, reflecting use-dependent plasticity. However, no difference in corticomotor plasticity (change in MEP or SICI) was observed between young and old adults. Further studies are needed to identify the experimental or behavioral factors that might contribute to the maintenance of corticomotor plasticity in older adults.

Age-related Differences in Pre- and Post-synaptic Motor Cortex Inhibition are Task Dependent

“…Within the current study, SICI in resting muscle was not different between young and old adults, suggesting that resting post-synaptic GABAA mediated inhibitory tone in motor cortex is maintained with age [7]. This supports previous investigations from within our lab [20,[34][35][36], and from elsewhere [37][38][39], but is in contrast to reports of reduced [23,40,41] or increased [19,42] SICI with age. The reasons for these inconsistencies are currently unclear, but likely relate to variations in subject characteristics and methodological approach [20].…”

Age-related Differences in Short- and Long-interval Intracortical Inhibition in a Human Hand Muscle

“…Furthermore, recent research suggests that the magnitude of intracortical inhibition in young subjects also depends on the proportion of the test alone MEP relative to the maximum muscle response (Mmax; 18,25). These findings suggest that comparisons of intracortical inhibition between young and older subjects, which typically exhibit different Mmax characteristics (20), may confound the estimate of SICI and LICI between subject groups. Furthermore, it is not clear whether changes in test TMS intensity or test MEP amplitude (absolute or normalised) have similar effects on the magnitude of SICI and LICI in young and older adults.…”

Intracortical Inhibition Assessed with Paired-Pulse Transcranial Magnetic Stimulation is Modulated during Shortening and Lengthening Contractions in Young and Old Adults

“…Previous literature suggests contradictory effects of age on SICI and LICI during both relaxation [27,29,[49][50][51][52][53][54][55][56][57][58][59][60][61][62] and isometric muscle activation [30,33,63], with variations in methodology and subject characteristics likely contributing to this heterogeneity. Despite this, as the conditioning stimulus intensity was adjusted to produce 50% inhibition of the test MEP during postural contractions, our findings could not have been confounded by any age-related differences in inhibition at baseline.…”