Corticomotor plasticity and learning of a ballistic thumb training task are diminished in older adults

Abstract

This study examined changes in corticomotor excitability and plasticity after a thumb abduction training task in young and old adults. Electromyographic (EMG) recordings were obtained from right abductor pollicis brevis (APB, target muscle) and abductor digiti minimi (ADM, control muscle) in 14 young (18-24 yr) and 14 old (61-82 yr) adults. The training task consisted of 300 ballistic abductions of the right thumb to maximize peak thumb abduction acceleration (TAAcc). Transcranial magnetic stimulation (TMS) of the left primary motor cortex was used to assess changes in APB and ADM motor evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) before, immediately after, and 30 min after training. No differences in corticomotor excitability (resting and active TMS thresholds, MEP input-output curves) or SICI were observed in young and old adults before training. Motor training resulted in improvements in peak TAAcc in young (177% improvement, P < 0.001) and old (124%, P = 0.005) subjects, with greater improvements in young subjects (P = 0.002). Different thumb kinematics were observed during task performance, with increases in APB EMG related to improvements in peak TAAcc in young (r(2) = 0.46, P = 0.008) but not old (r(2) = 0.09, P = 0.3) adults. After training, APB MEPs were 50% larger (P < 0.001 compared with before) in young subjects, with no change after training in old subjects (P = 0.49), suggesting reduced use-dependent corticomotor plasticity with advancing age. These changes were specific to APB, because no training-related change in MEP amplitude was observed in ADM. No significant association was observed between change in APB MEP and improvement in TAAcc with training in individual young and old subjects. SICI remained unchanged after training in both groups, suggesting that it was not responsible for the diminished use-dependent corticomotor plasticity for this task 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].…”

Imaging human brain networks to improve the clinical efficacy of non-invasive brain stimulation

“…Inconsistent treatment outcomes appear to be influenced by variability in individuals' responses to NIBS. Several factors have been shown to influence the reliability and effectiveness of NIBS (Ridding and Ziemann, 2010), including genetic factors (Cheeran et al, 2008, Antal et al, 2010, hormone and drug interactions (Sale et al, 2008), age (Rogasch et al, 2009;Todd et al, 2010;Bashir et al, 2014) and levels of physical activity ). Other important factors that appear critical in determining the brain's response to NIBS, and which will be discussed in detail in this review, are the site of stimulation (Fox et al, 2014) and the activity of the underlying networks during stimulation (Nitsche et al, 2003;Fregni et al, 2005;Hummel et al, 2005;Antal et al, 2007;Kujirai et al, 2006;Andrews et al, 2011).…”

“…Plasticity induced with NIBS is likely to be reduced in the aged human brain (Rogasch et al, 2009;Todd et al, 2010;Bashir et al, 2014). Further, there is decreased segregation of A c c e p t e d M a n u s c r i p t Sale et al,32 neural networks as a function of increasing age (Chan et al, 2014).…”

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.…”