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

Abstract

Abstract■ 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 motorevoked 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 shortinterval 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. ■

Age-related differences in corticospinal excitability during a choice reaction time task

“…In the current study, lower suppression of MEPs yet longer RTs in the older individuals may appear seemingly contradictory. Nonetheless, it has been documented that faster RTs in CRT or Go/NoGo tasks are associated with increased activation of inhibitory interneurons (often expressed by a gradual increase of SICIs) towards the expected onset of the imperative stimulus (Fujiyama et al 2011(Fujiyama et al , 2012bSoto et al 2010). This observation has been argued to reflect an increased recruitment of GABA A -ergic inhibitory circuits (Kujirai et al 1993), presumably to suppress premature responses during the preparation period.…”

“…Recently, TMS has been applied to study age-related differences in neuronal activity during response selection or event preparation for simple/go-no-go reaction tasks (Fujiyama et al 2011(Fujiyama et al , 2012bLevin et al 2011). However, there are virtually no studies examining agerelated changes of CS excitability during preparation and motor generation in CRT tasks.…”

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

“…As previous findings suggest that lengthening contractions are associated with disinhibition of contralateral M1 [7,8,25], we expected that lengthening movements would also be associated with a reduction in SICI and LICI. In addition, as the activity-dependent modulation of inhibitory tone is thought to be reduced in old adults [27,29,30], and this has been related to impaired motor performance in the elderly [27], we expected that old individuals would demonstrate less modulation of cortical inhibition during movement, and that this would be associated with greater motor deficits in older 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.…”

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

“…Furthermore, the nature of this modulation is thought to be task-dependent [17,18]. In contrast, task-related changes in inhibition in older adults have been limited to measurements made during tonic contractions [19,20] or in the period prior to contraction [21][22][23]. However, some evidence suggests that the task-dependency of inhibitory tone in M1 is modified by age [24].…”