Abstract
Motor behaviors often require refraining from selecting options that may be part of the repertoire of natural response tendencies but that are in conflict with ongoing goals. The presence of sensory conflict has a behavioral cost but the latter can be attenuated in contexts where control processes are recruited because conflict is expected in advance, producing a behavioral gain compared to contexts where conflict occurs in a less predictable way. In the present study, we investigated the corticospinal correlates of these behavioral effects (both conflict-driven cost and context-related gain). To do so, we measured motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) of young and healthy older adults performing the Eriksen Flanker Task. Subjects performed button-presses according to a central arrow, flanked by irrelevant arrows pointing in the same (congruent trial) or opposite direction (incongruent trial). Conflict expectation was manipulated by changing the probability of congruent and incongruent trials in a given block. It was either high (mostly incongruent blocks, MIB, 80% incongruent trials) or low (mostly congruent blocks, MCB, 80% congruent). The MEP data indicate that the conflict-driven behavioral cost is associated with a strong increase in inappropriate motor activity regardless of the age of individuals, as revealed by larger MEPs in the non-responding muscle in incongruent than in congruent trials. However, this aberrant facilitation disappeared in both groups of subjects when conflict could be anticipated (i.e., in the MIBs) compared to when it occurred in a less predictably way (MCBs), probably allowing the behavioral gain observed in both the young and the older individuals. Hence, the ability to overcome and anticipate conflict was surprisingly preserved in the older adults. Nevertheless, some control processes are likely to evolve with age because the behavioral gain observed in the MIB context was associated with an attenuated suppression of MEPs at the time of the imperative signal (i.e., before conflict is actually detected) in older individuals, suggesting altered motor inhibition, compared to young individuals. In addition, the behavioral analysis suggests that young and older adults rely on different strategies to cope with conflict, including a change in speed-accuracy tradeoff.
Highlights
Human beings are often faced with a multitude of stimuli in front of which they need to decide how to behave (Oliveira et al, 2010; Doya and Shadlen, 2012; Klein et al, 2012; Thura and Cisek, 2014; Zénon et al, 2015; Derosiere et al, 2016)
We investigated the corticospinal aspects of these behavioral effects by measuring Motor-evoked potential (MEP) at specific time epochs during action preparation, with an emphasis on: (1) the MEP correlates of the conflict-driven behavioral cost; and (2) the MEP correlates of the contextrelated behavioral gain, focusing on motor inhibitory changes occurring before and during action selection, in young and older adults
Reaction time (RT) in incongruent trials were shorter in the Mostly Incongruent Blocks’’ (MIB) context than in the Mostly Congruent Blocks’’ (MCB) context, both in young (p ≤ 0.02) and older adults (p ≤ 0.0001) whereas RTs in congruent trials were comparable in the two contexts (p > 0.27 in both groups)
Summary
Human beings are often faced with a multitude of stimuli in front of which they need to decide how to behave (Oliveira et al, 2010; Doya and Shadlen, 2012; Klein et al, 2012; Thura and Cisek, 2014; Zénon et al, 2015; Derosiere et al, 2016) In this context, irrelevant stimuli can occasionally induce a powerful activation of action representations that are not consistent with the ongoing goals. In young subjects, control processes recruited to deal more proficiently with conflict seem to involve, on the one hand, a strengthening of inhibitory influences directed at motor representations before action selection begins (i.e., before conflict is detected in the imperative signal) and, on the other hand, a reduced influence of conflicting irrelevant information on corticospinal activity, possibly through an enhanced motor inhibition, during action selection
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