Abstract
In everyday life, environmental cues are used to predict and respond faster to upcoming events. Similarly, in cueing paradigms (where, on cued trials, a cued target requires a speeded response), cues are known to speed up response times (RTs), suggesting that motor preparation has occurred. However, some studies using short cue-target intervals (<300 ms) have found slower RTs on cued, compared to uncued trials (namely, the “paradoxical warning cost”). One explanation of this paradoxical effect is proactive inhibition, a motor gating mechanism that prevents false alarms, also called “the default state of executive control.” Alternative hypotheses claim that, with such short cue-target delays, participants cannot fully prepare the motor response, thus producing slower RTs. In studies of action inhibition, it is often assumed that participants prepare a response on each trial, a prerequisite to induce and measure (proactive) motor inhibition. In this study, we psychophysically manipulated stimulus’ duration in a simple RT task, and measured a duration threshold at which participants responded on time on 80% of the trials. When participants are tested at their stimulus’ duration threshold, they are more likely to prepare the motor response on each trial. Furthermore, we directly measured participants’ readiness to respond by recording transcranial-magnetic stimulation (TMS)-elicited motor evoked potentials (MEPs), a direct measure of corticospinal excitability. Participants performed cued and uncued trials on a simple RT task with short cue-target intervals. We expected lower MEPs’ amplitude on cued than uncued trials with short cue-target intervals, as it would be predicted by the proactive inhibition account. However, when conditions are equated so that motor preparation is induced both under cued and uncued trials, the paradoxical warning cost disappears, as RTs were always faster on cued than uncued trials. Moreover, MEPs recorded from the task-relevant muscle were never suppressed at target onset compared to baseline, a result that does not support the proactive inhibition hypothesis. These results suggest that proactive inhibition is not active by default and that its activation depends on motor preparation.
Highlights
In everyday life, we often must quickly react to stimuli in the environment, such as catching a ball unexpectedly thrown at us
While a “competition-resolution” process is thought to contribute to action selection in choice response times (RTs) tasks by inhibiting alternative task-irrelevant responses (Burle et al, 2004; Ridderinkhof et al, 2004), proactive inhibition would exert “impulse control” to prevent false alarms (Duque et al, 2010)
Responses made during the stimulus-onset asynchrony (SOA), or in less than 100 ms were considered, together with late button presses delivered after the marble fell off the plane, as errors, and analyzed with non-parametrical ANOVAs with Wilcoxon post hoc corrections
Summary
We often must quickly react to stimuli in the environment, such as catching a ball unexpectedly thrown at us. To efficiently and promptly react to unexpected events, we seek for signals in the environment that might help us predict upcoming stimuli. To investigate the effects of cues on motor preparation and inhibition, instructed delayed simple or choice RT tasks are used in the lab. Whatever the underlying cause of the alerting benefit effect, executive control functions are thought to be necessary to prevent the premature execution of the, supposedly prepared, responses (Greenhouse et al, 2015; Duque et al, 2017). While a “competition-resolution” process is thought to contribute to action selection in choice RT tasks by inhibiting alternative task-irrelevant responses (Burle et al, 2004; Ridderinkhof et al, 2004), proactive inhibition would exert “impulse control” to prevent false alarms (Duque et al, 2010). Reactive forms of inhibitory control are thought to be engaged at the presentation of stop signals or NoGo stimuli (in stop signal and Go/NoGo paradigms, respectively; Aron, 2011; Swick et al, 2011), whereas proactive inhibitory mechanisms are activated “in anticipation of stimulation when the situation is unpredictable” (Criaud et al, 2017)
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