The paradoxical effect of warning on reaction time: Demonstrating proactive response inhibition with event-related potentials

Objective: Warning signals can induce a paradoxical increase in reaction time (RT) when detecting targets. Top-down inhibitory control intended to prevent undesired responses to non-target stimuli may account for this effect. This hypothesis assumes the existence of a gating mechanism during conditions of uncertainty that locks the initiation of movement before any stimulus is presented and thus increases the RT to any targets that are not preceded by warning cues. However, direct evidence for the involvement of inhibitory mechanisms in the paradoxical warning effect is still lacking. Methods: Event-related potentials (ERPs) were analyzed by means of a target detection task according to two experimental protocols: one requiring inhibition (warned and unwarned trials mixed in the same block) and the other not requiring inhibition (warned and unwarned trials separated into different blocks). Results: The probability that warning signals would be introduced in a block (mixed-block design) dramatically increased RT to unwarned targets and evoked target-locked markers of inhibition in these trials (N2 was evoked and P3 was delayed). Cue-locked ERPs also exhibited a strong N2 component whatever the design. Conclusions: The top-down control implemented in the pre-stimulus period to prevent undesired automatic responses to external events inhibits the triggering of movement. Significance: Understanding this gating mechanism may provide insight into various neurological or psychiatric disorders affecting movement initiation, such as akinesia or impulsivity that may both be viewed as disorders of higher order inhibitory control. (C) 2009 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.