Phasic alertness impairs cognitive control by amplifying competition between evidence accumulators.

Although phasic alertness generally benefits cognitive performance, it often increases the impact of distracting information, resulting in impaired decision-making and cognitive control. However, it is unclear why phasic alertness has these negative effects. Here, we present a novel, biologically-informed account, according to which phasic alertness generates a transient, evidence-independent input to the decision process. This shortens overall response times, but also amplifies competition between evidence accumulators, thus slowing down decision-making and impairing cognitive control. The key hypotheses of this account are supported with pupil measurements and electrophysiological data from human decision-makers of either sex performing an arrow flanker task. We also show that a computational model of the flanker task that incorporates a transient, evidence-independent input can reproduce the behavioral effects of phasic alertness, but only when the evidence accumulators compete with each other through lateral inhibition. Our results reveal a close interplay between dynamic changes in alertness, cognitive control and evidence accumulation.Significance Statement The human attention system is thought to consist of three fundamental components: alerting, orienting, and cognitive control, which shields goal-directed mental activity from distracting information. Although these attentional components are thought to be subserved by distinct brain systems, they seemingly work in concert to produce complicated patterns of behavior. Here, we focus on an interaction between alertness and cognitive control that has puzzled cognitive psychologists for two decades: although increased alertness generally benefits cognitive performance, it disrupts cognitive control. We propose a neurobiologically plausible mechanistic account of how alertness impairs control, and support this account with pupil and EEG measurements, as well as computer simulations.