Phasic and tonic arousal distinctly shape human decision bias

Neuroscientific theories hypothesize that arousal fluctuations influence human perception and behavior in two functionally distinct ways: through variations in baseline state (tonic arousal) and by transient task-evoked bursts (phasic arousal). We combined causal (pharmacology) and correlational (pupillometry) methods to test the hypothesis that tonic and phasic arousal differentially influence decision biases in human male participants performing a yes/no visual detection task. Computational modeling of choice behavior and analyses of neural data (EEG) revealed that experimentally induced shifts in decision bias were associated with changes in preparatory activity over motor cortex resembling a starting-point bias in the decision formation. The behavioral, computational, and neural effects of strategic shifts in decision bias were weakest on trials with high phasic pupil-linked arousal, but did not relate to tonic pupil-linked arousal or pharmacology. In sharp contrast, tonic pupil-linked arousal and pharmacological interventions were associated with more liberal decision-making (increased proportion of "yes" choices) independent of task context. Thus, in line with the hypothesized functional distinction, tonic arousal was associated with inherent decision bias, whereas phasic arousal was related to context-dependent strategic shifts in decision bias.