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. Generally, increased tonic arousal was associated with more liberal decision-making (increased proportion “yes” choices), but experimentally induced shifts in decision bias by task context were uniquely related to phasic (pupil-linked) arousal. Electroencephalography and computational evidence revealed a direct link between phasic arousal and biased prestimulus preparatory signals over motor cortex. 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.