Transcutaneous vagus nerve stimulation boosts evidence accumulation during perceptual decision making.

Ascending arousal systems, including the locus coeruleus-norepinephrine (LC-NE) system, may shape perceptual decision-making by enhancing sensory processing ("gain") or accelerating response commitment ("urgency"). We examined how arousal-related neuromodulatory activity shapes decisions and whether its effects depend on arousal state. We tested 62 participants in three groups, receiving transcutaneous vagus nerve stimulation (tVNS) at three timings during a random dot motion task. tVNS increased pupil-linked arousal, with early stimulation producing more sustained dilation into decision-making. Behaviorally, tVNS selectively improved accuracy under initially low performance without affecting response times. Drift diffusion modeling linked this improvement to increased drift rate, consistent with more efficient evidence accumulation. These effects occurred across groups but were most pronounced with early stimulation. Our findings support a gain-related enhancement of evidence accumulation under tVNS and suggest that pupil-linked arousal, potentially involving LC-NE among other ascending arousal pathways, may help stabilize attentional engagement when endogenous control is suboptimal.