Stress, Heartbeats, and Biases: Cardiac phase and acute stress modulate affective and spatial response bias.

Threatening situations often require rapid decisions which are particularly susceptible to response bias. The perception of threat also elicits an acute stress reaction, assumed to promote automatic action tendencies via several mechanisms. Cardioafferent signals during systole, for example, increase racially biased misidentifications of tools as weapons. Acute stress, linked to threat-biased decisions may both bias such decisions directly and alter cardiac effects on them. Here, we investigated the effects of cardiac phase and acute stress in a combined Weapon Identification and Simon Task. Thirty-one participants classified laterally or vertically presented weapons and tools with lateralized responses, such that target location and prime emotion provided task-irrelevant spatial and affective information. Target onsets were timed to systole or diastole, and in separate sessions participants were exposed to repeated feet cold pressor tests (CPT) and a warm-water control condition. The CPT reliably increased subjective, cardiovascular, and endocrine stress measures. Both systolic target onsets and CPT exposure strengthened the influence of emotional primes on weapon and tool classification, without evidence for general threat prioritization. For spatial compatibility, acute stress enlarged the Simon effect in sensitivity among fast responses, consistent with stronger initial automatic activation, and cardiac phase modulated the advantage of spatial correspondence over locations without dimensional overlap. Midfrontal theta showed prime-dependent target modulations with distinct patterns for cardiac phase and stress. Cardiac phase and stress did not interact, consistent with independent contributions rather than amplification or attenuation. Together, the findings indicate that acute stress and phasic cardioafferent signals both increase the influence of task-irrelevant affective and spatial information on action through partly distinct processes.