Response primacy in fear conditioning: Disentangling the contributions of UCS vs. UCR intensity

Debates have ensued over whether fear conditioning is mainly a function of the strength of the aversive unconditioned stimulus (UCS) or the severity a nd intensity of the unconditioned response (UCR). The present study introdu ces a novel and clinically relevant preparation to test these competing hyp otheses. Sex-balanced groups of undergraduate participants (N = 96) were as signed to one of three conditioned stimuli (CS) differing in fear relevance (snake, heart, and flowers) and within each CS, to either 20% or 13% CO2-e nriched air as UCSs. Autonomic (electrodermal, heart rate) and self-report (SUDS) conditioned responses (CRs) at acquisition and extinction were predi cted from (a) UCS intensity (20% vs. 13% CO2-enriched air), and (b) UCR int ensity. UCS intensity predicted autonomic CRs during acquisition and extinc tion, but not SUDS CRs during extinction. However, these UCS-CR relations w ere almost completely mediated by UCR intensity. Findings suggest that UCS intensity is limited as a predictor of conditioning, and that UCR intensity is a more robust predictor of fear conditioning. We discuss the conditions that may account for the differential predictive value of the UCS and UCR in explaining fear onset.