Document Type



Debates have ensued over whether fear conditioning is
mainly a function of the strength of the aversive unconditioned
stimulus (UCS) or the severity and intensity of the unconditioned
response (UCR). The present study introduces a novel and
clinically relevant preparation to test these competing hypotheses.
Sex-balanced groups of undergraduate participants (N = 96) were
assigned 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-enriched air as UCSs. Autonomic
(electrodermal, heart rate) and self-report (SUDS) conditioned
responses (CRs) at acquisition and extinction were predicted from
(a) UCS intensity (20% vs. 13% CO2-enriched air), and (b) UCR
intenSity. UCS intensity predicted autonomic CRs during
acquisition and extinction, but not SUDS CRs during extinction.
However, these UCS-CR relations were 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.