Response intensity in content-specific fear conditioning comparing 20% versus 13% CO₂-enriched air as unconditioned stimuli

This study examined the relation between the intensity of CO₂-induced psychophysiological responses and content-specific fear conditioning. Sex- balanced groups of undergraduates (N = 96) were assigned to 1 of 3 conditioned stimuli (CSs) differing in fear-relevance, and within each CS, to either 20% or 13% CO₂-enriched air (unconditioned stimuli [UCS]). Several psychophysiological measures were assessed before, during, and following conditioning phases. Consistent with expectation, electrodermal and cardiac conditioned responses were larger and more resistant to extinction when associated with fear-relevant compared with fear-irrelevant stimuli, and this overall effect of fear-relevance was more robust to the more intense UCS. Severity and frequency of DSM-IV (Diagnostic and Statistical Manual of Mental Disorders, 4th ed.) panic symptoms also varied reliably with UCS intensity, and women reported more distress and symptoms than men. Overall, the findings suggest that content-specific fear conditioning is mediated, in part, by the intensity of the bodily response. The authors discuss clinical and theoretical implications for understanding fear onset in the absence of obvious environmental pain or trauma.