Two fear-relevant imagery scripts were used as conditioned stimuli (CSs) in a differential learning paradigm with 5.5% CO2-enriched air as unconditioned stimulus (US). In another condition, 2 neutral scripts were used as CSs (N = 56). Within each condition, one script was imagined while breathing the CO2-enriched air (CS+/US trial), the other while breathing room air (CS- trial). Three CS+ and 3 CS- trials were run in an acquisition phase, followed by 2 CS+ and 2 CS- test trials (imagining the scripts while breathing air). Respiratory behavior, end-tidal CO2, and heart rate were measured throughout the experiment; subjective symptoms were measured after each trial. The type of imagery had strong effects on symptoms and physiological responses. A selective conditioning effect was also observed: CS+ imagery produced more symptoms and altered respiratory behavior compared with CS- imagery, but only in the fear-relevant script condition. The findings are discussed as to their relevance for panic and agoraphobic anxiety.

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