Systemic alarms in fear conditioning II: An experimental methodology using 20% carbon dioxide inhalation as an unconditioned stimulus

We present a conditioning methodology to examine the role of abrupt aversive systemic bodily responses (alarms) in human fear acquisition and extinction in a non-clinical student population (N = 64). Animated fear- relevant and fear-irrelevant video stimuli referring to either the body (internal) or environment (external) were paired with an unconditioned stimulus (UCS) of 20 sec inhalations of 20% carbon dioxide (CO₂)-enriched air. Measurements of psychophysiological responses, such as electrodermal, electromyograph (EMG), heart rate, and Subjective Units of Distress Scale (SUDS) to fear-relevant stimuli were more easily acquired, were of higher magnitude, and showed greater resistance to extinction compared with measurements for fear-irrelevant stimuli. Conditioning was more pronounced to external (snake) than to internal (heart) phobic stimuli. Findings support the viability of using CO₂-enriched air as an alternative to commonly used peripheral UCSs (e.g., shock) to mimic the role of alarms as conditioning events in human fear acquisition. We discuss the relevance of this methodology for understanding fears that develop without identifiable environmental trauma.