In search of the behavioural effects of fear: a paradigm to assess conditioned suppression in humans

Conditioned fear can substantially reduce the likelihood that an individual will engage in reward-related behaviour - a phenomenon coined conditioned suppression. Despite the unmistakable relevance of conditioned suppression for excessive fears and their adverse consequences, the phenomenon has primarily been observed in animal models and is not yet well understood. Here, we aimed to develop a conditioned suppression paradigm that enables a robust quantification of the effect of Pavlovian aversive stimuli on subsequent reward-related behaviour in humans and assess its potential relation to physiological measures of fear. In phase 1, an instrumental response was incentivized with monetary rewards. In phase 2, one of two conditioned stimuli (CS+) was reinforced with an aversive unconditioned stimulus (US, i.e., electric stimulus). During aversive Pavlovian learning we assessed differential skin conductance (SCR) and fear potentiated startle responses (FPS). Lastly, we tested the effect of the aversively conditioned CS+ on the response rate of the instrumental response in a transfer phase. Despite strong aversive Pavlovian conditioning, as indicated by large effect sizes in differential SCR and FPS, we did not find any evidence for conditioned suppression: i.e., there was no significant reduction of instrumental responding in the presence of the CS+ compared to a new control stimulus. This lack of conditioned suppression is in line with previous studies that reported difficulties inducing conditioned suppression and points towards a general challenge in investigating conditioned suppression in humans. Implications and directions for future research on the highly relevant behavioural effects of fear and anxiety are discussed