Fear generalization plays a major role in the acquisition and maintenance of anxiety disorders. In fear generalization, conditioned fear responses are observed for novel stimuli sharing perceptually or conceptually similar properties with the conditioned stimulus (CS). The reinforcement rates of the unconditioned stimulus (UCS) impact fear acquisition and generalization. Fear generalization can be studied using the category-based conditioning paradigm, where individuals are conditioned to members of a category rather than a single stimulus. The current study explored the effect of UCS reinforcement (i.e., continuous and partial) on conceptual fear generalization due to category-based similarity using a visual aversive UCS. The CSs were exemplars from four categories, i.e., animals, insects, household appliances, and tools. Thirty healthy participants underwent fear acquisition with exemplars from the four stimulus categories, presented with varying levels of UCS reinforcement (100%, 62.5%, and 37.5%). Subsequently, fear generalization was tested with novel unreinforced exemplars from each category. A significant effect of reinforcement on the UCS expectancy and CS-UCS contingency ratings was observed. UCS expectancy ratings increased with increasing certainty of UCS occurrence in the generalization phase. Our results may help to understand how fear generalizes to conceptually related stimuli based on the certainty of the UCS occurrence.
