Generalization of extinction with a generalization stimulus is determined by learnt threat beliefs.

Expectancy violation refers to the mismatch between an expected and the actual outcome. Maximizing expectancy violation is crucial for exposure-based treatment. Since the original stimulus of fear acquisition (CS+) is rarely available, stimuli that resemble the CS+ (generalization stimuli; GSs) are presented during treatment. A given GS may evoke either strong or weak generalized fear depending on an individual's threat beliefs. Presenting this GS in extinction would then evoke different levels of expectancy violation, which determines the strength of the subsequent generalization of extinction to other stimuli, including the CS+. After differential fear conditioning, participants exhibited discrete generalization gradients depending on their inferred relational rules (Linear vs Similarity). Crucially, the Linear group showed strong generalized fear to the GS used in extinction. This strong expectancy violation led to enhanced extinction learning and subsequently to strong generalization of extinction as characterized by a flat generalization gradient, and reduced conditioned fear to the CS+. In contrast, the Similarity group showed weak generalized fear to the same GS in extinction, and limited generalization of extinction. These results corroborate the importance of expectancy violation in exposure-based treatment, and suggest that exposure sessions designed to evoke strong threat beliefs may lead to better treatment outcome.