Bidirectional cross-modal fear generalization: semantic similarity in representation space as a foundational mechanism

Fear generalization is vital for human survival in the complex, multimodal environments. Despite extensive research on fear generalization within sensory modalities, our theoretical understanding of cross-modal generalization mechanisms remains unclear. To fill this gap, this study conducted two experiments (total N = 95) to investigate bidirectional cross-modality fear generalization in individuals with high and low anxiety levels: Experiment 1 used an auditory conditioned stimulus (CS) and visual generalization stimuli (GSs), while Experiment 2 used a visual CS and auditory GSs. Participants underwent fear conditioning and generalization tasks, measuring shock expectancy ratings, skin conductance responses, and prefrontal cortex activation via functional near-infrared spectroscopy. Critically, we demonstrate that fear generalized cross-modally in both directions-from auditory to visual and from visual to auditory-with auditory-to-visual generalization being significantly stronger than the reverse, indicating an asymmetric pattern. Furthermore, high-anxiety individuals exhibited elevated shock expectancy and skin conductance during generalization, alongside reduced medial prefrontal cortex activation. These findings demonstrate that semantic similarity in mental representational space fundamentally governs associative learning across sensory boundaries, and that bidirectional cross-modal fear generalization is asymmetrical. Moreover, we provide evidence of excessive cross-modality fear generalization in subclinical anxiety, enhancing understanding of anxiety mechanisms and fear-related disorders.