How does optical blur affect audiovisual speech perception and emotion perception?

This study investigated how optical blur affects the auditory-based perception of speech and emotion within a unified experimental paradigm. It is widely known that auditory judgments of speech and emotion are strongly influenced by visual information (e.g., the speaker's lip movements or facial expressions). However, it remains unclear to what extent such visual influence is modulated similarly across speech and emotion perception under visual degradation. We investigated this issue by using optical blur to degrade audiovisual stimuli, requiring participants to judge speech and emotion based on auditory information. The results showed that visual influence persisted in both tasks even when stimuli were blurred. Crucially, however, the patterns of visual influence differed between the tasks. Specifically, as blurring increased, the visual influence started to decrease at a slightly higher spatial frequency cutoff in speech perception (6.7 cycles per face-width; cpf) than in emotion perception (5 cpf), indicating that speech perception is more sensitive to optical blur. Interestingly, however, the visual influence remained significant even under the most severe blur (3.3 cpf) in speech perception but not in emotion perception, suggesting a relatively higher persistence of visual influence in speech perception. These findings suggest that optical blur differentially modulates visual influence across perceptual domains, leading us to propose that the multisensory perceptual system dynamically calibrates sensory weights based on visual reliability in a task-dependent manner. OPEN PRACTICES STATEMENT: The experiment stimuli, trial-level, data, and analysis code for the generalized linear models are provided at this repository: https://osf.io/73tby/overview?view_only=3be0b6ee895e4823a6d1466494a8aadf . The present study was not preregistered.