Did you see the sound? A Bayesian assessment on cross-modal perception in low vision.

Multisensory integration is often assumed to increase when visual input is degraded, yet it remains unclear whether low vision enhances susceptibility to cross-modal illusions or whether such effects depend on local variations in visual reliability. We tested low-vision and sighted control participants on the sound-induced flash illusion (SIFI) across 24 visual-field locations, concurrently measuring baseline flash-detection accuracy. While both groups showed the expected auditory-driven increase in perceived flash numerosity, sighted controls were significantly more susceptible to the classic "double-flash" percept. While illusion strength approached marginal significance for eccentricity, it was more robustly predicted by local flash-detection accuracy, indicating that sound-induced percepts depend on the availability of a reliable visual signal. Exploratory Bayesian causal inference modeling revealed that when fitted across all locations, model performance was systematically weaker and more variable for low-vision observers. Crucially, however, restricting the analysis to high-visibility locations yielded robust model fits by the Bayesian information criterion and close agreement between observed and predicted response proportions. This suggests that, like sighted controls, low-vision participants continue to behave in a manner broadly consistent with Bayesian-optimal inference when visual information is sufficiently reliable. Together, these findings demonstrate that low vision does not globally amplify audiovisual interactions; rather, multisensory enhancements are constrained by local visual reliability, and Bayesian-like inference remains most evident when provided with a sufficient threshold of sensory input.