Verticality is the perception of what’s upright relative to gravity. The vestibular system provides information about the head’s orientation relative to gravity, while visual cues influence the perception of external objects’ alignment with the vertical. According to Bayesian integration, the perception of verticality depends on the relative reliability of visual and vestibular cues. Ambiguities in vestibular signals are resolved through visual information, with the brain integrating these cues alongside prior knowledge of the upright orientation. While it is established that both vestibular and visual cues contribute to verticality perception, the precise mechanisms underlying this integration remain unclear. Here we investigated how the human brain combines vestibular and visual cues to perceive verticality based on their reliability. We assessed verticality perception using a signal detection theory based visual verticality detection task. Participants were shown lines that were either vertical or tilted and asked to judge their orientation. To manipulate cue reliability, we used optokinetic stimulation for visual cues, galvanic vestibular stimulation for vestibular cues, and a combined visual-vestibular condition by simultaneously delivering optokinetic and galvanic vestibular stimulation. Sham stimulations were administered to control for non-specific effects. Our findings demonstrate that reductions in the reliability of visual and vestibular cues impair sensitivity to verticality, with visual cues exerting a more pronounced influence. Importantly, no changes in response bias were observed. The observed pattern aligns with a model in which the relative contributions of visual and vestibular inputs are determined by linear weightings and their combined summation.
