Abstract
Accurate and precise estimates of direction of gravity are essential for spatial orientation. According to Bayesian theory, multisensory vestibular, visual and proprioceptive input is centrally integrated in a weighted fashion based on the reliability of the component sensory signals. For otolithic input, a decreasing signal-to-noise ratio was demonstrated with increasing roll-angle. We hypothesized that the weights of vestibular (otolithic) and extra-vestibular (visual/proprioceptive) sensors are roll-angle dependent and predicted an increased weight of extra-vestibular cues with increasing roll-angle, potentially following the Bayesian hypothesis. To probe this concept, the subjective visual vertical (SVV) was assessed in different roll-positions (≤±120°, steps=30°, n=10) with/without presenting an optokinetic stimulus (velocity=±60°/s). The optokinetic stimulus biased the SVV towards the direction of stimulus-rotation for roll-angles ≥±30° (p<0.005). Offsets grew from 3.9±1.8° (upright) to 22.1±11.8° (±120° roll-tilt, p<0.001). Trial-to-trial variability increased with roll-angle, demonstrating a non-significant increase when providing optokinetic stimulation. Variability and optokinetic bias were correlated (R(2)=0.71, slope=0.71, 95%-confidence-interval=0.57-0.86). An optimal-observer model combining an optokinetic bias with vestibular input reproduced measured errors closely. These findings support the hypothesis of a weighted multisensory-integration when estimating direction of gravity with optokinetic stimulation. Visual input was weighted more when vestibular input became less reliable, i.e., at larger roll-tilt angles. However, according to Bayesian theory, the variability of combined cues is always lower than the variability of each source cue. If the observed increase in variability -although non-significant- is true, either it must depend on an additional source of variability, added after SVV-computation, or it would conflict with the Bayesian hypothesis.
Ward, Bryan K