Explaining attractive and repulsive biases in the subjective visual vertical

Perception of gravity can be assessed by measuring the subjective visual vertical (SVV), the visually indicated spatial direction that appears earth-vertical to an observer. When the SVV is assessed in darkness while the observer is roll-tilted, it shows substantial biases. At tilts larger than 45°, the bias is attractive, that is, the visual indicator appears vertical when rotated toward the observer. At smaller tilts, however, a repulsive bias is observed. The attractive bias has been explained within the Bayesian framework as the effect of a prior for upright posture. The repulsive bias has so far been considered anti-Bayesian, suboptimal, or as the result of uncompensated ocular counterroll. Here we show that both biases can be explained within a purely Bayesian model. More specifically, the repulsive bias at small roll-tilts is a consequence of the known tilt-dependent variability of the SVV, which is hypothesized to reflect different levels of sensory noise of the otolith organs. We thus provide a solution to a century-old question of why there is a repulsive bias in vertical perception.Author summary: To judge whether a picture frame on the wall is hanging vertically, we use the direction of gravity as a fundamental reference. Our estimate of this direction depends on multiple sensory cues, most prominently the signals from the vestibular organs in the inner ear. Yet, for more than a century, it has been known that our perception of vertical is not fixed but can be systematically biased by the tilt of the head. For large head tilts, our perceived vertical shifts towards our own head axis – an attractive bias known as the Aubert effect. In contrast, for small head tilts, our perceived vertical shifts away from the head axis – a repulsive effect referred to as the Müller effect. Over the past two decades, the attractive bias has been successfully explained within Bayesian frameworks as the optimal perceptual strategy to process uncertain and noisy sensory information. The repulsive bias has typically been considered as suboptimal or inconsistent with a Bayesian interpretation. Here, by incorporating head-tilt-dependent sensory uncertainty, consistent with the known variation in sensory noise of the otolith organs, we show that both biases arise naturally within a single Bayesian framework.