Do visual and vestibular perceptual biases mutually compensate during the execution of a self-motion visuo-vestibular task?

gravity affects the motion of all objects we encounter in daily life by accelerating them toward the center of the earth. Its expectation has been shown to influence human perceptual judgments. In fact, downward-moving visual targets are perceived as faster than upward-moving ones, and a directional asymmetry emerges in the sensitivity of speed judgement of vertically-moving objects, both indicating that gravity is an internalized prior. similar results are observed during vertical self-motion: higher sensitivities are noted during the vestibular perception of downward compared to upward motion. It remains unclear whether the internalized gravity prior also affects the vestibular perception of vertical self-motion duration and how such prior influences visual and vestibular integration. to investigate this, we designed a task where participants estimated the duration of motion along the earth's vertical axis using visual targets (VI), passive whole-body movements (VE), or both (VV), in three separate blocks in which participants compared the durations of two stimuli with opposite motion directions. In both visual and vestibular blocks, downward-moving stimuli were perceived as faster than upward-moving ones, confirming the predictions of perceptual biases. however, when both visual and vestibular information were available, these perceptual biases were canceled, suggesting that the integration of information leads to a more accurate estimation of reality.