Investigating the time course of verticality sensitivity changes during visuo-vestibular conflict

Verticality perception is a multisensory process that relies primarily on visual and vestibular input. Previous studies show that sensory conflict induced by optic flow causes increased errors in verticality perception. However, the time course over which sensory conflict alters verticality perception and the processes through which perceptual stability is subsequently restored, remain unknown. Experiment 1 aimed to examine how verticality perception changes over time in response to sensory conflict and Experiment 2 aimed to determine whether verticality perception recovers following the removal of a sensory conflict. Sensory conflict was induced using optic flow, such that visual cues signalled self-motion while vestibular input provided no self-motion signal. Sensitivity to verticality was measured using a Vertical Detection Task (VDT) in which participants judged whether lines were vertically oriented or not. Signal detection theory was used to analyse responses, where

d′ was an index of perceptual sensitivity to verticality and criterion was a measure of response bias. In Experiment 1 participants completed the detection task whilst either roll optic flow or randomly moving dots were presented in the background. Results showed that d′ was significantly lower in the roll-optic flow condition, with this effect varying as a function of time. Specifically, d′ was significantly lower in the roll versus random visual stimulation condition for up to six minutes of exposure. Experiment 2 extended these findings by examining recovery from sensory conflict, comparing VDT performance during roll optic flow with performance at successive timepoints following its removal. Consistent with Experiment 1, d′ was significantly reduced during optic flow exposure. However, d′ returned to baseline levels by the first post-flow timepoint, indicating a rapid recovery of verticality sensitivity. Together, these findings demonstrate that sensory conflict reduces verticality sensitivity in a time-dependent manner, followed by rapid recovery within minutes after the conflict is removed.