Habituation of vestibular-evoked balance responses after repeated exposure to a postural threat.

Vestibular-evoked balance responses are facilitated when faced with threats to stability. However, the extent to which these sensorimotor adaptations covary with changes in emotional and autonomic state remains unclear. This study repeatedly exposed individuals to the same postural threat while vestibular-motor responses were probed using stochastic vestibular stimulation (SVS; 2-25 Hz). This allowed emotional and autonomic state to be manipulated within the same threat environment to determine if vestibular-evoked balance responses are coupled with the emotional/autonomic changes induced by the threat or are facilitated in a strictly context-dependent manner. Twenty-three young adults stood with their head turned 90° while receiving SVS at LOW (0.8 m above ground, away from edge) and HIGH (3.2 m above ground, at edge) conditions. LOW trials were completed before and after a block of 10 HIGH trials. Ground reaction forces (GRFs) and plantar flexor (soleus and medial gastrocnemius (MG)) EMG were recorded. Vestibular-evoked responses recorded from GRFs and EMG were quantified in terms of signal coupling (coherence and cumulant density) and gain, and emotional and autonomic state were assessed from self-reports and electrodermal activity. Vestibular-evoked balance responses were facilitated with initial threat exposure. After repeated exposure, there was significant habituation of the emotional response to threat, which was accompanied by reductions in vestibular-evoked balance responses, most notably for GRFs and MG-EMG. This suggests that threat-related changes in vestibular-motor function are tightly coupled with the emotional and autonomic changes induced by the threat, and not an invariant response to context-specific features of the threat. KEY POINTS: Balance corrective responses mediated through vestibular-motor pathways are facilitated when stability is threatened; however, the extent to which these sensorimotor adaptations covary with changes in emotional state remains unclear. By having young adults repeatedly stand at the edge of an elevated surface, this study examined how vestibular-evoked balance responses, probed using stochastic vestibular stimulation and recorded from ground reaction forces and plantar flexor EMG, changed alongside estimates of emotional state. Vestibular-evoked responses were facilitated when individuals were first exposed to the postural threat, but demonstrated marked habituation alongside estimates of emotional state after repeated exposure. This suggests that threat-related changes in vestibular-motor function are coupled to the emotional response to threat, and are not an invariant response to context-specific features of the threat. These changes in vestibular-motor function are likely part of a multisensory adaptation process that primes the nervous system to respond to sudden destabilizing forces when fearful of falling.