Modulation of human vestibular reflexes with increased postural threat

Key points The vestibular system is an important sensory contributor to the control of standing balance. Fear, anxiety and arousal are thought to influence the excitability of the vestibular system, but it is not clear if these changes lead to altered vestibular‐evoked balance reflexes. Low and high standing surface heights were used to manipulate fear and anxiety in this study, while stochastic vestibular stimulation was used to evoke balance reflexes. High surface heights lead to greater coupling between vestibular inputs and balance reflexes, as well as larger responses. These results support the idea that the manner in which vestibular information is processed is altered when people are exposed to a threat to their balance, and this altered processing may explain why normal balance behaviour is different in threatening scenarios.Abstract Anxiety and arousal have been shown to facilitate human vestibulo‐ocular reflexes, presumably through direct neural connections between the vestibular nuclei and emotional processing areas of the brain. However, the effects of anxiety, fear and arousal on balance‐relevant vestibular reflexes are currently unknown. The purpose of this study was to manipulate standing height to determine whether anxiety and fear can modulate the direct relationship between vestibular signals and balance reflexes during stance. Stochastic vestibular stimulation (SVS; 2–25 Hz) was used to evoke ground reaction forces (GRF) while subjects stood in both LOW and HIGH surface height conditions. Two separate experiments were conducted to investigate the SVS–GRF relationship, in terms of coupling (coherence and cumulant density) and gain, in the medio‐lateral (ML) and antero‐posterior (AP) directions. The short‐ and medium‐latency cumulant density peaks were both significantly increased in the ML and AP directions when standing in HIGH, compared to LOW, conditions. Likewise, coherence was statistically greater between 4.3 Hz and 6.7 Hz in the ML, and between 5.5 and 17.7 Hz in the AP direction. When standing in the HIGH condition, the gain of the SVS–GRF relationship was increased 81% in the ML direction, and 231% in the AP direction. The significant increases in coupling and gain observed in both experiments demonstrate that vestibular‐evoked balance responses are augmented in states of height‐induced postural threat. These data support the possibility that fear or anxiety‐mediated changes to balance control are affected by altered central processing of vestibular information.