Modulation of vestibular evoked reflexes in postural muscles during self-motion experiences in virtual environments

BACKGROUND & AIM: Maintaining posture requires the input from three primary sensory systems. These systems include somatosensory, visual, and vestibular components. Of these three systems, the visual and vestibular systems work closest to maintain balance and posture due to visual-vestibular interactions that take place during static and dynamic activities. They are mainly responsible for identifying stimuli and using sensory information to orient the body in space. Thus, understanding how visual and vestibular components integrate may be critical to understand how visual and vestibular sensory information is weighted to control balance. The purpose of this investigation was to determine if visual stimuli alters central vestibular function, modifying postural control when interacting with screen-simulated activities. METHODS: Fifteen young adults performed 45 randomized standing trials (60-seconds) consisting of 6 conditions (EYES CLOSED+GVS, VISUAL+GVS, EYES OPEN+GVS*, EYES CLOSED, EYES OPEN*, VISUAL ONLY*). While standing, subjects were positioned with the head turned at a 90- degree angle, while focusing gaze on a screen. Subjects were outfitted with lower-limb bilateral EMG (Delsys Inc., MA, USA) on the soleus, tibialis anterior, and gastrocnemius to assess short and medium latency reflexes elicited by 30 seconds of galvanic vestibular stimulation (GVS). Stimulus dependent muscle responses were analyzed to determine short and medium reflex latencies (onset) during activity. Force plate (AMTI, Watertown, Massachusetts) data were used to calculate deviations, and velocity shifts in COP over the trials. RESULTS: Results revealed a significant increase in COP velocity for the VISUAL ONLY condition (dynamic visual) (p =.002) when compared to the EYES OPEN* (static visual) condition. A similar increase (p = .001) was also observed when comparing the VISUAL ONLY condition to EYES OPEN+GVS* (static visual) condition. Finally, a significant difference was noted between the VISUAL + GVS and EYES OPEN + GVS* conditions with an increase observed in the VISUAL+GVS condition (p=.003). Overall, dynamic conditions (i.e. VISUAL+GVS & EYES CLOSED+GVS) elicited greater COP velocity when compared to the EYES OPEN* condition. CONCLUSIONS: The main findings from this study revealed that when presented with a dynamic visual stimulus, participants increased COP velocity about the base of support, regardless of whether or not GVS was administered. These findings are in line with previous literature, which indicates that visually induced linear vcction increases postural control regulation, thus resulting in the readjustments necessary to maintain balance. This suggests that although vestibular input is important to controlling balance and posture, visual sensory information is utilized to a greater extent when the visual field or visual stimuli is dynamic in nature. A closer look at vestibular evoked muscle responses may provide greater insight into how information from the vestibular system is utilized when presented with dynamic stimuli