Differential effects of vision upon the accuracy and precision of vestibular-evoked balance responses.

key pointsEffective balance control requires the transformation of vestibular signals from head- to foot-centred coordinates in order to move the body in an appropriate direction. This transformation process has previously been studied by analysing the directional accuracy of the averaged sway response to multiple electrical vestibular stimuli (EVS). Here we studied trial-by-trial variability of EVS responses to measure any changes in directional precision which may be masked by the averaging process. We found that vision increased directional variability without influencing the mean sway direction, demonstrating that response accuracy and precision are dissociable. These results emphasise the importance of single trial analysis in determining the efficacy of vestibular control of balance. ABSTRACT: Vestibular information must be transformed from head- to-foot-centred coordinates for balance control. This transformation process has previously been investigated using electrical vestibular stimulation (EVS), which evokes a sway response fixed in head coordinates. The craniocentric nature of the response has been demonstrated by analysing average responses to multiple stimuli. This approach misses any trial-by-trial variability which would reflect poor balance control. Here we performed single-trial analysis to measure this directional variability (precision), and compared this to mean performance (accuracy). We determined the effect of vision upon both parameters. Standing volunteers adopted various head orientations (0, ±30 and ±60 deg yaw) while EVS-evoked response direction was determined from ground reaction force vectors. As previously reported, mean force direction was orientated towards the anodal ear, and rotated in line with head yaw. Although vision caused a ∼50% reduction in response magnitude, it had no influence on the direction of the mean sway response, indicating that accuracy was unaffected. However, individual trial analysis revealed up to 30% increases in directional variability with the eyes open. This increase was inversely correlated with the size of the force response. The paradoxical observation that vision reduces the precision of the balance response may be explained by a multi-sensory integration process. As additional veridical sensory information becomes available, this lessens the relative contribution of vestibular input, causing a simultaneous reduction in both the magnitude and the precision of the response to EVS. Our novel approach demonstrates the importance of single-trial analysis in revealing the efficacy of vestibular reflexes.