Vestibular balance control is dynamically weighted during locomotion. this might result from a selective suppression of vestibular inputs in favor of a feed-forward balance regulation based on locomotor efference copies. The feasibility of such a feed-forward mechanism should however critically depend on the predictability of head movements (HMP) during locomotion. To test this, we studied in 10 healthy subjects the differential impact of a stochastic vestibular stimulation (SVS) on body sway (center-of-pressure, COP) during standing and walking at different speeds and compared it to activity-dependent changes in HMp. SVS-cop coupling was determined by correlation analysis in frequency and time domains. HMP was quantified as the proportion of head motion variance that can be explained by the average head trajectory across the locomotor cycle. SVS-COP coupling decreased from standing to walking and further dropped with faster locomotion. Correspondingly, HMP increased with faster locomotion. Furthermore, SVS-COP coupling depended on the gait-cycle-phase with peaks corresponding to periods of least HMP. These findings support the assumption that during stereotyped human self-motion, locomotor efference copies selectively replace vestibular cues, similar to what was previously observed in animal models.The vestibular system encodes head orientation and motion to facilitate balance reflexes that ensure postural equilibrium during passive as well as self-initiated movements 1 . During locomotion, i.e., stereotyped self-motion, vestibular influences on balance control appear to be dynamically up-or down-regulated in dependence on the phase and speed of the locomotor pattern. Accordingly, vestibulospinal reflexes exhibit phasic modulations across the locomotor cycle 2,3 with the result that balance is particularly sensitive to vestibular perturbations at specific phases of the gait cycle 4 . Furthermore, vestibular influences appear to be down-weighted during faster locomotion. Accordingly, the destabilizing impact of a vestibular loss or perturbation on the gait pattern decreases with increasing locomotion speeds 5-9 .It was previously assumed that activity-dependent modulations of vestibular balance reflexes might reflect an up-or down-regulation of a concurrent intrinsic feed-forward control of posture 10-13 . Accordingly, balance adjustments during self-motion might not solely rely on sensory feedback about how the body has moved, but also on predictions of resultant movements derived from efference copies of the motor command 14 . Physiological evidence for such a direct feed-forward control mode has recently been shown for animal locomotion. During Xenopus laevis tadpole swimming, intrinsic efference copies of the locomotor command deriving from spinal central pattern generators (CPG) were shown to directly trigger ocular adjustments for gaze stabilization and selectively cancel out any afferent (ex-and reafferent) vestibular inputs 10,15 . Thus, also during human stereotyped locomotion, efference copies might pro...

1Vestibular balance control is dynamically weighted during locomotion. This might result 2 from a selective suppression of vestibular inputs in favor of a feed-forward balance 3 regulation based on locomotor efference copies. The feasibility of such a feed-forward 4 mechanism should however critically depend on the predictability of head movements 5 (PHM) during locomotion. To test this, we studied in healthy subjects the differential 6 impact of a stochastic vestibular stimulation (SVS) on body sway (center-of-pressure, 7 COP) during standing and walking at different speeds using time-frequency analyses 8 and compared it to activity-dependent changes in PHM. SVS-COP coupling decreased 9 from standing to walking and further dropped with faster locomotion. Correspondingly, 10 PHM increased with faster locomotion. Furthermore, SVS-COP coupling depended on 11 the gait-cycle-phase with peaks corresponding to periods of least PHM. These findings 12 support the assumption that during stereotyped human self-motion, locomotor 13 efference copies selectively replace vestibular cues, similar to what was previously 14 observed in animal models. 15 16 17 18 19 20 21

Downbeat nystagmus (DBN) is a common form of acquired fixation nystagmus related to vestibulo-cerebellar impairments and associated with impaired vision and postural imbalance. DBN intensity becomes modulated by various factors such as gaze direction, head position, daytime, and resting conditions. Further evidence suggests that locomotion attenuates postural symptoms in DBN. Here, we examined whether walking might analogously influence ocular-motor deficits in DBN. Gaze stabilization mechanisms and nystagmus frequency were examined in 10 patients with DBN and 10 age-matched healthy controls with visual fixation during standing vs. walking on a motorized treadmill. Despite their central ocular-motor deficits, linear and angular gaze stabilization in the vertical plane were functional during walking in DBN patients and comparable to controls. Notably, nystagmus frequency in patients was considerably reduced during walking compared to standing (p < 0.001). The frequency of remaining nystagmus during walking was further modulated in a manner that depended on the specific phase of the gait cycle (p = 0.015). These attenuating effects on nystagmus intensity during walking suggest that ocular-motor control disturbances are selectively suppressed during locomotion in DBN. This suppression is potentially mediated by locomotor efference copies that have been shown to selectively govern gaze stabilization during stereotyped locomotion in animal models.

No abstract