Abstract
Human standing balance relies on self-motion estimates that are used by the nervous system to detect unexpected movements and enable corrective responses and adaptations in control. These estimates must accommodate for inherent delays in sensory and motor pathways. Here, we used a robotic system to simulate human standing about the ankles in the anteroposterior direction and impose sensorimotor delays into the control of balance. Imposed delays destabilized standing, but through training, participants adapted and re-learned to balance with the delays. Before training, imposed delays attenuated vestibular contributions to balance and triggered perceptions of unexpected standing motion, suggesting increased uncertainty in the internal self-motion estimates. After training, vestibular contributions partially returned to baseline levels and larger delays were needed to evoke perceptions of unexpected standing motion. Through learning, the nervous system accommodates balance sensorimotor delays by causally linking whole-body sensory feedback (initially interpreted as imposed motion) to self-generated balance motor commands.
Highlights
The nervous system learns and maintains motor skills by forming probabilistic estimates of self-motion
The peak velocity variance leading up to a perceptual detection was extracted for each perceived transition, and resulted in an average peak velocity variance of 8.42 ± 8.62 [°/s]2. These results indicate that the perception of unexpected motion and increased sway variability arising from an imposed delay are accompanied by a 70-90% attenuation of vestibular contributions to balance
Participants learned to balance with an imposed sensorimotor delay of 400 ms over 5 days (100 mins of training), showing decreased sway velocity variance and increased percent time balancing within the virtual limits, restored vestibular control of balance, and fewer unexpected movement detections while balancing with the delay
Summary
The nervous system learns and maintains motor skills by forming probabilistic estimates of self-motion. As a consequence of these relatively long delays, computational feedback models of upright standing predict that balance controllers cannot adjust their sensorimotor gains and stabilize balance in the anteroposterior direction with imposed delays larger than ~300-340 ms (Milton and Insperger 2019; van der Kooij and Peterka 2011). These predictions contrast the reported upper limb sensorimotor adaptation to imposed delays
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