Poster: Adaptability of human standing balance with unexpected sensorimotor delays

Poster presented at 2023 Neural Control of Movement Meeting in Victoria, British Columbia, Canada

Highlights:

· Using a unique robotic balance simulator, we investigate how humans adapt balance control and learn to maintain upright stance with unexpected sensorimotor delays

· Imposing long delays destabilizes upright equilibrium, but through training, humans learn to overcome these delays by re-associating delayed body motion with self-generated balancing motor commands

· Humans can generalize learned principles, suggesting that the brain estimates the source of unexpected balance motion and broadly updates its control to accommodate for added delays even in substantially different contexts

Justification:

Every animal has inherent sensorimotor delays when controlling movements. Consequently, understanding how delays influence the control and adaption of motor behaviour is of fundamental interest to the field of neural control of movement. For human standing balance, sensorimotor delays threaten our stability and increase our risk of falling. Aging and neurological diseases can lead to longer sensorimotor delays, further challenging our ability to balance upright. Here, we used a unique robotic balance simulator to investigate how imposing delays into human balance control disrupts the control of upright posture and determine the underlying mechanisms responsible for any sensorimotor adaptation and generalization of learned behaviours to other contexts.

Abstract:

To control movement, the brain must associate self-generated motor commands with resulting delayed sensory feedback. This is critical during bipedal activities like standing, as upright balance is mechanically unstable and sensorimotor delays decrease the region of stability. Here, we investigated how sensorimotor delays added to the sensory consequences of self-generated motor commands destabilize human standing balance, and explored the mechanisms underlying any subsequent adaptation, learning and generalization. Healthy adults balanced upright in a robotic balance simulator that recreates the physical sensations and sensorimotor signals for standing balance. In our first experiments, we found that lengthening the sensorimotor delay destabilized upright equilibrium, decreased vestibular contributions to balance and led to perceptions of unexpected motion. Through training with an imposed 400ms delay over 5 days, participants regained their ability to balance, which was accompanied by recovered vestibular contributions and perceptions of expected standing. These compensations were retained three months later, demonstrating long-term retention of learning. We next explored whether humans can generalize this sensorimotor adaptation across different balance contexts, such as varied delays, movement directions (anteroposterior vs mediolateral), and muscle effectors. Participants that trained with a 400ms imposed delay also improved their balance across delays between 200 and 500ms. Furthermore, when participants trained to balance with a 350ms delay in a single direction (e.g., anteroposterior), they generalized their adapted balance control to the orthogonal (e.g., mediolateral) direction. Importantly, this transfer was not dependent on biomechanical interactions across directions of standing balance, because participants also generalized learning across independent motor effectors (from the hand to the lower limb). Finally, we investigated the influence of visual feedback on adapting to imposed delays of 250ms. Although removing visual feedback increased postural oscillations with delays, blindfolded participants learned to maintain balance with imposed delays and exhibited similar learning rates as participants who learned with visual feedback. Collectively, these experiments show that the human brain can adapt the control of balance to accommodate for unexpected sensorimotor delays in a variety of contexts. While long delays are initially destabilizing, humans learn to overcome these sensorimotor delays by re-associating delayed whole-body motion with self-generated balancing motor commands. Furthermore, humans can generalize learned principles, suggesting that the brain estimates the source of unexpected balance motion (i.e., new motor command – body movement relationship) and broadly updates its control to accommodate for imposed delays even in substantially different contexts.

Preprint

Preprint

Poster: Adaptability of human standing balance with unexpected sensorimotor delays