Unperceived motor actions of the balance system interfere with the causal attribution of self-motion

Convergence of vestibular and proprioceptive signals in the cerebellar nodulus/uvula enhances the encoding of self-motion in primates

The integration of different sensory streams is required to dynamically estimate how our head and body are oriented and moving relative to gravity. This process is essential to continuously maintain stable postural control, autonomic regulation, and self-motion perception. The nodulus/uvula (NU) in the posterior cerebellar vermis is known to integrate canal and otolith vestibular input to signal angular and linear head motion in relation to gravity. However, estimating body orientation and motion requires integrating proprioceptive cues with vestibular signals. Lesion studies demonstrate that the NU is crucial for maintaining postural control, suggesting it could play an important role in combining multimodal sensory input. Using high-density extracellular recordings in rhesus monkeys, we found that the majority of vestibular-sensitive Purkinje cells also encoded dynamic neck proprioceptive input. Furthermore, Purkinje cells generally aligned their directional tuning to vestibular and proprioceptive stimulation such that self-motion encoding was enhanced. The heterogeneous response dynamics among Purkinje cells enabled their population activity to generate head or body motion encoding in the downstream nuclei neurons on which they converge. Strikingly, when we then experimentally altered the orientation of the head relative to the body, Purkinje cells modulated their responses to vestibular stimulation to account for the change in body motion in space. These findings reveal that the NU integrates proprioceptive and vestibular input synergistically to maintain robust postural control.

Inverse relation between motion perception and postural responses induced by motion of a touched object

Self vs. external attribution of motions based on vestibular cues is suggested to underlie our coherent perception of object motion and self-motion. However, it remains unclear whether such attribution also underlies sensorimotor responses. Here, we examined this issue in the context of touch. We asked participants to lightly touch a moving object with their thumb while standing still on an unstable surface. We measured both the accuracy of judging the object motion direction and the postural response. If the attribution underlies both object-motion perception and posture control, sensitivity of posture to object motion should decrease with motion speed since high speed motion is unlikely to reflect self-motion. Furthermore, when motion perception is erroneous, there should be a corresponding increase in postural responses. Our results are consistent with these predictions and suggest that self-external attribution of somatosensory motion underlies both object motion perception and postural responses.

Different mechanisms of contextual inference govern associatively learned and sensory-evoked postural responses

Human standing balance relies on the continuous monitoring and integration of sensory signals to infer our body's motion and orientation within the environment. However, when sensory information is no longer contextually relevant to balancing the body (e.g., when sensory and motor signals are incongruent), sensory-evoked balance responses are rapidly suppressed, much earlier than any conscious perception of changes in balance control. Here, we used a robotic balance simulator to assess whether associatively learned postural responses are similarly modulated by sensorimotor incongruence and contextual relevance to postural control. Twenty-nine participants in three groups were classically conditioned to generate postural responses to whole-body perturbations when presented with an initially neutral sound cue. During catch and extinction trials, participants received only the auditory stimulus but in different sensorimotor states corresponding to their group: 1) during normal active balance, 2) while immobilized, and 3) throughout periods where the computer subtly removed active control over balance. In the balancing and immobilized states, conditioned responses were either evoked or suppressed, respectively, according to the (in)ability to control movement. Following the immobilized state, conditioned responses were renewed when balance was restored, indicating that conditioning was retained but only expressed when contextually relevant. In contrast, conditioned responses persisted in the computer-controlled state even though there was no causal relationship between motor and sensory signals. These findings suggest that mechanisms responsible for sensory-evoked and conditioned postural responses do not share a single, central contextual inference and assessment of their relevance to postural control, and may instead operate in parallel.

Learning to stand with sensorimotor delays generalizes across directions and from hand to leg effectors

Humans receive sensory information from the past, requiring the brain to overcome delays to perform daily motor skills such as standing upright. Because delays vary throughout the body and change over a lifetime, it would be advantageous to generalize learned control policies of balancing with delays across contexts. However, not all forms of learning generalize. Here, we use a robotic simulator to impose delays into human balance. When delays are imposed in one direction of standing, participants are initially unstable but relearn to balance by reducing the variability of their motor actions and transfer balance improvements to untrained directions. Upon returning to normal standing, aftereffects from learning are observed as small oscillations in control, yet they do not destabilize balance. Remarkably, when participants train to balance with delays using their hand, learning transfers to standing with the legs. Our findings establish that humans use experience to broadly update their neural control to balance with delays.

Age-related impairments and influence of visual feedback when learning to stand with unexpected sensorimotor delays

Background

While standing upright, the brain must accurately accommodate for delays between sensory feedback and self-generated motor commands. Natural aging may limit adaptation to sensorimotor delays due to age-related decline in sensory acuity, neuromuscular capacity and cognitive function. This study examined balance learning in young and older adults as they stood with robot-induced sensorimotor delays.

Methods

A cohort of community dwelling young (mean = 23.6 years, N = 20) and older adults (mean = 70.1 years, N = 20) participated in this balance learning study. Participants stood on a robotic balance simulator which was used to artificially impose a 250 ms delay into their control of standing. Young and older adults practiced to balance with the imposed delay either with or without visual feedback (i.e., eyes open or closed), resulting in four training groups. We assessed their balance behavior and performance (i.e., variability in postural sway and ability to maintain upright posture) before, during and after training. We further evaluated whether training benefits gained in one visual condition transferred to the untrained condition.

Results

All participants, regardless of age or visual training condition, improved their balance performance through training to stand with the imposed delay. Compared to young adults, however, older adults had larger postural oscillations at all stages of the experiments, exhibited less relative learning to balance with the delay and had slower rates of balance improvement. Visual feedback was not required to learn to stand with the imposed delay, but it had a modest effect on the amount of time participants could remain upright. For all groups, balance improvements gained from training in one visual condition transferred to the untrained visual condition.

Conclusion

Our study reveals that while advanced age partially impairs balance learning, the older nervous system maintains the ability to recalibrate motor control to stand with initially destabilizing sensorimotor delays under differing visual feedback conditions.