Vestibular Contributions to Primate Neck Postural Muscle Activity during Natural Motion

Sensorimotor Transformations for Postural Control in the Vermis of the Cerebellum

The cerebellar vermis plays an essential role in maintaining posture and balance by integrating sensory inputs from multiple modalities to effectively coordinate movement. By transforming convergent sensory information into precise motor commands, it ensures smooth, adaptive motor control, enabling the body to maintain stability in dynamic environments. This review examines recent findings that investigate the distinct neural computations performed by the anterior vermis and posterior vermis (nodulus/uvula). Specifically, we examine how Purkinje cells in these regions integrate vestibular and proprioceptive signals to convert self-motion information from a head-centered to a body-centered reference frame, which is essential for maintaining precise postural control in response to unexpected movements. Additionally, we consider recent findings showing that, during voluntary self-motion, Purkinje cells in the anterior vermis selectively suppress responses in the vestibulospinal pathway by integrating motor inputs with sensory signals. Given the anterior vermis's role in maintaining balance during voluntary behaviors such as locomotion, its suppression prevents counterproductive stabilizing reflexes, enabling goal-directed movement through space. In contrast, the posterior vermis, encompassing the nodulus and uvula, integrates vestibular inputs from both the otoliths and semicircular canals to maintain equilibrium relative to gravitational forces. We thus hypothesize that Purkinje cells in the nodulus/uvula do not generate suppression signals like those observed in the anterior vermis but instead continuously compute our orientation in space, regardless of whether movement is voluntarily generated or unexpected. If our hypothesis is correct, the nodulus/uvula would effectively provide consistent "ground truth" information about self-motion relative to gravity.

Reference Frames for Encoding of Translation and Tilt in the Caudal Cerebellar Vermis

Many daily behaviors rely on estimates of our body's motion and orientation in space. Vestibular signals are essential for such estimates, but to contribute appropriately, two key computations are required. First, ambiguous motion information from otolith organs must be combined with spatially transformed rotational signals (e.g., from the canals) to distinguish head translation from tilt. Second, tilt and translation estimates must be transformed from a head- to a body-centered reference frame to correctly interpret the body's motion. Studies have shown that cells in the caudal cerebellar vermis (nodulus and ventral uvula, NU) reflect the output of the first set of computations to estimate translation and tilt. However, it remains unknown whether these estimates are encoded exclusively in head-centered coordinates or whether they reflect further transformation toward body-centered coordinates. Here, we addressed this question by examining how the 3D spatial tuning of otolith and canal signals on translation- and tilt-selective NU Purkinje cells in male rhesus monkeys varies with changes in head-re-body and body-re-gravity orientation. We show that NU cell tuning properties are consistent with head-centered otolith signal coding during translation. Furthermore, while canal signals in the NU have been transformed into a specific world-referenced rotation signal indicating reorientation relative to gravity (tilt), as needed to resolve the tilt/translation ambiguity, the resulting tilt estimates are encoded in head-centered coordinates. Our results thus suggest that body-centered motion and orientation estimates required for postural control, navigation, and reaching are computed elsewhere, either by further transforming NU outputs or via computations in other parallel pathways.

Habituation of vestibular-evoked balance responses after repeated exposure to a postural threat

Vestibular-evoked balance responses are facilitated when faced with threats to stability. However, the extent to which these sensorimotor adaptations covary with changes in emotional and autonomic state remains unclear. This study repeatedly exposed individuals to the same postural threat while vestibular-motor responses were probed using stochastic vestibular stimulation (SVS; 2-25 Hz). This allowed emotional and autonomic state to be manipulated within the same threat environment to determine if vestibular-evoked balance responses are coupled with the emotional/autonomic changes induced by the threat or are facilitated in a strictly context-dependent manner. Twenty-three young adults stood with their head turned 90° while receiving SVS at LOW (0.8 m above ground, away from edge) and HIGH (3.2 m above ground, at edge) conditions. LOW trials were completed before and after a block of 10 HIGH trials. Ground reaction forces (GRFs) and plantar flexor (soleus and medial gastrocnemius (MG)) EMG were recorded. Vestibular-evoked responses recorded from GRFs and EMG were quantified in terms of signal coupling (coherence and cumulant density) and gain, and emotional and autonomic state were assessed from self-reports and electrodermal activity. Vestibular-evoked balance responses were facilitated with initial threat exposure. After repeated exposure, there was significant habituation of the emotional response to threat, which was accompanied by reductions in vestibular-evoked balance responses, most notably for GRFs and MG-EMG. This suggests that threat-related changes in vestibular-motor function are tightly coupled with the emotional and autonomic changes induced by the threat, and not an invariant response to context-specific features of the threat. KEY POINTS: Balance corrective responses mediated through vestibular-motor pathways are facilitated when stability is threatened; however, the extent to which these sensorimotor adaptations covary with changes in emotional state remains unclear. By having young adults repeatedly stand at the edge of an elevated surface, this study examined how vestibular-evoked balance responses, probed using stochastic vestibular stimulation and recorded from ground reaction forces and plantar flexor EMG, changed alongside estimates of emotional state. Vestibular-evoked responses were facilitated when individuals were first exposed to the postural threat, but demonstrated marked habituation alongside estimates of emotional state after repeated exposure. This suggests that threat-related changes in vestibular-motor function are coupled to the emotional response to threat, and are not an invariant response to context-specific features of the threat. These changes in vestibular-motor function are likely part of a multisensory adaptation process that primes the nervous system to respond to sudden destabilizing forces when fearful of falling.

Commissural and monosynaptic inputs to medial vestibular nucleus GABAergic neurons in mice

Objective

MVN GABAergic neurons is involved in the rebalance of commissural system contributing to alleviating acute peripheral vestibular dysfunction syndrome. This study aims to depict monosynaptic inputs to MVN GABAergic neurons.

Methods

The modified rabies virus-based retrogradation method combined with the VGAT-IRES-Cre mice was used in this study. Moreover, the commissural connections with MVN GABAergic neurons were analyzed.

Results

We identified 60 nuclei projecting to MVN GABAergic neurons primarily distributed in the cerebellum and the medulla. The uvula-nodulus, gigantocellular reticular nucleus, prepositus nucleus, intermediate reticular nucleus, and three other nuclei sent dense inputs to MVN GABAergic neurons. The medial (fastigial) cerebellar nucleus, dorsal paragigantocellular nucleus, lateral paragigantocellular nucleus and 10 other nuclei sent moderate inputs to MVN GABAergic neurons. Sparse inputs to MVN GABAergic neurons originated from the nucleus of the solitary tract, lateral reticular nucleus, pedunculopontine tegmental nucleus and 37 other nuclei. The MVN GABAergic neurons were regulated by the contralateral MVN, lateral vestibular nucleus, superior vestibular nucleus, and inferior vestibular nucleus.

Conclusion

Our study contributes to further understanding of the vestibular dysfunction in terms of neural circuits and search for new strategies to facilitate vestibular compensation.

Human proprioceptive gaze stabilization during passive body rotations underneath a fixed head

The present study explored the presence of torsional gaze-stabilization to proprioceptive neck activation in humans. Thirteen healthy subjects (6 female, mean age 25) were exposed to passive body rotations while maintaining a head-fixed, gravitationally upright, position. Participants were seated in a mechanical sled, their heads placed in a chin rest embedded in a wooden beam while wearing an eye tracker attached to the beam using strong rubber bands to ensure head stability. The body was passively rotated underneath the head both in darkness and while viewing a projected visual scene. Static torsional gaze positions were compared between the baseline position prior to the stimulation, and immediately after the final body tilt had been reached. Results showed that passive neck flexion produced ocular torsion when combined with a visual background. The eyes exhibited rotations in the opposite direction of the neck's extension, matching a hypothetical head tilt in the same direction as the sled. This corresponded with a predicted head rotation aimed at straightening the head in relation to the body. No such response was seen during trials in darkness. Altogether, these findings suggest that proprioception may produce a predictive gaze-stabilizing response in humans.

Effect of vestibular loss on head-on-trunk stability in individuals with vestibular schwannoma

The vestibulo-collic reflex generates neck motor commands to produce head-on-trunk movements that are essential for stabilizing the head relative to space. Here we examined the effects of vestibular loss on head-on-trunk kinematics during voluntary behavior. Head and trunk movements were measured in individuals with vestibular schwannoma before and then 6 weeks after unilateral vestibular deafferentation via surgical resection of the tumor. Movements were recorded in 6 dimensions (i.e., 3 axes of rotation and 3 axes of translation) using small light-weight inertial measurement units while participants performed balance and gait tasks. Kinematic measures differed between individuals with vestibular schwannoma (at both time points) and healthy controls for the more challenging exercises, namely those performed in tandem position or on an unstable surface without visual input. Quantitative assessment of the vestibulo-ocular reflex (VOR) revealed a reduction in VOR gain for individuals with vestibular schwannoma compared to control subjects, that was further reduced following surgery. These findings indicated that the impairment caused by either the tumor or subsequent surgical tumor resection altered head-on-trunk kinematics in a manner that is not normalized by central compensation. In contrast, we further found that head-on-trunk kinematics in individuals with vestibular schwannoma were actually comparable before and after surgery. Thus, taken together, our results indicate that vestibular loss impacts head-on-trunk kinematics during voluntary balance and gait behaviors, and suggest that the neural mechanisms mediating adaptation alter the motion strategies even before surgery in a manner that may be maladaptive for long-term compensation.

Electrical stimulation of the peripheral and central vestibular system

PURPOSE OF REVIEW

Electrical stimulation of the peripheral and central vestibular system using noninvasive (galvanic vestibular stimulation, GVS) or invasive (intracranial electrical brain stimulation, iEBS) approaches have a long history of use in studying self-motion perception and balance control. The aim of this review is to summarize recent electrophysiological studies of the effects of GVS, and functional mapping of the central vestibular system using iEBS in awake patients.

RECENT FINDINGS

The use of GVS has become increasingly common in the assessment and treatment of a wide range of clinical disorders including vestibulopathy and Parkinson's disease. The results of recent single unit recording studies have provided new insight into the neural mechanisms underlying GVS-evoked improvements in perceptual and motor responses. Furthermore, the application of iEBS in patients with epilepsy or during awake brain surgery has provided causal evidence of vestibular information processing in mostly the middle cingulate cortex, posterior insula, inferior parietal lobule, amygdala, precuneus, and superior temporal gyrus.

SUMMARY

Recent studies have established that GVS evokes robust and parallel activation of both canal and otolith afferents that is significantly different from that evoked by natural head motion stimulation. Furthermore, there is evidence that GVS can induce beneficial neural plasticity in the central pathways of patients with vestibular loss. In addition, iEBS studies highlighted an underestimated contribution of areas in the medial part of the cerebral hemispheres to the cortical vestibular network.