Asymmetric short-term adaptation of the vertical vestibulo-ocular reflex in humans

Unidirectional Vertical Vestibuloocular Reflex Adaptation in Humans Using 1D and 2D Scenes

HYPOTHESIS

The vertical vestibuloocular reflex (VOR) in response to pitch head impulses can be optimally trained to increase in one direction using a two-dimensional (2D) visual training target with minimal effect on the horizontal VOR.

BACKGROUND

We modified the incremental VOR adaptation (IVA) technique, shown to increase the horizontal VOR in patients with vestibular hypofunction, to drive vertical VOR adaptation in healthy control subjects.

METHODS

We measured the horizontal and vertical active (self-generated) and passive (imposed) head impulse VOR gains (eye velocity/head velocity) before and after 15 minutes of unidirectional downward IVA training. IVA training consisted of two sessions, one using a single-dot one-dimensional (1D) target, the other a grid-of-dots 2D target.

RESULTS

The downward head impulse VOR gain significantly increased because of training by 13.3%, whereas the upward VOR gain did not change. The addition of extraretinal (2D) feedback did not result in greater adaptation, i.e., 1D and 2D gain increases were 15.5% and 10.6%, respectively. The vertical VOR gain increase resulted in a 3.2% decrease in horizontal VOR gain.

CONCLUSION

This preliminary study is the first to show that physiologically relevant (high frequency) unidirectional increases in vertical VOR gain are possible with just 15 minutes of training. This study sets the basis for future clinical trials examining vertical IVA training in patients, which may provide the first practical rehabilitation treatment to functionally improve the vertical VOR.

Gaze-Stabilizing Central Vestibular Neurons Project Asymmetrically to Extraocular Motoneuron Pools

Within reflex circuits, specific anatomical projections allow central neurons to relay sensations to effectors that generate movements. A major challenge is to relate anatomical features of central neural populations, such as asymmetric connectivity, to the computations the populations perform. To address this problem, we mapped the anatomy, modeled the function, and discovered a new behavioral role for a genetically defined population of central vestibular neurons in rhombomeres 5–7 of larval zebrafish. First, we found that neurons within this central population project preferentially to motoneurons that move the eyes downward. Concordantly, when the entire population of asymmetrically projecting neurons was stimulated collectively, only downward eye rotations were observed, demonstrating a functional correlate of the anatomical bias. When these neurons are ablated, fish failed to rotate their eyes following either nose-up or nose-down body tilts. This asymmetrically projecting central population thus participates in both upward and downward gaze stabilization. In addition to projecting to motoneurons, central vestibular neurons also receive direct sensory input from peripheral afferents. To infer whether asymmetric projections can facilitate sensory encoding or motor output, we modeled differentially projecting sets of central vestibular neurons. Whereas motor command strength was independent of projection allocation, asymmetric projections enabled more accurate representation of nose-up stimuli. The model shows how asymmetric connectivity could enhance the representation of imbalance during nose-up postures while preserving gaze stabilization performance. Finally, we found that central vestibular neurons were necessary for a vital behavior requiring maintenance of a nose-up posture: swim bladder inflation. These observations suggest that asymmetric connectivity in the vestibular system facilitates representation of ethologically relevant stimuli without compromising reflexive behavior.

SIGNIFICANCE STATEMENT Interneuron populations use specific anatomical projections to transform sensations into reflexive actions. Here we examined how the anatomical composition of a genetically defined population of balance interneurons in the larval zebrafish relates to the computations it performs. First, we found that the population of interneurons that stabilize gaze preferentially project to motoneurons that move the eyes downward. Next, we discovered through modeling that such projection patterns can enhance the encoding of nose-up sensations without compromising gaze stabilization. Finally, we found that loss of these interneurons impairs a vital behavior, swim bladder inflation, that relies on maintaining a nose-up posture. These observations suggest that anatomical specialization permits neural circuits to represent relevant features of the environment without compromising behavior.

A bi-hemispheric neuronal network model of the cerebellum with spontaneous climbing fiber firing produces asymmetrical motor learning during robot control

To acquire and maintain precise movement controls over a lifespan, changes in the physical and physiological characteristics of muscles must be compensated for adaptively. The cerebellum plays a crucial role in such adaptation. Changes in muscle characteristics are not always symmetrical. For example, it is unlikely that muscles that bend and straighten a joint will change to the same degree. Thus, different (i.e., asymmetrical) adaptation is required for bending and straightening motions. To date, little is known about the role of the cerebellum in asymmetrical adaptation. Here, we investigate the cerebellar mechanisms required for asymmetrical adaptation using a bi-hemispheric cerebellar neuronal network model (biCNN). The bi-hemispheric structure is inspired by the observation that lesioning one hemisphere reduces motor performance asymmetrically. The biCNN model was constructed to run in real-time and used to control an unstable two-wheeled balancing robot. The load of the robot and its environment were modified to create asymmetrical perturbations. Plasticity at parallel fiber-Purkinje cell synapses in the biCNN model was driven by error signal in the climbing fiber (cf) input. This cf input was configured to increase and decrease its firing rate from its spontaneous firing rate (approximately 1 Hz) with sensory errors in the preferred and non-preferred direction of each hemisphere, as demonstrated in the monkey cerebellum. Our results showed that asymmetrical conditions were successfully handled by the biCNN model, in contrast to a single hemisphere model or a classical non-adaptive proportional and derivative controller. Further, the spontaneous activity of the cf, while relatively small, was critical for balancing the contribution of each cerebellar hemisphere to the overall motor command sent to the robot. Eliminating the spontaneous activity compromised the asymmetrical learning capabilities of the biCNN model. Thus, we conclude that a bi-hemispheric structure and adequate spontaneous activity of cf inputs are critical for cerebellar asymmetrical motor learning.

Vestibular-related frontal cortical areas and their roles in smooth-pursuit eye movements: representation of neck velocity, neck-vestibular interactions, and memory-based smooth-pursuit

Smooth-pursuit eye movements are voluntary responses to small slow-moving objects in the fronto-parallel plane. They evolved in primates, who possess high-acuity foveae, to ensure clear vision about the moving target. The primate frontal cortex contains two smooth-pursuit related areas; the caudal part of the frontal eye fields (FEF) and the supplementary eye fields (SEF). Both areas receive vestibular inputs. We review functional differences between the two areas in smooth-pursuit. Most FEF pursuit neurons signal pursuit parameters such as eye velocity and gaze-velocity, and are involved in canceling the vestibulo-ocular reflex by linear addition of vestibular and smooth-pursuit responses. In contrast, gaze-velocity signals are rarely represented in the SEF. Most FEF pursuit neurons receive neck velocity inputs, while discharge modulation during pursuit and trunk-on-head rotation adds linearly. Linear addition also occurs between neck velocity responses and vestibular responses during head-on-trunk rotation in a task-dependent manner. During cross-axis pursuit-vestibular interactions, vestibular signals effectively initiate predictive pursuit eye movements. Most FEF pursuit neurons discharge during the interaction training after the onset of pursuit eye velocity, making their involvement unlikely in the initial stages of generating predictive pursuit. Comparison of representative signals in the two areas and the results of chemical inactivation during a memory-based smooth-pursuit task indicate they have different roles; the SEF plans smooth-pursuit including working memory of motion-direction, whereas the caudal FEF generates motor commands for pursuit eye movements. Patients with idiopathic Parkinson's disease were asked to perform this task, since impaired smooth-pursuit and visual working memory deficit during cognitive tasks have been reported in most patients. Preliminary results suggested specific roles of the basal ganglia in memory-based smooth-pursuit.

Unidirectional rotations produce asymmetric changes in horizontal VOR gain before and after unilateral labyrinthectomy in macaques

Unilateral vestibular lesions cause marked asymmetry in the horizontal vestibulo-ocular reflex (VOR) during rapid head rotations, with VOR gain being lower for head rotations toward the lesion than for rotations in the opposite direction. Reducing this gain asymmetry by enhancing ipsilesional responses would be an important step toward improving gaze stability following vestibular lesions. To that end, there were two goals in this study. First, we wanted to determine whether we could selectively increase VOR gain in only one rotational direction in normal monkeys by exposing them to a training session comprised of a 3-h series of rotations in only one direction (1,000°/s² acceleration to a plateau of 150°/s for 1 s) while they wore 1.7 × magnifying spectacles. Second, in monkeys with unilateral vestibular lesions, we designed a paradigm intended to reduce the gain asymmetry by rotating the monkeys toward the side of the lesion in the same way as above but without spectacles. There were three main findings (1) unidirectional rotations with magnifying spectacles result in gain asymmetry in normal monkeys, (2) gain asymmetry is reduced when animals are rotated towards the side of the labyrinthectomy via the ipsilesional rotation paradigm, and (3) repeated training causes lasting reduction in VOR gain asymmetry.

Directional preponderance of vertical eye movements induced by cross-axis adaptation of the vestibulo-ocular reflex in the cat

OBJECTIVE

We examined the emerging vertical component after cross-axis vestibulo-ocular reflex (VOR) adaptation.

METHODS

For the adaptive conditioning, the whole body of a subject was rotated horizontally while a visual pattern was rotated vertically for 1h.

RESULTS

During the conditioning period, the vertical component in response to the vertical visual stimulation was an up-down asymmetric, and its amplitude gradually increased as the stimulus progressed. After conditioning, the direction of VOR was changed via an emerging vertical component during horizontal head rotation in total darkness. Even though the retinal slip-velocity was greater for downward than for upward during the conditioning period, the emerging upward component was significantly larger than the emerging downward component.

CONCLUSION

These results may be helpful for developing an optimal vestibular-compensation system by visual-vestibular mismatch/optokinetic training.

Ocular motor syndromes of the brainstem and cerebellum

PURPOSE OF REVIEW

The brainstem and cerebellum contain many neuronal types that play a critical role in eye movement control. In a physiological approach, understanding how these neuronal assemblies cooperate provides strong insight into general brain functions. Furthermore, eye movements provide an interesting model for understanding neural mechanisms of sensorimotor learning, and a knowledge of the mechanisms underlying oculomotor plasticity is essential for correctly diagnosing and effectively managing patients. Finally, knowledge of the ocular motor syndromes frequently helps localize the pathological abnormality.

RECENT FINDINGS

We review the recently published works dealing with the physiological organization and pathology of slow and rapid eye movements at a brainstem and cerebellar level.

SUMMARY

The main recent findings of great interest for clinical practice or research concern the physiopathology of head shaking nystagmus, downbeat nystagmus and oculopalatal tremor; the neural substrates of three-dimensional control of eye movements and of optokinetic nystagmus; the understanding of saccade generation and of its consequences on physiological and pathological eye oscillations; and, finally, the physiological basis of saccadic adaptation.