Visual and Vestibular Integration Express Summative Eye Movement Responses and Reveal Higher Visual Acceleration Sensitivity than Previously Described

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.

Visually induced vertical vergence as a motion processing biomarker associated with postural instability

The present study explored visually induced vertical vergence (VIVV) as non-specific motion processing response. Healthy participants (7 male, mean age 28.57 ± 2.30; 9 female, mean age 27.67 ± 3.65) were exposed to optokinetic stimuli in an HTC VIVE virtual reality headset while VIVV, pupil-size, and postural sway was recorded. The methodology was shown to produce VIVV in the roll plane at 30 deg/s. Subsequent trials consisted of 40 s optokinetic motion in yaw, pitch, and roll directions at 60 deg/s, and radial optic flow; optokinetic directions were inverted after 20 s of motion. Median VIVV amplitude changes were normalized to the clockwise roll rotation, analysed, and correlated with changes in pupil-size and body sway. VIVV, pupil-size, and body sway were all affected by changes in optokinetic direction. Post-hoc analyses showed significant VIVV responses during optokinetic yaw and pitch rotations, as well as during radial optic flow stimulations. VIVV magnitudes were universally correlated with pupil-size and body sway. In conclusion, VIVV was expressed in all tested dimensions and may consequently serve as a visual motion processing biomarker. Failing to support binocularity while responding to optokinetic directionality, VIVV may reflect an eye-movement response associated with increased postural instability and stress, similar to a dorsal light reflex.

Temporal dynamics of ocular torsion and vertical vergence during visual, vestibular, and visuovestibular rotations

Ocular torsion and vertical divergence reflect the brain's sensorimotor integration of motion through the vestibulo-ocular reflex (VOR) and the optokinetic reflex (OKR) to roll rotations. Torsion and vergence however express different response patterns depending on several motion variables, but research on their temporal dynamics remains limited. This study investigated the onset times of ocular torsion (OT) and vertical vergence (VV) during visual, vestibular, and visuovestibular motion, as well as their relative decay rates following prolonged optokinetic stimulations. Temporal characteristics were retrieved from three separate investigations where the level of visual clutter and acceleration were controlled. Video eye-tracking was used to retrieve the eye-movement parameters from a total of 41 healthy participants across all trials. Ocular torsion consistently initiated earlier than vertical vergence, particularly evident under intensified visual information density, and higher clutter levels were associated with more balanced decay rates. Additionally, stimulation modality and accelerations affected the onsets of both eye movements, with visuovestibular motion triggering earlier responses compared to vestibular motion, and increased accelerations leading to earlier onsets for both movements. The present study showed that joint visuovestibular responses produced more rapid onsets, indicating a synergetic sensorimotor process. It also showed that visual content acted as a fusional force during the decay period, and imposed greater influence over the torsional onset compared to vergence. Acceleration, by contrast, did not affect the temporal relationship between the two eye movements. Altogether, these findings provide insights into the sensorimotor integration of the vestibulo-ocular and optokinetic reflex arcs.

Cervicocephalic kinaesthesia reveals novel subgroups of motor control impairments in patients with neck pain

Cervical-spine sensorimotor control is associated with chronicity and recurrence of neck pain (NP). Tests used to measure sensorimotor impairments lack consistency in studied parameters. Interpretation is often based on either a handful or numerous parameters, without considering their possible interrelation. Different aspects of motor-control could be studied with different parameters, but this has not yet been addressed. The aim of this study was to determine if different parameters of cervical position (JPE) and movement (Butterfly) sense tests represent distinct components of motor-control strategies in patients with chronic NP. Principal component analysis performed on 135 patients revealed three direction-specific (repositioning from flexion, extension or rotations) and one parameter-specific (variability of repositioning) component for JPE, two difficulty-specific (easy or medium and difficult trajectory) and one movement-specific (undershooting a target) component for Butterfly test. Here we report that these components could be related to central (neck repositioning and control of cervical movement) and peripheral sensorimotor adaptations (variability of repositioning) present in NP. New technologies allow extraction of greater number of parameters of which hand-picking could lead to information loss. This study adds towards better identification of diverse groups of parameters offering potentially clinically relevant information and improved functional diagnostics for patients with NP.

A Proposed Mechanism for Visual Vertigo: Post-Concussion Patients Have Higher Gain From Visual Input Into Subcortical Gaze Stabilization

Purpose

Post-concussion syndrome (PCS) is commonly associated with dizziness and visual motion sensitivity. This case-control study set out to explore altered motion processing in PCS by measuring gaze stabilization as a reflection of the capacity of the brain to integrate motion, and it aimed to uncover mechanisms of injury where invasive subcortical recordings are not feasible.

Methods

A total of 554 eye movements were analyzed in 10 PCS patients and nine healthy controls across 171 trials. Optokinetic and vestibulo-ocular reflexes were recorded using a head-mounted eye tracker while participants were exposed to visual, vestibular, and visuo-vestibular motion stimulations in the roll plane. Torsional and vergence eye movements were analyzed in terms of slow-phase velocities, gain, nystagmus frequency, and sensory-specific contributions toward gaze stabilization.

Results

Participants expressed eye-movement responses consistent with expected gaze stabilization; slow phases were fastest for visuo-vestibular trials and slowest for visual stimulations (P < 0.001) and increased with stimulus acceleration (P < 0.001). Concussed patients demonstrated increased gain from visual input to gaze stabilization (P = 0.005), faster slow phases (P = 0.013), earlier nystagmus beats (P = 0.003), and higher relative visual influence over the gaze-stabilizing response (P = 0.001), presenting robust effect sizes despite the limited population size.

Conclusions

The enhanced neural responsiveness to visual motion in PCS, combined with semi-intact visuo-vestibular integration, presented a subcortical hierarchy for altered gaze stabilization. Drawing on comparable animal trials, findings suggest that concussed patients may suffer from diffuse injuries to inhibiting pathways for optokinetic information, likely early in the visuo-vestibular hierarchy of sensorimotor integration. These findings offer context for common but elusive symptoms, presenting a neurological explanation for motion sensitivity and visual vertigo in PCS.

Eye Tracking in Optometry: A Systematic Review

This systematic review examines the use of eye-tracking devices in optometry, describing their main characteristics, areas of application and metrics used. Using the PRISMA method, a systematic search was performed of three databases. The search strategy identified 141 reports relevant to this topic, indicating the exponential growth over the past ten years of the use of eye trackers in optometry. Eye-tracking technology was applied in at least 12 areas of the field of optometry and rehabilitation, the main ones being optometric device technology, and the assessment, treatment, and analysis of ocular disorders. The main devices reported on were infrared light-based and had an image capture frequency of 60 Hz to 2000 Hz. The main metrics mentioned were fixations, saccadic movements, smooth pursuit, microsaccades, and pupil variables. Study quality was sometimes limited in that incomplete information was provided regarding the devices used, the study design, the methods used, participants' visual function and statistical treatment of data. While there is still a need for more research in this area, eye-tracking devices should be more actively incorporated as a useful tool with both clinical and research applications. This review highlights the robustness this technology offers to obtain objective information about a person's vision in terms of optometry and visual function, with implications for improving visual health services and our understanding of the vision process.

Concussed patients with visually induced dizziness exhibit increased ocular torsion and vertical vergence during optokinetic gaze-stabilization

Visually Induced Dizziness (VID) is a common post-concussion sequalae that remains poorly understood and difficult to quantify. The present study aims to identify biomarkers for VID in the form of gaze-stabilizing eye movements. Nine patients with post-commotio VID and nine age-matched healthy controls were recruited by physiotherapists at a local neurorehabilitation centre. Torsional and vergence eye movements were recorded while participants viewed a series of optokinetic rotations where the central- and peripheral regions moved coherently, incoherently, or semi-randomly. Results showed that vergence and torsional velocities were increased in VID patients, reflecting increased oculomotor gain to visual motion, and that responses correlated with symptom severity. Coherent stimulation produced fastest torsional slow-phases across all participants; when faced with confliction directional information, eye movements tended to follow the direction of the central visual field, albeit at slower velocities than during coherent motion, meaning that while torsion was sensitive to visual content of the entire visual field it expressed directional preference to the central stimulation. In conclusion, post-commotio VID was associated with faster slow-phases during optokinetic gaze-stabilization, with both vergence and torsion being correlated to symptom intensity. As torsional tracking remains inaccessible using commercial eye-trackers, vertical vergence may prove particularly accessible for clinical utility.

Conserved subcortical processing in visuo-vestibular gaze control

Gaze stabilization compensates for movements of the head or external environment to minimize image blurring. Multisensory information stabilizes the scene on the retina via the vestibulo-ocular (VOR) and optokinetic (OKR) reflexes. While the organization of neuronal circuits underlying VOR is well-described across vertebrates, less is known about the contribution and evolution of the OKR and the basic structures allowing visuo-vestibular integration. To analyze these neuronal pathways underlying visuo-vestibular integration, we developed a setup using a lamprey eye-brain-labyrinth preparation, which allowed coordinating electrophysiological recordings, vestibular stimulation with a moving platform, and visual stimulation via screens. Lampreys exhibit robust visuo-vestibular integration, with optokinetic information processed in the pretectum that can be downregulated from tectum. Visual and vestibular inputs are integrated at several subcortical levels. Additionally, saccades are present in the form of nystagmus. Thus, all basic components of the visuo-vestibular control of gaze were present already at the dawn of vertebrate evolution.

Here, the authors show that gaze stabilization relies on a visuo-vestibular network conserved from lamprey to primates. This primordial blueprint highlights how visual and vestibular streams are organized to control fundamental aspects of eye movements.

Alertness and Visual Attention Impact Different Aspects of the Optokinetic Reflex

Purpose

Assessing visual attention and alertness is of great importance in visual and cognitive neuroscience, providing objective measures valuable for both researchers and clinicians. This study investigates how the optokinetic response differs between levels of visual attention in healthy adults while controlling for alertness.

Methods

Twelve healthy subjects (8 men and 4 women; mean age = 33 ± 9.36) with intact gaze-stability, visual acuity, and binocularity were recruited. Subjects viewed a rotating visual scene provoking torsional optokinetic nystagmus (OKN) while wearing a video eye tracker in a seated head-fixed position. Tasks requiring focused, neutral, and divided visual attention were issued to each subject and the OKN was recorded. Pupil sizes were monitored as a proxy for alertness.

Results

Pupil dilation was increased for both focused and divided visual attention. The number of nystagmus beats was highest for the focused condition and lowest for the divided attentional task. OKN gain was increased during both focused and divided attention. The distribution of nystagmus beats over time showed that only focused attention produced a reliable adaptation of the OKN.

Conclusions

Results consequently indicate that OKN frequency is adaptive to a viewer's level of visual attention, whereas OKN gain is influenced by alertness levels. This pattern offers insight into the neural processes integrating visual input with reflexive motor responses. For example, it contextualizes why attention to visual stimuli can cause dizziness, as the OKN frequency reflects activity of the velocity storage mechanism. Additionally, the OKN could offer a possible venue for differentiating between visual attention and alertness during psychometric testing.

Optokinetic stimulation induces vertical vergence, possibly through a non-visual pathway

Vertical vergence is generally associated with one of three mechanisms: vestibular activation during a head tilt, induced by vertical visual disparity, or as a by-product of ocular torsion. However, vertical vergence can also be induced by seemingly unrelated visual conditions, such as optokinetic rotations. This study aims to investigate the effect of vision on this latter form of vertical vergence. Eight subjects (4m/4f) viewed a visual scene in head erect position in two different viewing conditions (monocular and binocular). The scene, containing white lines angled at 45° against a black background, was projected at an eye-screen distance of 2 m, and rotated 28° at an acceleration of 56°/s2. Eye movements were recorded using a Chronos Eye-Tracker, and eye occlusions were carried out by placing an infrared-translucent cover in front of the left eye during monocular viewing. Results revealed vergence amplitudes during binocular viewing to be significantly lower than those seen for monocular conditions (p = 0.003), while torsion remained unaffected. This indicates that vertical vergence to optokinetic stimulation, though visually induced, is visually suppressed during binocular viewing. Considering that vertical vergence is generally viewed as a vestibular signal, the findings may reflect a visually induced activation of a vestibular pathway.