Short-term learning of the vestibulo-ocular reflex induced by a custom interactive computer game.

Retinal image slip during head rotation drives motor learning in the rotational vestibulo-ocular reflex (VOR) and forms the basis of gaze-stability exercises that treat vestibular dysfunction. Clinical exercises, however, are unengaging, cannot easily be titrated to the level of impairment, and provide neither direct feedback nor tracking of the patient's adherence, performance, and progress. To address this, we have developed a custom application for VOR training based on an interactive computer game. In this study, we tested the ability of this game to induce VOR learning in individuals with normal vestibular function, and we compared the efficacy of single-step and incremental learning protocols. Eighteen participants played the game twice on different days. All participants tolerated the game and were able to complete both sessions. The game scenario incorporated a series of brief head rotations, similar to active head impulses, that were paired with a dynamic acuity task and with a visual-vestibular mismatch (VVM) intended to increase VOR gain (single-step: 300 successful trials at ×1.5 viewing; incremental: 100 trials each of ×1.13, ×1.33, and ×1.5 viewing). Overall, VOR gain increased by 15 ± 4.7% (mean ± 95% CI, P < 0.001). Gains increased similarly for active and passive head rotations, and, contrary to our hypothesis, there was little effect of the learning strategy. This study shows that an interactive computer game provides robust VOR training and has the potential to deliver effective, engaging, and trackable gaze-stability exercises to patients with a range of vestibular dysfunctions.NEW & NOTEWORTHY This study demonstrates the feasibility and efficacy of a customized computer game to induce motor learning in the high-frequency rotational vestibulo-ocular reflex. It provides a physiological basis for the deployment of this technology to clinical vestibular rehabilitation.

Covert Tracking to Immersive Stimuli in Traumatic Brain Injury Subjects With Disorders of Consciousness.

Eye tracking assessments are clinician dependent and can contribute to misclassification of coma. We investigated responsiveness to videos with and without audio in traumatic brain injury (TBI) subjects using video eye-tracking (VET). We recruited 20 healthy volunteers and 10 unresponsive TBI subjects. Clinicians were surveyed whether the subject was tracking on their bedside assessment. The Coma Recovery Scale-Revised (CRS-R) was also performed. Eye movements in response to three different 30-second videos with and without sound were recorded using VET. The videos consisted of moving characters (a dancer, a person skateboarding, and Spiderman). Tracking on VET was defined as visual fixation on the character and gaze movement in the same direction of the character on two separate occasions. Subjects were classified as "covert tracking" (tracking using VET only), "overt tracking" (VET and clinical exam by clinicians), and "no tracking". A k-nearest-neighbors model was also used to identify tracking computationally. Thalamocortical connectivity and structural integrity were evaluated with EEG and MRI. The ability to obey commands was evaluated at 6- and 12-month follow-up. The average age was 29 (± 17) years old. Three subjects demonstrated "covert tracking" (CRS-R of 6, 8, 7), two "overt tracking" (CRS-R 22, 11), and five subjects "no tracking" (CRS-R 8, 6, 5, 6, 7). Among the 84 tested trials in all subjects, 11 trials (13%) met the criteria for "covert tracking". Using the k-nearest approach, 14 trials (17%) were classified as "covert tracking". Subjects with "tracking" had higher thalamocortical connectivity, and had fewer structures injured in the eye-tracking network than those without tracking. At follow-up, 2 out of 3 "covert" and all "overt" subjects recovered consciousness versus only 2 subjects in the "no tracking" group. Immersive stimuli may serve as important objective tools to differentiate subtle tracking using VET.

Covert Tracking to Visual Stimuli in Comatose Patients With Traumatic Brain Injury.

OBJECTIVES: This study investigated video eye tracking (VET) in comatose patients with traumatic brain injury (TBI). METHODS: We recruited healthy participants and unresponsive patients with TBI. We surveyed the patients' clinicians on whether the patient was tracking and performed the Coma Recovery Scale-Revised (CRS-R). We recorded eye movements in response to motion of a finger, a face, a mirror, and an optokinetic stimulus using VET glasses. Patients were classified as covert tracking (tracking on VET alone) and overt tracking (VET and clinical examination). The ability to obey commands was evaluated at 6-month follow-up. RESULTS: We recruited 20 healthy participants and 10 patients with TBI. The use of VET was feasible in all participants and patients. Two patients demonstrated covert tracking (CRS-R of 6 and 8), 2 demonstrated overt tracking (CRS-R of 22 and 11), and 6 patients had no tracking (CRS-R of 8, 6, 5, 7, 6, and 7). Five of 56 (9%) tracking assessments were missed on clinical examination. All patients with tracking recovered consciousness at follow-up, whereas only 2 of 6 patients without tracking recovered at follow-up. DISCUSSION: VET is a feasible method to measure covert tracking. Future studies are needed to confirm the prognostic value of covert tracking.

Subthalamic deep brain stimulation affects heading perception in Parkinson's disease.

Parkinson's disease (PD) presents with visuospatial impairment and falls. It is critical to understand how subthalamic deep brain stimulation (STN DBS) modulates visuospatial perception. We hypothesized that DBS has different effects on visual and vestibular perception of linear motion (heading), a critical aspect of visuospatial navigation; and such effects are specific to modulated STN location. Two-alternative forced-choice experiments were performed in 14 PD patients with bilateral STN DBS and 19 age-matched healthy controls (HC) during passive en bloc linear motion and 3D optic-flow in immersive virtual reality measured vestibular and visual heading. Objective measure of perception with Weibull psychometric function revealed that PD has significantly lower accuracy [L: 60.71 (17.86)%, R: 74.82 (17.44)%] and higher thresholds [L: 16.68 (12.83), R: 10.09 (7.35)] during vestibular task in both directions compared to HC (p < 0.05). DBS significantly improved vestibular discrimination accuracy [81.40 (14.36)%] and threshold [4.12 (5.87), p < 0.05] in the rightward direction. There were no DBS effects on the slopes of vestibular psychometric curves. Visual heading perception was better than vestibular and it was comparable to HC. There was no significant effect of DBS on visual heading response accuracy or discrimination threshold (p > 0.05). Patient-specific DBS models revealed an association between change in vestibular heading perception and the modulation of the dorsal STN. In summary, DBS may have different effects on vestibular and visual heading perception in PD. These effects may manifest via dorsal STN putatively by its effects on the cerebellum.

Vestibular heading perception in Parkinson's disease.

Postural instability and falls are common causes of morbidity and mortality in the second most prevalent neurodegenerative condition, Parkinson's disease (PD). Poor understanding of balance dysfunction in PD has hampered the development of novel therapeutic measures for postural instability and balance dysfunction. We aimed to determine how the ability to perceive one's own linear motion in the absence of visual cues, i.e., vestibular heading, is affected in PD. We examined vestibular heading function using a two-alternative forced choice task performed on a six-degree-of-freedom motion platform. Sensitivity of the vestibular system to subtle variations in heading direction and systematic errors in accuracy of responses were assessed for each subject using a Gaussian cumulative distribution psychometric function. Compared to healthy subjects, PD presented with higher angular thresholds to detect vestibular heading direction. These results confirm the potential of our study to provide valuable insight to the vestibular system's role in spatial navigation deficits in PD.

A model-based study of internuclear ophthalmoparesis and ocular-motor fatigue in multiple sclerosis.

Internuclear ophthalmoparesis (INO) in multiple sclerosis (MS) is due to demyelination of the medial longitudinal fasciculus (MLF). INO is typically modeled as an increased peak-velocity and peak-acceleration ratio of abducting to adducting eye (pulse-size ratio, PSR). PSR can be affected by fatigue during prolonged ocular-motor tasks (ocular-motor fatigue). We propose that an important component of horizontal disconjugacy in INO is due to a delayed delivery of the saccadic pulse to the adducting eye (pulse-time delay, PTD). We expanded a control-system model to account for both abnormal PSR and PTD reflecting faulty axonal transmission in INO and to provide a better understanding of possible changes induced by fatigue. Saccades were measured in 19 MS patients with INO and 10 controls, using a 10-min saccadic "fatigue test" consisting of repetitive back-to-back 20° saccades. In the horizontal saccades model the unitary MLF connection was partitioned into parallel sub-tracts representing progressive degrees of disease effect. INO patients showed baseline abnormal PSR and PTD with some changes during the fatigue test. Manipulations of gain and transmission delay in the model provided simulated saccades that closely resembled those of INO. Ocular-motor fatigue may be a heterogeneous phenomenon that involves inter-saccadic fluctuation of PSR and PTD and adaptation during demanding ocular-motor tasks. INO as a model of abnormal axonal conduction has a potential role in assessing efficacy of reparative therapies in MS.

Practice guideline: Cervical and ocular vestibular evoked myogenic potential testing: Report of the Guideline Development, Dissemination, and Implementation Subcommittee of the American Academy of Neurology.

OBJECTIVE: To systematically review the evidence and make recommendations with regard to diagnostic utility of cervical and ocular vestibular evoked myogenic potentials (cVEMP and oVEMP, respectively). Four questions were asked: Does cVEMP accurately identify superior canal dehiscence syndrome (SCDS)? Does oVEMP accurately identify SCDS? For suspected vestibular symptoms, does cVEMP/oVEMP accurately identify vestibular dysfunction related to the saccule/utricle? For vestibular symptoms, does cVEMP/oVEMP accurately and substantively aid diagnosis of any specific vestibular disorder besides SCDS? METHODS: The guideline panel identified and classified relevant published studies (January 1980-December 2016) according to the 2004 American Academy of Neurology process. RESULTS AND RECOMMENDATIONS: Level C positive: Clinicians may use cVEMP stimulus threshold values to distinguish SCDS from controls (2 Class III studies) (sensitivity 86%-91%, specificity 90%-96%). Corrected cVEMP amplitude may be used to distinguish SCDS from controls (2 Class III studies) (sensitivity 100%, specificity 93%). Clinicians may use oVEMP amplitude to distinguish SCDS from normal controls (3 Class III studies) (sensitivity 77%-100%, specificity 98%-100%). oVEMP threshold may be used to aid in distinguishing SCDS from controls (3 Class III studies) (sensitivity 70%-100%, specificity 77%-100%). Level U: Evidence is insufficient to determine whether cVEMP and oVEMP can accurately identify vestibular function specifically related to the saccule/utricle, or whether cVEMP or oVEMP is useful in diagnosing vestibular neuritis or Ménière disease. Level C negative: It has not been demonstrated that cVEMP substantively aids in diagnosing benign paroxysmal positional vertigo, or that cVEMP or oVEMP aids in diagnosing/managing vestibular migraine.

Vertical head translation impairs dynamic visual acuity during near viewing.

Dynamic visual acuity is an important clinical tool for assessment of the rotational vestibulo-ocular reflex (rVOR). It is based on the fact that the normal rVOR stabilizes vision and maintains visual acuity during head rotation. The translational VOR (tVOR) generates eye movements during linear head motion. Studies in humans have shown that gaze stabilization during translation is incomplete and that there is a strong effect of the visual environment: eye velocity is much greater in the light than in the dark. In this study, we measured visual acuity during vertical translation in 11 subjects and asked whether a more complex visual background would enhance the response and improve acuity. During 2 Hz whole-body translation, tumbling-E optotypes (0.0-0.9 logMAR in steps of 0.1 logMAR, six trials of each size randomly ordered) were flashed on a screen that was 30 cm in front of the subject's eyes. The subject reported the optotype's orientation with a joystick. Based on a threshold of 75% trials correctly identified, the group dynamic acuity was 0.72 logMAR, compared to a static acuity of 0.0 logMAR. When the background was enhanced with a stationary dot pattern, dynamic acuity improved to 0.42 logMAR. Our findings show that vertical head translation degrades vision more than head rotation. This may limit the use of dynamic acuity as a clinical measure of otolith function.

Static and dynamic postural stability in veterans with combat-related mild traumatic brain injury.

Persistent post-concussive symptoms are reported by 10-15% of individuals who suffer mild traumatic brain injury (mTBI), but their basis is often uncertain. One such symptom is disequilibrium, a sensation of impaired balance during standing and walking. The hypothesis for this study was that this subjective symptom is associated with objective and measurable deficits in static and dynamic postural stability. An infrared motion tracking system was used to record body motion during quiet standing and in response to waist perturbations in fourteen veterans (age 22-40 years, 13 male) of the Operations Enduring Freedom (OEF) and Iraqi Freedom (OIF), who had a history of mTBI that occurred 7 months to 7 years prior to testing. We compared body sway between veterans with mTBI reporting persistent disequilibrium (TD, n=8) and those with no vestibular symptoms (n=6), as well as to a group of non-veterans with no balance symptoms (n=10). Static postural stability was reduced in TD veterans in comparison to each of the other two groups (p<0.0002), most notably on a compliant surface with eyes closed. The TD group also had decreased dynamic stability of the upper trunk (p<0.05) and enhanced postural oscillations (p<0.02) following waist perturbations. Our findings support a physiological basis for persistent disequilibrium after mTBI and are consistent with impaired vestibular processing. Disruption of semicircular canal inputs is unlikely to be the cause, as head impulse responses were normal in all groups. The unexpected finding of dynamic postural oscillations requires further study but may indicate enhanced instability in sensorimotor networks responsible for postural control.

Cross-coupled eye movement supports neural origin of pattern strabismus.

PURPOSE: Pattern strabismus describes vertically incomitant horizontal strabismus. Conventional theories emphasized the role of orbital etiologies, such as abnormal fundus torsion and misaligned orbital pulleys as a cause of the pattern strabismus. Experiments in animal models, however, suggested the role of abnormal cross-connections between the neural circuits. We quantitatively assessed eye movements in patients with pattern strabismus with a goal to delineate the role of neural circuits versus orbital etiologies. METHODS: We measured saccadic eye movements with high-precision video-oculography in 14 subjects with pattern strabismus, 5 with comitant strabismus, and 15 healthy controls. We assessed change in eye position in the direction orthogonal to that of the desired eye movement (cross-coupled responses). We used fundus photography to quantify the fundus torsion. RESULTS: We found cross-coupling of saccades in all patients with pattern strabismus. The cross-coupled responses were in the same direction in both eyes, but larger in the nonviewing eye. All patients had clinically apparent inferior oblique overaction with abnormal excylotorsion. There was no correlation between the amount of the fundus torsion or the grade of oblique overaction and the severity of cross-coupling. The disconjugacy in the saccade direction and amplitude in pattern strabismics did not have characteristics predicted by clinically apparent inferior oblique overaction. CONCLUSIONS: Our results validated primate models of pattern strabismus in human patients. We found no correlation between ocular torsion or oblique overaction and cross-coupling. Therefore, we could not ascribe cross-coupling exclusively to the orbital etiology. Patients with pattern strabismus could have abnormalities in the saccade generators.

Post-traumatic headaches.

Chronic pain, especially headache, is an exceedingly common complication of traumatic brain injury (TBI). In fact, paradoxically, the milder the TBI, the more likely one is to develop headaches. The environment of injury and the associated comorbidities can have a significant impact on the frequency and severity of headaches and commonly serve to direct management of the headaches. Trauma likely contributes to the development of headaches via alterations in neuronal signaling, inflammation, and musculoskeletal changes. The clinical picture of the patient with post-traumatic headaches is often that of a mixed headache disorder with features of tension-type headaches as well as migrainous headaches. Treatment of these headaches is thus often guided by the predominant characteristics of the headaches and can include pharmacologic and nonpharmacologic strategies. Pharmacologic therapies include both abortive and prophylactic agents with prophylaxis targeting comorbidities, primarily impaired sleep. Nonpharmacologic interventions for post-traumatic headaches include thermal and physical modalities as well as cognitive behavioral approaches. As with many postconcussive symptoms, headaches can lessen with time but in up to 25% of patients, chronic headaches are long-term residua.

Behavior of the human translational vestibulo-ocular reflex during simultaneous head translation and rotation.

BACKGROUND: During head translations, vestibular eye movements are ∼ 60% of those required to hold the line of sight on target but, during translation of the orbits due to head rotation about an eccentric axis, the eyes are held %eye on target. OBJECTIVE: To resolve this paradoxical behavior of vestibulo-ocular reflexes. METHODS: Subjects sat on a moving platform viewing a near target and were: (1) rotated en bloc in yaw about a vertical axis centered on the head at 1 Hz; (2) rotated with their head displaced ∼ 10 cm anterior (eccentric rotation) at 1 Hz; (3) translated along the inter-aural axis at 1.9 Hz; (4) rotated with the head centered at 1 Hz while they were translated along the inter-aural axis at 1.9 Hz. We calculated compensation ratio (CR): Eye velocity/eye velocity geometrically required to hold the eye on target. RESULTS: During yaw, mean CR was 0.88 and during eccentric rotation CR was 0.93. During translation at 1.9 Hz, CR was 0.65. During combined rotation at 1.0 Hz and translation at 1.9 Hz, CR was 0.81 for head rotations and 0.74 for head translations. CONCLUSIONS: Translations of the orbits due to head rotation are better compensated for than translations of the orbits due to head translation. These different behaviors may be determined by context, the important difference being whether the subject is moving through the environment.

Amplitude and frequency prediction in the translational vestibulo-ocular reflex.

The goal of this study was to assess the effect of amplitude and frequency predictability on the performance of the translational vestibulo-ocular reflex (tVOR). Eye movements were recorded in 5 subjects during continuous vertical translation that consisted of a series of segments with: 1) 3 amplitudes at constant frequency (2 Hz) or 2) 3 different frequencies (1.6, 2, 2.5 Hz). Stimulus changes were presented in a pseudo-random order. We found that there was little change in the tVOR immediately after an unexpected stimulus change, as if eye velocity were being driven more by an expectation based on previous steady-state motion than by current head translation. For amplitude transitions, only about 30% of the eventual response change was seen in the first half cycle. Similarly, a sudden change in translation frequency did not appear in eye velocity for 70 ms, compared to a 8 ms lag during similar yaw rotation. Finally, after a sudden large decrease in frequency, the eyes continued to track at the original higher frequency, resulting initially in an anti-compensatory tVOR acceleration. Our results elucidate further the complexity of the tVOR and show that motion prediction based on prior experience plays an important role in its response.

Three-dimensional kinematics of saccadic and pursuit eye movements in humans: relationship between Donders' and Listing's laws.

For Listing's law to be obeyed during eye movements, the "half-angle rule" must be satisfied: the eye velocity axis must tilt away from Listing's plane by half the angle of eye position eccentricity from primary position. We aimed to determine if this rule is satisfied during horizontal and vertical pursuit compared with saccades. Three-dimensional (3-d) eye rotation data were acquired from five normal head-fixed humans using the search coil technique. Saccades were recorded in response to 40° horizontal or vertical steps in target position, at different elevations and azimuths. Pursuit was recorded while tracking a target moving horizontally or vertically at 20°/s, with peak-to-peak amplitude of 40°, at the same elevations and azimuths. First- and second-order surfaces were fitted to 3-d eye position data from periods of fixation. In all subjects, eye positions did not lie on a planar surface, but on a twisted surface in 3-d space. The tilt-angle coefficient (TAC) during saccades and pursuit was calculated as the ratio of the angle of eye velocity axis tilt to the angle of eye position eccentricity. During horizontal saccades and pursuit, mean TACs were 0.58 and 0.64, respectively. During vertical saccades and pursuit, mean TACs were 0.35 and 0.43, respectively, and lower than their horizontal counterparts (p<0.05). These findings suggest that Listing's law is not perfectly satisfied during saccades or pursuit. On the basis of model simulations, we propose that the discrepancy in horizontal and vertical TACs causes eye positions to lie on a twisted rather than a planar surface.

Three-dimensional kinematics of saccadic eye movements in humans: is the "half-angle rule" obeyed?

The "half-angle rule" must be satisfied for Listing's law to be obeyed during saccades: the eye velocity axis must tilt away from Listing's plane by half the angle of eye position eccentricity from primary position. We aimed to determine if this rule is satisfied during saccades. Horizontal and vertical saccades were recorded using the search coil technique, at different elevations and azimuths, in five normal humans. In all, eye positions were located on a twisted rather than planar surface. The mean tilt-angle coefficients (TACs; ratio of angle of eye velocity axis tilt to angle of eye position eccentricity) were 0.57 and 0.34 for horizontal and vertical saccades, respectively. TACs were significantly lower for vertical saccades. Thus, Listing's law is not perfectly obeyed during saccades. We suggest that the discrepancy in horizontal and vertical TACs causes eye positions to lie on a twisted rather than planar surface in three-dimensional space.

The cerebellar nodulus/uvula integrates otolith signals for the translational vestibulo-ocular reflex.

BACKGROUND: The otolith-driven translational vestibulo-ocular reflex (tVOR) generates compensatory eye movements to linear head accelerations. Studies in humans indicate that the cerebellum plays a critical role in the neural control of the tVOR, but little is known about mechanisms of this control or the functions of specific cerebellar structures. Here, we chose to investigate the contribution of the nodulus and uvula, which have been shown by prior studies to be involved in the processing of otolith signals in other contexts. METHODOLOGY/PRINCIPAL FINDINGS: We recorded eye movements in two rhesus monkeys during steps of linear motion along the interaural axis before and after surgical lesions of the cerebellar uvula and nodulus. The lesions strikingly reduced eye velocity during constant-velocity motion but had only a small effect on the response to initial head acceleration. We fit eye velocity to a linear combination of head acceleration and velocity and to a dynamic mathematical model of the tVOR that incorporated a specific integrator of head acceleration. Based on parameter optimization, the lesion decreased the gain of the pathway containing this new integrator by 62%. The component of eye velocity that depended directly on head acceleration changed little (gain decrease of 13%). In a final set of simulations, we compared our data to the predictions of previous models of the tVOR, none of which could account for our experimental findings. CONCLUSIONS/ SIGNIFICANCE: Our results provide new and important information regarding the neural control of the tVOR. Specifically, they point to a key role for the cerebellar nodulus and uvula in the mathematical integration of afferent linear head acceleration signals. This function is likely to be critical not only for the tVOR but also for the otolith-mediated reflexes that control posture and balance.

The linear vestibulo-ocular reflex, locomotion and falls in neurological disorders.

PURPOSE: During locomotion, head perturbations, consisting of rotations and translations (linear movements), occur with predominant frequencies of 0.5-5.0 Hz. The vestibular reflexes act at short latency to safeguard clear vision and stable posture during locomotion. Much is known about the angular vestibulo-ocular reflex (aVOR) in response to head rotations, which depend on the semicircular canals of the vestibular labyrinth. However, the means to test reliably the linear or translational vestibulo-ocular reflex (tVOR), which depends on the otolithic organs, has only become available more recently. METHODS: We used a moving platform to translate normal human subjects vertically at frequencies similar to those occurring during locomotion, under ambient illumination. RESULTS: Our findings suggested that, whereas aVOR is concerned with stabilizing images of visual targets on the retina to optimize visual acuity, tVOR seems best suited to minimize retinal image motion between objects lying in different depth planes, in order to optimize motion parallax information. We then asked whether the tVOR functioned abnormally in patients with two neurological disorders that often cause falls: progressive supranuclear palsy (PSP) and cerebellar ataxia. We found that patients with PSP cannot adjust tVOR responses appropriately during viewing of near objects, nor converge their eyes. Vestibular-evoked myogenic potentials (VEMPs), an otolith-spinal reflex, are also impaired in PSP patients. Patients with cerebellar ataxia also lack the ability to adjust tVOR for near viewing, even though they may be able to converge. CONCLUSIONS: Taken together, our studies suggest that abnormal otolithic vestibular reflexes contribute to postural instability in PSP and cerebellar ataxia, and deserve further investigation.

The human vertical translational vestibulo-ocular reflex. Normal and abnormal responses.

Geometric considerations indicate that the human translational vestibulo-ocular reflex (tVOR) should have substantially different properties than the angular vestibulo-ocular reflex (aVOR). Specifically, tVOR cannot simultaneously stabilize images of distant and near objects on the retina. Most studies make the tacit assumption that tVOR acts to stabilize foveal images even though, in humans, tVOR is reported to compensate for less than 60% of foveal image motion. We have determined that the compensation gain (eye rotational velocity/required eye rotational velocity to maintain foveal target fixation) of tVOR is held steady at approximately 0.6 during viewing of either near or distant targets during vertical (bob) translations in ambient illumination. We postulate that tVOR evolved not to stabilize the image of the target on the fovea, but rather to minimize retinal image motion between objects lying in different depth planes, in order to optimize motion parallax information. Such behavior is optimized when binocular visual cues of both near and distant targets are available in ambient light. Patients with progressive supranuclear palsy or cerebellar ataxia show impaired ability to increase tVOR responses appropriately when they view near targets. In cerebellar patients, impaired ability to adjust tVOR responses to viewing conditions occurs despite intact ability to converge at near. Loss of the ability to adjust tVOR according to viewing conditions appears to represent a distinct disorder of vestibular function.

Enhancement of the bias component of downbeat nystagmus after lesions of the nodulus and uvula.

In two monkeys, we recorded spontaneous eye movements before and after ablation of the cerebellar nodulus and uvula (Nod/Uv). In both monkeys, there was an increase in upward ocular drift (downbeat nystagmus [DBN]) in darkness (M1: 1.5 degrees/s pre, 3.4 degrees/s post; M2: 1.3 degrees/s pre, 7.0 degrees/s post), but not in light. There was little effect of orbital position on drift velocity. These findings suggest that the Nod/Uv may play a role in the bias component of DBN.