Signals used to compute errors in monkey vestibuloocular reflex: possible role of flocculus

1984 ◽  
Vol 52 (6) ◽  
pp. 1140-1153 ◽  
Author(s):  
S. G. Lisberger ◽  
F. A. Miles ◽  
D. S. Zee
Keyword(s):  
Eye Movement ◽  
Visual Stimulation ◽  
Species Difference ◽  
Head Movements ◽  
Vestibuloocular Reflex ◽  
Full Field ◽  
Adaptive Changes ◽  
Ventral Paraflocculus ◽  
Sinusoidal Optokinetic Stimulation ◽  
Eye Velocity

Adaptive changes were induced in the vestibuloocular reflex (VOR) of monkeys by oscillating them while they viewed the visual scene through optical devices (“spectacles”) that required changes in the amplitude of eye movement during head turns. The “gain” of the VOR (eye velocity divided by head velocity) during sinusoidal oscillation in darkness underwent gradual changes that were appropriate to reduce the motion of images on the retina during the adapting procedures. Bilateral ablation of the flocculus and ventral paraflocculus caused a complete and enduring loss of the ability to undergo adaptive changes in the VOR. Partial lesions caused a substantial but incomplete loss of the adaptive capability. We conclude that the flocculus is necessary for adaptive changes in the monkey's VOR. Further experiments in normal animals determined the types of stimuli that were necessary and/or sufficient to cause changes in VOR gain. Full-field visual stimulation was not necessary to induce adaptive changes in the VOR. Monkeys tracked a small spot in conditions that elicited the same combination of eye and head movements seen during passive oscillation with spectacles. The gain of the VOR showed changes 50-70% as large as those produced by the same duration of oscillation with spectacles. Since the effective tracking conditions cause a consistent correlation of floccular output with vestibular inputs, these data are compatible with our previous suggestion that the flocculus may provide signals used by the central nervous system to compute errors in the gain of the VOR. Prolonged sinusoidal optokinetic stimulation with the head stationary caused only a slight increase in VOR gain. Left-right reversal of vision and eye movement during sinusoidal vestibular oscillation caused decreases in VOR gain. In rabbits, both of these stimulus conditions produced large increases in the gain of the VOR, which implied that eye velocity signals were used instead of vestibular inputs to compute errors in the VOR. Our different results argue that vestibular signals are necessary for computing errors in VOR gain in the monkey. The species difference may reflect the additional role that smooth pursuit eye movements play in stabilizing gaze during head turns in monkeys.

Neural basis for motor learning in the vestibuloocular reflex of primates. III. Computational and behavioral analysis of the sites of learning

1994 ◽  
Vol 72 (2) ◽  
pp. 974-998 ◽  
Author(s):  
S. G. Lisberger
Keyword(s):  
Motor Learning ◽  
Eye Movement ◽  
Computer Model ◽  
Visual Motion ◽  
Transfer Functions ◽  
Vestibular Nucleus ◽  
Vestibuloocular Reflex ◽  
Neural Basis ◽  
Ventral Paraflocculus ◽  
Eye Velocity

1. We have used a combination of eye movement recordings and computer modeling to study long-term adaptive modification (motor learning) in the vestibuloocular reflex (VOR). The eye movement recordings place constraints on possible sites for motor learning. The computer model abides by these constraints, as well as constraints provided by data in previous papers, to formalize a new hypothesis about the sites of motor learning. The model was designed to reproduce as much of the existing neural and behavioral data as possible. 2. Motor learning was induced in monkeys by fitting them with spectacles that caused the gain of the VOR (eye speed divided by head speed) to increase to values > 1.6 or to decrease to values < 0.4. We elicited pursuit by providing ramp motion of a small target at 30 degrees/s along the horizontal axis. Changes in the gain of the VOR caused only small and inconsistent changes in the eye acceleration in the first 100 ms after the onset of pursuit and had no effect on the eye velocity during tracking of steady target motion. Electrical stimulation in the flocculus and ventral paraflocculus with single pulses or trains of pulses caused smooth eye movement toward the side of stimulation after latencies of 9–11 ms. Neither the latency, the peak eye velocity, nor the initial eye acceleration varied as a consistent function of the gain of the VOR. 3. The computer model contained nodes that represented position-vestibular-pause cells (PVP-cells) and flocculus target neurons (FTNs) in the vestibular nucleus, and horizontal gaze-velocity Purkinje cells (HGVP-cells) in the cerebellar flocculus and ventral paraflocculus. Node FTN represented only the “E-c FTNs,” which show increased firing for eye motion away from the side of recording. The transfer functions in the model included dynamic elements (filters) as well as static elements (summing junctions, gain elements, and time delays). Except for the transfer functions that converted visual motion inputs into commands for smooth eye movement, the model was linear. 4. The performance of the model was determined both by computer simulation and, for the VOR in the dark, by analytic solution of linear equations. For simulation, we adjusted the parameters by hand to match the output of the model to the eye velocity of monkeys and to match the activity of the relevant nodes in the model to the firing of HGVP-cells, FTNs, and PVP-cells when the gain of the VOR was 0.4, 1.0, and 1.6.(ABSTRACT TRUNCATED AT 400 WORDS)

Context Compensation in the Vestibuloocular Reflex During Active Head Rotations

2000 ◽  
Vol 84 (6) ◽  
pp. 2904-2917 ◽  
Author(s):  
W. P. Medendorp ◽  
J.A.M. Van Gisbergen ◽  
S. Van Pelt ◽  
C.C.A.M. Gielen
Keyword(s):  
Human Subjects ◽  
Linear Regression Analysis ◽  
Head Movements ◽  
Target Distance ◽  
General Context ◽  
Vestibuloocular Reflex ◽  
Gaze Stabilization ◽  
Retinal Slip ◽  
Eye Velocity ◽  
Head Rotations

The vestibuloocular reflex (VOR) needs to modulate its gain depending on target distance to prevent retinal slip during head movements. We investigated gain modulation (context compensation) for binocular gaze stabilization in human subjects during voluntary yaw and pitch head rotations. Movements of each eye were recorded, both when attempting to maintain gaze on a small visual target at straight-ahead in a darkened room and after its disappearance (remembered target). In the analysis, we relied on a binocular coordinate system yielding a version and a vergence component. We examined how frequency and target distance, approached here by using vergence angle, affected the gain and phase of the version component of the VOR and compared the results to the requirements for ideal performance. Linear regression analysis on the version gain-vergence relationship yielded a slope representing the influence of target proximity and an intercept corresponding to the response at zero vergence (“default gain”). The slope of the fitted relationship, divided by the geometrically required slope, provided a measure for the quality of version context compensation (“context gain”). In both yaw and pitch experiments, we found default version gains close to one even for the remembered target condition, indicating that the active VOR for far targets is already close to ideal without visual support. In near target experiments, the presence of visual feedback yielded near unity context gains, indicating close to optimal performance (retinal slip <0.4°/s). For remembered targets, the context gain deteriorated but was still superior to performance in corresponding passive studies reported in the literature. In general, context compensation in the remembered target paradigm was better for vertical than for horizontal head rotations. The phase delay of version eye velocity relative to head velocity was small (∼2°) for both horizontal and vertical head movements. Analysis of the vergence data from the near target experiments showed that context compensation took into account that the two eyes require slightly different VORs. In thediscussion, comparison of the present default VOR gains and context gains with data from earlier passive studies has led us to propose a limited role for efference copies during self-generated movements. We also discuss how our analysis can provide a framework for evaluating two different hypotheses for the generation of binocular VOR eye movements.

Eye Movements in Cerebellar and Combined Cerebellobrainstem Diseases

1983 ◽  
Vol 92 (2) ◽  
pp. 165-171 ◽  
Author(s):  
Carsten Wennmo ◽  
Bengt Hindfelt ◽  
Ilmari Pyykkö
Keyword(s):  
Quantitative Analysis ◽  
Eye Movements ◽  
Eye Movement ◽  
Smooth Pursuit ◽  
Optokinetic Nystagmus ◽  
Vestibular Nystagmus ◽  
Full Field ◽  
Eye Velocity ◽  
Voluntary Saccades ◽  
Cerebellar Disorders

We report a quantitative analysis of eye movement disturbances in patients with isolated cerebellar disorders and patients with cerebellar disorders and concomitant brainstem involvement. The most characteristic abnormalities in the exclusively cerebellar patients were increased velocities of the slow phases of vestibular nystagmus induced by rotation in the dark and increased peak velocities of the fast phases of optokinetic nystagmus induced by full-field optokinetic stimuli. Dysmetria of saccades was found in three of six cerebellar patients and gaze nystagmus in all six patients. The typical findings in the combined cerebellobrainstem group were reduced peak velocities of voluntary saccades, defective smooth pursuit and reduced peak velocities of the fast component of nystagmus during rotation in both the dark and light. All patients with combined cerebellobrainstem disorder had dysmetric voluntary saccades and gaze nystagmus. The numbers of superimposed saccades during smooth pursuit were uniformly increased. Release of inhibition in cerebellar disorders may explain the hyperresponsiveness and inaccuracy of eye movements found in this study. In addition, when lesions also involve the brainstem, however, integrative centers coding eye velocity are affected, leading to slow and inaccurate eye movements. These features elicited clinically may be useful in the diagnosis of cerebellar and brainstem disorders.

Eye movement responses to active, high-frequency pitch and yaw head rotations in subjects with unilateral vestibular loss or posterior semicircular canal occlusion

2003 ◽  
Vol 13 (2-3) ◽  
pp. 131-141 ◽  
Author(s):  
Claire C. Gianna-Poulin ◽  
Valerie Stallings ◽  
F. Owen Black
Keyword(s):  
Eye Movement ◽  
Head Rotation ◽  
Normal Subjects ◽  
Head Movements ◽  
Fixed Target ◽  
Posterior Semicircular Canal ◽  
Vestibular Loss ◽  
Semicircular Canal Occlusion ◽  
Eye Velocity ◽  
Head Rotations

This study assessed the eye movement responses to active head rotation in six subjects with complete unilateral vestibular loss (UVL), five subjects with posterior canal plugging (PCP) and age- and sex-matched normal subjects. Subjects performed head rotations in the pitch and yaw planes at frequencies ranging from 2 to 6 Hz, while looking at an earth-fixed target. Vertical eye movement gains obtained in UVL, PCP and normal subjects were not significantly different. Vertical phases decreased with increasing head movement frequencies in both UVL and PCP subjects. Although this decrease produced significantly different vertical phases between UVL and normal subjects for head movements above 3.9 Hz, vertical phases in some normal subjects were similar to those obtained in UVL subjects. We conclude that active head oscillations in the pitch plane are not clinically useful for the detection of vertical canal impairment limited to one ear. As expected, UVL subjects showed reduced horizontal gains, and eye velocity asymmetries during active head rotation in the yaw plane. Results in some PCP subjects suggested possible minor impairments of horizontal vestibulo-ocular reflexes.

Vestibuloocular Reflex Signal Modulation During Voluntary and Passive Head Movements

2002 ◽  
Vol 87 (5) ◽  
pp. 2337-2357 ◽  
Author(s):  
Jefferson E. Roy ◽  
Kathleen E. Cullen
Keyword(s):  
Eye Movement ◽  
Head Movements ◽  
Body Motion ◽  
Whole Body ◽  
Type I ◽  
Vestibuloocular Reflex ◽  
Body Rotation ◽  
Gaze Shifts ◽  
Head Velocity ◽  
Wide Range

The vestibuloocular reflex (VOR) effectively stabilizes the visual world on the retina over the wide range of head movements generated during daily activities by producing an eye movement of equal and opposite amplitude to the motion of the head. Although an intact VOR is essential for stabilizing gaze during walking and running, it can be counterproductive during certain voluntary behaviors. For example, primates use rapid coordinated movements of the eyes and head (gaze shifts) to redirect the visual axis from one target of interest to another. During these self-generated head movements, a fully functional VOR would generate an eye-movement command in the direction opposite to that of the intended shift in gaze. Here, we have investigated how the VOR pathways process vestibular information across a wide range of behaviors in which head movements were either externally applied and/or self-generated and in which the gaze goal was systematically varied (i.e., stabilize vs. redirect). VOR interneurons [i.e., type I position-vestibular-pause (PVP) neurons] were characterized during head-restrained passive whole-body rotation, passive head-on-body rotation, active eye-head gaze shifts, active eye-head gaze pursuit, self-generated whole-body motion, and active head-on-body motion made while the monkey was passively rotated. We found that regardless of the stimulation condition, type I PVP neuron responses to head motion were comparable whenever the monkey stabilized its gaze. In contrast, whenever the monkey redirected its gaze, type I PVP neurons were significantly less responsive to head velocity. We also performed a comparable analysis of type II PVP neurons, which are likely to contribute indirectly to the VOR, and found that they generally behaved in a quantitatively similar manner. Thus our findings support the hypothesis that the activity of the VOR pathways is reduced “on-line” whenever the current behavioral goal is to redirect gaze. By characterizing neuronal responses during a variety of experimental conditions, we were also able to determine which inputs contribute to the differential processing of head-velocity information by PVP neurons. We show that neither neck proprioceptive inputs, an efference copy of neck motor commands nor the monkey's knowledge of its self-motion influence the activity of PVP neurons per se. Rather we propose that efference copies of oculomotor/gaze commands are responsible for the behaviorally dependent modulation of PVP neurons (and by extension for modulation of the status of the VOR) during gaze redirection.

Characterization and adaptive modification of the goldfish vestibuloocular reflex by sinusoidal and velocity step vestibular stimulation

1992 ◽  
Vol 68 (6) ◽  
pp. 2003-2015 ◽  
Author(s):  
A. M. Pastor ◽  
R. R. de la Cruz ◽  
R. Baker
Keyword(s):  
Electrical Stimulation ◽  
Eye Movement ◽  
Constant Velocity ◽  
Directional Asymmetry ◽  
Vestibuloocular Reflex ◽  
Dynamic Component ◽  
Head Velocity ◽  
Vor Adaptation ◽  
Eye Velocity ◽  
Stimulation Of

1. The normal and adapted vestibuloocular reflex (VOR) of goldfish was characterized by means of sinusoidal, velocity step, and position step head rotations about the vertical axis. VOR adaptation was induced by short-term, 1- to 4-h, presentation of visual and vestibular stimuli that altered the ratio of eye to head velocity. 2. The VOR response measured with sinusoidal oscillations in the dark was close to ideal compensatory values over 2 decades (1/32-2 Hz). Gain approximated unity, and phase, in relation to the head, was nearly 180 degrees. The VOR was linear within the range of head velocity tested (4-64 degrees/s). 3. Head velocity steps from 1/8 to 1 Hz produced steplike eye velocity profiles that could be divided into an early acceleration-related "dynamic" component and a later constant-velocity "sustained" period frequently separated by a sag at approximately 0.1-0.15 s from the initiation of eye movement. The sustained response exhibited no decay during the constant-velocity component of the step. 4. Higher temporal resolution of the dynamic response showed the adducting eye movement to have a shorter latency, faster rise time, and larger peak gain than the abducting eye movement. The characteristics of this directional asymmetry were similar for position steps and electrical stimulation of the vestibular nerve. However, the asymmetry was not observed during sinusoidal head rotation, the sustained component of the step response, or after electrical stimulation of the VIth and IIIrd nerves. We conclude that this directional asymmetry is of central origin and may be largely due to the parallel vestibular and abducens internuclear neuron pathways onto medial rectus motoneurons. 5. The VOR adaptation process for both higher and lower eye velocity exhibited an exponential time course with time constants of 55 and 45 min, respectively. After continuous sinusoidal training for 4 h, VOR gain reached an asymptotic level 5% away from perfect suppression in the low-gain training, but 19% away from the actual performance in the high-gain paradigm. The time constant for VOR gain reversal was 5 h, and an asymptotic level 40% less than performance was reached within 10 h. 6. Adapted VOR gain was symmetrical for both directions of eye movement measured either during sinusoidal rotation or the sustained part of the velocity step. VOR adaptation also produced a comparable gain change in the nasal and temporal directions of the dynamic component, but this reflected the asymmetric characteristics observed in the preadapted condition.(ABSTRACT TRUNCATED AT 400 WORDS)

A review of the effects of space flight on the asymmetry of vertical optokinetic and vestibulo-ocular reflexes

2003 ◽  
Vol 13 (4-6) ◽  
pp. 255-263
Author(s):  
Gilles Clément
Keyword(s):  
Phase Velocity ◽  
Optokinetic Nystagmus ◽  
Visual Stimulation ◽  
Head Movements ◽  
Slow Phase ◽  
Eye Position ◽  
Slow Phase Velocity ◽  
Ocular Reflex ◽  
Vestibulo Ocular Reflex ◽  
Eye Velocity

Prolonged microgravity during orbital flight is a unique way to modify the otolith inputs and to determine the extent of their contribution to the vertical vestibulo-ocular reflex (VOR) and optokinetic nystagmus (OKN). This paper reviews the data collected on 10 astronauts during several space missions and focuses on the changes in the up-down asymmetry. Both the OKN elicited by vertical visual stimulation and the active VOR elicited by voluntary pitch head movements showed an asymmetry before flight, with upward slow phase velocity higher than downward slow phase velocity. Early in-flight, this asymmetry was inverted, and a symmetry of both responses was later observed. An upward shift in the vertical mean eye position in both OKN and VOR suggests that these effects may be related to otolith-dependent changes in eye position which, in themselves, affect slow phase eye velocity.

Vertical vestibuloocular reflex in cat: asymmetry and adaptation

1988 ◽  
Vol 59 (2) ◽  
pp. 279-298 ◽  
Author(s):  
L. H. Snyder ◽  
W. M. King
Keyword(s):  
Eye Movement ◽  
Head Movement ◽  
Time Course ◽  
Percentage Change ◽  
Slow Phase ◽  
High Temporal Resolution ◽  
Vestibuloocular Reflex ◽  
Movement Trajectory ◽  
Head Velocity ◽  
Eye Velocity

1. We studied eye velocity during the first 2 s of the vertical vestibuloocular reflex (VOR) elicited from cats placed on their sides (90 degrees roll position) and rotated about an earth vertical axis. Vestibular stimuli were presented in the dark and consisted of brief trapezoidal velocity profiles. Eye movements were recorded with a magnetic search coil, and eye velocity was analyzed with high temporal resolution. 2. The first 2 s of upward or downward eye velocity after the onset of head rotation was characterized and compared. Adaptive changes in VOR gain (eye/head velocity) were then induced, and upward and downward eye velocity responses were again compared. 3. The early time course of the vertical VOR was complex. After a latency of approximately 15 ms, eye velocity increased rapidly until it was equal in magnitude and opposite in direction to head velocity. The peak eye velocity decayed within less than 1 s to a plateau of slow-phase eye velocity (SPEV) equal to approximately -0.6 times the head velocity. Peak upward and downward eye velocity was symmetric. The transition from peak to plateau was more rapid for the downward VOR (slow phases downward) than for the upward VOR (slow phases upward). The plateau attained by upward SPEV was approximately 15% higher than the plateau attained by downward SPEV. 4. VOR gain adaptation was symmetric. The percentage change in adapted upward eye velocity equalled the percentage change in adapted downward eye velocity. Both peak and plateau SPEV adapted, but peak eye velocity adapted less than plateau eye velocity. VOR latency was unchanged by adaptation. 5. The trajectory of the VOR response to steps of head velocity could be divided into an invariant and a variant interval. The invariant interval consisted of the initial approximately 15 ms of the eye movement. Neither direction of head movement (upward vs. downward) nor adaptation of the VOR gain effected the eye movement trajectory during the invariant interval. The variant interval began approximately 30 ms after the onset of head movement and approximately 15 ms after the onset of eye movement. In unadapted animals, downward eye speed exceeded upward eye speed during the variant interval. In adapted animals, eye speed during the variant interval, but not during the invariant interval, diverged from eye speed in the unadapted state. We suggest that the initial invariant interval (approximately 15 ms) of the eye movement response trajectory may represent the direct response of the classically described three-neuron arc.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of motor learning in the response of the primate vestibuloocular reflex pathway to electrical stimulation

1992 ◽  
Vol 67 (6) ◽  
pp. 1493-1508 ◽  
Author(s):  
D. M. Broussard ◽  
H. M. Bronte-Stewart ◽  
S. G. Lisberger
Keyword(s):  
Electrical Stimulation ◽  
Eye Movements ◽  
Motor Learning ◽  
Eye Movement ◽  
Image Motion ◽  
Vestibuloocular Reflex ◽  
Horizontal Canal ◽  
Vestibular Afferents ◽  
Eye Velocity ◽  
Stimulation Of

1. The vestibuloocular reflex (VOR) undergoes long-term adaptive changes in the presence of persistent retinal image motion during head turns. Previous experiments using natural stimuli have provided evidence that the VOR is subserved by parallel pathways, including some that are modified during learning and some that are not. We have used electrical stimulation of the vestibular labyrinth to investigate the temporal properties of the signals that are transmitted through the modified pathways. 2. Electrodes were implanted chronically in the superior semi-circular canal, the horizontal canal, or the vestibule for electrical activation of the vestibular afferents. Learning was induced by fitting the monkeys with spectacles that magnified or miniaturized vision. Before, during, and after motor learning, we measured the eye movements evoked by electrical stimulation of the labyrinth as well as the gain of the VOR, defined as eye speed divided by head speed during natural vestibular stimulation in the dark. 3. Trains of pulses applied to the labyrinth caused the eyes to move away from the side of stimulation with an initial rapid change in eye velocity followed by a steady-state plateau. Changes in the gain of the VOR caused large changes in the trajectory and magnitude of eye velocity during the plateau, showing that our stimulating electrodes had access to the modified pathways. 4. A single, brief current pulse applied to the labyrinth evoked an eye movement that had a latency of 5 ms and consisted of a pulse of eye velocity away from the side of the stimulation followed by a rebound toward the side of stimulation. To quantify the effect of motor learning on these eye movements, we pooled the data across different VOR gains and computed the slope of the relationship between eye velocity and VOR gain at each millisecond after the stimulus. We refer to the slope as the "modification index." 5. In comparison with the evoked eye velocity, the modification index took longer to return to baseline and showed a large peak at the time of the rebound in eye velocity. Increases in stimulus current increased both the amplitude and the duration of the modification index and revealed several later peaks. These observations suggest that the full expression of motor learning requires activation of multisynaptic pathways and recruitment of primary vestibular afferents with higher thresholds for electrical stimulation. 6. The modification index was almost always positive during the initial deflection in eye velocity, and the latency of the first change in the modification index was usually the same as the latency of the evoked eye movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuron activity in monkey vestibular nuclei during vertical vestibular stimulation and eye movements

1984 ◽  
Vol 52 (4) ◽  
pp. 724-742 ◽  
Author(s):  
M. C. Chubb ◽  
A. F. Fuchs ◽  
C. A. Scudder
Keyword(s):  
Eye Movements ◽  
Eye Movement ◽  
Unit Activity ◽  
Vestibular Nuclei ◽  
Vestibular Stimulation ◽  
Head Movements ◽  
Medial Longitudinal Fasciculus ◽  
Eye Position ◽  
Head Velocity ◽  
Eye Velocity

To elucidate how information is processed in the vestibuloocular reflex (VOR) pathways subserving vertical eye movements, extracellular single-unit recordings were obtained from the vestibular nuclei of alert monkeys trained to track a visual target with their eyes while undergoing sinusoidal pitch oscillations (0.2-1.0 Hz). Units with activity related to vertical vestibular stimulation and/or eye movements were classified as either vestibular units (n = 53), vestibular plus eye-position units (n = 30), pursuit units (n = 10), or miscellaneous units (n = 5), which had various combinations of head- and eye-movement sensitivities. Vestibular units discharged in relation to head rotation, but not to smooth eye movements. On average, these units fired approximately in phase with head velocity; however, a broad range of phase shifts was observed. The activities of 8% of the vestibular units were related to saccades. Vestibular plus eye-position units fired in relation to head velocity and eye position and, in addition, usually to eye velocity. Their discharge rates increased for eye and head movements in opposite directions. During combined head and eye movements, the modulation in unit activity was not significantly different from the sum of the modulations during each alone. For saccades, the unit firing rate either decreased to zero or was unaffected. Pursuit units discharged in relation to eye position, eye velocity, or both, but not to head movements alone. For saccades, unit activity usually either paused or was unaffected. The eye-movement-related activities of the vestibular plus eye-position and pursuit units were not significantly different. A quantitative comparison of their firing patterns suggests that vestibular, vestibular plus eye-position, and pursuit neurons in the vestibular nucleus could provide mossy fiber inputs to the flocculus. In addition, the vertical vestibular plus eye-position neurons have discharge patterns similar to those of fibers recorded rostrally in the medial longitudinal fasciculus. Therefore, our data support the view that vertical vestibular plus eye-position neurons are interneurons of the VOR.

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