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.

Signals in vestibular nucleus mediating vertical eye movements in the monkey

1984 ◽  
Vol 51 (6) ◽  
pp. 1121-1136 ◽  
Author(s):  
R. D. Tomlinson ◽  
D. A. Robinson
Keyword(s):  
Eye Movements ◽  
Vestibular Nucleus ◽  
Vestibular Nuclei ◽  
Total Sample ◽  
Medial Longitudinal Fasciculus ◽  
Eye Position ◽  
Velocity Signal ◽  
Head Velocity ◽  
Position Signal ◽  
Eye Velocity

The action potentials of single neurons were recorded extracellularly throughout the rostral vestibular nuclei and subadjacent reticular formation in three alert, juvenile, rhesus monkeys. Neuronal responses were tested during a) sinusoidal pitch oscillations in darkness, b) cancellation of the vestibuloocular reflex (VOR) during similar oscillations by fixation of a target moving with the head, c) sinusoidal vertical smooth pursuit, d) vertical saccades, and e) fixation with the head stationary. Eye movements were measured using the magnetic field-search coil technique. Of the 527 neurons isolated, 318 responded to pitch oscillation and/or vertical eye movements. The latter cells could be classified into six categories. Of this group, 273 cells were recorded from for sufficient time to allow them to be fully tested and form the basis of this report. Cells were classified as follows: pure-vestibular cells with firing rates modulated only by head velocity (15%), vestibular-pause cells that were similar to the pure-vestibular cells but paused for saccades in all directions (10%), gaze-velocity cells that modulated their rates in proportion to vertical eye velocity in space (7%), position cells with rates modulated by changes in eye position in the head but that did not burst or pause during saccades (33%), position-burst cells that also carried an eye-position signal but did burst during saccades in one direction and paused in the opposite direction (15%), and position-vestibular-pause cells that carried signals proportional to eye position in the head and head velocity and paused during all saccades (20%). Most cells that carried an eye-position signal also carried an eye-velocity signal during pursuit. Position and position-burst cells could be divided into two subcategories. Position cells that also reported head velocity represented 20% of the total sample, while those without head-velocity signals made up the remaining 13%. Position-burst cells were divided into two subcategories based on their behavior during pitch oscillation in darkness. Both carried eye-velocity signals during pursuit, but only one type (8% of the total sample) also carried an eye-velocity signal during vestibular eye movements in the dark, while the other (7%) did not. Some cells in all six categories except the pure-vestibular cells responded antidromically to stimulation of the medial longitudinal fasciculus (MLF). Only the position-vestibular-pause, position-burst, and gaze-velocity cells, however, were judged to be commonly antidromically activated, suggesting that these three cell types are the major contributors to the MLF from the rostral vestibular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Discharge patterns in nucleus prepositus hypoglossi and adjacent medial vestibular nucleus during horizontal eye movement in behaving macaques

1992 ◽  
Vol 68 (1) ◽  
pp. 319-332 ◽  
Author(s):  
J. L. McFarland ◽  
A. F. Fuchs
Keyword(s):  
Eye Movements ◽  
Eye Movement ◽  
Smooth Pursuit ◽  
Vestibular Nucleus ◽  
Medial Vestibular Nucleus ◽  
Eye Position ◽  
Head Velocity ◽  
Firing Rates ◽  
Prepositus Hypoglossi ◽  
Eye Velocity

1. Monkeys were trained to perform a variety of horizontal eye tracking tasks designed to reveal possible eye movement and vestibular sensitivities of neurons in the medulla. To test eye movement sensitivity, we required stationary monkeys to track a small spot that moved horizontally. To test vestibular sensitivity, we rotated the monkeys about a vertical axis and required them to fixate a target rotating with them to suppress the vestibuloocular reflex (VOR). 2. All of the 100 units described in our study were recorded from regions of the medulla that were prominently labeled after injections of horseradish peroxidase into the abducens nucleus. These regions include the nucleus prepositus hypoglossi (NPH), the medial vestibular nucleus (MVN), and their common border (the “marginal zone”). We report here the activities of three different types of neurons recorded in these regions. 3. Two types responded only during eye movements per se. Their firing rates increased with eye position; 86% had ipsilateral “on” directions. Almost three quarters (73%) of these medullary neurons exhibited a burst-tonic discharge pattern that is qualitatively similar to that of abducens motoneurons. There were, however, quantitative differences in that these medullary burst-position neurons were less sensitive to eye position than were abducens motoneurons and often did not pause completely for saccades in the off direction. The burst of medullary burst position neurons preceded the saccade by an average of 7.6 +/- 1.7 (SD) ms and, on average, lasted the duration of the saccade. The number of spikes in the burst was well correlated with saccade size. The second type of eye movement neuron displayed either no discernible burst or an inconsistent one for on-direction saccades and will be referred to as medullary position neurons. Neither the burst-position nor the position neurons responded when the animals suppressed the VOR; hence, they displayed no vestibular sensitivity. 4. The third type of neuron was sensitive to both eye movement and vestibular stimulation. These neurons increased their firing rates during horizontal head rotation and smooth pursuit eye movements in the same direction; most (76%) preferred ipsilateral head and eye movements. Their firing rates were approximately in phase with eye velocity during sinusoidal smooth pursuit and with head velocity during VOR suppression; on average, their eye velocity sensitivity was 50% greater than their vestibular sensitivity. Sixty percent of these eye/head velocity cells were also sensitive to eye position. 5. The NPH/MVN region contains many neurons that could provide an eye position signal to abducens neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of fibers in medial longitudinal fasciculus (MLF) of alert monkeys during horizontal and vertical conjugate eye movements evoked by vestibular or visual stimuli

1976 ◽  
Vol 39 (6) ◽  
pp. 1135-1149 ◽  
Author(s):  
W. M. King ◽  
S. G. Lisberger ◽  
A. F. Fuchs
Keyword(s):  
Eye Movements ◽  
Vestibular Stimulation ◽  
Head Movements ◽  
Medial Rectus ◽  
Discharge Pattern ◽  
Medial Longitudinal Fasciculus ◽  
Eye Position ◽  
Behavioral Paradigm ◽  
Discharge Patterns ◽  
Almost All

Extracellular recordings were obtained from 37 histologically identified MLF fibers near the trochlear nucleus in alert monkeys trained to perform a visual tracking task and subjected to adequate horizontal and vertical vestibular stimulation. The behavioral paradigm permitted independent quantitative assessment of a fiber's response to eye or head movements. 2. According to their discharge pattern, almost all MLF fibers were placed in one of two classes: 1) Horizontal burst-tonic fibers (n = 20). During both vestibular and visually evoked eye movements, burst-tonic fibers discharged in strict relation to horizontal eye movements, exhibiting a burst of activity prior to on-direction saccades and steady firing related to horizontal eye position during fixations. 2) Vertical vestibular plus eye-position fibers (n = 14). Vertical fibers discharged in relation to vertical head velocity in the absence of eye movements and in relation to vertical eye position in the absence of head movement, and paused with saccades in any direction. 3. The quantitative similarity of horizontal burst-tonic fiber discharge to that of ipsilateral medial rectus oculomotoneurons suggests that burst-tonic fibers provide the major excitatory synaptic drive to medial rectus motoneurons during conjugate eye movements of vestibular or visual origin. 4. The discharge pattern of vertical vestibular plus eye-position fibers is significantly different from that of oculomotoneurons, suggesting that additional neural processing of vertical fiber information must occur in the mesencephalon. 5. The functional dichotomy of horizontal and vertical MLF fibers and their contrasting discharge patterns provide new evidence for the anatomic and functional separation of horizontal and vertical eye movement mechanisms in the pons and mesencephalon, respectively.

scholarly journals The vestibular-related frontal cortex and its role in smooth-pursuit eye movements and vestibular-pursuit interactions

2006 ◽  
Vol 16 (1-2) ◽  
pp. 1-22 ◽  
Author(s):  
Junko Fukushima ◽  
Teppei Akao ◽  
Sergei Kurkin ◽  
Chris R.S. Kaneko ◽  
Kikuro Fukushima
Keyword(s):  
Eye Movements ◽  
Frontal Cortex ◽  
Smooth Pursuit ◽  
Vestibular Stimulation ◽  
Target Motion ◽  
Head Movements ◽  
Image Motion ◽  
Target Velocity ◽  
Eye Velocity ◽  
Pursuit Neurons

In order to see clearly when a target is moving slowly, primates with high acuity foveae use smooth-pursuit and vergence eye movements. The former rotates both eyes in the same direction to track target motion in frontal planes, while the latter rotates left and right eyes in opposite directions to track target motion in depth. Together, these two systems pursue targets precisely and maintain their images on the foveae of both eyes. During head movements, both systems must interact with the vestibular system to minimize slip of the retinal images. The primate frontal cortex contains two pursuit-related areas; the caudal part of the frontal eye fields (FEF) and supplementary eye fields (SEF). Evoked potential studies have demonstrated vestibular projections to both areas and pursuit neurons in both areas respond to vestibular stimulation. The majority of FEF pursuit neurons code parameters of pursuit such as pursuit and vergence eye velocity, gaze velocity, and retinal image motion for target velocity in frontal and depth planes. Moreover, vestibular inputs contribute to the predictive pursuit responses of FEF neurons. In contrast, the majority of SEF pursuit neurons do not code pursuit metrics and many SEF neurons are reported to be active in more complex tasks. These results suggest that FEF- and SEF-pursuit neurons are involved in different aspects of vestibular-pursuit interactions and that eye velocity coding of SEF pursuit neurons is specialized for the task condition.

Effects of Viewing Distance on the Responses of Vestibular Neurons to Combined Angular and Linear Vestibular Stimulation

1999 ◽  
Vol 81 (5) ◽  
pp. 2538-2557 ◽  
Author(s):  
Chiju Chen-Huang ◽  
Robert A. McCrea
Keyword(s):  
Eye Movements ◽  
Eye Movement ◽  
Vestibular Stimulation ◽  
The Other ◽  
Viewing Distance ◽  
Vestibuloocular Reflex ◽  
Horizontal Canal ◽  
Head Velocity ◽  
Vestibular Neurons ◽  
Axis Of Rotation

Effects of viewing distance on the responses of vestibular neurons to combined angular and linear vestibular stimulation. The firing behavior of 59 horizontal canal–related secondary vestibular neurons was studied in alert squirrel monkeys during the combined angular and linear vestibuloocular reflex (CVOR). The CVOR was evoked by positioning the animal’s head 20 cm in front of, or behind, the axis of rotation during whole body rotation (0.7, 1.9, and 4.0 Hz). The effect of viewing distance was studied by having the monkeys fixate small targets that were either near (10 cm) or far (1.3–1.7 m) from the eyes. Most units (50/59) were sensitive to eye movements and were monosynaptically activated after electrical stimulation of the vestibular nerve (51/56 tested). The responses of eye movement–related units were significantly affected by viewing distance. The viewing distance–related change in response gain of many eye-head-velocity and burst-position units was comparable with the change in eye movement gain. On the other hand, position-vestibular-pause units were approximately half as sensitive to changes in viewing distance as were eye movements. The sensitivity of units to the linear vestibuloocular reflex (LVOR) was estimated by subtraction of angular vestibuloocular reflex (AVOR)–related responses recorded with the head in the center of the axis of rotation from CVOR responses. During far target viewing, unit sensitivity to linear translation was small, but during near target viewing the firing rate of many units was strongly modulated. The LVOR responses and viewing distance–related LVOR responses of most units were nearly in phase with linear head velocity. The signals generated by secondary vestibular units during voluntary cancellation of the AVOR and CVOR were comparable. However, unit sensitivity to linear translation and angular rotation were not well correlated either during far or near target viewing. Unit LVOR responses were also not well correlated with their sensitivity to smooth pursuit eye movements or their sensitivity to viewing distance during the AVOR. On the other hand there was a significant correlation between static eye position sensitivity and sensitivity to viewing distance. We conclude that secondary horizontal canal–related vestibuloocular pathways are an important part of the premotor neural substrate that produces the LVOR. The otolith sensory signals that appear on these pathways have been spatially and temporally transformed to match the angular eye movement commands required to stabilize images at different distances. We suggest that this transformation may be performed by the circuits related to temporal integration of the LVOR.

scholarly journals Matching the Oculomotor Drive During Head-Restrained and Head-Unrestrained Gaze Shifts in Monkey

2010 ◽  
Vol 104 (2) ◽  
pp. 811-828 ◽  
Author(s):  
Bernard P. Bechara ◽  
Neeraj J. Gandhi
Keyword(s):  
Mean Squared Error ◽  
Vestibular Nuclei ◽  
Cell Types ◽  
Head Movements ◽  
Abducens Nucleus ◽  
Gaze Shifts ◽  
Gaze Shift ◽  
Head Velocity ◽  
Burst Neurons ◽  
Eye Velocity

High-frequency burst neurons in the pons provide the eye velocity command (equivalently, the primary oculomotor drive) to the abducens nucleus for generation of the horizontal component of both head-restrained (HR) and head-unrestrained (HU) gaze shifts. We sought to characterize how gaze and its eye-in-head component differ when an “identical” oculomotor drive is used to produce HR and HU movements. To address this objective, the activities of pontine burst neurons were recorded during horizontal HR and HU gaze shifts. The burst profile recorded on each HU trial was compared with the burst waveform of every HR trial obtained for the same neuron. The oculomotor drive was assumed to be comparable for the pair yielding the lowest root-mean-squared error. For matched pairs of HR and HU trials, the peak eye-in-head velocity was substantially smaller in the HU condition, and the reduction was usually greater than the peak head velocity of the HU trial. A time-varying attenuation index, defined as the difference in HR and HU eye velocity waveforms divided by head velocity [α = ( Ḣhr − Ėhu)/ Ḣ] was computed. The index was variable at the onset of the gaze shift, but it settled at values several times greater than 1. The index then decreased gradually during the movement and stabilized at 1 around the end of gaze shift. These results imply that substantial attenuation in eye velocity occurs, at least partially, downstream of the burst neurons. We speculate on the potential roles of burst-tonic neurons in the neural integrator and various cell types in the vestibular nuclei in mediating the attenuation in eye velocity in the presence of head movements.

Discharge characteristics of medial rectus and abducens motoneurons in the goldfish

1991 ◽  
Vol 66 (6) ◽  
pp. 2125-2140 ◽  
Author(s):  
A. M. Pastor ◽  
B. Torres ◽  
J. M. Delgado-Garcia ◽  
R. Baker
Keyword(s):  
Eye Movements ◽  
Firing Rate ◽  
Eye Movement ◽  
Vestibular Stimulation ◽  
Medial Rectus ◽  
Eye Position ◽  
Sensory Stimuli ◽  
Time Constants ◽  
Recruitment Threshold ◽  
Eye Blink

1. The discharge of antidromically identified medial rectus and abducens motoneurons was recorded in restrained unanesthesized goldfish during spontaneous eye movements and in response to vestibular and optokinetic stimulation. 2. All medial rectus and abducens motoneurons exhibited a similar discharge pattern. A burst of spikes accompanied spontaneous saccades and fast phases during vestibular and optokinetic nystagmus in the ON-direction. Firing rate decreased for the same eye movements in the OFF-direction. All units showed a steady firing rate proportional to eye position beyond their recruitment threshold. 3. Motoneuronal position (ks) and velocity (rs) sensitivity for spontaneous eye movements were calculated from the slope of the rate-position and rate-velocity linear regression lines, respectively. The averaged ks and rs values of medial rectus motoneurons were higher than those of abducens motoneurons. The differences in motoneuronal sensitivity coupled with structural variations in the lateral versus the medial rectus muscle suggest that symmetric nasal and temporal eye movements are preserved by different motor unit composition. Although the abducens nucleus consists of distinct rostral and caudal subgroups, mean ks and rs values were not significantly different between the two populations. 4. Every abducens and medial rectus motoneuron fired an intense burst of spikes during its corresponding temporal or nasal activation phase of the "eye blink." This eye movement consisted of a sequential, rather than a synergic, contraction of both vertical and horizontal extraocular muscles. The eye blink could act neither as a protective reflex nor as a goal-directed eye movement because it could not be evoked in response to sensory stimuli. We propose a role for the blink in recentering eye position. 5. Motoneuronal firing rate after ON-directed saccades decreased exponentially before reaching the sustained discharge proportional to the new eye position. Time constants of the exponential decay ranged from 50 to 300 ms. Longer time constants after the saccade were associated with backward drifts of eye position and shorter time constants with onward drifts. These postsaccadic slide signals are suggested to encode the transition of eye position to the new steady level. 6. Motoneurons modulated sinusoidally in response to sinusoidal head rotation in the dark, but for a part of the cycle they went into cutoff, dependent on their eye position recruitment threshold. Eye position (kv) and velocity (rv) sensitivity during vestibular stimulation were measured at frequencies between 1/16 and 2 Hz. Motoneuronal time constants (tau v = rv/kv) decreased on the average by 25% with the frequency of vestibular stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Precerebellar Hindbrain Neurons Encoding Eye Velocity During Vestibular and Optokinetic Behavior in the Goldfish

2006 ◽  
Vol 96 (3) ◽  
pp. 1370-1382 ◽  
Author(s):  
James C. Beck ◽  
Paul Rothnie ◽  
Hans Straka ◽  
Susan L. Wearne ◽  
Robert Baker
Keyword(s):  
Firing Rate ◽  
Eye Movement ◽  
Time Course ◽  
Mossy Fiber ◽  
Vestibular Stimulation ◽  
Neuronal Firing ◽  
Velocity Storage ◽  
Head Velocity ◽  
Motor Error ◽  
Eye Velocity

Elucidating the causal role of head and eye movement signaling during cerebellar-dependent oculomotor behavior and plasticity is contingent on knowledge of precerebellar structure and function. To address this question, single-unit extracellular recordings were made from hindbrain Area II neurons that provide a major mossy fiber projection to the goldfish vestibulolateral cerebellum. During spontaneous behavior, Area II neurons exhibited minimal eye position and saccadic sensitivity. Sinusoidal visual and vestibular stimulation over a broad frequency range (0.1–4.0 Hz) demonstrated that firing rate mirrored the amplitude and phase of eye or head velocity, respectively. Table frequencies >1.0 Hz resulted in decreased firing rate relative to eye velocity gain, while phase was unchanged. During visual steps, neuronal discharge paralleled eye velocity latency (∼90 ms) and matched both the build-up and the time course of the decay (∼19 s) in eye velocity storage. Latency of neuronal discharge to table steps (40 ms) was significantly longer than for eye movement (17 ms), but firing rate rose faster than eye velocity to steady-state levels. The velocity sensitivity of Area II neurons was shown to equal (±10%) the sum of eye- and head-velocity firing rates as has been observed in cerebellar Purkinje cells. These results demonstrate that Area II neuronal firing closely emulates oculomotor performance. Conjoint signaling of head and eye velocity together with the termination pattern of each Area II neuron in the vestibulolateral lobe presents a unique eye-velocity brain stem-cerebellar pathway, eliminating the conceptual requirement of motor error signaling.

Vertical eye movement-related secondary vestibular neurons ascending in medial longitudinal fasciculus in cat I. Firing properties and projection pathways

1990 ◽  
Vol 63 (4) ◽  
pp. 902-917 ◽  
Author(s):  
Y. Iwamoto ◽  
T. Kitama ◽  
K. Yoshida
Keyword(s):  
Eye Movements ◽  
Firing Rate ◽  
Vestibular Nucleus ◽  
Vestibular Nerve ◽  
Medial Longitudinal Fasciculus ◽  
Eye Position ◽  
Phase Lag ◽  
Antidromic Activation ◽  
Head Velocity ◽  
Stimulation Of

1. The firing characteristics and projection patterns of secondary vestibular nucleus neurons involved in the vertical vestibuloocular pathways were investigated in alert cats. Single-unit recordings were made in the medial longitudinal fasciculus (MLF) near the trochlear nucleus from axons that were monosynaptically activated after electrical stimulation of the vestibular nerve. In a total of 253 identified secondary neurons, 225 discharged in relation to vertical eye movements; 189 of these increased their firing rate for downward eye movements and 36 for upward movements. The activity of the remaining 28 axons was not related to eye movements when the head was still. 2. Virtually all of the secondary neurons with downward on-direction displayed tonic activity that was primarily related to steady eye position during fixation (DPV neurons). The slope of the relationship between firing rate and vertical eye position ranged from 1.2 to 9.1 (spikes/s)/deg with a mean of 3.2 (spikes/s)/deg. The regularity of firing was quantified by calculating the coefficient of variation (CV) of interspike intervals. A comparison of the CV in the population units indicated that DPV neurons could be classified as either regular or irregular neurons. There was a tendency for regular neurons to have higher firing rates and higher correlation coefficients for the rate-position relationships than irregular neurons. 3. During pitch rotation in the light, all the DPV neurons tested increased their firing rate with upward head rotation. Both the phase and the amplitude of the response indicated that DPV neurons discharged not only in relation to eye position but also in relation to head velocity, suggesting that they received monosynaptic input from the posterior semicircular canal. The gain and phase lag of the response relative to head velocity were measured at 0.5 Hz. The range of the gain was 1.1-5.1 (spikes/s)/(deg/s), and that of the phase lag was 18.3-62.4 degrees. There was a tendency for irregular DPV neurons to have a larger gain and smaller phase lag than regular DPV neurons. 4. Ascending and descending projection pathways were determined for 147 DPV axons. Of these, 69 ascended in the contralateral MLF with respect to their soma (crossed-DPV axons), and 78 in the ipsilateral MLF (uncrossed-DPV axons), as revealed by their monosynaptic activation from the contralateral or ipsilateral vestibular nerve. Stimulation of the caudal MLF at the level of the obex evoked direct responses caused by antidromic activation of descending collaterals in approximately 70% (49/69) of the crossed-DPV axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Compensatory and Orienting Eye Movements Induced By Off-Vertical Axis Rotation (OVAR) in Monkeys

2002 ◽  
Vol 88 (5) ◽  
pp. 2445-2462 ◽  
Author(s):  
Keisuke Kushiro ◽  
Mingjia Dai ◽  
Mikhail Kunin ◽  
Sergei B. Yakushin ◽  
Bernard Cohen ◽  
...  
Keyword(s):  
Eye Movements ◽  
Vertical Axis ◽  
Velocity Storage ◽  
Eye Position ◽  
Rotational Axis ◽  
Vestibuloocular Reflex ◽  
Time Constants ◽  
Head Velocity ◽  
Axis Rotation ◽  
Eye Velocity

Nystagmus induced by off-vertical axis rotation (OVAR) about a head yaw axis is composed of a yaw bias velocity and modulations in eye position and velocity as the head changes orientation relative to gravity. The bias velocity is dependent on the tilt of the rotational axis relative to gravity and angular head velocity. For axis tilts <15°, bias velocities increased monotonically with increases in the magnitude of the projected gravity vector onto the horizontal plane of the head. For tilts of 15–90°, bias velocity was independent of tilt angle, increasing linearly as a function of head velocity with gains of 0.7–0.8, up to the saturation level of velocity storage. Asymmetries in OVAR bias velocity and asymmetries in the dominant time constant of the angular vestibuloocular reflex (aVOR) covaried and both were reduced by administration of baclofen, a GABAB agonist. Modulations in pitch and roll eye positions were in phase with nose-down and side-down head positions, respectively. Changes in roll eye position were produced mainly by slow movements, whereas vertical eye position changes were characterized by slow eye movements and saccades. Oscillations in vertical and roll eye velocities led their respective position changes by ≈90°, close to an ideal differentiation, suggesting that these modulations were due to activation of the orienting component of the linear vestibuloocular reflex (lVOR). The beating field of the horizontal nystagmus shifted the eyes 6.3°/ g toward gravity in side down position, similar to the deviations observed during static roll tilt (7.0°/ g). This demonstrates that the eyes also orient to gravity in yaw. Phases of horizontal eye velocity clustered ∼180° relative to the modulation in beating field and were not simply differentiations of changes in eye position. Contributions of orientating and compensatory components of the lVOR to the modulation of eye position and velocity were modeled using three components: a novel direct otolith-oculomotor orientation, orientation-based velocity modulation, and changes in velocity storage time constants with head position re gravity. Time constants were obtained from optokinetic after-nystagmus, a direct representation of velocity storage. When the orienting lVOR was combined with models of the compensatory lVOR and velocity estimator from sequential otolith activation to generate the bias component, the model accurately predicted eye position and velocity in three dimensions. These data support the postulates that OVAR generates compensatory eye velocity through activation of velocity storage and that oscillatory components arise predominantly through lVOR orientation mechanisms.

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