Eye-position dependence of torsional velocity during interaural translation, horizontal pursuit, and yaw-axis rotation in humans

Three-dimensional analysis of otolith-ocular reflex during eccentric rotation in humans

When a participant is rotated while displaced from the axis of rotation (eccentric rotation, ER), both rotational stimulation and linear acceleration are applied to the participant. As linear acceleration stimulates the otolith, the vestibulo-ocular reflex (VOR) caused by the otolith (linear VOR; lVOR) would be induced during ER. Ten participants were rotated sinusoidally at a maximum angular velocity of 50°/s and at frequencies of 0.1, 0.3, 0.5, and 0.7Hz. The radius of rotation during ER was 90cm. The participants sat on a chair at three different positions: on the axis (center rotation, CR), at 90cm backward from the axis (nose-in ER, NI-ER) and at 90cm forward from the axis (nose-out ER, NO-ER). Their eye movements during rotation were recorded and analyzed three-dimensionally. The VOR gain during NI-ER was lower at 0.5 and 0.7Hz, and that during NO-ER was higher at 0.3, 0.5, and 0.7Hz than during CR. These results indicate that lVOR actually worked at 0.5 and 0.7Hz during ER and that the enhancement and decline of the VOR gain relative to the VOR gain during CR was seen in humans. Thus, we suggest that otolith function can be assessed via rotational testing of NI-ER and NO-ER.

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.

The human translational vestibulo-ocular reflex in response to complex motion

We studied the translational vestibulo-ocular reflex (tVOR) in four healthy human subjects during complex, unpredictable sum-of-sines head motion (combination of 0.73, 1.33, 1.93, and 2.93 Hz), while subjects viewed a target 15 cm away. Ideal eye velocity was calculated from recorded head motion; actual eye velocity was measured with scleral coils. The gain and phase for each frequency component was determined by least-squares optimization. Gain averaged approximately 40% and did not change with frequency; phase lag increased with frequency to a maximum of 66°. Fitting actual to ideal eye velocity predicted a tVOR latency of 48 m/s for vertical and 38 m/s for horizontal translation. These findings provide further evidence that the normal tVOR is considerably undercompensatory, even at low frequencies if the stimulus is not predictable. The similarity of this behavior to that of pursuit suggests that these two eye movements may share some aspects of neural processing.

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.

Frequency-dependent spatiotemporal tuning properties of non-eye movement related vestibular neurons to three-dimensional translations in squirrel monkeys

Responses of vestibular-only translation sensitive (VOTS) neurons in vestibular nuclei of two squirrel monkeys were studied at multiple frequencies to three-dimensional translations and rotations. A novel frequency-dependent spatiotemporal analysis examined in each neuron whether complex models, with unrestricted response dynamics in three-dimensional (3D) space, provided significantly better fits than restricted models following simple, cosine rule. Subsequently, the statistically selected optimal model was used to predict the maximum translation direction, expressed as a unitary vector, Vt(max), and its associated sensitivity and phase across frequencies. Simple models were sufficient to quantify the 3D translational responses of 66% of neurons. Most VOTS neurons, complex or simple, exhibited flat-gain or low-pass response dynamics. The Vt(max) of simple neurons was fixed, whereas that of complex neurons changed with frequency. The spatial distribution of Vt(max) in simple neurons, which fell within 30 degrees of either the horizontal plane or/and the sagittal plane, was closely aligned with Vt(max) of vestibular afferents. In contrast, the frequency-dependent Vt(max) of most complex neurons migrated from the dorsoventral axis at higher frequency toward the horizontal plane, especially the interaural axis, at lower frequency. When the maximum rotation direction was estimated from responses of the same VOTS neurons to 1.2 Hz yaw, pitch, and roll rotations, complex neurons were more likely to respond to rotations activating vertical canals. Responses to 0.15-0.3 Hz linear accelerations produced by inertial or gravitational forces were indistinguishable in most complex neurons but significantly different in most simple neurons. These observations suggest that simple and complex VOTS neurons constitute distinctive vestibular pathways where complex neurons, exhibiting a novel spatiotemporal filtering mechanism in processing otolith-related signals, are well suited to drive tilt-related responses, whereas simple neurons probably mediate pure translation related responses.

Kinematics of the Rotational Vestibuloocular Reflex: Role of the Cerebellum

We studied the effect of cerebellar lesions on the 3-D control of the rotational vestibuloocular reflex (RVOR) to abrupt yaw-axis head rotation. Using search coils, three-dimensional (3-D) eye movements were recorded from nine patients with cerebellar disease and seven normal subjects during brief chair rotations (200°/s2 to 40°/s) and manual head impulses. We determined the amount of eye-position dependent torsion during yaw-axis rotation by calculating the torsional-horizontal eye-velocity axis for each of three vertical eye positions (0°, ±15°) and performing a linear regression to determine the relationship of the 3-D velocity axis to vertical eye position. The slope of this regression is the tilt angle slope. Overall, cerebellar patients showed a clear increase in the tilt angle slope for both chair rotations and head impulses. For chair rotations, the effect was not seen at the onset of head rotation when both patients and normal subjects had nearly head-fixed responses (no eye-position-dependent torsion). Over time, however, both groups showed an increasing tilt-angle slope but to a much greater degree in cerebellar patients. Two important conclusions emerge from these findings: the axis of eye rotation at the onset of head rotation is set to a value close to head-fixed (i.e., optimal for gaze stabilization during head rotation), independent of the cerebellum and once the head rotation is in progress, the cerebellum plays a crucial role in keeping the axis of eye rotation about halfway between head-fixed and that required for Listing's Law to be obeyed.

Three dimensional kinematics of rapid compensatory eye movements in humans with unilateral vestibular deafferentation

Saccades executed with the head stationary have kinematics conforming to Listing's law (LL), confining the ocular rotational axis to Listing's plane (LP). In unilateral vestibular deafferentation (UVD), the vestibulo-ocular reflex (VOR), which does not obey LL, has at high head acceleration a slow phase that has severely reduced velocity during ipsilesional rotation, and mildly reduced velocity during contralesional rotation. Studying four subjects with chronic UVD using 3D magnetic search coils, we investigated kinematics of stereotypic rapid eye movements that supplement the impaired VOR. We defined LP with the head immobile, and expressed eye and head movements as quaternions in LP coordinates. Subjects underwent transient whole body yaw at peak acceleration 2,800 degrees /s(2) while fixating targets centered, or 20 degrees up or down prior to rotation. The VOR shifted ocular torsion out of LP. Vestibular catch-up saccades (VCUS) occurred with mean latency 90 +/- 44 ms (SD) from ipsilesional rotation onset, maintained initial non-LL torsion so that their quaternion trajectories paralleled LP, and had velocity axes changing by half of eye position. During contralesional rotation, rapid eye movements occurred at mean latency 135 +/- 36 ms that were associated with abrupt decelerations (ADs) of the horizontal slow phase correcting 3D deviations in its velocity axis, with quaternion trajectories not paralleling LP. Rapid eye movements compensating for UVD have two distinct kinematics. VCUS have velocity axis dependence on eye position consistent with LL, so are probably programmed in 2D by neural circuits subserving visual saccades. ADs have kinematics that neither conform to LL nor match the VOR axis, but appear instead programmed in 3D to correct VOR axis errors.

Rotational and translational optokinetic nystagmus have different kinematics

We studied the dependence of ocular torsion on eye position during horizontal optokinetic nystagmus (OKN) elicited by random-dot translational motion (tOKN) and prolonged rotation in the light (rOKN). For slow and quick phases, we fit the eye-velocity axis to vertical eye position to determine the tilt angle slope (TAS). The TAS for tOKN was 0.48 for both slow and quick phases, close to what is found during translational motion of the head. The TAS for rOKN was less for both slow (0.11) and quick phases (0.26), close to what is found during rotational motion of the head. Our findings are consistent with the notion that translational and rotational optic flow are processed differently by the brain and that they produce different 3-D eye movement commands that are comparable to the different commands generated in response to vestibular signals when the head is actually translating or rotating.

Is an internal model of head orientation necessary for oculomotor control?

In order to test whether the control of eye movement in response to head movement requires an internal model of head orientation or instead can rely on directly sensing information about head orientation and movement, perceived gravity was separated from physical gravity to see which dominated the eye-movement response. Internal model theory suggests that the oculomotor response should be driven by perceived, internalized gravity, whereas the direct sensing theory predicts it should always be driven by vestibularly sensed gravity. Subjects lay on an airbed either supine or on their side and were sinusoidally translated along their dorsoventral body axis. The direction of perceived gravity was separated from physical gravity by performing the experiments in a room built on its side with the direction of its "floor" orthogonal to both physical gravity and the subject's translation. The swinging sum of the imposed sinusoidal acceleration with physical gravity was thus in a plane orthogonal to its sum with perceived gravity. Oculomotor responses to these swinging vectors were looked for and responses were found only to the sum of the acceleration with physical gravity, not perceived gravity. It was concluded that an internal model is not used to drive these compensatory eye movements.

Human angular vestibulo-ocular reflex initiation: relationship to Listing's law

An ideal vestibulo-ocular reflex (VOR) generates ocular rotations compensatory for head motion. During visually guided movements, Listing's law (LL) constrains eye rotation to axes in Listing's plane (LP). Recently, it has been reported that the VOR axis is not collinear with the rotation axis of the head, but is influenced by eye position in the orbit. Elaborate models have been proposed suggesting dynamic neural control of the VOR axis. By examining the variability and time course of changes in VOR axis orientation, we sought to test plausibility of these models. Binocular LPs were defined in eight humans. The VOR was evoked by a highly repeatable, transient, whole-body yaw rotation in darkness at peak acceleration 2800 deg/s2. Immediately prior to rotation, subjects regarded targets at eye level, 20 degrees up, or 20 degrees down. Eye and head positions were expressed in LP coordinates for comparison with LL. Eye position generally followed head position and departed LP when the head axis tilted out of LP. In the velocity domain the VOR axis tilted 28 +/- 9% of the change in vertical eye position, but there was significant intrasubject variation (14% to 41%). This roughly "quarter-angle" behavior began with the earliest detectable VOR. Given the brief latency and marked interindividual variability of the eye position dependence of the VOR rotational axis, and the small deviation of the VOR from LL in the position domain, it is speculated that this behavior is largely due to orbital mechanics interacting with the basic neural commands that initiate the VOR.

Temporal dynamics of ocular position dependence of the initial human vestibulo-ocular reflex

PURPOSE

While an ideal vestibulo-ocular reflex (VOR) generates ocular rotations compensatory for head motion, during visually guided movements, Listing's Law (LL) constrains the eye to rotational axes lying in Listing's Plane (LP). The present study was conducted to explore the recent proposal that the VOR's rotational axis is not collinear with the head's, but rather follows a time-dependent strategy intermediate between LL and an ideal VOR.

METHODS

Binocular LPs were defined during visual fixation in eight normal humans. The VOR was evoked by a highly repeatable transient whole-body yaw rotation in darkness at a peak acceleration of 2800 deg/s2. Immediately before rotation, subjects regarded targets 15 or 500 cm distant located at eye level, 20 degrees up, or 20 degrees down. Eye and head responses were compared with LL predictions in the position and velocity domains.

RESULTS

LP orientation varied both among subjects and between individual subject's eyes, and rotated temporally with convergence by 5 +/- 5 degrees (+/-SEM). In the position domain, the eye compensated for head displacement even when the head rotated out of LP. Even within the first 20 ms from onset of head rotation, the ocular velocity axis tilted relative to the head axis by 30% +/- 8% of vertical gaze position. Saccades increased this tilt. Regardless of vertical gaze position, the ocular rotation axis tilted backward 4 degrees farther in abduction than in adduction. There was also a binocular vertical eye velocity transient and lateral tilt of the ocular axis.

CONCLUSIONS

These disconjugate, short-latency axis perturbations appear intrinsic to the VOR and may have neural or mechanical origins.

Axis of eye rotation changes with head-pitch orientation during head impulses about earth-vertical

The goal of this study was to assess how the axis of head rotation, Listing's law, and eye position influence the axis of eye rotation during brief, rapid head rotations. We specifically asked how the axis of eye rotation during the initial angular vestibuloocular reflex (VOR) changed when the pitch orientation of the head relative to Earth-vertical was varied, but the initial position of the eye in the orbit and the orientation of Listing's plane with respect to the head were fixed. We measured three-dimensional eye and head rotation axes in eight normal humans using the search coil technique during head-and-trunk (whole-body) and head-on-trunk (head-only) "impulses" about an Earth-vertical axis. The head was initially oriented at one of five pitch angles (30 degrees nose down, 15 degrees nose down, 0 degrees, 15 degrees nose up, 30 degrees nose up). The fixation target was always aligned with the nasooccipital axis. Whole-body impulses were passive, unpredictable, manual, rotations with peak-amplitude of approximately 20 degrees , peak-velocity of approximately 80 degrees /s, and peak-acceleration of approximately 1000 degrees /s2. Head-only impulses were also passive, unpredictable, manual, rotations with peak-amplitude of approximately 20 degrees , peak-velocity of approximately 150 degrees /s, and peak-acceleration of approximately 3000 degrees /s2. During whole-body impulses, the axis of eye rotation tilted in the same direction, and by an amount proportional (0.51 +/- 0.09), to the starting pitch head orientation (P < 0.05). This proportionality constant decreased slightly to 0.39 +/- 0.08 (P < 0.05) during head-only impulses. Using the head-only impulse data, with the head pitched up, we showed that only 50% of the tilt in the axis of eye rotation could be predicted from vectorial summation of the gains (eye velocity/head velocity) obtained for rotations about the pure yaw and roll head axes. Thus, even when the orientation of Listing's plane and eye position in the orbit are fixed, the axis of eye rotation during the VOR reflects a compromise between the requirements of Listing's law and a perfectly compensatory VOR.

Eye-position dependence of torsional velocity during step-ramp pursuit and transient yaw rotation in humans

The time course of eye-position-dependent torsion during transient horizontal pursuit and yaw rotation was examined in seven normal human subjects. The stimuli consisted of step-ramp target motion (25, 40 degrees /s) and brief chair rotation (approximately 200 degrees /s(2) accelerated to 40 degrees /s) at three different vertical positions (center 0 degrees , up or down 15 degrees ). Three-dimensional eye movements were recorded with dual search coils. The kinematics of pursuit and the rotational vestibulo-ocular reflex (rVOR) were assessed by determining the tilt-angle slope, a measure of the variation of the axis of eye-velocity with vertical eye position. We found that the tilt-angle slope during pursuit was initially 0.4+/-0.07 (mean+/-95% confidence interval) and then gradually rose to 0.64+/-0.04, at about the time that the steady-state eye-velocity was reached. The rVOR began with a nearly head-fixed axis (0.08+/-0.04), appropriate for full retinal image stabilization, followed by a gradual increase of the tilt-angle slope to 0.31+/-0.02. Thus, differences between pursuit and the rVOR with respect to Listing's law can be seen from the onset of transient responses, although in both cases eye-position-dependent torsion increases with time. This temporal evolution of the axis of eye-velocity may involve the velocity-storage mechanism.

Phoria adaptation after sustained symmetrical convergence: influence of saccades

We recorded divergence eye movements after short (4 s) and long (36 s) periods of sustained symmetrical convergence (30 degrees) in nine normal human subjects using the search coil technique. Following the long period of convergence, alignment after the initial 1,250 ms of divergence was more converged than after the short period of convergence, showing short-term "phoria adaptation". The first 1,000 ms of divergence, however, could be slower, faster or relatively unchanged, depending upon the subject. A change in the timing and/or amplitude of associated saccades (which accelerate ongoing vergence) between the long and short stimuli accounted for much of the difference in the rate of divergence. The differences in saccade pattern during early divergence following the long and short periods of convergence may reflect changes in attentional focus (to near or to far).

Kinematics of vertical saccades during the yaw vestibulo-ocular reflex in humans

PURPOSE

Listing's law (LL) constrains the rotational axes of saccades and pursuit eye movements to Listing's plane (LP). In the velocity domain, LL is ordinarily equivalent to a tilt in the ocular velocity axis equal to half the change in eye position, giving a tilt angle ratio (TAR) of 0.5. This study was undertaken to investigate vertical saccade behavior after the yaw vestibulo-ocular reflex (VOR) had driven eye torsion out of LP, an initial condition causing the position and velocity domain formulations of LL to differ.

METHODS

Binocular eye and head motions were recorded with magnetic search coils in eight humans. With the head immobile, LP was determined for each eye, and mean TAR was 0.50 +/- 0.07 (mean +/- SD) for horizontal and 0.45 +/- 0.11 for vertical saccades. The VOR was evoked by transient, whole-body yaw at 2800 deg/s2 peak acceleration, capable of evoking large, uninterrupted VOR slow phases. Before rotation, subjects viewed a target at eye level, 20 degrees up, or 20 degrees down. In two thirds of the trials, the target moved upward or downward at systematically varying times, triggering a vertical saccade during the horizontal VOR slow phase.

RESULTS

Because the head rotation axis was generally misaligned with LP, the eye averaged 3.6 degrees out of LP at vertical saccade onset. During the saccade, eye position continued to depart LP by an average 0.8 degrees. The horizontal TAR at saccade onset was 0.29 +/- 0.07. At peak saccade velocity 35 +/- 3 ms later, the vertical TAR was 0.45 +/- 0.07, statistically similar to that of head fixed saccades. Saccades did not return to LP.

CONCLUSIONS

Although they did not observe the position domain formulation of LL, vertical saccades, during the VOR, observed the half-angle velocity domain formulation of LL.

Perceiving a stable world during active rotational and translational head movements

When a person moves through the world, the associated visual displacement of the environment in the opposite direction is not usually seen as external movement but rather as a changing view of a stable world. We measured the amount of visual motion that can be tolerated as compatible with the perception of moving within a stable world during active, sinusoidal, translational and rotational head movement. Head movements were monitored by means of a low-latency, mechanical head tracker and the information was used to update a helmet-mounted visual display. A variable gain was introduced between the head tracker and the display. Ten subjects adjusted this gain until the visual display appeared stable during sinusoidal yaw, pitch and roll head rotations and naso-occipital, inter-aural and dorso-ventral translations at 0.5 Hz. Each head movement was tested with movement either orthogonal to or parallel with gravity. A wide spread of gains was accepted as stable (0.8 to 1.4 for rotation and 1.1 to 1.8 for translation). The gain most likely to be perceived as stable was greater than that required by the geometry (1.2 for rotation; 1.4 for translation). For rotational motion, the mean gains were the same for all axes. For translation there was no effect of whether the movement was inter-aural (mean gain 1.6) or dorso-ventral (mean gain 1.5) and no effect of the relative orientation of the translation direction relative to gravity. However translation in the naso-occipital direction was associated with more closely veridical settings (mean gain 1.1) and narrower standard deviations than in other directions. These findings are discussed in terms of visual and non-visual contributions to the perception of an earth-stable environment during active head movement.

Binocular coordination in fore/aft motion

Stabilization of images on the fovea during either fore/aft translation of a subject or fore/aft movement of a visual target in front of a stationary observer imposes complex geometrical requirements that depend upon the eccentricity of the object of interest with respect to the eyes. Each eye needs to be rotated independently with varying proportions of conjugate (version) and disconjugate (vergence) eye movements to maintain fixation of the target. Here, we describe binocular coordination in the early response to translational movements of normal subjects along their naso-occipital axis. We recorded the responses evoked by small (about 4 cm), abrupt (about 0.7 g), fore/aft translations in four normal subjects while they viewed a near target. In the forward and backward starting positions the target was 15 or 10.5 cm away, respectively. Each subject was tested with the target centered between the eyes, aligned on the right eye, and placed to the right of the right eye by approximately 3 cm. The three conditions differed only in the lateral eccentricity of the target, yet the geometrical requirements for image stabilization are very different: pure vergence, one eye still, or mostly version. We found that the eye-movement responses closely matched what was needed for visual stabilization of the target, though responses to stimuli calling for divergence were less accurate than those for convergence. The latency of these responses ranged from 40 to 65 ms and achieved about 80% of the ideal response by 90 to 100 ms after the onset of the stimulus. Next, we asked whether these eye movements were generated by the vestibular system or by high-level strategies for image stabilization, such as pursuit. Thus, in a second set of experiments we used the mean profile of fore\aft body motion computed for each subject to drive a small visual target across the same distances and in the same eccentricities used during body translations. We found that visually driven responses had longer latencies (by at least 80 ms, ranging from 144 to 155 ms) and slower dynamics (with significantly lower peak eye velocities), highlighting the different subsystems producing the two types of responses. Saccades were also an important component of the response to both visual and vestibular stimuli, less frequent during the centered-target configuration and more frequent during viewing of eccentric targets. Visual stimuli evoked saccadic corrections more often and at shorter latencies than did vestibular stimuli. Both smooth and saccadic eye movements were appropriately disconjugate and their pattern depended on whether the eyes were converging or diverging.

Human angular vestibulo-ocular reflex axis disconjugacy: relationship to magnetic resonance imaging evidence of globe translation

Magnetic resonance imaging (MRI) demonstrates that the lateral rectus pulley shifts 0.5 mm inferiorly relative to the medial rectus in 20 degrees upgaze, but 0.5 mm superiorly in 20 degrees downgaze, whereas the globe translates 0.7 mm nasally in adduction and 0.2 mm nasally in abduction. If pulleys influence ocular kinematics, these effects would predict disconjugate alterations of the yaw vestibulo-ocular reflex (VOR) rotational axes. Binocular eye and head movements were recorded using three-dimensional search coils in 8 humans (age 24 +/- 2 years, mean +/- SE) undergoing directionally randomized, transient, whole-body yaw (2800 degrees /s2 peak) in darkness while fixating straight ahead, as well as +/- 18 degrees vertically. Eye and head rotational velocity axes were expressed as quaternions in Listing coordinates. In the initial 70 ms, the ocular axis varied with vertical gaze by one-quarter the angle of target elevation, but this effect summed significantly with a disconjugate effect of horizontal duction. In central gaze, the mean adducting eye (AD) rotational axis tilted 3.4 +/- 0.8 degrees forward relative to the head axis, while that of the abducting eye (AB) tilted 0.6 +/- 0.8 degrees backward. In downgaze, the AD rotational axis tilted 8.6 +/- 1.0 degrees forward, and AB 5.7 +/- 1.2 degrees forward. In upgaze, the AD rotational axis tilted backward by 0.1 +/- 0.7 degrees, and AB backward 3.4 +/- 0.9 degrees. We suggest that nasal globe translation relative to the fixed trochlea produces binocular extorsion accounting for yaw VOR axis disconjugacy, and thus a horizontal duction dependence in VOR rotational axis summating with classic dependence of VOR axis on vertical gaze. Confirmation of predicted duction-dependent VOR disconjugacy supports the idea that rectus pulleys influence kinematics for all eye movements.

Similar kinematic properties for ocular following and smooth pursuit eye movements

Ocular following (OFR) is a short-latency visual stabilization response to the optic flow experienced during self-motion. It has been proposed that it represents the early component of optokinetic nystagmus (OKN) and that it is functionally linked to the vestibularly driven stabilization reflex during translation (translational vestibuloocular reflex, TVOR). Because no single eye movement can eliminate slip from the whole retina during translation, the OFR and the TVOR appear to be functionally related to maintaining visual acuity on the fovea. Other foveal-specific eye movements, like smooth pursuit and saccades, exhibit an eye-position-dependent torsional component, as dictated by what is known as the "half-angle rule" of Listing's law. In contrast, eye movements that stabilize images on the whole retina, such as the rotational vestibuloocular reflex (RVOR) and steady-state OKN do not. Consistent with the foveal stabilization hypothesis, it was recently shown that the TVOR is indeed characterized by an eye-position-dependent torsion, similar to pursuit eye movements. Here we have investigated whether the OFR exhibits three-dimensional kinematic properties consistent with a foveal response (i.e., similar to the TVOR and smooth pursuit eye movements) or with a whole-field stabilization function (similar to steady-state OKN). The OFR was elicited using 100-ms ramp motion of a full-field random dot pattern that moved horizontally at 20, 62, or 83 degrees/s. To study if an eye-position-dependent torsion is generated during the OFR, we varied the initial fixation position vertically within a range of +/-20 degrees . As a control, horizontal smooth pursuit eye movements were also elicited using step-ramp target motion (10, 20, or 30 degrees/s) at similar eccentric positions. We found that the OFR followed kinematic properties similar to those seen in pursuit and the TVOR with the eye-position-dependent torsional tilt of eye velocity having slopes that averaged 0.73 +/- 0.16 for OFR and 0.57 +/- 0.12 (means +/- SD) for pursuit. These findings support the notion that the OFR, like the TVOR and pursuit, are foveal image stabilization systems.

Initiation and cancellation of the human heave linear vestibulo-ocular reflex after unilateral vestibular deafferentation

The effect of unilateral vestibular deafferentation (UVD) on the linear vestibulo-ocular reflex (LVOR) was studied in 11 humans an average of 52 months following surgical UVD. Controls consisted of seven healthy age-matched subjects. The LVOR was evoked by directionally random, transient whole body interaural (heave) translation with a peak acceleration of 0.5 g while subjects viewed earth-fixed (LVOR) and head-fixed (cancellation) targets 15, 25, and 200 cm distant. The magnitude of the LVOR slow phase was inversely proportional to target distance for both subject groups. Neither latency nor the magnitude of the LVOR significantly differed in the ipsi- vs contralesional directions (P>0.1) in UVD. When the target disappeared at heave onset, subjects with UVD had LVOR slow phase displacement 100 ms later that was 5% of ideal at 15 cm, 6% at 25 cm, and 16% at 200 cm. This was significantly less than corresponding control values of 41, 43, and 50%. During cancellation the LVOR magnitude 100 ms from heave onset was reduced at all target distances by an average of 40+/-4%, and the relative reduction did not significantly differ between controls and subjects with UVD (P>0.1). Cancellation latency did not vary significantly among target distances or subject groups. It is concluded that after UVD, the LVOR is bilaterally and symmetrically reduced but remains modulated by viewing distance and cancellation effort.