Influence of sensorimotor context on the linear vestibulo-ocular reflex

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.

Directional asymmetry of nystagmus elicitation in humans during step and sinusoidal modes of lateral linear acceleration

We investigated nystagmus elicitation in 50 normal subjects who were exposed interaurally to linear acceleration with step (rectangular) and sinusoidal modes of oscillation using a linear accelerator. Relatively strong G-loads of 0.3-0.5 G at a 10 m stroke were applied to subjects who looked at a memorized target in darkness, with the head and trunk tightly restrained in the upright sitting position. Horizontal and vertical eye movements were recorded by electrooculography (EOG). Various levels of G-directional preponderance (DP), including completely one-sided, were observed similarly in either stimulus mode, strongly suggesting that directional asymmetry in nystagmus elicitation may be a functional characteristic in the otolith-ocular response, in contrast to the canal-ocular response. The effects of G-load increase were less congruent between the two stimulus modes. In the step-mode oscillation, the desaccaded slow eye position which corresponds to the vestibuloocular reflex (VOR) was saw-toothed in shape as was the stimulus velocity curve, but the baseline often drifted slowly and DP-dependently in the direction opposite to the fast phase of nystagmus. When the slow phase velocity (SPV), a slope of the saw-tooth, was adjusted mathematically for such slow drift, it revealed that the adjusted SPVs were almost symmetrical between rightward and leftward G-directions. These results suggest that DP generation is separate from VOR generation which is primarily symmetrical.

Horizontal linear vestibulo-ocular reflex testing in patients with peripheral vestibular disorders

Horizontal eye movements in response to lateral head translation [linear vestibulo-ocular reflex (LVOR)] in normal subjects and in patients with bilateral vestibular failure (n = 14), unilateral vestibular nerve section (n = 9), and benign positional vertigo (n = 14), were studied. LVORs were elicited in darkness by step acceleration (0.24 g) of the whole body along the interaural axis.

RESULTS AND CONCLUSIONS

(1) In patients with bilateral vestibular failure, LVORs were either absent or abnormal with asymmetries, diminished velocities, and prolonged latencies. Measurements of dynamic visual acuity during linear self-motion showed decreased performance in patients at 1.0 and 1.5 Hz, which correlated with absent or delayed LVORs. These findings demonstrate the functional role of LVORs for dynamic visual acuity. (2) Early after vestibular nerve section, LVORs were diminished or absent with head acceleration toward the operated ear and normal in the opposite direction. After 6-10 weeks, responses were symmetrical again. Thus, a single utricle appears to be polarized with respect to the LVOR early after unilateral vestibular loss generating mostly contraversive responses. (3) Patients with benign positional vertigo showed mostly normal LVORs, which can be explained by minor utricular damage or central compensation of a chronic unilateral deficit.

Functional organization of primate translational vestibulo-ocular reflexes and effects of unilateral labyrinthectomy

Translational vestibulo-ocular reflexes (trVORs) are characterized by distinct spatio-temporal properties and sensitivities that are proportional to the inverse of viewing distance. Anodal (inhibitory) labyrinthine stimulation (100 microA, < 2 s) during motion decreased the high-pass filtered dynamics, as well as horizontal trVOR sensitivity and its dependence on viewing distance. Cathodal (excitatory) currents had opposite effects. Translational VORs were also affected after unilateral labyrinthectomy. Animals lost their ability to modulate trVOR sensitivity as a function of viewing distance acutely after the lesion. These deficits partially recovered over time, albeit a significant reduction in trVOR sensitivity as a function of viewing distance remained in compensated animals. During fore-aft motion, the effects of unilateral labyrinthectomy were more dramatic. Both acute and compensated animals permanently lost their ability to modulate fore-aft trVOR responses as a function of target eccentricity. These results suggest that (1) the dynamics and viewing distance-dependent properties of the trVORs are very sensitive to changes in the resting firing rate of vestibular afferents and, consequently, vestibular nuclei neurons; (2) the most irregularly firing primary otolith afferents that are most sensitive to labyrinthine electrical stimulation might contribute to reflex dynamics and sensitivity; (3) inputs from both labyrinths are necessary for the generation of the translational VORs.

Effects of tilt of the gravito-inertial acceleration vector on the angular vestibuloocular reflex during centrifugation

Effects of tilt of the gravito-inertial acceleration vector on the angular vestibuloocular reflex during centrifugation. Interaction of the horizontal linear and angular vestibuloocular reflexes (lVOR and aVOR) was studied in rhesus and cynomolgus monkeys during centered rotation and off-center rotation at a constant velocity (centrifugation). During centered rotation, the eye velocity vector was aligned with the axis of rotation, which was coincident with the direction of gravity. Facing and back to motion centrifugation tilted the resultant of gravity and linear acceleration, gravito-inertial acceleration (GIA), inducing cross-coupled vertical components of eye velocity. These components were upward when facing motion and downward when back to motion and caused the axis of eye velocity to reorient from alignment with the body yaw axis toward the tilted GIA. A major finding was that horizontal time constants were asymmetric in each monkey, generally being longer when associated with downward than upward cross coupling. Because of these asymmetries, accurate estimates of the contribution of the horizontal lVOR could not be obtained by simply subtracting horizontal eye velocity profiles during facing and back to motion centrifugation. Instead, it was necessary to consider the effects of GIA tilts on velocity storage before attempting to estimate the horizontal lVOR. In each monkey, the horizontal time constant of optokinetic after-nystagmus (OKAN) was reduced as a function of increasing head tilt with respect to gravity. When variations in horizontal time constant as a function of GIA tilt were included in the aVOR model, the rising and falling phases of horizontal eye velocity during facing and back to motion centrifugation were closely predicted, and the estimated contribution of the compensatory lVOR was negligible. Beating fields of horizontal eye position were unaffected by the presence or magnitude of linear acceleration during centrifugation. These conclusions were evaluated in animals in which the low-frequency aVOR was abolished by canal plugging, isolating the contribution of the lVOR. Postoperatively, the animals had normal ocular counterrolling and horizontal eye velocity modulation during off-vertical axis rotation (OVAR), suggesting that the otoliths were intact. No measurable horizontal eye velocity was elicited by centrifugation with angular accelerations </=40 degrees /s2 and angular velocities </=400 degrees /s. We conclude that in rhesus and cynomolgus monkeys, differences between horizontal eye velocities recorded during facing and back to motion constant velocity centrifugation can be explained by orienting effects of the GIA tilt on the time constants of the horizontal aVOR and not by a superposed lVOR.

Neural control of rotational kinematics within realistic vestibuloocular coordinate systems

Previous theoretical investigations of the three-dimensional (3-D) angular vestibuloocular reflex (VOR) have separately modeled realistic coordinate transformations in the direct velocity path or the nontrivial problems of converting angular velocity into a 3-D orientation command. We investigated the physiological and behavioral implications of combining both approaches. An ideal VOR was simulated using both a plant model with head-fixed eye muscle actions (standard plant) and one with muscular position dependencies that facilitate Listing's law (linear plant). In contrast to saccade generation, stabilization of the eye in space required a 3-D multiplicative (tensor) interaction between the various components of velocity and position in both models: in the indirect path of the standard plant version, but also in the direct path of the linear plant version. We then incorporated realistic nonorthogonal coordinate transformations (with the use of matrices) into both models. Each now malfunctioned, predicting ocular drift/retinal destabilization during and/or after the head movement, depending on the plant version. The problem was traced to the standard multiplication tensor, which was only defined for right-handed, orthonormal coordinates. We derived two solutions to this problem: 1) separating the brain stem coordinate transformation into two (sensory and motor) transformations that reordered and "undid" the nonorthogonalities of canals and muscle transformations, thus ensuring orthogonal brain stem coordinates, or 2) computing the correct tensor components for velocity-orientation multiplication in arbitrary coordinates. Both solutions provided an ideal VOR. A similar problem occurred with partial canal or muscle damage. Altering a single brain stem transformation was insufficient because the resulting coordinate changes rendered the multiplication tensor inappropriate. This was solved by either recomputing the multiplication tensor, or recomputing the appropriate internal sensory or motor matrix to normalize and reorthogonalize the brain stem. In either case, the multiplication tensor had to be correctly matched to its coordinate system. This illustrates that neural coordinate transformations affect not only serial/parallel projections in the brain, but also lateral projections associated with computations within networks/nuclei. Consequently, a simple progression from sensory to motor coordinates may not be optimal. We hypothesize that the VOR uses a dual coordinate transformation (i.e., both sensory and motor) to optimize intermediate brain stem coordinates, and then sets the appropriate internal tensor for these coordinates. We further hypothesize that each of these processes should optimally be capable of specific, experimentally identifiable adjustments for motor learning and recovery from damage.

Human vestibuloocular reflex and its interactions with vision and fixation distance during linear and angular head movement

Human vestibuloocular reflex and its interactions with vision and fixation distance during linear and angular head movement. J. Neurophysiol. 80: 2391-2404, 1998. The vestibuloocular reflex (VOR) maintains visual image stability by generating eye movements that compensate for both angular (AVOR) and linear (LVOR) head movements, typically in concert with visual following mechanisms. The VORs are generally modulated by the "context" in which head movements are made. Three contextual influences on VOR performance were studied during passive head translations and rotations over a range of frequencies (0.5-4 Hz) that emphasized shifting dynamics in the VORs and visual following, primarily smooth pursuit. First, the dynamic characteristics of head movements themselves ("stimulus context") influence the VORs. Both the AVOR and LVOR operate with high-pass characteristics relative to a head velocity input, although the cutoff frequency of the AVOR (<0.1 Hz) is far below that of the LVOR ( approximately 1 Hz), and both perform well at high frequencies that exceed, but complement, the capabilities of smooth pursuit. Second, the LVOR and AVOR are modulated by fixation distance, implemented with a signal related to binocular vergence angle ("fixation context"). The effect was quantified by analyzing the response during each trial as a linear relationship between LVOR sensitivity (in deg/cm), or AVOR gain, and vergence (in m-1) to yield a slope (vergence influence) and an intercept (response at 0 vergence). Fixation distance (vergence) was modulated by presenting targets at different distances. The response slope rises with increasing frequency, but much more so for the LVOR than the AVOR, and reflects a positive relationship for all but the lowest stimulus frequencies in the AVOR. A third influence is the context of real and imagined targets on the VORs ("visual context"). This was studied in two ways-when targets were either earth-fixed to allow visual enhancement of the VOR or head-fixed to permit visual suppression. The VORs were assessed by extinguishing targets for brief periods while subjects continued to "fixate" them in darkness. The influences of real and imagined targets were most robust at lower frequencies, declining as stimulus frequency increased. The effects were nearly gone at 4 Hz. These properties were equivalent for the LVOR and AVOR and imply that the influences of real and imagined targets on the VORs generally follow low-pass and pursuit-like dynamics. The influence of imagined targets accounts for roughly one-third of the influence of real targets on the VORs at 0.5 Hz.

Hypothesis for shared central processing of canal and otolith signals

A common goal of the translational vestibuloocular reflex (TVOR) and the rotational vestibuloocular reflex (RVOR) is to stabilize visual targets on the retinae during head movement. However, these reflexes differ significantly in their dynamic characteristics at both sensory and motor levels, implying a requirement for different central processing of canal and otolith signals. Semicircular canal afferents carry a signal proportional to angular head velocity, whereas primary otolith afferents modulate approximately in phase with linear head acceleration. Behaviorally, the RVOR exhibits a robust response down to approximately 0.01 Hz, yet the TVOR is only significant above approximately 0.5 Hz. Several hypotheses were proposed to address central processing in the TVOR pathways. All rely on a central filtering process that precedes a "neural integrator" shared with the RVOR. We propose an alternative hypothesis for the convergence of canal and otolith signals that does not impose the requirement for additional low-pass filters for the TVOR. The approach is demonstrated using an anatomically based, simple model structure that reproduces the general dynamic characteristics of the RVOR and TVOR at both ocular and central levels. Differential dynamic processing of otolith and canal signals is achieved by virtue of the location at which sensory information enters a shared but distributed neural integrator. As a result, only the RVOR is provided with compensation for the eye plant. Hence canal and otolith signals share a common central integrator, as in previous hypotheses. However, we propose that the required additional filtering of otolith signals is provided by the eye plant.

Dynamics of squirrel monkey linear vestibuloocular reflex and interactions with fixation distance

Horizontal, vertical, and torsional eye movements were recorded using the magnetic search-coil technique during linear accelerations along the interaural (IA) and dorsoventral (DV) head axes. Four squirrel monkeys were translated sinusoidally over a range of frequencies (0.5-4.0 Hz) and amplitudes (0.1-0.7 g peak acceleration). The linear vestibuloocular reflex (LVOR) was recorded in darkness after brief presentations of visual targets at various distances from the subject. With subjects positioned upright or nose-up relative to gravity, IA translations generated conjugate horizontal (IA horizontal) eye movements, whereas DV translations with the head nose-up or right-side down generated conjugate vertical (DV vertical) responses. Both were compensatory for linear head motion and are thus translational LVOR responses. In concert with geometric requirements, both IA-horizontal and DV-vertical response sensitivities (in deg eye rotation/cm head translation) were related linearly to reciprocal fixation distance as measured by vergence (in m-1, or meter-angles, MA). The relationship was characterized by linear regressions, yielding sensitivity slopes (in deg.cm-1.MA-1) and intercepts (sensitivity at 0 vergence). Sensitivity slopes were greatest at 4.0 Hz, but were only slightly more than half the ideal required to maintain fixation. Slopes declined with decreasing frequency, becoming negligible at 0.5 Hz. Small responses were observed when vergence was zero (intercept), although no response is required. Like sensitivity slope, the intercept was largest at 4.0 Hz and declined with decreasing frequency. Phase lead was near zero (compensatory) at 4.0 Hz, but increased as frequency declined. Changes in head orientation, motion axis (IA vs. DV), and acceleration amplitude produced slight and sporadic changes in LVOR parameters. Translational LVOR response characteristics are consistent with high-pass filtering within LVOR pathways. Along with horizontal eye movements, IA translation generated small torsional responses. In contrast to the translational LVORs, IA-torsional responses were not systematically modulated by vergence angle. The IA-torsional LVOR is not compensatory for translation because it cannot maintain image stability. Rather, it likely compensates for the effective head tilt simulated by translation. When analyzed in terms of effective head tilt, torsional responses were greatest at the lowest frequency and declined as frequency increased, consistent with low-pass filtering of otolith input. It is unlikely that IA-torsional responses compensate for actual head tilt, however, because they were similar for both upright and nose-up head orientations. The IA-torsional and -horizontal LVORs seem to respond only to linear acceleration along the IA head axis, and the DV-vertical LVOR to acceleration along the head's DV axis, regardless of gravity.

Modulation of vergence by off-vertical yaw axis rotation in the monkey: normal characteristics and effects of space flight

Horizontal movements of both eyes were recorded simultaneously using scleral search coils in 2 rhesus monkeys before and after the COSMOS 2229 space-flight of 1992-1993. Another 9 monkeys were tested at comparable time intervals and served as controls. Ocular vergence, defined as the difference in horizontal position between the left and right eyes, was measured during off-vertical yaw axis rotation (OVAR) in darkness. Vergence was modulated sinusoidally as a function of head position with regard to gravity during OVAR. The amplitude of peak-to-peak modulation increased with increments in tilt of the angle of the rotational axis (OVAR tilt angle) that ranged from 15 degrees to 90 degrees. Of the 11 monkeys tested, 1 had no measurable modulation in vergence. In the other 10, the mean amplitude of the peak to peak modulation was 5.5 degrees +/- 1.3 degrees at 90 degrees tilt. Each of these monkeys had maximal vergence when its nose was pointed close to upward (gravity back; mean phase: -0.9 degree +/- 26 degrees). After space flight, the modulation in vergence was reduced by over 50% for the two flight monkeys, but the phase of vergence modulation was not altered. The reduction in vergence modulation was sustained for the 11-day postflight testing period. We conclude that changes in vergence are induced in monkeys by the sinusoidal component of gravity acting along the naso-occipital axis during yaw axis OVAR, and that the modulation of the vergence reflex is significantly less sensitive to linear acceleration after space flight.

Modeling the organization of the linear and angular vestibulo-ocular reflexes

A one-dimensional mathematical model of the compensatory linear vestibuloocular reflex (lVOR) was developed. The model was based on the concept that to effect oculomotor compensation, linear head acceleration sensed by the otoliths must be integrated twice to form the angular position-related signal required by the motoneurons. This contradicts the postulate that linear acceleration is differentiated to generate "jerk," which is then used to drive the compensatory lVOR. The transfer characteristics of different otolith afferent classes were modeled by a transfer function with a common modal structure and different degrees of compensation. Both the time and frequency domain behavior of regular and irregular otolith afferents were simulated. The outputs of the various afferent classes were superposed by a linear filter to generate the velocity command which drives the oculomotor velocity-position integrator. The model was used to simulate the dominant gain and phase characteristics of the compensatory lVOR in monkey and the dynamic characteristics of the compensatory human lVOR response for brief periods of linear acceleration on a sled. The model was then combined with the velocity storage-based model of the angular vestibulo-ocular reflex (aVOR) to simulate the eye velocity response to centrifugation in monkey and man. The model suggests that the orientation response that modifies the time constants of the velocity storage integrator is the dominant aspect of the response to linear acceleration in monkey. Human responses, on the other hand, are dominated by an effect of the beating field, which modifies the eye velocity command to the oculomotor system.