Dynamic otolith stimulation improves the low frequency horizontal vestibulo-ocular reflex

Semicircular Canal Influences on the Developmental Tuning of the Translational Vestibulo-Ocular Reflex

Vestibulo-ocular reflexes (VORs) rely on neuronal computations that transform vestibular sensory signals into spatio-temporally appropriate extraocular motor commands. The motoneuronal discharge for contractions of the superior oblique eye muscle during linear translation derives from a utricular epithelial sector that is spatially aligned with the pulling direction of this muscle. In Xenopus laevis, the alignment is gradually achieved during larval development and requires motion-related semicircular canal afferent activity. Here, we studied the origin of semicircular canal and utricular signals responsible for the establishment and maturation of the extraocular motor response vector. Experiments were conducted on semi-intact preparations of Xenopus tadpoles before and after unilateral transection of the VIIIth nerve and in preparations of animals in which semicircular canal formation was prevented on one side by the injection of hyaluronidase into the otic capsule prior to the establishment of the tubular structures. Unilateral VIIIth nerve sections revealed that the excitation underlying the contraction of the superior oblique eye muscle during horizontal linear acceleration and clockwise/counter-clockwise roll motion derives exclusively from the utricle and the posterior semicircular canal on the ipsilateral side. In contrast, the developmental constriction of the otolith response vector depends on signals from the posterior semicircular canal on the contralateral side. These latter signals suppress directionally incorrect components that derive from the utricular sector perpendicular to the superior oblique eye muscle. This directional tuning complies with a stabilization of spatially correct utricular inputs that are aligned with the extraocular motor target muscle. In addition, misaligned signals are concurrently suppressed by semicircular canal-related commissural pathways from the contralateral side and through local interneuronal inhibitory circuits within the ipsilateral vestibular nuclei.

Potential solutions to several vestibular challenges facing clinicians

Among other problems, patients with vestibular problems suffer imbalance, spatial disorientation, and blurred vision. These problems lead to varying degrees of disability and can be debilitating. Unfortunately, a large number of patients with vestibular complaints cannot be diagnosed with the clinical tests available today. Nor do we have treatments for all patients that we can diagnose. These clinical problems provide challenges to and opportunities for the field of vestibular research. In this paper, we discuss some new diagnostic and treatment options that could become available for tomorrow's patients. As a new diagnostic, we have begun measuring patient's perceptual direction-detection thresholds. Preliminary results appear encouraging; patients diagnosed with bilateral loss have yaw rotation thresholds almost ten times greater than normals, while patients diagnosed with migraine associated vertigo have roll tilt thresholds well below normal at 0.1 Hz. As a new treatment, we have performed animal studies looking at responses evoked by electrical stimulation provided by a vestibular prosthesis. Results measuring the VOR demonstrate promise and preliminary studies of balance and perception are also encouraging. While electrical stimulation is a standard means of stimulation, optical stimulation is also being investigated as a way to improve prosthetic stimulation specificity.

Canal and otolith contributions to compensatory tilt responses in pigeons

Gaze-stabilizing eye and head responses compensate more effectively for low-frequency rotational motion when such motion stimulates the otolith organs, as during earth-horizontal axis rotations. However, the nature of the otolith signal responsible for this improvement in performance has not been previously determined. In this study, we used combinations of earth-horizontal axis rotational and translational motion to manipulate the magnitude of net linear acceleration experienced by pigeons, under both head-fixed and head-free conditions. We show that phase enhancement of eye and head responses to low-frequency rotational motion was causally related to the magnitude of dynamic net linear acceleration and not the gravitational acceleration component. We also show that canal-driven and otolith-driven eye responses were both spatially and temporally appropriate to combine linearly, and that a simple linear model combining canal- and otolith-driven components predicted eye responses to complex motion that were consistent with our experimental observations. However, the same model did not predict the observed head responses, which were spatially but not temporally appropriate to combine according to the same linear scheme. These results suggest that distinct vestibular processing substrates exist for eye and head responses in pigeons and that these are likely different from the vestibular processing substrates observed in primates.

Recovery of gaze stability during vestibular regeneration

Many motion related behaviors, such as gaze stabilization, balance, orientation, and navigation largely depend on a properly functioning vestibular system. After vestibular insult, many of these responses are compromised but can return during the regeneration of vestibular receptors and afferents as is known to occur in birds, reptiles, and amphibians. Here we characterize gaze stability in pigeons to rotational motion during regeneration after complete bilateral vestibular loss via an ototoxic antibiotic. Immediate postlesion effects included severe head oscillations, postural ataxia, and total lack of gaze control. We found that these abnormal behaviors gradually subsided, and gaze stability slowly returned to normal function according to a temporal sequence that lasted several months. We also found that the dynamic recovery of gaze function during regeneration was not homogeneous for all types of motion. Instead high-frequency motion stability was first achieved, followed much later by slow movement stability. In addition, we found that initial gaze stability was established using almost exclusive head-response components with little eye-movement contribution. However, that trend reversed as recovery progressed so that when gaze stability was complete, the eye component had increased and the head response had decreased to levels significantly different from that observed in normal birds. This was true even though the head-fixed VOR response recovered normally. Recovery of gaze stability coincided well with the three stage temporal sequence of morphologic regeneration previously described by our laboratory.

Otolith Deprivation Induces Optokinetic Compensation

According to the multisensory integration theory vestibular, optokinetic and proprioceptive inputs act in concert to maintain a stable retinal image of the visual world. Yet, it remains elusive to what extent the otolith organs contribute to this process and whether a specific loss of otolith input is compensated for. Here we investigated the compensatory eye movements in tilted mice, which lack otoconia because of a mutation in otopetrin 1. Tilted mice showed very small displacements of the eyes in the orbit during static roll paradigms, suggesting the absence of functional otolith organs. Independent of head position with respect to gravity, the gain and phase lead of angular vestibuloocular reflex of tilted mice were decreased and increased, respectively (frequencies 0.2 to 1 Hz and peak accelerations 8 to 197°/s2, respectively). Furthermore, lack of otolith input increases the dependency of the vestibular system on stimulus frequency. In contrast, the gain of optokinetic reflex in tilted mice was significantly higher in the low-frequency range than in control mice, regardless of the position of the mice in space or the plane of the eye movements. To explain these results, a simple model was used in which a multisensory integration unit was embedded. With this model, we were able to simulate all the behaviors observed. Thus our data and the model support the presence of the multisensory integration system and revealed a compensatory enhanced optokinetic reflex in tilted mice, indicating an adaptive synergism in the processing of otolith and visually driven signals.

Gravity and perceptual stability during translational head movement on earth and in microgravity

We measured the amount of visual movement judged consistent with translational head movement under normal and microgravity conditions. Subjects wore a virtual reality helmet in which the ratio of the movement of the world to the movement of the head (visual gain) was variable. Using the method of adjustment under normal gravity 10 subjects adjusted the visual gain until the visual world appeared stable during head movements that were either parallel or orthogonal to gravity. Using the method of constant stimuli under normal gravity, seven subjects moved their heads and judged whether the virtual world appeared to move "with" or "against" their movement for several visual gains. One subject repeated the constant stimuli judgements in microgravity during parabolic flight. The accuracy of judgements appeared unaffected by the direction or absence of gravity. Only the variability appeared affected by the absence of gravity. These results are discussed in relation to discomfort during head movements in microgravity.

Asymmetric adaptive gain changes of the vertical vestibulo-ocular reflex in cats

The present study was conducted to examine adaptive gain changes of vertical vestibulo-ocular reflex (VOR) after exposure to a vertical visual-vestibular mismatch in cats. The visual-vestibular mismatch was induced by oscillating the animals for an hour about an inter-aural axis at frequencies of 0.16 and 0.32 Hz with the peak velocity of 20 degrees/s, coupled with either in-phase ("gain decrease" conditioning) or out-of-phase ("gain increase" conditioning) sinusoidal rotation of a random-dot pattern. Eye movements were measured with a magnetic search coil system. Before conditioning, vertical VOR showed up-down asymmetric responses. That is, upward slow phase eye velocity (SPV) in response to downward head rotation was significantly larger than downward SPV in response to upward head rotation. After adaptation to "gain increase" conditioning, VOR gain increased in both stimulus directions. The increase in VOR gain was significantly larger for upward SPV than for downward SPV. After adaptation to "gain decrease" conditioning, VOR gain decreased in both stimulus directions. The decrease in VOR gain was, however, significantly larger for downward SPV than for upward SPV. Our results indicate that VOR in the vertical plane adaptively changes but that the gain change shows a directional asymmetry. This asymmetry was dependent on the direction of the slip of visual image rather than the direction of head rotation, and the gain change was smaller when the retinal slip was generated downward. Possible explanations for the asymmetry are discussed on a physiological and anatomical basis.

Vestibular gaze stabilization: different behavioral strategies for arboreal and terrestrial avians

In birds, it is thought that head movements play a major role in the reflexive stabilization of gaze and vision. In this study, we investigated the contributions of the eye and head to gaze stabilization during rotations under both head-fixed [vestibuloocular (VOR)] and head-free conditions in two avian species: pigeons and quails. These two species differ both in ocular anatomy (the pigeon has 2 distinct foveal regions), as well as in behavioral repertoires. Pigeons are arboreal, fly extended distances, and can navigate. Quails are primarily engrossed in terrestrial niches and fly only short distances. Unlike the head-fixed VOR gains that were under-compensatory for both species, gaze gains under head-free conditions were completely compensatory at high frequencies. This compensation was achieved primarily with head movements in pigeons, but with combined head and eye-in-head contributions in the quail. In contrast, eye-in-head motion, which was significantly reduced for head-free compared with head-fixed conditions, contributed very little to overall gaze stability in pigeons. These results suggest that disparity between the stabilization strategies employed by these two birds may be attributed to differences in species-specific behavior and anatomy.

Human 3-D aVOR with and without otolith stimulation

We describe in detail the frequency response of the human three-dimensional angular vestibulo-ocular response (3-D aVOR) over a frequency range of 0.05-1 Hz. Gain and phase of the human aVOR were determined for passive head rotations in the dark, with the rotation axis either aligned with or perpendicular to the direction of gravity (earth-vertical or earth-horizontal). In the latter case, the oscillations dynamically stimulated both the otolith organs and the semi-circular canals. We conducted experiments in pitch and yaw, and compared the results with previously-published roll data. Regardless of the axis of rotation and the orientation of the subject, the gain in aVOR increased with frequency to about 0.3 Hz, and was approximately constant from 0.3 to 1 Hz. The aVOR gain during pitch and yaw rotations was larger than during roll rotations. Otolith and canal cues combined differently depending upon the axis of rotation: for torsional and pitch rotations, aVOR gain was higher with otolith input; for yaw rotations the aVOR was not affected by otolith stimulation. There was a phase lead in all three dimensions for frequencies below 0.3 Hz when only the canals were stimulated. For roll and pitch rotations this phase lead vanished with dynamic otolith stimulation. In contrast, the horizontal phase showed no improvement with additional otolith input during yaw rotations. The lack of a significant otolith contribution to the yaw aVOR was observed when subjects were supine, prone or lying on their sides. Our results confirm studies with less-natural stimuli (off-vertical axis rotation) that the otoliths contribute a head-rotation signal to the aVOR. However, the magnitude of the contribution depends on the axis of rotation, with the gain in otolith-canal cross-coupling being smallest for yaw axis rotations. This could be because, in humans, typical yaw head movements will stimulate the otoliths to a much lesser extent then typical pitch and roll head movements.

Cerebellar signatures of vestibulo-ocular reflex motor learning

The vestibulo-ocular reflex (VOR) comprises an outstanding system to perform studies that probe possible cerebellar roles in motor learning. Novel VOR gains can be induced (learned) by the wearing of minifying or magnifying lenses, and learning requires the presence of the cerebellum. Previously, it was shown that Purkinje cells change their head velocity sensitivities with learning and that this change was thought to be inappropriate to be causal for the changed behavior. We now demonstrate that Purkinje cells also change their eye position, eye velocity, and head velocity sensitivities after learning. These combined changes at the Purkinje cell level contribute to a net modulation that is appropriate to support the new VOR gains. Importantly, the changes in the eye position parameter, reported for the first time, suggest the involvement of the neuronal integrator pathways in VOR learning. We provide evidence that all of these changes are necessary for VOR behavior and can explain learning deficits after cerebellar removal.

The vestibulo ocular reflex (VOR) in otoconia deficient head tilt (het) mutant mice versus wild type C57BL/6 mice

The horizontal and vertical vestibulo ocular reflex (VOR) of head tilted (het) mutant mice was compared to C57BL/6 controls. Eye movements were recorded in darkness using a temporarily attached search coil. Contributions of semicircular canal versus otolith organ signals were investigated by providing a canal only (vertical axis) or canal plus otolith organ (horizontal axis) stimulus. In controls, rotations that stimulated only the canals (upright yaw and on tail roll) produced accurate VOR timing during middle frequency rotations at 0.5 Hz (gain 0.27, phase error 6 degrees), and a phase advanced VOR during low frequency rotations at 0.05 Hz (0.05, 115 degrees). In het mutant mice, these rotations produced a highly attenuated VOR response and phase errors at both 0.5 Hz (0.11, 42 degrees) and 0.05 Hz (0.01, 36 degrees). In controls, rotations that stimulated both the otolith organs and semicircular canals (upright roll and on tail yaw) produced higher VOR gains overall than were elicited during vertical axis rotations, with phase accurate VOR at both 0.5 Hz (0.52, 4 degrees) and 0.05 Hz (0.34, 9 degrees). In het mutant mice, these rotations produced a highly attenuated VOR response and phase errors at both 0.5 Hz (0.14, 51 degrees) and 0.05 Hz (0.01, 43 degrees). During constant velocity rotations about an earth horizontal axis, eye velocity bias and modulation were virtually absent in het mutant mice, while robust in controls. Control mice produced compensatory ocular deviations in response to static head tilt, but responses in het mice were weak and inconsistent. These results show that het mice not only lack all aspects of otolith mediated VOR, but also are deficient in canal mediated VOR. Because semicircular canals are normal in het mice, we conclude that central neurons of the canal VOR are dependent on otolith organ signals for normal performance.

Otolith function: basis for modern testing

The major challenge in developing a robust test of otolith function, particularly with regard to linear vestibulo-ocular reflex (LVOR) and perceptual measures, is to find a way in which graded lesions are reflected in graded response properties and abnormalities. The ability of the vestibulo-ocular reflex (VOR) to compensate and adapt to dysfunction and pathology presents formidable challenges for registering localizing clinical findings, whether in the angular vestibulo-ocular reflex (AVOR), the LVOR, or both. Based on a variety of considerations, various forms of eccentric rotation seem to provide the most convenient, and potentially the most useful, means to generate motion profiles from which otolith function can be directly assessed. Both translational and tilt responses can be recorded depending on the stimulus profile. The near-centric version is particularly enticing because of the ability to study one labyrinth at a time, much like calorics. In that case and in others in which the tilt-LVOR is prominent, measures of the perceived visual vertical are useful and by all accounts similar to ocular torsion. The latter does hold the important advantage of being an objective measure, requiring no intervention on the part of the patient. The translational-LVOR can be derived from eccentric rotation responses with the head displaced forward as well as backward, while viewing near targets in hopes of generating a large addition or subtraction (even inversion) of an otherwise AVOR-driven reflex. These considerations provide an impetus to pursue improved methods of quantifying otolith function in a clinical population. The sobering caveat is that the diagnosis of total unilateral vestibular loss presents little challenge either clinically or by classic testing (e.g., calorics), and yet most of our efforts in developing quantifiable measures of dysfunction over the years have yielded results that are modest and hardly compelling.

Principles of linear and angular vestibuloocular reflex organization in the frog

We compared the spatial organization patterns of linear and angular vestibuloocular reflexes in frogs by recording the multiunit spike activity from cranial nerve branches innervating the lateral rectus, the inferior rectus, or the inferior obliquus eye muscles. Responses were evoked by linear horizontal and/or vertical accelerations on a sled or by angular accelerations about an earth-vertical axis on a turntable. Before each sinusoidal oscillation test in darkness, the static head position was systematically altered to determine those directions of horizontal linear acceleration and those planes of angular head oscillation that were associated with minimal response amplitudes. Inhibitory response components during angular accelerations were clearly present, whereas inhibitory response components during linear accelerations were absent. Likewise was no contribution from the vertical otolith organs (i.e., lagena and saccule) observed during vertical linear acceleration. Horizontal linear acceleration evoked responses that originated from eye muscle-specific sectors on the contralateral utricular macula. The sectors of the inferior obliquus and lateral rectus muscles on the utricle had an opening angle of 45 and 60 degrees, respectively and overlapped to a large extent in the laterorostral part of the utricle. Both sectors were coplanar with the horizontal semicircular canals. The sector of the inferior rectus muscle was narrow (opening 5 degrees), laterocaudally oriented, and slightly pitched up by 6 degrees. Angular acceleration evoked maximal responses in the inferior obliquus muscle nerve that originated from the ipsilateral horizontal and the contralateral anterior vertical canals in a ratio of 50:50. Lateral rectus excitation originated from the contralateral horizontal and anterior vertical semicircular canals in a ratio of 80:20. The excitatory responses of the inferior rectus muscle nerve originated exclusively from the contralateral posterior vertical canal. Measured data and known semicircular canal plane vectors were used to calculate the spatial orientation of maximum sensitivity vectors for the investigated eye muscle nerves in semicircular canal coordinates. Comparison of the directions of maximal sensitivity vectors of responses evoked by linear or angular accelerations in a given eye muscle nerve showed that the two vector directions were oriented about orthogonally with respect to each other. With this arrangement the linear and the angular vestibuloocular reflex can support each other dynamically whenever they are co-activated without a change in the spatial response characteristics. The mutual adaptation of angular and linear vestibuloocular reflexes as well as the differences in their organization described here for frogs may represent a basic feature common for vertebrates in general.

Three-dimensional organization of vestibular related eye movements to rotational motion in pigeons

During rotational motions, compensatory eye movement adjustments must continually occur in order to maintain objects of visual interest as stable images on the retina. In the present study, the three-dimensional organization of the vestibulo-ocular reflex in pigeons was quantitatively examined. Rotations about different head axes produced horizontal, vertical, and torsional eye movements, whose component magnitude was dependent upon the cosine of the stimulus axis relative to the animal's visual axis. Thus, the three-dimensional organization of the VOR in pigeons appears to be compensatory for any direction of head rotation. Frequency responses of the horizontal, vertical, and torsional slow phase components exhibited high pass filter properties with dominant time constants of approximately 3 s.

Characteristics of the VOR in response to linear acceleration

The primate linear VOR (LVOR) includes two forms. First, eye-movement responses to translation [e.g., horizontal responses to interaural (i.a.) motion] help maintain binocular fixation on targets, and therefore a stable bifoveal image. The translational LVOR is strongly modulated by fixation distance, and operates with high-pass dynamics (> 1 Hz). Second, other LVOR responses occur that cannot be compensatory for translation and instead seem compensatory for head tilt. This reflects an otolith response ambiguity--that is, an inability to distinguish head translation from head tilt relative to gravity. Thus, ocular torsion is appropriately compensatory for head roll-tilt, but also occurs during IA translation, since both stimuli entail IA acceleration. Unlike the IA-horizontal response, IA torsion behaves with low-pass dynamics (with respect to "tilt"), and is uninfluenced by fixation distance. Interestingly, roll-tilt, like IA translation, also produces both horizontal (a translational reflex) and torsional (a tilt reflex) responses, further emphasizing the ambiguity problem. Early data from subjects following unilateral labyrinthectomy, which demonstrates a general immediate decline in translational LVOR responses, are also presented, followed by only modest recovery over several months. Interestingly, the usual high-pass dynamics of these reflexes shift to an even higher cutoff. Both eyes respond roughly equally, suggesting that unilateral otolith input generates a binocularly symmetric LVOR.

Three-dimensional organization of otolith-ocular reflexes in rhesus monkeys. III. Responses To translation

The three-dimensional (3-D) properties of the translational vestibulo-ocular reflexes (translational VORs) during lateral and fore-aft oscillations in complete darkness were studied in rhesus monkeys at frequencies between 0.16 and 25 Hz. In addition, constant velocity off-vertical axis rotations extended the frequency range to 0.02 Hz. During lateral motion, horizontal responses were in phase with linear velocity in the frequency range of 2-10 Hz. At both lower and higher frequencies, phase lags were introduced. Torsional response phase changed more than 180 degrees in the tested frequency range such that torsional eye movements, which could be regarded as compensatory to "an apparent roll tilt" at the lowest frequencies, became anticompensatory at all frequencies above approximately 1 Hz. These results suggest two functionally different frequency bandwidths for the translational VORs. In the low-frequency spectrum (<<0.5 Hz), horizontal responses compensatory to translation are small and high-pass-filtered whereas torsional response sensitivity is relatively frequency independent. At higher frequencies however, both horizontal and torsional response sensitivity and phase exhibit a similar frequency dependence, suggesting a common role during head translation. During up-down motion, vertical responses were in phase with translational velocity at 3-5 Hz but phase leads progressively increased for lower frequencies (>90 degrees at frequencies <0.2 Hz). No consistent dependence on static head orientation was observed for the vertical response components during up-down motion and the horizontal and torsional response components during lateral translation. The frequency response characteristics of the translational VORs were fitted by "periphery/brain stem" functions that related the linear acceleration input, transduced by primary otolith afferents, to the velocity signals providing the input to the velocity-to-position neural integrator and the oculomotor plant. The lowest-order, best-fit periphery/brain stem model that approximated the frequency dependence of the data consisted of a second order transfer function with two alternating poles (at 0.4 and 7.2 Hz) and zeros (at 0.035 and 3.4 Hz). In addition to clearly differentiator dynamics at low frequencies (less than approximately 0.5 Hz), there was no frequency bandwidth where the periphery/brain stem function could be approximated by an integrator, as previously suggested. In this scheme, the oculomotor plant dynamics are assumed to perform the necessary high-frequency integration as required by the reflex. The detailed frequency dependence of the data could only be precisely described by higher order functions with nonminimum phase characteristics that preclude simple filtering of afferent inputs and might be suggestive of distributed spatiotemporal processing of otolith signals in the translational VORs.

Chronic recording of the vestibulo-ocular reflex in the restrained rat using a permanently implanted scleral search coil

A technique is described which allows accurate long-term monitoring of eye movements in the rat using permanently implanted scleral search coils. Search coils permanently sutured around the sclera yield vestibulo-ocular reflex (VOR) gain and phase values which are comparable to those reported previously in the literature using acutely implanted coils or electrooculographic electrodes. Considerations related to strain, sex and surgical procedures which permit measurement of responses in the chronically restrained rat are described. VOR gain and phase show a time course to their recovery following the implant surgery, with asymptotic performance typically attained approximately 10 days post-surgically. This technique, with the ability to monitor eye movements over weeks to months, appears ideal for development of rodent models of reflex adaptation which require observation of reflex behavior over extended periods of time. Development of a chronic procedure for monitoring eye movement in rodents is especially important given their initial response to restraint (extensive struggling). Finally, adaptation of this technique to smaller species (e.g., mouse) appears technically feasible which should permit the application of transgenic and knockout techniques to the determination of various vestibular reflex functions requiring long-term monitoring.

Canal-otolith interactions driving vertical and horizontal eye movements in the squirrel monkey

The vestibulo-ocular reflex (VOR) was studied in three squirrel monkeys subjected to rotations with the head either centered over, or displaced eccentrically from, the axis of rotation. This was done for several different head orientations relative to gravity in order to determine how canal-mediated angular (aVOR) and otolith-mediated linear (IVOR) components of the VOR are combined to generate eye movement responses in three-dimensional space. The aVOR was stimulated in isolation by rotating the head about the axis of rotation in the upright (UP), right-side down (RD), or nose-up (NU) orientations. Horizontal and vertical aVOR responses were compensatory for head rotation over the frequency range 0.25-4.0 Hz, with mean gains near 0.9. The horizontal aVOR was relatively constant across the frequency range, while vertical aVOR gains increased with increasing stimulation frequency. In the NU orientation, compensatory torsional aVOR responses were of relatively low gain (0.54) compared with horizontal and vertical responses, and gains remained constant over the frequency range. When the head was displaced eccentrically, rotation provided the same angular stimuli but added linear stimulus components, due to the centripetal and tangential accelerations acting on the head. By manipulating the orientation of the head relative to gravity and relative to the axis of rotation, the IVOR response could be combined with, or isolated from, the aVOR response. Eccentric rotation in the UP and RD orientations generated aVOR and IVOR responses which acted in the same head plane. Horizontal aVOR-IVOR interactions were recorded when the head was in the UP orientation and facing toward ("nose-in") or away from ("nose-out") the rotation axis. Similarly, vertical responses were recorded with the head RD and in the nose-out or nose-in positions. For both horizontal and vertical responses, gains were dependent on both the frequency of stimulation and the directions and relative amplitudes of the angular and linear motion components. When subjects were positioned nose-out, the angular and linear stimuli produced synergistic interactions, with the IVOR driving the eyes in the same direction as the aVOR. Gains increased with increasing frequency, consistent with an addition of broad-band aVOR and high-pass IVOR components. When subjects were nose-in, angular and linear stimuli generated eye movements in opposing directions, and gains declined with increasing frequency, consistent with a subtraction of the IVOR from the aVOR. This response pattern was identical for horizontal and vertical eye movements. aVOR and IVOR interactions were also assessed when the two components acted in orthogonal response planes. By rotating the monkeys into the NU orientation, the aVOR acted primarily in the roll plane, generating torsional ocular responses, while the translational (IVOR) component generated horizontal or vertical ocular responses, depending on whether the head was oriented such that linear accelerations acted along the interaural or dorsoventral axes, respectively. Horizontal and vertical IVOR responses were negligible at 0.25 Hz and increased dramatically with increasing frequency. Comparison of the combined responses (UP and RD orientations) with the isolated aVOR (head-centered) and IVOR (NU orientation) responses, indicates that these VOR components sum in a linear fashion during complex head motion.

Vestibular perception of passive whole-body rotation about horizontal and vertical axes in humans: goal-directed vestibulo-ocular reflex and vestibular memory-contingent saccades

This study was aimed at complementing the existing knowledge about vestibular perception of self-motion in humans. Both goal-directed vestibulo-ocular reflex and vestibular memory-contingent saccade (VMCS) tasks were used, respectively as concurrent and retrospective magnitude estimators for passive whole-body rotation. Rotations were applied about the earth-vertical and earth-horizontal axes to study the effect of the otolith signal in self-rotation evaluation, and both in yaw and pitch to examine the horizontal and vertical semi-circular canals. Two different magnitudes of constant angular acceleration (50 degrees/s2 and 100 degrees/s2) were used. The main findings were (1) strong correlation between both oculomotor responses of both tasks, (2) greater accuracy with rotations about the earth-vertical than the earth: -horizontal axis, (3) greater accuracy for yaw than for pitch rotations, (4) greater accuracy for high acceleration than for low, and (5) no effect of the delay (2 s or 12 s) in the VMCS task. Adequacy of both tasks as subjective magnitude estimators of vestibular perception of self-motion is discussed.

Excitation of the extraocular muscles in decerebrate cats during the vestibulo-ocular reflex in three-dimensional space

(1) Vestibulo-ocular reflex excitation of the six extraocular muscles was studied by recording their electromyographic activity in decerebrate cats during oscillations about horizontal and vertical axes, at frequencies from 0.07 to 4 Hz. Animals were oriented in many different positions and rotated about axes that lay in the horizontal, frontal, or sagittal planes defined by our coordinate system. (2) The strengths of modulation (gains) of the responses of all extraocular muscles were a sinusoidal function of the orientation of the rotation axis within a coordinate plane, and this function was nearly independent of rotation frequency. (3) The responses were used to determine an axis of maximal excitation for each of the extraocular muscles by the vestibulo-ocular reflex. Antagonistic muscle pairs were found to have best axes in nearly opposite directions, confirming their operation as pairs. (4) Excitation of the medial and lateral rectus could be explained by input from the paired horizontal semicircular canals, with essentially no convergent input from vertical canals. (5) Excitation of the vertical rectus and oblique muscles could be explained by convergent inputs from the vertical canals with little or no horizontal canal input.

Compensatory eye movements during head and body rotation in infants

Eye movements were measured in 2- and 4-month-old infants during rotation in the light and in the dark using a specially constructed infant seat that was rotated sinusoidally at 0.1, 0.25 and 0.5 Hz. As in adults, the phase of the vestibulo-ocular reflex (VOR) was nearly compensatory with respect to head position, and the gain values were generally in the range between 0.4 and 1.4. There were two types of phases for the cervico-ocular (COR) responses: compensatory and anticompensatory. The gain of the COR was 4-6 times larger than that reported for adults. In addition, it was both frequency- and age-dependent. These characteristics of the COR may reflect developmental processes in the infant nervous system. The interaction of the two reflexes (VOR + COR) gave responses similar to when the VOR was elicited alone. The subjects' use of a fixation light raised the gain of the VOR and VOR + COR to about 1.67 and 1.44, respectively. These values were higher than most gains reported for adults. However, they were close to what was predicted (1.53) for the experimental conditions wherein fixation was on a target 25 cm in front of the eyes.