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All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

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.

NASA Discipline Neuroscience↗
NASA NTRS2000

Role of muscle pulleys in producing eye position-dependence in the angular vestibuloocular reflex: a model-based study

It is well established that the head and eye velocity axes do not always align during compensatory vestibular slow phases. It has been shown that the eye velocity axis systematically tilts away from the head velocity axis in a manner that is dependent on eye-in-head position. The mechanisms responsible for producing these axis tilts are unclear. In this model-based study, we aimed to determine whether muscle pulleys could be involved in bringing about these phenomena. The model presented incorporates semicircular canals, central vestibular pathways, and an ocular motor plant with pulleys. The pulleys were modeled so that they brought about a rotation of the torque axes of the extraocular muscles that was a fraction of the angle of eye deviation from primary position. The degree to which the pulleys rotated the torque axes was altered by means of a pulley coefficient. Model input was head velocity and initial eye position data from passive and active yaw head impulses with fixation at 0 degrees, 20 degrees up and 20 degrees down, obtained from a previous experiment. The optimal pulley coefficient required to fit the data was determined by calculating the mean square error between data and model predictions of torsional eye velocity. For active head impulses, the optimal pulley coefficient varied considerably between subjects. The median optimal pulley coefficient was found to be 0.5, the pulley coefficient required for producing saccades that perfectly obey Listing's law when using a two-dimensional saccadic pulse signal. The model predicted the direction of the axis tilts observed in response to passive head impulses from 50 ms after onset. During passive head impulses, the median optimal pulley coefficient was found to be 0.21, when roll gain was fixed at 0.7. The model did not accurately predict the alignment of the eye and head velocity axes that was observed early in the response to passive head impulses. We found that this alignment could be well predicted if the roll gain of the angular vestibuloocular reflex was modified during the initial period of the response, while pulley coefficient was maintained at 0.5. Hence a roll gain modification allows stabilization of the retinal image without requiring a change in the pulley effect. Our results therefore indicate that the eye position-dependent velocity axis tilts could arise due to the effects of the pulleys and that a roll gain modification in the central vestibular structures may be responsible for countering the pulley effect.

NASA Discipline Neuroscience↗
NASA NTRS2000

Low-frequency otolith and semicircular canal interactions after canal inactivation

During sustained constant velocity and low-frequency off-vertical axis rotations (OVAR), otolith signals contribute significantly to slow-phase eye velocity. The adaptive plasticity of these responses was investigated here after semicircular canal plugging. Inactivation of semicircular canals results in a highly compromised and deficient vestibulo-ocular reflex (VOR). Based on the VOR enhancement hypothesis, one could expect an adaptive increase of otolith-borne angular velocity signals due to combined otolith/canal inputs after inactivation of the semicircular canals. Contrary to expectations, however, the steady-state slow-phase velocity during constant velocity OVAR decreased in amplitude over time. A similar progressive decrease in VOR gain was also observed during low-frequency off-vertical axis oscillations. This response deterioration was present in animals with either lateral or vertical semicircular canals inactivated and was limited to the plane(s) of the plugged canals. The results are consistent with the idea that the low-frequency otolith signals do not simply enhance VOR responses. Rather, the nervous system appears to correlate vestibular sensory information from the otoliths and the semicircular canals to generate an integral response to head motion.

Non-NASA Center↗
NASA NTRS2000

Anatomic patterning in the expression of vestibulosympathetic reflexes

To investigate the possibility that expression of vestibulosympathetic reflexes (VSR) is related to a nerve's anatomic location rather than its target organ, we compared VSR recorded from the same type of postganglionic fiber [muscle vasoconstrictor (MVC)] located at three different rostrocaudal levels: hindlimb, forelimb, and face. Experiments were performed on chloralose-anesthetized cats, and vestibular afferents were stimulated electrically. Single MVC unit activity was extracted by spike shape analysis of few-fiber recordings, and unit discrimination was confirmed by autocorrelation. Poststimulus time histogram analysis revealed that about half of the neurons were initially inhibited by vestibular stimulation (type 1 response), whereas the other MVC fibers were initially strongly excited (type 2 response). MVC units with types 1 and 2 responses were present in the same nerve fascicle. Barosensitivity was equivalent in the two groups, but fibers showing type 1 responses fired significantly faster than those giving type 2 responses (0.29 +/- 0.04 vs. 0.20 +/- 0.02 Hz). Nerve fibers with type 1 responses were most common in the hindlimb (21 of 29 units) and least common in the face (2 of 11 units), the difference in relative proportion being significant (P < 0.05, chi(2) test). These results support the hypothesis that VSR are anatomically patterned.

Non-NASA Center↗
NASA NTRS2000

Primate translational vestibuloocular reflexes. IV. Changes after unilateral labyrinthectomy

The effects of unilateral labyrinthectomy on the properties of the translational vestibuloocular reflexes (trVORs) were investigated in rhesus monkeys trained to fixate near targets. Translational motion stimuli consisted of either steady-state lateral and fore-aft sinusoidal oscillations or short-lasting transient displacements. During small-amplitude, steady-state sinusoidal lateral oscillations, a small decrease in the horizontal trVOR sensitivity and its dependence on viewing distance was observed during the first week after labyrinthectomy. These deficits gradually recovered over time. In addition, the vertical response component increased, causing a tilt of the eye velocity vector toward the lesioned side. During large, transient lateral displacements, the deficits were larger and longer lasting. Responses after labyrinthectomy were asymmetric, with eye velocity during movements toward the side of the lesion being more compromised. The most profound effect of the lesions was observed during fore-aft motion. Whereas responses were kinematically appropriate for fixation away from the side of the lesion (e.g., to the left after right labyrinthectomy), horizontal responses were anticompensatory during fixation at targets located ipsilateral to the side of the lesion (e.g., for targets to the right after right labyrinthectomy). This deficit showed little recovery during the 3-mo post-labyrinthectomy testing period. These results suggest that inputs from both labyrinths are important for the proper function of the trVORs, although the details of how bilateral signals are processed and integrated remain unknown.

NASA Discipline Neuroscience↗
NASA NTRS2000

Horizontal vestibuloocular reflex evoked by high-acceleration rotations in the squirrel monkey. III. Responses after labyrinthectomy

The horizontal angular vestibuloocular reflex (VOR) evoked by high-frequency, high-acceleration rotations was studied in four squirrel monkeys after unilateral labyrinthectomy. Spontaneous nystagmus was measured at the beginning and end of each testing session. During the period that animals were kept in darkness (4 days), the nystagmus at each of these times measured approximately 20 degrees /s. Within 18-24 h after return to the light, the nystagmus (measured in darkness) decreased to 2.8 +/- 1.5 degrees /s (mean +/- SD) when recorded at the beginning but was 20.3 +/- 3.9 degrees /s at the end of the testing session. The latency of the VOR measured from responses to steps of acceleration (3,000 degrees /s(2) reaching a velocity of 150 degrees /s) was 8.4 +/- 0.3 ms for responses to ipsilesional rotations and 7.7 +/- 0.4 ms for contralesional rotations. During the period that animals were kept in darkness after the labyrinthectomy, the gain of the VOR measured during the steps of acceleration was 0.67 +/- 0.12 for contralesional rotations and 0.39 +/- 0.04 for ipsilesional rotations. Within 18-24 h after return to light, the VOR gain for contralesional rotations increased to 0.87 +/- 0.08, whereas there was only a slight increase for ipsilesional rotations to 0.41 +/- 0. 06. A symmetrical increase in the gain measured at the plateau of head velocity was noted after the animals were returned to light. The VOR evoked by sinusoidal rotations of 2-15 Hz, +/-20 degrees /s, showed a better recovery of gain at lower (2-4 Hz) than at higher (6-15 Hz) frequencies. At 0.5 Hz, gain decreased symmetrically when the peak amplitude was increased from 20 to 100 degrees /s. At 10 Hz, gain was decreased for ipsilesional half-cycles and increased for contralesional half-cycles when velocity was raised from 20 to 50 degrees /s. A model incorporating linear and nonlinear pathways was used to simulate the data. Selective increases in the gain for the linear pathway accounted for the recovery in VOR gain for responses at the velocity plateau of the steps of acceleration and for the sinusoidal rotations at lower peak velocities. The increase in gain for contralesional responses to steps of acceleration and sinusoidal rotations at higher frequencies and velocities was due to an increase in the contribution of the nonlinear pathway. This pathway was driven into cutoff and therefore did not affect responses for rotations toward the lesioned side.

NASA Discipline Neuroscience↗
NASA NTRS2000

Convergent properties of vestibular-related brain stem neurons in the gerbil

Three classes of vestibular-related neurons were found in and near the prepositus and medial vestibular nuclei of alert or decerebrate gerbils, those responding to: horizontal translational motion, horizontal head rotation, or both. Their distribution ratios were 1:2:2, respectively. Many cells responsive to translational motion exhibited spatiotemporal characteristics with both response gain and phase varying as a function of the stimulus vector angle. Rotationally sensitive neurons were distributed as Type I, II, or III responses (sensitive to ipsilateral, contralateral, or both directions, respectively) in the ratios of 4:6:1. Four tested factors shaped the response dynamics of the sampled neurons: canal-otolith convergence, oculomotor-related activity, rotational Type (I or II), and the phase of the maximum response. Type I nonconvergent cells displayed increasing gains with increasing rotational stimulus frequency (0.1-2.0 Hz, 60 degrees /s), whereas Type II neurons with convergent inputs had response gains that markedly decreased with increasing translational stimulus frequency (0.25-2.0 Hz, +/-0.1 g). Type I convergent and Type II nonconvergent neurons exhibited essentially flat gains across the stimulus frequency range. Oculomotor-related activity was noted in 30% of the cells across all functional types, appearing as burst/pause discharge patterns related to the fast phase of nystagmus during head rotation. Oculomotor-related activity was correlated with enhanced dynamic range compared with the same category that had no oculomotor-related response. Finally, responses that were in-phase with head velocity during rotation exhibited greater gains with stimulus frequency increments than neurons with out-of-phase responses. In contrast, for translational motion, neurons out of phase with head acceleration exhibited low-pass characteristics, whereas in-phase neurons did not. Data from decerebrate preparations revealed that although similar response types could be detected, the sampled cells generally had lower background discharge rates, on average one-third lower response gains, and convergent properties that differed from those found in the alert animals. On the basis of the dynamic response of identified cell types, we propose a pair of models in which inhibitory input from vestibular-related neurons converges on oculomotor neurons with excitatory inputs from the vestibular nuclei. Simple signal convergence and combinations of different types of vestibular labyrinth information can enrich the dynamic characteristics of the rotational and translational vestibuloocular responses.

Non-NASA Center↗
NASA NTRS2000

Primate translational vestibuloocular reflexes. III. Effects of bilateral labyrinthine electrical stimulation

The effects of functional, reversible ablation and potential recruitment of the most irregular otolith afferents on the dynamics and sensitivity of the translational vestibuloocular reflexes (trVORs) were investigated in rhesus monkeys trained to fixate near and far targets. Translational motion stimuli consisted of either steady-state lateral and fore-aft sinusoidal oscillations or short-lasting transient lateral head displacements. Short-duration (usually <2 s) anodal (inhibitory) and cathodal (excitatory) currents (50-100 microA) were delivered bilaterally during motion. In the presence of anodal labyrinthine stimulation, trVOR sensitivity and its dependence on viewing distance were significantly decreased. In addition, anodal currents significantly increased phase lags. During transient motion, anodal stimulation resulted in significantly lower initial eye acceleration and more sluggish responses. Cathodal currents tended to have opposite effects. The main characteristics of these results were simulated by a simple model where both regularly and irregularly discharging afferents contribute to the trVORs. Anodal labyrinthine currents also were found to decrease eye velocity during long-duration, constant velocity rotations, although results were generally more variable compared with those during translational motion.

NASA Program Biomedical Research and Countermeasur↗
NASA NTRS2000

Primate translational vestibuloocular reflexes. II. Version and vergence responses to fore-aft motion

To maintain binocular fixation on near targets during fore-aft translational disturbances, largely disjunctive eye movements are elicited the amplitude and direction of which should be tuned to the horizontal and vertical eccentricities of the target. The eye movements generated during this task have been investigated here as trained rhesus monkeys fixated isovergence targets at different horizontal and vertical eccentricities during 10 Hz fore-aft oscillations. The elicited eye movements complied with the geometric requirements for binocular fixation, although not ideally. First, the corresponding vergence angle for which the movement of each eye would be compensatory was consistently less than that dictated by the actual fixation parameters. Second, the eye position with zero sensitivity to translation was not straight ahead, as geometrically required, but rather exhibited a systematic dependence on viewing distance and vergence angle. Third, responses were asymmetric, with gains being larger for abducting and downward compared with adducting and upward gaze directions, respectively. As frequency was varied between 4 and 12 Hz, responses exhibited high-pass filter properties with significant differences between abduction and adduction responses. As a result of these differences, vergence sensitivity increased as a function of frequency with a steeper slope than that of version. Despite largely undercompensatory version responses, vergence sensitivity was closer to ideal. Moreover, the observed dependence of vergence sensitivity on vergence angle, which was varied between 2.5 and 10 MA, was largely linear rather than quadratic (as geometrically predicted). We conclude that the spatial tuning of eye velocity sensitivity as a function of gaze and viewing distance follows the general geometric dependencies required for the maintenance of foveal visual acuity. However, systematic deviations from ideal behavior exist that might reflect asymmetric processing of abduction/adduction responses perhaps because of different functional dependencies of version and vergence eye movement components during translation.

NASA Program Biomedical Research and Countermeasur↗
NASA NTRS2000

Primate translational vestibuloocular reflexes. I. High-frequency dynamics and three-dimensional properties during lateral motion

The dynamics and three-dimensional (3-D) properties of the primate translational vestibuloocular reflex (trVOR) for high-frequency (4-12 Hz, +/-0.3-0.4 g) lateral motion were investigated during near-target viewing at center and eccentric targets. Horizontal response gains increased with frequency and depended on target eccentricity. The larger the horizontal and vertical target eccentricity, the steeper the dependence of horizontal response gain on frequency. In addition to horizontal eye movements, robust torsional response components also were present at all frequencies. During center-target fixation, torsional response phase was opposite (anticompensatory) to that expected for an "apparent" tilt response. Instead torsional response components depended systematically on vertical-target eccentricity, increasing in amplitude when looking down and reversing phase when looking up. As a result the trVOR eye velocity vector systematically tilted away from a purely horizontal direction, through an angle that increased with vertical eccentricity with a slope of approximately 0.7. This systematic dependence of torsional eye velocity tilt on vertical eye position suggests that the trVOR might follow the 3-D kinematic requirements that have been shown to govern visually guided eye movements and near-target fixation.

NASA Discipline Neuroscience↗
NASA NTRS1999

Three-dimensional organization of vestibular-related eye movements to off-vertical axis rotation and linear translation in pigeons

During linear accelerations, compensatory reflexes should 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 during linear accelerations produced by constant velocity off-vertical axis yaw rotations and translational motion in darkness. With off-vertical axis rotations, sinusoidally modulated eye-position and velocity responses were observed in all three components, with the vertical and torsional eye movements predominating the response. Peak torsional and vertical eye positions occurred when the head was oriented with the lateral visual axis of the right eye directed orthogonal to or aligned with the gravity vector, respectively. No steady-state horizontal nystagmus was obtained with any of the rotational velocities (8-58 degrees /s) tested. During translational motion, delivered along or perpendicular to the lateral visual axis, vertical and torsional eye movements were elicited. No significant horizontal eye movements were observed during lateral translation at frequencies up to 3 Hz. These responses suggest that, in pigeons, all linear accelerations generate eye movements that are compensatory to the direction of actual or perceived tilt of the head relative to gravity. In contrast, no translational horizontal eye movements, which are known to be compensatory to lateral translational motion in primates, were observed under the present experimental conditions.

NASA Discipline Neuroscience↗
NASA NTRS1999

Effect of viewing distance on the generation of vertical eye movements during locomotion

Vertical head and eye coordination was studied as a function of viewing distance during locomotion. Vertical head translation and pitch movements were measured using a video motion analysis system (Optotrak 3020). Vertical eye movements were recorded using a video-based pupil tracker (Iscan). Subjects (five) walked on a linear treadmill at a speed of 1.67 m/s (6 km/h) while viewing a target screen placed at distances ranging from 0.25 to 2.0 m at 0. 25-m intervals. The predominant frequency of vertical head movement was 2 Hz. In accordance with previous studies, there was a small head pitch rotation, which was compensatory for vertical head translation. The magnitude of the vertical head movements and the phase relationship between head translation and pitch were little affected by viewing distance, and tended to orient the naso-occipital axis of the head at a point approximately 1 m in front of the subject (the head fixation distance or HFD). In contrast, eye velocity was significantly affected by viewing distance. When viewing a far (2-m) target, vertical eye velocity was 180 degrees out of phase with head pitch velocity, with a gain of 0. 8. This indicated that the angular vestibulo-ocular reflex (aVOR) was generating the eye movement response. The major finding was that, at a close viewing distance (0.25 m), eye velocity was in phase with head pitch and compensatory for vertical head translation, suggesting that activation of the linear vestibulo-ocular reflex (lVOR) was contributing to the eye movement response. There was also a threefold increase in the magnitude of eye velocity when viewing near targets, which was consistent with the goal of maintaining gaze on target. The required vertical lVOR sensitivity to cancel an unmodified aVOR response and generate the observed eye velocity magnitude for near targets was almost 3 times that previously measured. Supplementary experiments were performed utilizing body-fixed active head pitch rotations at 1 and 2 Hz while viewing a head-fixed target. Results indicated that the interaction of smooth pursuit and the aVOR during visual suppression could modify both the gain and phase characteristics of the aVOR at frequencies encountered during locomotion. When walking, targets located closer than the HFD (1.0 m) would appear to move in the same direction as the head pitch, resulting in suppression of the aVOR. The results of the head-fixed target experiment suggest that phase modification of the aVOR during visual suppression could play a role in generating eye movements consistent with the goal of maintaining gaze on targets closer than the HFD, which would augment the lVOR response.

NASA Discipline Neuroscience↗
NASA NTRS1999

Short-latency primate vestibuloocular responses during translation

Short-lasting, transient head displacements and near target fixation were used to measure the latency and early response gain of vestibularly evoked eye movements during lateral and fore-aft translations in rhesus monkeys. The latency of the horizontal eye movements elicited during lateral motion was 11.9 +/- 5.4 ms. Viewing distance-dependent behavior was seen as early as the beginning of the response profile. For fore-aft motion, latencies were different for forward and backward displacements. Latency averaged 7.1 +/- 9.3 ms during forward motion (same for both eyes) and 12.5 +/- 6.3 ms for the adducting eye (e.g., left eye during right fixation) during backward motion. Latencies during backward motion were significantly longer for the abducting eye (18.9 +/- 9.8 ms). Initial acceleration gains of the two eyes were generally larger than unity but asymmetric. Specifically, gains were consistently larger for abducting than adducting eye movements. The large initial acceleration gains tended to compensate for the response latencies such that the early eye movement response approached, albeit consistently incompletely, that required for maintaining visual acuity during the movement. These short-latency vestibuloocular responses could complement the visually generated optic flow responses that have been shown to exhibit much longer latencies.

Non-NASA Center↗
NASA NTRS1999

Horizontal vestibuloocular reflex evoked by high-acceleration rotations in the squirrel monkey. II. Responses after canal plugging

The horizontal angular vestibuloocular reflex (VOR) evoked by high-frequency, high-acceleration rotations was studied in four squirrel monkeys after unilateral plugging of the three semicircular canals. During the period (1-4 days) that animals were kept in darkness after plugging, the gain during steps of acceleration (3, 000 degrees /s(2), peak velocity = 150 degrees /s) was 0.61 +/- 0.14 (mean +/- SD) for contralesional rotations and 0.33 +/- 0.03 for ipsilesional rotations. Within 18-24 h after animals were returned to light, the VOR gain for contralesional rotations increased to 0. 88 +/- 0.05, whereas there was only a slight increase in the gain for ipsilesional rotations to 0.37 +/- 0.07. A symmetrical increase in the gain measured at the plateau of head velocity was noted after animals were returned to light. The latency of the VOR was 8.2 +/- 0. 4 ms for ipsilesional and 7.1 +/- 0.3 ms for contralesional rotations. The VOR evoked by sinusoidal rotations of 0.5-15 Hz, +/-20 degrees /s had no significant half-cycle asymmetries. The recovery of gain for these responses after plugging was greater at lower than at higher frequencies. Responses to rotations at higher velocities for frequencies >/=4 Hz showed an increase in contralesional half-cycle gain, whereas ipsilesional half-cycle gain was unchanged. A residual response that appeared to be canal and not otolith mediated was noted after plugging of all six semicircular canals. This response increased with frequency to reach a gain of 0.23 +/- 0.03 at 15 Hz, resembling that predicted based on a reduction of the dominant time constant of the canal to 32 ms after plugging. A model incorporating linear and nonlinear pathways was used to simulate the data. The coefficients of this model were determined from data in animals with intact vestibular function. Selective increases in the gain for the linear and nonlinear pathways predicted the changes in recovery observed after canal plugging. An increase in gain of the linear pathway accounted for the recovery in VOR gain for both responses at the velocity plateau of the steps of acceleration and for the sinusoidal rotations at lower peak velocities. The increase in gain for contralesional responses to steps of acceleration and sinusoidal rotations at higher frequencies and velocities was due to an increase in the gain of the nonlinear pathway. This pathway was driven into inhibitory cutoff at low velocities and therefore made no contribution for rotations toward the ipsilesional side.

NASA Discipline Developmental Biology↗
NASA NTRS1999

Horizontal vestibuloocular reflex evoked by high-acceleration rotations in the squirrel monkey. I. Normal responses

The horizontal angular vestibuloocular reflex (VOR) evoked by high-frequency, high-acceleration rotations was studied in five squirrel monkeys with intact vestibular function. The VOR evoked by steps of acceleration in darkness (3,000 degrees /s(2) reaching a velocity of 150 degrees /s) began after a latency of 7.3 +/- 1.5 ms (mean +/- SD). Gain of the reflex during the acceleration was 14.2 +/- 5.2% greater than that measured once the plateau head velocity had been reached. A polynomial regression was used to analyze the trajectory of the responses to steps of acceleration. A better representation of the data was obtained from a polynomial that included a cubic term in contrast to an exclusively linear fit. For sinusoidal rotations of 0.5-15 Hz with a peak velocity of 20 degrees /s, the VOR gain measured 0.83 +/- 0.06 and did not vary across frequencies or animals. The phase of these responses was close to compensatory except at 15 Hz where a lag of 5.0 +/- 0.9 degrees was noted. The VOR gain did not vary with head velocity at 0.5 Hz but increased with velocity for rotations at frequencies of >/=4 Hz (0. 85 +/- 0.04 at 4 Hz, 20 degrees /s; 1.01 +/- 0.05 at 100 degrees /s, P < 0.0001). No responses to these rotations were noted in two animals that had undergone bilateral labyrinthectomy indicating that inertia of the eye had a negligible effect for these stimuli. We developed a mathematical model of VOR dynamics to account for these findings. The inputs to the reflex come from linear and nonlinear pathways. The linear pathway is responsible for the constant gain across frequencies at peak head velocity of 20 degrees /s and also for the phase lag at higher frequencies being less than that expected based on the reflex delay. The frequency- and velocity-dependent nonlinearity in VOR gain is accounted for by the dynamics of the nonlinear pathway. A transfer function that increases the gain of this pathway with frequency and a term related to the third power of head velocity are used to represent the dynamics of this pathway. This model accounts for the experimental findings and provides a method for interpreting responses to these stimuli after vestibular lesions.

NASA Program Biomedical Research and Countermeasur↗
NASA NTRS1999

Yaw and pitch visual-vestibular interaction in weightlessness

Both yaw and pitch visual-vestibular interactions at two separate frequencies of chair rotation (0.2 and 0.8 Hz) in combination with a single velocity of optokinetic stimulus (36 degrees/s) were used to investigate the effects of sustained weightlessness on neural strategies adopted by astronaut subjects to cope with the stimulus rearrangement of spaceflight. Pitch and yaw oscillation in darkness at 0.2 and 0.8 Hz without optokinetic stimulation, and constant velocity linear optokinetic stimulation at 18, 36, and 54 degrees/s presented relative to the head with the subject stationary, were used as controls for the visual-vestibular interactions. The results following 8 days of space flight showed no significant changes in: (1) either the horizontal and vertical vestibulo-ocular reflex (VOR) gain, phase, or bias; (2) the yaw visual-vestibular response (VVR); or (3) the horizontal or vertical optokinetic (OKN) slow phase velocity (SPV). However, significant changes were observed: (1) when during pitch VVR at 0.2 Hz late inflight, the contribution of the optokinetic input to the combined oculomotor response was smaller than during the stationary OKN SPV measurements, followed by an increased contribution during the immediate postflight testing; and (2) when during pitch VVR at 0.8 Hz, the component of the combined oculomotor response due to the underlying vertical VOR was more efficiently suppressed early inflight and less suppressed immediately postflight compared with preflight observations. The larger OKN response during pitch VVR at 0.2 Hz and the better suppression of VOR during pitch VVR at 0.8 Hz postflight are presumably due to the increased role of vision early inflight and immediately after spaceflight, as previously observed in various studies. These results suggest that the subjects adopted a neural strategy to structure their spatial orientation in weightlessness by reweighting visual, otolith, and perhaps tactile/somatic signals.

NASA Center JSC↗
NASA NTRS1999

Directional abnormalities of vestibular and optokinetic responses in cerebellar disease

Directional abnormalities of vestibular and optokinetic responses in patients with cerebellar degeneration are reported. Three-axis magnetic search-coil recordings of the eye and head were performed in eight cerebellar patients. Among these patients, examples of directional cross-coupling were found during (1) high-frequency, high-acceleration head thrusts; (2) constant-velocity chair rotations with the head fixed; (3) constant-velocity optokinetic stimulation; and (4) following repetitive head shaking. Cross-coupling during horizontal head thrusts consisted of an inappropriate upward eye-velocity component. In some patients, sustained constant-velocity yaw-axis chair rotations produced a mixed horizontal-torsional nystagmus and/or an increase in the baseline vertical slow-phase velocity. Following horizontal head shaking, some patients showed an increase in the slow-phase velocity of their downbeat nystagmus. These various forms of cross-coupling did not necessarily occur to the same degree in a given patient; this suggests that different mechanisms may be responsible. It is suggested that cross-coupling during head thrusts may reflect a loss of calibration of brainstem connections involved in the direct vestibular pathways, perhaps due to dysfunction of the flocculus. Cross-coupling during constant-velocity rotations and following head shaking may result from a misorientation of the angular eye-velocity vector in the velocity-storage system. Finally, responses to horizontal optokinetic stimulation included an inappropriate torsional component in some patients. This suggests that the underlying organization of horizontal optokinetic tracking is in labyrinthine coordinates. The findings are also consistent with prior animal-lesion studies that have shown a role for the vestibulocerebellum in the control of the direction of the VOR.

NASA Discipline Neuroscience↗