Vestibuloocular reflex inhibition and gaze saccade control characteristics during eye-head orientation in humans

1988 ◽  
Vol 59 (3) ◽  
pp. 997-1013 ◽  
Author(s):  
D. Pelisson ◽  
C. Prablanc ◽  
C. Urquizar
Keyword(s):  
Constant Velocity ◽  
Pulse Generator ◽  
Head Rotation ◽  
Systematic Difference ◽  
Head Motion ◽  
Head Orientation ◽  
Vestibuloocular Reflex ◽  
Gaze Saccade ◽  
Gaze Saccades ◽  
Saccade Duration

1. In natural conditions, gaze (i.e., eye + head) orientation is a complex behavior involving simultaneously the eye and head motor systems. Thus one of the key problems of gaze control is whether or not the vestibuloocular reflex (VOR) elicited by head rotation and saccadic eye movement linearly add. 2. Kinematics of human gaze saccades within the oculomotor range (OMR) were quantified under different conditions of head motion. Saccades were visually triggered while the head was fixed or passively moving at a constant velocity (200 deg/s) either in the same direction as, or opposite to, the saccade. Active eye-head coordination was also studied in a session in which subjects were trained to actively rotate their head at a nearly constant velocity during the saccade and, in another session, during natural gaze responses. 3. When the head was passively rotated toward the visual target, both maximum and mean gaze velocities increased with respect to control responses with the head fixed; these effects increased with gaze saccade amplitude. In addition, saccade duration was reduced so that corresponding gaze accuracy, although poorer than for control responses, was not dramatically affected by head motion. 4. The same effects on gaze velocity were present during active head motion when a constant head velocity was maintained throughout saccade duration, and gaze saccades were as accurate as with the head fixed. 5. During natural gaze responses, an increased gaze velocity and a decreased saccade duration with respect to control responses became significant only for gaze displacement larger than 30 degrees, due to the negligible contribution of head motion for smaller responses. 6. When the head was passively rotated in the opposite direction to target step, gaze saccades were slower than those obtained with the head fixed; but their average accuracy was still maintained. 7. These results confirm a VOR inhibition during saccadic eye movements within the OMR. This inhibition, present in all 16 subjects studied, ranged from 40 to 96% (for a 40 degree target step) between subjects and increased almost linearly with target step amplitude. Furthermore, the systematic difference between instantaneous VOR gain estimated at the time of maximum gaze velocity and mean VOR gain estimated over the whole saccadic duration indicates a decay of VOR inhibition during the ongoing saccade. 8. A simplified model is proposed with a varying VOR inhibition during the saccade. It suggests that VOR inhibition is not directly controlled by the saccadic pulse generator.(ABSTRACT TRUNCATED AT 400 WORDS)

Neural basis for motor learning in the vestibuloocular reflex of primates. I. Changes in the responses of brain stem neurons

1994 ◽  
Vol 72 (2) ◽  
pp. 928-953 ◽  
Author(s):  
S. G. Lisberger ◽  
T. A. Pavelko ◽  
D. M. Broussard
Keyword(s):  
Eye Movements ◽  
Motor Learning ◽  
Firing Rate ◽  
Brain Stem ◽  
Head Rotation ◽  
Head Motion ◽  
Vestibuloocular Reflex ◽  
Neural Basis ◽  
Ventral Paraflocculus ◽  
The Brain

1. We recorded from neurons in the brain stem of monkeys before and after they had worn magnifying or miniaturizing spectacles to cause changes in the gain of the vestibuloocular reflex (VOR). The gain of the VOR was estimated as eye speed divided by head speed during passive horizontal head rotation in darkness. Electrical stimulation in the cerebellum was used to identify neurons that receive inhibition at monosynaptic latencies from the flocculus and ventral paraflocculus (flocculus target neurons or FTNs). Cells were studied during smooth pursuit eye movements with the head stationary, fixation of different positions, cancellation of the VOR, and the VOR evoked by rapid changes in head velocity. 2. FTNs were divided into two populations according to their responses during pursuit with the head stationary. The two groups showed increased firing during smooth eye motion toward the side of recording (Eye-ipsiversive or E-i) or away from the side of recording (Eye-contraversive or E-c). A higher percentage of FTNs showed increased firing rate for contraversive pursuit when the gain of the VOR was high (> or = 1.6) than when the gain of the VOR was low (< or = 0.4). 3. Changes in the gain of the VOR had a striking effect on the responses during the VOR for the FTNs that were E-c during pursuit with the head stationary. Firing rate increased during contraversive VOR eye movements when the gain of the VOR was high or normal and decreased during contraversive VOR eye movements when the gain of the VOR was low. Changes in the gain of the VOR caused smaller changes in the responses during the VOR of FTNs that were E-i during pursuit with the head stationary. We argue that motor learning in the VOR is the result of changes in the responses of individual FTNs. 4. The responses of E-i and E-c FTNS during cancellation of the VOR depended on the gain of the VOR. Responses tended to be in phase with contraversive head motion when the gain of the VOR was low and in phase with ipsiversive head motion when the gain of the VOR was high. Comparison of the effect of motor learning on the responses of FTNs during cancellation of the VOR with the results of similar experiments on horizontal-gaze velocity Purkinje cells in the flocculus and ventral paraflocculus suggests that the brain stem vestibular inputs to FTNs are one site of motor learning in the VOR.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Does Head Orientation Influence 3D Facial Imaging? A Study on Accuracy and Precision of Stereophotogrammetric Acquisition

2021 ◽  
Vol 18 (8) ◽  
pp. 4276
Author(s):  
Giuditta Battistoni ◽  
Diana Cassi ◽  
Marisabel Magnifico ◽  
Giuseppe Pedrazzi ◽  
Marco Di Blasio ◽  
...  
Keyword(s):  
Head Rotation ◽  
Anthropometric Measurements ◽  
Head Orientation ◽  
3D Images ◽  
Face Shape ◽  
Operator Error ◽  
Accuracy And Precision ◽  
Error Magnitude ◽  
Repeatability And Reproducibility ◽  
Head Positions

This study investigates the reliability and precision of anthropometric measurements collected from 3D images and acquired under different conditions of head rotation. Various sources of error were examined, and the equivalence between craniofacial data generated from alternative head positions was assessed. 3D captures of a mannequin head were obtained with a stereophotogrammetric system (Face Shape 3D MaxiLine). Image acquisition was performed with no rotations and with various pitch, roll, and yaw angulations. On 3D images, 14 linear distances were measured. Various indices were used to quantify error magnitude, among them the acquisition error, the mean and the maximum intra- and inter-operator measurement error, repeatability and reproducibility error, the standard deviation, and the standard error of errors. Two one-sided tests (TOST) were performed to assess the equivalence between measurements recorded in different head angulations. The maximum intra-operator error was very low (0.336 mm), closely followed by the acquisition error (0.496 mm). The maximum inter-operator error was 0.532 mm, and the highest degree of error was found in reproducibility (0.890 mm). Anthropometric measurements from alternative acquisition conditions resulted in significantly equivalent TOST, with the exception of Zygion (l)–Tragion (l) and Cheek (l)–Tragion (l) distances measured with pitch angulation compared to no rotation position. Face Shape 3D Maxiline has sufficient accuracy for orthodontic and surgical use. Precision was not altered by head orientation, making the acquisition simpler and not constrained to a critical precision as in 2D photographs.

Interstitial Nucleus of Cajal Encodes Three-Dimensional Head Orientations in Fick-Like Coordinates

2007 ◽  
Vol 97 (1) ◽  
pp. 604-617 ◽  
Author(s):  
Eliana M. Klier ◽  
Hongying Wang ◽  
J. Douglas Crawford
Keyword(s):  
Coordinate System ◽  
Degrees Of Freedom ◽  
Three Dimensional ◽  
Semicircular Canals ◽  
Head Rotation ◽  
Head Orientation ◽  
Interstitial Nucleus Of Cajal ◽  
The Gaze ◽  
Behavioral Constraints ◽  
The Right

Two central, related questions in motor control are 1) how the brain represents movement directions of various effectors like the eyes and head and 2) how it constrains their redundant degrees of freedom. The interstitial nucleus of Cajal (INC) integrates velocity commands from the gaze control system into position signals for three-dimensional eye and head posture. It has been shown that the right INC encodes clockwise (CW)-up and CW-down eye and head components, whereas the left INC encodes counterclockwise (CCW)-up and CCW-down components, similar to the sensitivity directions of the vertical semicircular canals. For the eyes, these canal-like coordinates align with Listing’s plane (a behavioral strategy limiting torsion about the gaze axis). By analogy, we predicted that the INC also encodes head orientation in canal-like coordinates, but instead, aligned with the coordinate axes for the Fick strategy (which constrains head torsion). Unilateral stimulation (50 μA, 300 Hz, 200 ms) evoked CW head rotations from the right INC and CCW rotations from the left INC, with variable vertical components. The observed axes of head rotation were consistent with a canal-like coordinate system. Moreover, as predicted, these axes remained fixed in the head, rotating with initial head orientation like the horizontal and torsional axes of a Fick coordinate system. This suggests that the head is ordinarily constrained to zero torsion in Fick coordinates by equally activating CW/CCW populations of neurons in the right/left INC. These data support a simple mechanism for controlling head orientation through the alignment of brain stem neural coordinates with natural behavioral constraints.

Effect of Adaptation to Telescopic Spectacles on the Initial Human Horizontal Vestibuloocular Reflex

2000 ◽  
Vol 83 (1) ◽  
pp. 38-49 ◽  
Author(s):  
Benjamin T. Crane ◽  
Joseph L. Demer
Keyword(s):  
Head Rotation ◽  
Head Movements ◽  
Target Distance ◽  
Whole Body ◽  
Rotational Axis ◽  
Vestibuloocular Reflex ◽  
Head Acceleration ◽  
Gain Enhancement ◽  
Lower Head ◽  
Distant Target

Gain of the vestibuloocular reflex (VOR) not only varies with target distance and rotational axis, but can be chronically modified in response to prolonged wearing of head-mounted magnifiers. This study examined the effect of adaptation to telescopic spectacles on the variation of the VOR with changes in target distance and yaw rotational axis for head velocity transients having peak accelerations of 2,800 and 1,000°/s2. Eye and head movements were recorded with search coils in 10 subjects who underwent whole body rotations around vertical axes that were 10 cm anterior to the eyes, centered between the eyes, between the otoliths, or 20 cm posterior to the eyes. Immediately before each rotation, subjects viewed a target 15 or 500 cm distant. Lighting was extinguished immediately before and was restored after completion of each rotation. After initial rotations, subjects wore 1.9× magnification binocular telescopic spectacles during their daily activities for at least 6 h. Test spectacles were removed and measurement rotations were repeated. Of the eight subjects tolerant of adaptation to the telescopes, six demonstrated VOR gain enhancement after adaptation, while gain in two subjects was not increased. For all subjects, the earliest VOR began 7–10 ms after onset of head rotation regardless of axis eccentricity or target distance. Regardless of adaptation, VOR gain for the proximate target exceeded that for the distant target beginning at 20 ms after onset of head rotation. Adaptation increased VOR gain as measured 90–100 ms after head rotation onset by an average of 0.12 ± 0.02 (SE) for the higher head acceleration and 0.19 ± 0.02 for the lower head acceleration. After adaptation, four subjects exhibited significant increases in the canal VOR gain only, whereas two subjects exhibited significant increases in both angular and linear VOR gains. The latencies of linear and early angular target distance effects on VOR gain were unaffected by adaptation. The earliest significant change in angular VOR gain in response to adaptation occurred 50 and 68 ms after onset of the 2,800 and 1,000°/s2 peak head accelerations, respectively. The latency of the adaptive increase in linear VOR gain was ∼50 ms for the peak head acceleration of 2,800°/s2, and 100 ms for the peak head acceleration of 1,000°/s2. Thus VOR gain changes and latency were consistent with modification in the angular VOR in most subjects, and additionally in the linear VOR in a minority of subjects.

Dynamics of Squirrel Monkey Linear Vestibuloocular Reflex and Interactions with Fixation Distance

1997 ◽  
Vol 78 (4) ◽  
pp. 1775-1790 ◽  
Author(s):  
Laura Telford ◽  
Scott H. Seidman ◽  
Gary D. Paige
Keyword(s):  
Eye Movements ◽  
Squirrel Monkey ◽  
Head Tilt ◽  
Linear Acceleration ◽  
Head Orientation ◽  
Vestibuloocular Reflex ◽  
Response Characteristics ◽  
Fixation Distance ◽  
Eye Rotation ◽  
Low Pass

Telford, Laura, Scott H. Seidman, and Gary D. Paige. Dynamics of squirrel monkey linear vestibuloocular reflex and interactions with fixation distance. J. Neurophysiol. 78: 1775–1790, 1997. 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.

Rotational kinematics of the human vestibuloocular reflex. II. Velocity steps

1994 ◽  
Vol 72 (5) ◽  
pp. 2480-2489 ◽  
Author(s):  
D. Tweed ◽  
M. Fetter ◽  
D. Sievering ◽  
H. Misslisch ◽  
E. Koenig
Keyword(s):  
Cross Talk ◽  
Human Subjects ◽  
Head Rotation ◽  
Horizontal Axis ◽  
Main Diagonal ◽  
Velocity Storage ◽  
Vestibuloocular Reflex ◽  
Simple Explanation ◽  
Time Constants ◽  
Axis Rotation

1. Gain matrices were used to quantify the three-dimensional vestibuloocular reflex (VOR) in five human subjects who were accelerated over 1 s and then spun at a constant 150 degrees/s for 29 s in darkness. Rotations were torsional, vertical and horizontal, about earth-vertical and earth-horizontal axes. 2. Elements on the main diagonal of the gain matrices were much smaller than the optimal value of -1, and torsional gain was weaker than vertical or horizontal. Off-diagonal elements, indicating cross talk, were minimal except for a small but consistent horizontal response to torsional head rotation. 3. Downward slow phases were more than twice as fast as upward at the start of rotation about both earth-vertical and earth-horizontal axes, but the asymmetry vanished later in the rotation. 4. During earth-vertical-axis rotation, all matrix elements decayed to zero. The main-diagonal torsional and vertical gains waned with time constants close to that of the cupula (6.7 and 7.3 s). Velocity storage prolonged the horizontal response to horizontal head rotation (time constant 14.2 s) but not the horizontal response to torsion (7.7 s). A simple explanation is that velocity storage acts on a central estimate of head motion that accurately distinguishes horizontal from torsional and that the inappropriate horizontal eye velocity response to torsion occurs because of cross talk downstream from velocity storage. 5. During earth-horizontal-axis rotation, the torsional, vertical, and horizontal main-diagonal elements declined, with time constants of 7.6, 8.2, and 7.9 s, to maintained nonzero values, all equal to about -0.1. Off-diagonal elements, including the horizontal response to torsion, decayed to zero, so that the otolith-driven reflex, late in the rotation, was equally strong in all dimensions and almost free of detectable cross talk. 6. The difference between gain curves over the course of earth-vertical- and earth-horizontal-axis rotations was not constant but increased with time, suggesting that the VOR response to earth-horizontal-axis rotation is not a simple sum of canal and otolith reflexes.

Vestibuloocular Reflex Dynamics During High-Frequency and High-Acceleration Rotations of the Head on Body in Rhesus Monkey

2002 ◽  
Vol 88 (1) ◽  
pp. 13-28 ◽  
Author(s):  
Marko Huterer ◽  
Kathleen E. Cullen
Keyword(s):  
Eye Movements ◽  
High Frequency ◽  
The Body ◽  
Whole Body ◽  
Head Motion ◽  
Similar Response ◽  
Vestibuloocular Reflex ◽  
Frequency Range ◽  
Response Dynamics ◽  
Passive Rotation

For frequencies >10 Hz, the vestibuloocular reflex (VOR) has been primarily investigated during passive rotations of the head on the body in humans. These prior studies suggest that eye movements lag head movements, as predicted by a 7-ms delay in the VOR reflex pathways. However, Minor and colleagues recently applied whole-body rotations of frequencies ≤15 Hz in monkeys and found that eye movements were nearly in phase with head motion across all frequencies. The goal of the present study was to determine whether VOR response dynamics actually differ significantly for whole-body versus head-on-body rotations. To address this question, we evaluated the gain and phase of the VOR induced by high-frequency oscillations of the head on the body in monkeys by directly measuring both head and eye movements using the magnetic search coil technique. A torque motor was used to rotate the heads of three Rhesus monkeys over the frequency range 5–25 Hz. Peak head velocity was held constant, first at ±50°/s and then ±100°/s. The VOR was found to be essentially compensatory across all frequencies; gains were near unity (1.1 at 5 Hz vs. 1.2 at 25 Hz), and phase lag increased only slightly with frequency (from 2° at 5 Hz to 11° at 25 Hz, a marked contrast to the 63° lag at 25 Hz predicted by a 7-ms VOR latency). Furthermore, VOR response dynamics were comparable in darkness and when viewing a target and did not vary with peak velocity. Although monkeys offered less resistance to the initial cycles of applied head motion, the gain and phase of the VOR did not vary for early versus late cycles, suggesting that an efference copy of the motor command to the neck musculature did not alter VOR response dynamics. In addition, VOR dynamics were also probed by applying transient head perturbations with much greater accelerations (peak acceleration >15,000°/s2) than have been previously employed. The VOR latency was between 5 and 6 ms, and mean gain was close to unity for two of the three animals tested. A simple linear model well described the VOR responses elicited by sinusoidal and transient head on body rotations. We conclude that the VOR is compensatory over a wide frequency range in monkeys and has similar response dynamics during passive rotation of the head on body as during passive rotation of the whole body in space.

scholarly journals Unilateral Head Impulses Training in Uncompensated Vestibular Hypofunction

2017 ◽  
Vol 2017 ◽  
pp. 1-6 ◽  
Author(s):  
Ana Carolina Binetti ◽  
Andrea Ximena Varela ◽  
Dana Lucila Lucarelli ◽  
Daniel Héctor Verdecchia
Keyword(s):  
Young Woman ◽  
Head Rotation ◽  
Rehabilitation Program ◽  
Vestibuloocular Reflex ◽  
Blurred Vision ◽  
Head Impulse ◽  
Vestibular Hypofunction ◽  
Affected Side ◽  
Head Impulses

The aim of this paper is to report a case of a young woman with unilateral vestibular chronic failure with a poorly compensated vestibuloocular reflex during rapid head rotation. Additionally, she developed migraine symptoms during the treatment with associated chronic dizzy sensations and blurred vision. Her report of blurred vision only improved after she completed a rehabilitation program using fast head impulse rotations towards the affected side for 5 consecutive days. We discuss why we elected this form of treatment and how this method may be useful for different patients.

scholarly journals Two distinct types of eye-head coupling in freely moving mice

2020 ◽  
Author(s):  
Arne F. Meyer ◽  
John O’Keefe ◽  
Jasper Poort
Keyword(s):  
Eye Movements ◽  
Ground Plane ◽  
Sensory Information ◽  
Head Tilt ◽  
Head Rotation ◽  
Head Movements ◽  
Head Motion ◽  
List Type ◽  
Visually Guided ◽  
Freely Moving

SummaryAnimals actively interact with their environment to gather sensory information. There is conflicting evidence about how mice use vision to sample their environment. During head restraint, mice make rapid eye movements strongly coupled between the eyes, similar to conjugate saccadic eye movements in humans. However, when mice are free to move their heads, eye movement patterns are more complex and often non-conjugate, with the eyes moving in opposite directions. Here, we combined eye tracking with head motion measurements in freely moving mice and found that both observations can be explained by the existence of two distinct types of coupling between eye and head movements. The first type comprised non-conjugate eye movements which systematically compensated for changes in head tilt to maintain approximately the same visual field relative to the horizontal ground plane. The second type of eye movements were conjugate and coupled to head yaw rotation to produce a “saccade and fixate” gaze pattern. During head initiated saccades, the eyes moved together in the same direction as the head, but during subsequent fixation moved in the opposite direction to the head to compensate for head rotation. This “saccade and fixate” pattern is similar to that seen in humans who use eye movements (with or without head movement) to rapidly shift gaze but in mice relies on combined eye and head movements. Indeed, the two types of eye movements very rarely occurred in the absence of head movements. Even in head-restrained mice, eye movements were invariably associated with attempted head motion. Both types of eye-head coupling were seen in freely moving mice during social interactions and a visually-guided object tracking task. Our results reveal that mice use a combination of head and eye movements to sample their environment and highlight the similarities and differences between eye movements in mice and humans.HighlightsTracking of eyes and head in freely moving mice reveals two types of eye-head couplingEye/head tilt coupling aligns gaze to horizontal planeRotational eye and head coupling produces a “saccade and fixate” gaze pattern with head leading the eyeBoth types of eye-head coupling are maintained during visually-guided behaviorsEye movements in head-restrained mice are related to attempted head movements

Gravity-Specific Adaptation of the Angular Vestibuloocular Reflex: Dependence on Head Orientation With Regard to Gravity

2003 ◽  
Vol 89 (1) ◽  
pp. 571-586 ◽  
Author(s):  
Sergei B. Yakushin ◽  
Theodore Raphan ◽  
Bernard Cohen
Keyword(s):  
Head Tilt ◽  
Vertical Axis ◽  
Continuous Functions ◽  
Head Position ◽  
Head Orientation ◽  
Vestibuloocular Reflex ◽  
Specific Adaptation ◽  
Gain Adaptation ◽  
Bias Level ◽  
Single State

The gain of the vertical angular vestibuloocular reflex (aVOR) was adaptively altered by visual-vestibular mismatch during rotation about an interaural axis, using steps of velocity in three head orientations: upright, left-side down, and right-side down. Gains were decreased by rotating the animal and visual surround in the same direction and increased by visual and surround rotation in opposite directions. Gains were adapted in one head position (single-state adaptation) or decreased with one side down and increased with the other side down (dual-state adaptation). Animals were tested in darkness using sinusoidal rotation at 0.5 Hz about an interaural axis that was tilted from horizontal to vertical. They were also sinusoidally oscillated from 0.5 to 4 Hz about a spatial vertical axis in static tilt positions from yaw to pitch. After both single- and dual-state adaptation, gain changes were maximal when the monkeys were in the position in which the gain had been adapted, and the gain changes progressively declined as the head was tilted away from that position. We call this gravity-specific aVOR gain adaptation. The spatial distribution of the specific aVOR gain changes could be represented by a cosine function that was superimposed on a bias level, which we called gravity-independent gain adaptation. Maximal gravity-specific gain changes were produced by 2–4 h of adaptation for both single- and dual-state adaptations, and changes in gain were similar at all test frequencies. When adapted while upright, the magnitude and distribution of the gravity-specific adaptation was comparable to that when animals were adapted in side-down positions. Single-state adaptation also produced gain changes that were independent of head position re gravity particularly in association with gain reduction. There was no bias after dual-state adaptation. With this difference, fits to data obtained by altering the gain in separate sessions predicted the modulations in gain obtained from dual-state adaptations. These data show that the vertical aVOR gain changes dependent on head position with regard to gravity are continuous functions of head tilt, whose spatial phase depends on the position in which the gain was adapted. From their different characteristics, it is likely that gravity-specific and gravity-independent adaptive changes in gain are produced by separate neural processes. These data demonstrate that head orientation to gravity plays an important role in both orienting and tuning the gain of the vertical aVOR.

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