The role of gravity in adaptation of the vertical angular vestibulo-ocular reflex

Nondestructive and objective assessment of the vestibular function in rodent models: A review

The normal function of the vestibular system is crucial for the sense of balance. The techniques used to assess the vestibular function plays a vital role in the research of the vestibular system. In this article, we have systematically reviewed some popular methods employing vestibular reflexes and vestibular evoked potentials for assessing the vestibular function in rodent models. These vestibular reflexes and vestibular evoked potentials to effective stimuli have been used as nondestructive and objective functional measures. The main types of vestibular reflexes include the vestibulo-ocular reflex (VOR), vestibulocollic reflex (VCR), and vestibulo-sympathetic reflex (VSR). They are all capable of indicating the functions of the semicircular canals and otoliths. However, the VOR assessment is much more prevalently used because of the relatively stereotypical inputoutput relationship and simple motion pattern of the ocular response. In contrast, the complicated motion pattern and small gain of the VCR response, as well as the undesired component possibly contributed from the acceleration receptors outside the labyrinths in the VSR response, restrict the widespread applications of VCR and VSR in the assessment of the vestibular system. The vestibular evoked myogenic potentials (VEMPs) and vestibular sensory evoked potentials (VsEPs) are the two typical evoked potentials that have been also employed for evaluating the vestibular function. Through exploiting different types of the VEMPs, the saccular and utricular functions can be evaluated separately. The sound-induced VEMPs, moreover, are capable of noninvasively assessing the unilateral vestibular function. The VsEPs, via the morphology of their signal waveforms, enable the access to the location-specific information that indicates the functional statuses of different components within the vestibular neural pathway.

Modeling spatial tuning of adaptation of the angular vestibulo-ocular reflex

Gain adaptation of the yaw angular vestibular ocular reflex (aVOR) induced in side-down positions has gravity-independent (global) and -dependent (localized) components. When the head oscillation angles are small during adaptation, localized gain changes are maximal in the approximate position of adaptation. Concurrently, polarization vectors of canal-otolith vestibular neurons adapt their orientations during these small-angle adaptation paradigms. Whether there is orientation adaptation with large amplitude head oscillations, when the head is not localized to a specific position, is unknown. Yaw aVOR gains were decreased by oscillating monkeys about a yaw axis in a side-down position in a subject-stationary visual surround for 2 h. Amplitudes of head oscillation ranged from 15° to 180°. The yaw aVOR gain was tested in darkness at 0.5 Hz, with small angles of oscillation (±15°) while upright and in tilted positions. The peak value of the gain change was highly tuned for small angular oscillations during adaptation and significantly broadened with larger oscillation angles during adaptation. When the orientation of the polarization vectors associated with the gravity-dependent component of the neural network model was adapted toward the direction of gravity, it predicted the localized learning for small angles and the broadening when the orientation adaptation was diminished. The model-based analysis suggests that the otolith orientation adaptation plays an important role in the localized behavior of aVOR as a function of gravity and in regulating the relationship between global and localized adaptation.

Tuning of gravity-dependent and gravity-independent vertical angular VOR gain changes by frequency of adaptation

The gain of the vertical angular vestibulo-ocular reflex (aVOR) was adaptively increased and decreased in a side-down head orientation for 4 h in two cynomolgus monkeys. Adaptation was performed at 0.25, 1, 2, or 4 Hz. The gravity-dependent and -independent gain changes were determined over a range of head orientations from left-side-down to right-side-down at frequencies from 0.25 to 10 Hz, before and after adaptation. Gain changes vs. frequency data were fit with a Gaussian to determine the frequency at which the peak gain change occurred, as well as the tuning width. The frequency at which the peak gravity-dependent gain change occurred was approximately equal to the frequency of adaptation, and the width increased monotonically with increases in the frequency of adaptation. The gravity-independent component was tuned to the adaptive frequency of 0.25 Hz but was uniformly distributed over all frequencies when the adaptation frequency was 1-4 Hz. The amplitude of the gravity-independent gain changes was larger after the aVOR gain decrease than after the gain increase across all tested frequencies. For the aVOR gain decrease, the phase lagged about 4° for frequencies below the adaptation frequency and led for frequencies above the adaptation frequency. For gain increases, the phase relationship as a function of frequency was inverted. This study demonstrates that the previously described dependence of aVOR gain adaptation on frequency is a property of the gravity-dependent component of the aVOR only. The gravity-independent component of the aVOR had a substantial tuning curve only at an adaptation frequency of 0.25 Hz.

Orientation adaptation of eye movement-related vestibular neurons due to prolonged head tilt

Sixteen neurons, including vestibular-only (VO), eye-head velocity (EHV), and position-vestibular-pause (PVP) neurons sensitive to head tilt were recorded in the rostromedial and in superior vestibular nuclei. Projection of the otolith polarization vector to the horizontal plane (response vector orientation [RVO]) was determined before and after prolonged head orientation in side-down position. The RVO of VO neurons shifted toward alignment with the axis of gravity when the head was in the position of adaptation. PVP neurons had similar changes in RVO. There were also changes in RVO in some EHV neurons, but generally in directions not related to gravity. Modeling studies have suggested that the tendency to align RVOs with gravity leads to tuning of gravity-dependent angular vestibular ocular reflex (aVOR) gain changes to the position of adaptation. Thus, coding of orientation in PVP neurons would contribute significantly to the gravity-dependent adaptation of the aVOR.

Dependence of the roll angular vestibuloocular reflex (aVOR) on gravity

Little is known about the dependence of the roll angular vestibuloocular reflex (aVOR) on gravity or its gravity-dependent adaptive properties. To study gravity-dependent characteristics of the roll aVOR, monkeys were oscillated about a naso-occipital axis in darkness while upright or tilted. Roll aVOR gains were largest in the upright position and decreased by 7-15% as animals were tilted from the upright. Thus the unadapted roll aVOR gain has substantial gravitational dependence. Roll gains were also decreased or increased by 0.25 Hz, in- or out-of-phase rotation of the head and the visual surround while animals were prone, supine, upright, or in side-down positions. Gain changes, determined as a function of head tilt, were fit with a sinusoid; the amplitudes represented the amount of the gravity-dependent gain change, and the bias, the gravity-independent gain change. Gravity-dependent gain changes were absent or substantially smaller in roll (approximately 5%) than in yaw (25%) or pitch (17%), whereas gravity-independent gain changes were similar for roll, pitch, and yaw (approximately 20%). Thus the high-frequency roll aVOR gain has an inherent dependence on head orientation re gravity in the unadapted state, which is different from the yaw/pitch aVORs. This inherent gravitational dependence may explain why the adaptive circuits are not active when the head is tilted re gravity during roll aVOR adaptation. These behavioral differences support the idea that there is a fundamental difference in the central organization of canal-otolith convergence of the roll and yaw/pitch aVORs.

Retention of VOR gain following short-term VOR adaptation

Motor learning in the vestibular system can be differentially obtained depending upon the context for which the vestibulo-ocular reflex (VOR) has been exposed. Manipulating head orientation relative to gravity is an example of a contextual cue that can elicit independent VOR gains. We were interested in examining retention of short-term VOR adaptation when the adapting stimulus was paired with a novel contextual cue. Two sets of non-human primate VOR adaptation experiments were designed to assess the influence of head position relative to gravity on retention of the pitch VOR. First, the pitch VOR of three squirrel monkeys was adapted for 3 h using minimizing (x0.45) spectacles and a sum-of-sines stimulus (20 degrees /s at 0.5, 1.1, 2.3, and 3.7 Hz) while the animals were positioned left ear down (LED adaptation). Pitch VOR gains were measured in the adapted position (LED) and two non-adapted positions (upright, UP) or right ear down (RED). In the second set of experiments, the pitch VOR was adapted in an upright head position (same adapting stimulus as used in LED) and tested in UP, LED or RED. No head immobility or darkness restrictions were imposed on the animals after the initial adaptation exposure. The pitch VOR gains were measured during the acceleration (G (A)) and constant velocity (G (V)) portions of 1,000 degrees /s(2)-150 degrees /s step responses and during 0.5, 2.0, and 4.0 Hz sinusoids with velocities varying from 20 to 100 degrees /s. All measures of VOR gain for UP, LED, and RED were done immediately after the adaptation and for three subsequent days and at post-adaptation day 7 (PAD 7). When tested in the adapting position, all experiments showed immediate reduction in G (A) and G (V) compared with pre-adaptation levels. For LED adaptation experiments, the pitch G (A) and G (V) gains were significantly reduced for as long as 7 days. Some retention of the LED-adapted VOR gain also occurred when testing in the RED position. No retention of pitch VOR G (A) or G (V) existed for the UP position after adaptation in LED. After the UP-adapt experiments, no retention of the G (A) or G (V) was found when tested in the adapting position. Interestingly, however, some retention of G (A) and G (V) did exist when the UP-adapted animals were tested in LED or RED. Data from sinusoidal rotations followed a similar adaptation pattern as the step responses. Our findings show that after only 3 h of adaptation exposure, adaptation of the pitch VOR gain is retained for several days. This long-term retention of VOR adaptation after short-term exposure appears to be the result of inducing adaptation with an atypical combination of movement and position for the monkey (LED-adapt). Our results indicate that head orientation relative to gravity is an effective context for retaining learned VOR gains in addition to restricting mobility or keeping animals in the dark. We also show that the adapting head position determines the magnitude of VOR adaptation.

Modeling Gravity-Dependent Plasticity of the Angular Vestibuloocular Reflex With a Physiologically Based Neural Network

A neural network model was developed to explain the gravity-dependent properties of gain adaptation of the angular vestibuloocular reflex (aVOR). Gain changes are maximal at the head orientation where the gain is adapted and decrease as the head is tilted away from that position and can be described by the sum of gravity-independent and gravity-dependent components. The adaptation process was modeled by modifying the weights and bias values of a three-dimensional physiologically based neural network of canal–otolith-convergent neurons that drive the aVOR. Model parameters were trained using experimental vertical aVOR gain values. The learning rule aimed to reduce the error between eye velocities obtained from experimental gain values and model output in the position of adaptation. Although the model was trained only at specific head positions, the model predicted the experimental data at all head positions in three dimensions. Altering the relative learning rates of the weights and bias improved the model-data fits. Model predictions in three dimensions compared favorably with those of a double-sinusoid function, which is a fit that minimized the mean square error at every head position and served as the standard by which we compared the model predictions. The model supports the hypothesis that gravity-dependent adaptation of the aVOR is realized in three dimensions by a direct otolith input to canal–otolith neurons, whose canal sensitivities are adapted by the visual-vestibular mismatch. The adaptation is tuned by how the weights from otolith input to the canal–otolith-convergent neurons are adapted for a given head orientation.