Adaptation Of Horizontal Vestibulo-ocular Reflex Research Articles

Angular vestibuloocular reflex responses in Otop1 mice. I. Otolith sensor input is essential for gravity context-specific adaptation

The role of the otoliths in mammals in the angular vestibuloocular reflex (VOR) has been difficult to determine because there is no surgical technique that can reliably ablate them without damaging the semicircular canals. The Otopetrin1 (Otop1) mouse lacks functioning otoliths because of failure to develop otoconia but seems to have otherwise normal peripheral anatomy and neural circuitry. By using these animals we sought to determine the role of the otoliths in angular VOR baseline function and adaptation. In six Otop1 mice and six control littermates we measured baseline ocular countertilt about the three primary axes in head coordinates; baseline horizontal (rotation about an Earth-vertical axis parallel to the dorsal-ventral axis) and vertical (rotation about an Earth-vertical axis parallel to the interaural axis) sinusoidal (0.2-10 Hz, 20-100°/s) VOR gain (= eye/head velocity); and the horizontal and vertical VOR after gain-increase (1.5×) and gain-decrease (0.5×) adaptation training. Countertilt responses were significantly reduced in Otop1 mice. Baseline horizontal and vertical VOR gains were similar between mouse types, and so was horizontal VOR adaptation. For control mice, vertical VOR adaptation was evident when the testing context, left ear down (LED) or right ear down (RED), was the same as the training context (LED or RED). For Otop1 mice, VOR adaptation was evident regardless of context. Our results suggest that the otolith translational signal does not contribute to the baseline angular VOR, probably because the mouse VOR is highly compensatory, and does not alter the magnitude of adaptation. However, we show that the otoliths are important for gravity context-specific angular VOR adaptation. NEW & NOTEWORTHY This is the first study examining the role of the otoliths (defined here as the utricle and saccule) in adaptation of the angular vestibuloocular reflex (VOR) in an animal model in which the otoliths are reliably inactivated and the semicircular canals preserved. We show that they do not contribute to adaptation of the normal angular VOR. However, the otoliths provide the main cue for gravity context-specific VOR adaptation.

Motor learning in common marmosets: Vestibulo-ocular reflex adaptation and its sensitivity to inhibitors of Purkinje cell long-term depression

Adaptation of the horizontal vestibulo-ocular reflex (HVOR) provides an experimental model for cerebellum-dependent motor learning. We developed an eye movement measuring system and a paradigm for induction of HVOR adaptation for the common marmoset. The HVOR gain in dark measured by 10° (peak-to-peak amplitude) and 0.11–0.5Hz turntable oscillation was around unity. The gain-up and gain-down HVOR adaptation was induced by 1h of sustained out-of-phase and in-phase 10°–0.33Hz combined turntable-screen oscillation in the light, respectively. To examine the role of long-term depression (LTD) of parallel fiber-Purkinje cell synapses, we intraperitonially applied T-588 or nimesulide, which block the induction of LTD in vitro or in vivo preparations, 1h before the test of HVOR adaptation. T-588 (3 and 5mg/kg body weight) did not affect nonadapted HVOR gains, and impaired both gain-up and gain-down HVOR adaptation. Nimesulide (3 and 6mg/kg) did not affect nonadapted HVOR gains, and impaired gain-up HVOR adaptation dose-dependently; however, it very little affected gain-down HVOR adaptation. These findings are consistent with the results of our study of nimesulide on the adaptation of horizontal optokinetic response in mice (Le et al., 2010), and support the view that LTD underlies HVOR adaptation.

Adaptive modifications of post-saccadic smooth pursuit eye movements and their interaction with saccades and the vestibulo-ocular reflex in the primate

Adaptation of the horizontal smooth pursuit eye movement was examined using step-ramp moving target paradigms in chronically prepared macaca fuscata. Monkeys were trained to pursue a small target which moved in the horizontal plane in a 3° step-10°/s ramp or a 0° step-10°/s ramp mode for 300–400 ms. When the target moved from central fixation point in a step-ramp mode, the monkeys usually responded with an initial pursuit eye movement (latency, 100–120 ms) which reached to a nearly constant velocity of 4°/s in 50–100 ms, followed by a 1–3.5° catch-up saccade (latency, 170–230 ms). The catch-up saccade was followed by a 7–8°/s post-saccadic pursuit. The post-saccadic pursuit velocity was measured 40–90 ms after the end of the catch-up saccade. When the target velocity was doubled (20°/s) for 100–200 ms immediately after the onset of the catch-up saccade, the post-saccadic pursuit velocity increased by 40%. When the target velocity was decreased by half (5°/s) immediately after the onset of the catch-up saccade for 150 ms, the post-saccadic pursuit velocity decreased by 30%. These increases or decreases of post-saccadic pursuit were observable within just 50–150 trials. The adaptation of post-saccadic pursuit occurred independent of the position of the target. The amplitude and latency of the catch-up saccade also increased correspondingly when the post-saccadic pursuit velocity was adaptively increased. Adaptation of smooth pursuit did not affect the dynamics of reflex eye movements, including the horizontal vestibulo-ocular reflex (HVOR) gain and phase measured by 0.33 Hz-10° (peak-to-peak) turntable oscillations in darkness. Conversely, adaptation of the HVOR gain induced by a 2 h sustained oscillation of the turntable and screen in reversed direction at 0.33 Hz-10° affected little the velocity of post-saccadic pursuit or the amplitude of the catch-up saccade. These results suggest that different neural mechanisms are respectively involved for the adaptation of horizontal smooth pursuit and HVOR in the primate.

2211 Dynamic characteristics and adaptation of horizontal vestibulo-ocular reflex and optokinetic response in mice

2211 Dynamic characteristics and adaptation of horizontal vestibulo-ocular reflex and optokinetic response in mice

Dynamic characteristics and adaptation of horizontal vestibulo-ocular reflex and optokinetic response in mice

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The Role of Cerebellar Flocculus in Adaptive Gain Control of Ocular Reflexes

Sustained oscillation of the head of an alert animal with inphase or outphase combination of screen oscillation in the horizontal plane induced marked adaptive changes in the gain of the horizontal vestibulo-ocular reflex (HVOR). These changes are assumed to be a prototype of motor learning by the cerebellar flocculus. Several lines of experimental evidence, using rabbits or monkeys as experimental material, have consistently suggested that the plastic changes of neuronal activities of a particular group of floccular Purkinje cells (H-cells) are the sources of the HVOR adaptation.

Behavior of floccular Purkinje cells correlated with adaptation of vestibulo-ocular reflex in pigmented rabbits.

The responsiveness of floccular Purkinje cells to head oscillations was examined in alert pigmented rabbits subjected to adaptation of horizontal vestibulo-ocular reflex (HVOR) under three different combinations of turntable and screen oscillations. Purkinje cells involved in the HVOR control (H-zone cells) were identified by local stimulation effects that induced horizontal eye movements. In control states, simple spike discharges of H-zone cells were modulated predominantly out of phase with the velocity of sinusoidal turntable oscillation (0.1Hz, 5 degrees peak-to-peak). A sustained 180 degrees outphase combination (5 degrees turntable and 5 degrees screen oscillation) was found to increase the average HVOR gain by 0.16, at which point the majority of H-zone cells increased the outphase simple spike modulation. A sustained inphase combination (5 degrees turntable and 5 degrees screen oscillation) decreased the average HVOR gain by 0.09, with the majority of H-zone cells decreasing the outphase simple spike modulation or becoming converted to the inphase modulation. With a vision-reversal combination (5 degrees turntable and 10 degrees screen oscillation), there was no change in the gain of the HVOR, but a moderate advancement in the phase. In this case, H-zone cells showed no appreciable changes in their simple spike modulation. Complex spike discharges of all H-zone cells tested were modulated in response to optokinetic stimuli involved in the combinations of turntable and screen oscillations. These results support the hypothesis that H-zone cells adaptively control HVOR dynamic characteristics through modification of mossy fiber responsiveness to head oscillation under influences of retinal error signals conveyed by climbing fiber afferents.

Role of cerebellar flocculus in adaptive interaction between optokinetic eye movement response and vestibulo-ocular reflex in pigmented rabbits.

Sustained sinusoidal oscillation of a striped cylindrical screen around a stationary, alert pigmented rabbit with certain parameters (for 4 h. 5 degrees, 7.5 degrees, 10 degrees peak-to-peak, 0.1 or 0.2 Hz) adaptively modified not only the horizontal optokinetic reflex (HOKR) but also the horizontal vestibuloocular reflex (HVOR). The major effects thus obtained during 4 h were an increase in the HOKR gain by 0.23, and that of the HVOR gain by 0.18. Bilateral destruction of floccular Purkinje cells with microinjection of kainic acid abolished these effects on both HOKR and HVOR. Single unit activities of floccular Purkinje cells were recorded from the floccular areas related to horizontal eye movements (H-zone) with local stimulus effects. Most H-zone Purkinje cells normally exhibited modulation of simple spike discharge in phase with screen velocity and out of phase with turntable velocity. Sustained screen oscillation (7.5 degrees, 0.1 Hz) for 1 h increased the simple spike responses not only to screen but also to turntable oscillation. No such changes were observed in other floccular areas. These observations suggest that sustained optokinetic stimulations induce adaptation of HVOR through an interaction of retinal slip and head velocity signals within the flocculus or its related neuronal tissues.

Role of the primate flocculus in adaptation of the vestibulo-ocular reflex

Neuronal events associated with adaptation of the horizontal vestibulo-ocular reflex (HVOR) induced by sustained vestibular-visual mismatching were investigated in the primate flocculus. The floccular area related to the HVOR (H-zone) was identified by electrical micro-stimulation which induced ipsilaterally directed horizontal eye movement. It was thus found that Purkinje cells in the H-zone consistently changed their simple spike responses to head rotation in parallel with the adaptive HVOR gain change. This was demonstrated by observing the change of simple spike firing of Purkinje cells during adaptation of HVOR either in a population study or an individual study. Since similar changes occurred even after bilateral lesioning of vestibular nuclei had extinguished the HVOR, these changes appear to represent vestibular, but not eye velocity, mossy fiber responsiveness. The complex spike discharge, on the other hand, modulated during vestibular-visual stimulation with a reciprocal pattern to the adaptive changes in the simple spike discharge. These results are consistent with the hypothesis that the flocculus Purkinje cells adaptively control the HVOR through their simple spike activity under influences of retinal error signals conveyed by visual climbing fiber pathways.

Loss of vision-guided adaptation of the vestibulo-ocular reflex after depletion of brain serotonin in the rabbit

Brain serotonin in pigmented rabbits was depleted by intraventricular injection of 5,7-dihydroxytryptamine (5,7-DHT). Before depletion, an adaptive increase in the gain of the horizontal vestibuloocular reflex (HVOR) could be induced regularly by continuously rotating the animals in combination with optokinetic stimulus. After the depletion, such an increase in HVOR gain did not occur, even though dynamic characteristics of the HVOR and of the optokinetic eye movement were not altered. 5,7-DHT treatment also reduced brain norepinephrine, but depletion of norepinephrine to a similar extent by intraventricular injection of 6-hydroxydopamine in other rabbits did not affect the HVOR adaptation. These results suggest that brain serotonin plays an important role in maintaining adaptive modifiability of the HVOR.