Journal of Vestibular Research

Vestibular hair cells, type I and II, with membrane potentials around -64 mV were prepared from guinea pig ampullar cristae and maculae. In type I cells, current injection, application of voltage steps during membrane patch-clamping, or extracellular alternating current (ac) fields evoked fast length changes of 50 nm to 500 nm of the cell “neck”. Mechanical responses were determined by computerized video techniques with contrast-enhanced digital image subtraction (DIS) and interpeak pixel counts (IPPC) or by double photodiode measurements. These techniques allowed spatial resolutions of 300 nm, 120 nm, and 50 nm, respectively. In contrast to measurements of high-frequency movements of auditory outer hair cells (OHCs), the mechanical responses of type I VHCs were restricted to low frequencies below 85 Hz. In addition to recently reported slow motility of VHCs, the present results suggest that fast mechanical VHC responses could significantly influence macular and cupular mechanics. Isometric and isotonic variants are discussed. The observed frequency maxima gap between VHCs and OHCs is suggested to contribute to a clear separation of the auditory and the vestibular sensory modality.

Head and eye movements of human subjects have been recorded during head-free pursuit in the horizontal plane of a target executing sinusoidal motion at a frequency of 0.26 to 0.78 Hz and a peak velocity of ±96^∘/s. The target was not presented continuously but was exposed for brief durations of 120 to 320 ms as it passed through the centre of the visual field at peak velocity. This technique allowed the timing of each response to be assessed in relation to the onset of target appearance. During the first 3 to 4 target presentations, there was a progressive buildup of both head velocity and the smooth component of gaze velocity, while, simultaneously, the responses became more phase-advanced with respect to target onset. In the steady state, similar temporal response trajectories were observed for head and gaze velocity, which were initiated approximately 500 ms prior to target on-set, rose to a peak that increased with the duration of target exposure, and then decayed with a time constant of 0.5 to 1 s. Whenever the target failed to appear as expected, the gaze and head velocity trajectories continued to be made, indicating that predictive suppression of the vestibulo-ocular reflex (VOR) was taking place in darkness. In a further experiment, subjects attempted to suppress the VOR during whole body oscillation at 0.2 or 0.4 Hz on a turntable by fixating, a head-fixed target that appeared for 10 to 160 ms at the time of peak head velocity. Again, VOR suppression was initiated prior to target appearance in the same manner as for natural head movements, and when the target suddenly disappeared but rotation continued, predictive VOR suppression was observed in darkness. The similarity of these predictive effects to those obtained previously for head-fixed pursuit provides further support for the hypothesis that both pursuit and visual suppression of the VOR are controlled primarily by identical visual feedback mechanisms.

Four normal subjects underwent ocular counterrolling testing in a tiltable chair. Measurements were taken in 62 different body positions in steps of 30^∘ varied rolls and pitches. In each body position the eyes were recorded on video and their roll angle was determined automatically by computer analysis. The ocular counterrolling profile showed a periodic characteristic with maximal amplitude at roll tilts of 60^∘. In this study we can clearly show that the eyes’ rolling response is not systematically affected when lateral body tilts are combined with any tilts in the pitch direction. This undoubtedly implies that the ocular counterrolling was mainly stimulated by the subject’s roll angle. As an empirical contribution, this study provides new data specially to be used in modelling and simulating the function of otolith organs.

There is a need to develop bedside tests of the vestibulo-ocular reflex (VOR) that could be used in the clinical situation to screen patients who may be candidates for further evaluation. In 1984 Barber described the oscillopsia test, which compared visual acuity with and without head movement. Barber indicated that head movement should occur at greater than 1 Hz. This study was performed to evaluate the oscillopsia test at higher frequencies (2 to 7 Hertz) in the hope of improving its performance. The sensitivity and specificity of this test were evaluated using three examiners (the authors) and were referenced to clinical electronystagmographic results in 115 patients and 17 control subjects. The oscillopsia test evaluated in this study was highly specific, but not highly sensitive. It did not detect vestibular loss or subjective dizziness in more than 50% of cases. The high frequency oscillopsia test does not appear to be an effective screening test for VOR abnormalities or vestibular loss.

Mechanical aspects of the ampullar diaphragm, that is the crista ampullaris and the cupula, related to its thickness, are studied by a numerical method. Numerical methods are able to go beyond the limits of analytical approaches and are the only methods able to take into account this thickness. A finite elements method is applied to the median plane slice of the ampullar diaphragm. One assumes that the cupula sticks firmly without slipping, to the ampullar wall and to the crista ampullaris. The computation takes into account the pressures on the liquid interfaces and the deformations of the ampulla. So the volume swept over by the cupula during quasi-static deformations can be evaluated and the global elasticity coefficient of the human cupula can be calculated. The related value of the long time constant of the semicircular canal is close to the value obtained when measuring, in vivo, the activity on the vestibular nerve in animals. The thick cupula model clearly shows two different spatial distributions of strain on the hairs of the sensory cells, leading to a discrimination between the vestibular inflating pressure and the transcupular pressure difference. This result matches recent neurophysiological data and brings a new insight in the mechanics of the vestibular angular accelerometer and its regulation.

Electrical stimulation in the monkey vestibulocerebellum has previously been shown to produce ocular nystagmus, but large stimulating current values were used. Using long duration (⩽10-second) stimulus pulse trains and low current values (<50 μA), we studied the nystagmus evoked by microstimulation in the uvular/nodular regions of the cerebellum. In doing this, we found quantitative differences in the nystagmus evoked from these two regions. Stimulation of the nodulus typically produced a vigorous nystagmus with a contralateral slow phase and a prolonged afternystagmus in the same direction. In contrast, stimulation of the uvula typically produced a regular ipsilateral nystagmus pattern with a very short, if any, afternystagmus in the same direction. In addition, at some stimulation sites in the uvula we observed an adaptation in the slow phase eye velocity during the time that the stimulation remained on. This effect could result in a secondary nystagmus, with a slow phase velocity direction opposite to that first evoked by the stimulation, followed by a prolonged afternystagmus in the direction of the secondary nystagmus at stimulus offset. The nystagmus evoked by these cerebellar stimulations differs from both natural nystagmus produced by large field visual motion and from the nystagmus produced by electrical stimulation of the nucleus of the optic tract. The nystagmus produced by uvular and nodular stimulation shows a shorter latency and a more rapid slow phase eye velocity buildup. The uvula stimulations also showed a much shorter afternystagmus. Also, the same nystagmus was evoked whether the animal was in a lighted or dark surround. These characteristics and recent single-unit recording studies in the uvula seem to suggest that the uvula acts not as a direct input to the velocity storage mechanism, but instead perhaps as part of an internal regulator for balance between the bilateral vestibular nuclei which are normally part of the nystagmus response. On the other hand, the nodulus, with its prolonged afternystagmus in the same direction as the evoked nystagmus, may be involved as a part of the velocity storage mechanism.

A longitudinal study on the effects of age on the susceptibility to motion sickness in the squirrel monkey was carried out over a lO-year period (1982 to 1991). The typical life span of squirrel monkeys is 15 years. Ten mature male squirrel monkeys of the Bolivian subspecies were found to be susceptible to motion sickness induced by a combination of vertical oscillation at 0.5 Hz and rotation in the horizontal plane at 25 rotations per minute (RPM) in a visually unrestricted environment. Signs of motion sickness were quantified according to a rating scale based on Graybiel’s diagnostic criteria. Latency to vomiting/retching and severity of sickness obtained from year 1 (baseline), 3, 5, 7 and 10 were subjected to repeated-measures design analysis. There were no significant differences in the susceptibility level (as measured by latency to vomiting/retching and cumulative sickness scores) in the monkeys throughout the 10-year period. The habituation to 7 consecutive daily exposures remained the same throughout the same period. We conclude that, in the squirrel monkeys from maturity to near the end of their life span, there is no change in susceptibility to motion sickness with aging.