Convergence of ampullar and macular inputs on vestibular nuclei unit of the rat

Head position modulates optokinetic nystagmus

Orientation and movement relies on both visual and vestibular information mapped in separate coordinate systems. Here, we examine how coordinate systems interact to guide eye movements of rabbits. We exposed rabbits to continuous horizontal optokinetic stimulation (HOKS) at 5°/s to evoke horizontal eye movements, while they were statically or dynamically roll-tilted about the longitudinal axis. During monocular or binocular HOKS, when the rabbit was roll-tilted 30° onto the side of the eye stimulated in the posterior → anterior (P → A) direction, slow phase eye velocity (SPEV) increased by 3.5-5°/s. When the rabbit was roll-tilted 30° onto the side of the eye stimulated in the A → P direction, SPEV decreased to ~2.5°/s. We also tested the effect of roll-tilt after prolonged optokinetic stimulation had induced a negative optokinetic afternystagmus (OKAN II). In this condition, the SPEV occurred in the dark, "open loop." Modulation of SPEV of OKAN II depended on the direction of the nystagmus and was consistent with that observed during "closed loop" HOKS. Dynamic roll-tilt influenced SPEV evoked by HOKS in a similar way. The amplitude and the phase of SPEV depended on the frequency of vestibular oscillation and on HOKS velocity. We conclude that the change in the linear acceleration of the gravity vector with respect to the head during roll-tilt modulates the gain of SPEV depending on its direction. This modulation improves gaze stability at different image retinal slip velocities caused by head roll-tilt during centric or eccentric head movement.

Zonal organization of the vestibulo-cerebellar pathways controlling the horizontal eye muscles using two recombinant strains of pseudorabies virus

Many studies have documented the influence of the flocculus upon vestibulo-ocular reflex eye movements. Electrical stimulation of Purkinje cells in a central longitudinal zone evoked slow ipsilateral eye movements in the horizontal plane. Recently, the organization of neurons in the vestibulo-cerebellar pathways controlling single lateral rectus and medial rectus muscles was identified in rats using the transynaptic transport of pseudorabies virus. Overlapping distributions of neurons innervating single muscles were located predominantly in a central longitudinal zone of ventral paraflocculi/dorsal flocculi, and the rostral half of ventral flocculi. This study used two isogenic pseudorabies virus recombinants to determine whether individual cells in those brain regions have collateralized projections to motoneuron pools innervating the right lateral rectus and the left medial rectus muscles using different survival times and dual injection paradigms. The infected neurons were detected using dual-labeling immunofluorescence. Three populations of labeled neurons were observed: two populations replicated only one reporter while a third contained both viruses (i.e. dual-labeled). Most dual-labeled cells were located in a central longitudinal zone of the ventral paraflocculus, ipsilateral to the injection into the medial rectus, whereas very few were in the flocculus. This finding suggests that the flocculus and ventral paraflocculus may exert influence upon distinct vestibulo-cerebellar pathways. Most Purkinje cells in the ventral paraflocculus may influence the vestibulo-ocular reflex pathways through collateralization, whereas those in the flocculus may instead provide a monocular control of eye movements.

Cerebellum-dependent learning: the role of multiple plasticity mechanisms

The cerebellum is an evolutionarily conserved structure critical for motor learning in vertebrates. The model that has influenced much of the work in the field for the past 30 years suggests that motor learning is mediated by a single plasticity mechanism in the cerebellum: long-term depression (LTD) of parallel fiber synapses onto Purkinje cells. However, recent studies of simple behaviors such as the vestibulo-ocular reflex (VOR) indicate that multiple plasticity mechanisms contribute to cerebellum-dependent learning. Multiple plasticity mechanisms may provide the flexibility required to store memories over different timescales, regulate the dynamics of movement, and allow bidirectional changes in movement amplitude. These plasticity mechanisms must act in combination with appropriate information-coding strategies to equip motor-learning systems with the ability to express learning in correct contexts. Studies of the patterns of generalization of motor learning in the VOR provide insight about the coding of information in neurons at sites of plasticity. These principles emerging from studies of the VOR are consistent with results concerning more complex behaviors and thus may reflect general principles of cerebellar function.

Gravity-specific adaptation of the angular vestibuloocular reflex: dependence on head orientation with regard to gravity

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.

Vestibular compensation in infants and children with congenital and acquired vestibular loss in both ears

In children with semicircular canal anomalies, vestibular compensation during their development and growth was studied. The damped rotation test elicited 'absence or poor per-rotatory nystagmus and absence of post-rotatory nystagmus in all cases. Development of gross motor and balance function was seriously delayed in each case during the first 2 or 3 years of life. Thereafter, during the pre-school age, all children could achieve most landmarks of motor development, such as head control, independent walking and running. However, balance functions at the age of entrance of the elementary school (6 years old) were variously impaired in each case. The better case could swim under water but the poor case could not maintain static balance with eyes closed. These motor skills due to vestibular compensation presumably depend on integration of the compensatory input from visual, somatosensory and proprioceptive senses, and the maturation of motor control systems in the cerebellum, basal ganglia and motor cortex.

Delayed motor function and results of vestibular function tests in children with inner ear anomalies

The relation between the results of vestibular function tests and gross motor development was examined in 4 children with inner ear anomalies. CT scans demonstrated the absence of lateral semicircular canals in both ears in all 4 cases. None responded to caloric stimulation using 40 ml of icewater. In contrast, the damped rotation test elicited per-rotatory nystagmus in all cases. Per-rotatory nystagmus was provoked in only two cases by the Bárány rotation test. Development of gross motor function, especially independent walking, was more delayed in the two children in whom the Bárány rotation test failed to elicit per-rotatory nystagmus.

The influence of gravity on horizontal and vertical vestibulo-ocular and optokinetic reflexes in the rabbit

The influence of the linear acceleration of gravity on the vertical and horizontal vestibulo-ocular reflexes (VVOR, HVOR) as well as the vertical and horizontal optokinetic reflexes (VOKR, HOKR) has been examined in rabbits. Rabbits were mounted in a biaxial rate table in front of a rear projection tangent screen. Eye movements were measured with a light projection technique. The HVOR, VVOR, HOKR and VOKR were measured in rabbits which were maintained both prone and supine. The gain of the HVOR for the supine orientation was reduced at all frequencies tested (0.01-0.80 Hz). Similarly there was a reduction in the gain of the HOKR. By contrast, the gain of the VVOR in the supine orientation was enhanced over a lower range of frequencies (0.02-0.04 Hz) and reduced at higher frequencies (0.10-0.80 Hz). The gain of the VOKR was not reduced in the supine orientation. The range of eye positions over which compensatory eye movements occurred was restricted in the supine orientation. The altered orientation of the medio-laterally polarized hair cells of the utricular maculae with respect to gravity in the supine orientation may cause postural instability and facilitate 'righting reflexes'. A reduction in the gains of the HVOR, VVOR and HOKR caused by linear accelerations in the sagittal plane during locomotion may decrease automatic postural responses during certain movements in which these automatic postural adjustments would not necessarily be adaptive.

Response of central vestibular neurons to horizontal linear acceleration in the rat

Responses of central vestibular neurons to horizontal sinusoidal translation (F:0.25Hz) were recorded in albino rat. 57.5% of vestibular neurons were responding to this stimulation by a modulation of their firing rate, the mean phase angle of the response, averaged from the whole population being 22 +/- 79 deg. lag, relative to the peak of contralateral acceleration. Dynamic characteristics of phase and gain were studied and appeared to be different from previous reports on primary afferents: the gain decreased or was flat with increasing acceleration at one frequency, and the phase lag which was flat in the same conditions increased with increasing frequency. A phase lead of some units has been observed at low frequency (0.1 Hz). Regarding the convergence between otolith and canal inputs on nuclear vestibular neurons, it was shown that the major pattern of convergence is between canal and otolith inputs of same polarity.

Effect of off-vertical tilt and macular ablation on postrotatory nystagmus in the squirrel monkey

The slow phase eye velocity (SPEV) and duration of post-rotatory nystagmus (PRN) were studied in squirrel monkeys (Saimiri sciureus) after a ramp speed rotation (0-200 degrees/sec, with 1 degree/sec2 angular acceleration). When the results were compared between straight upright vertical rotation, 9 degrees tilt rotation, and 18 degrees tilt rotation, faster decay both in SPEV and in duration was found in the tilt rotation situations. Difference in nystagmic decay curves by tilting rotation axis could be from the convergence of macula-semicircular canal inputs. Subsequently bilateral macular ablation (two-stage) was performed. The difference in nystagmus decay curves between three different rotations was reduced; therefore, the change of gravity direction perceived through gravity receptors other than macular endorgans was minimal and did not produce a difference in three different rotations.

The postnatal development of functional properties of central vestibular neurons in the rat

The postnatal development of the responses of rat central vestibular neurons to horizontal angular acceleration was studied in the time and frequency domain. The resting discharge was very low and irregular during the first postnatal days, increased gradually and became more regular throughout the first month and reached adult values approximately by the end of the first month. The relative distribution of type I and type II units was the same in all age groups. Threshold for frequency increase to angular acceleration and sensitivity of unit responses became lower and higher, respectively, as time elapsed after birth. Adult values were reached approximately by the end of the first month. There was a slight tendency towards shorter time constants and smaller phase lags in one-month-old animals when compared with the younger animals. The results are discussed in conjunction with similar work performed in vestibular afferents and correlated with known morphological and behavioral studies.

Electronystagmography in the laboratory rat

A method is described for obtaining electronystagmograms from the awake laboratory rat. Threshold valves for rotation impulse and oscillatory acceleration were determined, as well as the time constant for the horizontal semicircular canal. The time constant appeared to be small. This might be attributed to the rapid natural head movements in this species. No visual suppression of vestibular nystagmus was found in the rat.