Tetsuo Himi

The role of saccular afferents in vertical optokinetic nystagmus in primates. A study in relation to optokinetic nystagmus in microgravity

SummaryAfter bilateral sacculectomies, the squirrel monkeys vertical optokinetic nystagmus showed a gain increase of slow-phase down-nystagmus and a gain decrease of the slow-phase up-nystagmus. The nystagmus beating field of the former shifted upward but that of the latter did not change clearly. These results are in general agreement with the findings in man during the recent shuttle flight and indicate the functional linkage of saccular afferents to the vertical optokinetic nystagmus.

Asymmetry of Vertical Optokinetic After-nystagmus in Squirrel Monkeys

Asymmetry of vertical optokinetic after-nystagmus (OKAN) was studied in 6 squirrel monkeys. The slow-phase eye velocity (SPEV) of upward OKAN first-phase (OKAN-I) increased with increasing stimulus velocity, whereas the SPEV of downward OKAN-I diminished. The time constant of OKAN-I was shortened with the increase in stimulus speed in both directions. With a downward stimulus, the short stimulus duration failed to produce OKAN second-phase (OKAN-II) (upward slow-phase); however, with an increase in stimulus duration, the percentage appearance increased. There was no change in percentage appearance, regardless of the duration of upward stimulus. The asymmetry of OKAN-I and that of OKAN-II differed to a certain degree.

Autonomic Indexes During the Vestibular-Visual Conflict Exposure: A Squirrel Monkey Study

To assess the dynamic changes of the autonomic neural responses evoked by vestibular-visual conflict (VVC) in the squirrel monkey, we analyzed the changes of salivary amount and sodium concentration, and the coefficient of variance (CV) of R-R intervals (RRI) along the time course of VVC exposure given in pitch plane. The sodium concentration and the amount of saliva showed clear increases at the first and second 15 min stimulation periods. The CV of RRI was found to increase over the entire periods of stimulation, and there was a significant difference between the rest value and those in stimulated periods. The correlation between the CV of RRI, and amount of salivation was highly positive. These results indicate the existence of a common pattern of autonomic neural response during VVC, and suggest usefulness of these indexes for objectively monitoring the severity of motion sickness.

Comparative Effect of Some Neurotropic Agents on Balance Compensation After Unilateral and Bilateral (Two-Staged) Labyrinthectomy in Squirrel Monkeys

The dynamic repair process of body equilibrium after vestibular injury is a typical representation of nervous system plasticity. The body equilibrium system needs to re-establish a coordinated sensorimotor balance. For this purpose, the input from the remaining vestibular organs is believed to carry the primacy for balance compensation with its receptor characteristics. In addition, afferents from other sensory modalities are mobilized, modulated, and efficiently integrated at many levels of the central nervous system, particularly after the total vestibular loss; during the Bechterew’s compensation. Thus, the global and behavioral measurements in animal models is advantageous, for it can assess the integrated function of the entire neural system.

Effect of vestibulo-cerebellar lesions on asymmetry of vertical optokinetic functions in the squirrel monkey

The role of the cerebellar uvula and nodulus in vertical optokinetic after-nystagmus (OKAN) was studied in 4 squirrel monkeys. Aspiration ablation of the uvula and nodulus resulted in no significant change in the initial or peak gain of vertical optokinetic nystagmus (OKN) during the 24-week post-operative observation. However, the asymmetry of vertical OKAN was significantly altered. Using a protracted upward OK stimulus, slow phase-down OKAN-II, which was not seen pre-operatively, was significantly increased. In contrast, a downward OK stimulus produced little change in slow phase-up OKAN-II. Thus, the asymmetric degree of vertical OKAN-II was decreased after uvulonodulectomy. In addition, there was a post-operative reduction in the vertical oculomotor stability. When slow-phase eye velocity of OKAN was plotted along the time scale, the amplitude and frequency of the sinusoidal pattern was increased. OKAN-III and OKAN-IV were found in 50% of the monkeys after uvulonodulectomy. It is therefore thought that inhibition and directional control from the uvula and nodulus influence the stability and asymmetrical behaviour of the leaky integrator in the second order output system.

Temporal Bone Findings in Craniodiaphyseal Dysplasia

To our knowledge, this is the first description of the histopathology of the temporal bone from a case of craniodiaphyseal dysplasia. The air spaces of the mastoid, external auditory canal, and middle ear cavity are reduced by hyperostotic bone. The ossicles are also deformed by the bony overgrowth. Anomalous ossicles with hyperostosis could affect the air conduction. Even though the internal auditory canal is somewhat elongated and narrow, no labyrinthine abnormalities can be attributed to the abnormal bone. Facial nerves run through abnormal courses but the geniculate ganglion cells are not involved. The VIIIth nerve dysfunction in this case could be due to mechanical damage of the nerve fibers and/or impaired vascular supply by the hyperostosis.

Second-phase Optokinetic After-nystagmus and Vestibular Compensation

To evaluate the direction-specific effect of optokinetic storage function on balance compensation after unilateral labyrinthectomy, squirrel monkeys (n = 12) were exposed to a prolonged (30 min) daily optokinetic stimulus (horizontal, constant speed-90 degrees/sec) for 14 days; either in the ipsilateral direction to the slow phase of spontaneous nystagmus, or in the contralateral direction, or without any optokinetic stimulus. The effect of optokinetic stimulus in ipsilateral direction (slow phase of OKAN-II in the opposite direction) was significantly more (p less than 0.001) in the vestibulo-oculomotor balance (static) regainment than the other two groups, but this group showed the severest gait deviation in the vestibulospinal balance function (p less than 0.05).

Asymmetry of Vertical Optokinetic Nystagmus and Afternystagmus

The sensorimotor mechanism to stabilize the visual field in upright posture is not the same in earth-horizontal plane and vertical plane, because gravity possesses a directionality which is sensed by gravity-receptors. In this communication, asymmetric behavior of the vertical optokinetic nystagmus and afternystagmus is discussed, in conjunction with lesion placements within the peripheral and central neuro-networks (macula sacculi and cerebellar uvula and nodulus) in squirrel monkeys.

Change in Susceptibility to Vestibular-Visual Conflict Sickness in Monkeys by Repeated Exposure

Squirrel monkeys, susceptible to the vestibular-visual conflict sickness in pitch (with frank vomiting), were subjected to repeated exposure to pitch conflict in various modes and in a randomly mixed order (30 min daily, for 10 consecutive days). Immediately after the training, a significant decline in susceptibility was found, represented by reduced vomiting rates, reduced sickness scores, reduced salivation and improved regularity of vertical oculomotor responses. Susceptibility at the pre-training level returned when the test was repeated 10 days later. Temporary suppression of pitch conflict sickness susceptibility indicates the possibility of training crew members prior to their space flight missions to control the space motion sickness, particularly vomiting.