Effect of macular ablation on vertical optokinetic nystagmus in the squirrel monkey.

After macular ablation in squirrel monkeys, a significant improvement of the slow-phase eye speed of vertical optokinetic nystagmus (VOKN) was found. This findings is in contrast to the result of our previous study in which horizontal optokinetic nystagmus (HOKN) was not improved following macular ablation. The different influence of macular ablation on HOKN and VOKN could be due to the directional characteristics of vestibular and visual interaction relative to the direction of gravity and to the difference in horizontal and vertical vestibulo-oculomotor neural pathways, including vestibular end-organs. The vertical directional dominance which was observed preoperatively was similarly observed in the postablative status; the upward slow-phase nystagmus was easier to provoke than the downward nystagmus by application of an identical stimulus.     

Comparison of vertical and horizontal optokinetic nystagmus in the squirrel monkey.

Horizontal and vertical optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN) of squirrel monkeys were compared with those of rabbits, cats and humans that were previously described. Squirrel monkeys showed similar findings to cats, in which vertical optokinetic nystagmus (VOKN) is not as well elicited as horizontal optokinetic nystagmus (HOKN) and down-pursuit OKN is poorer than up-pursuit OKN. As to the reasons that bring about different responses of OKN and OKAN (and vestibular nystagmus) in different planes, we speculated two possibilities: compensatory activation of horizontal eye movement for narrowed visual field accompanied by frontally positioned eyes, and the gravity that restricts and modifies posture and locomotion. Directional difference of VOKN may be caused by a physiological mechanism that makes visual fixation not susceptible to downward movement of the ground surface during forward locomotion.     

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.     

Effect of otolith end organ ablation on horizontal optokinetic nystagmus, and optokinetic afternystagmus in the squirrel monkey.

Horizontal optokinetic nystagmus (OKN) and optokinetic afternystagmus (OKAN) (stimulus speed 0-200 degrees/sec with 1 degree/sec constant angular acceleration) were examined before and after utriculo-sacculectomy (bilateral, two-stage) in squirrel monkeys. OKN exhibited a slight decline only after bilateral otolith and organ ablations. OKAN showed a minimal decline after unilateral operation but no change after bilateral operations. Severe OKN reduction and disappearance of OKAN after bilateral labyrinthectomy in primates should basically reflect the elimination of inputs from the cristae ampullares, and not from the maculae.     

The effect of saccular ablation on vertical optokinetic after-nystagmus in squirrel monkeys

Summary The effect of bilateral saccular ablation on the asymmetry of vertical optokinetic after-nystagmus (OKAN) was studied in squirrel monkeys. No significant changes occurred in the initial slow-phase eye velocity (SPEV) or the time constant of the upward or downward OKAN first phase (OKAN-I) under various stimulus conditions. However, with a protracted downward stimulus, the maximum SPEV and the number of beats of the slow-phase-up OKAN second phase (OKAN-II) significantly increased. This increase should be the result from enhancement of the downward optokinetic input. In contrast, there was only minimal change in the slow-phase-down OKAN-II. Thus, the asymmetrical dominance of the vertical OKAN (dominance upward) remained the same after saccular deafferentation. Offprint request to: M. Igarashi     

Velocity and asymmetry of optokinetic nystagmus in the evaluation of vestibular lesions

Optokinetic nystagmus (OKN) at a constant stimulus velocity of 90 degrees/s was studied in 50 healthy subjects and three different groups of patients: 10 with vestibular neuritis (VN), 17 with unilateral infratentorial lesion (UI) and 13 patients with bilateral infratentorial lesion (BI). Among healthy subjects, OKN decreased with age in those over 42 and 44 years for maximum and mean velocity, respectively. The maximum velocity of OKN discriminated between the healthy subjects and the groups of patients rather better than did mean velocity of OKN. Asymmetry of either mean or maximum velocity contributed only marginally to correct group assignment. By subjecting test data on velocity and asymmetry variables to linear discriminant function analysis, 80% of subjects could be correctly classified as healthy/not healthy, and 64% assigned to the correct group according to diagnosis. Analysis of OKN data satisfactorily discriminated BI and UI patients from VN patients and healthy subjects, and is thus useful in assessing infratentorial vestibular lesions.     

A review of the effects of space flight on the asymmetry of vertical optokinetic and vestibulo-ocular reflexes

Prolonged microgravity during orbital flight is a unique way to modify the otolith inputs and to determine the extent of their contribution to the vertical vestibulo-ocular reflex (VOR) and optokinetic nystagmus (OKN). This paper reviews the data collected on 10 astronauts during several space missions and focuses on the changes in the up-down asymmetry. Both the OKN elicited by vertical visual stimulation and the active VOR elicited by voluntary pitch head movements showed an asymmetry before flight, with upward slow phase velocity higher than downward slow phase velocity. Early in-flight, this asymmetry was inverted, and a symmetry of both responses was later observed. An upward shift in the vertical mean eye position in both OKN and VOR suggests that these effects may be related to otolith-dependent changes in eye position which, in themselves, affect slow phase eye velocity.     

Compensation of vertical vestibulo-ocular functions in squirrel monkeys after unilateral labyrinthectomy

Modifications in the vertical vestibulo-ocular reflex (VVOR) were studied in four squirrel monkeys (Saimiri sciureus) over time after unilateral labyrinthectomy. In the dark, with head upright, animals were exposed to sinusoidal rotation in the pitch plane. The magnetic field-pericorneal search coli technique was used to detect eye movement. The upward and downward slow-phase eye velocity (SPEV) of the VVOR and the maximum SPEV of the vertical component of spontaneous nystagmus (SPN) were studied. The mean preoperative gain of VVOR was symmetric. After unilateral labyrinthectomy, the maximum reduction of VVOR gain showed an asymmetry. The recovery of VVOR gain to the preoperative level took about 2 weeks, and the vertical component of SPN was present for about 3 weeks.     

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.     

Effect of amphetamine on optokinetically evoked head and eye movements in the squirrel monkey

The effect of amphetamine sulfate injection on head and eye movements evoked by optokinetic stimulus was studied in squirrel monkeys. No change was found in the gain of optokinetic-oculomotor and optokinetic-caputomotor functions, even after the maximum dose (0.5 mg/kg) of amphetamine. Also, no change in eye-head coordination was found. However, amphetamine facilitated the frequency of head nystagmus, whereas that of eye nystagmus did not change. When the poststimulus responses were studied in the dark, both nystagmoid head movement and optokinetic after eye nystagmus showed first phase and second phase. A different effect was found in the duration of these two phases suggesting the existence of distinct generators. Moreover, the synchronized onsets of head and eye nystagmus in both phases, and synchronized occurrence of quick phases indicated the existence of shared neural circuitry between head and eye movements.     

Quantitative analysis of horizontal and vertical optokinetic nystagmus and optokinetic after-nystagmus in the cat

This study reported on the horizontal optokinetic nystagmus (OKN) and vertical OKN of cats under the same conditions with quantitative parameters. Using the search coil method, the horizontal and vertical OKN was investigated in 5 alert cats in an upright position. As the optokinetic stimulus, a stepped random dot pattern was used. We recorded the quantitative parameters in both the horizontal and vertical OKN (for the direct pathway parameters, initial fast rise and fast fall; for the indirect pathway parameters, steady state slow phase velocity [SPV] and the optokinetic after-nystagmus [OKAN] area) in cats. The SPV of the horizontal OKN increased with the stimulus amplitude up to 40-60 degrees/s but saturated thereafter (in some cats even more). Right and left OKN were almost symmetrical. The SPV of the downward OKN increased with the stimulus amplitude up to 20 degrees/s but saturated thereafter. This was lower than the horizontal OKN. On the other hand, the SPV of the upward OKN was weak and irregular. As for the OKAN, the right and left OKAN was also almost symmetrical. A downward OKAN was also observed but was weaker than the horizontal OKAN. A fast fall in the SPV of the OKAN was observed in the horizontal and downward OKN. On the other hand, there was little upward OKAN. OKN in cats was composed of both a direct pathway and an indirect pathway. This study suggested that directional differences of OKN were mainly responsible for the indirect pathway. Both the direct and indirect pathways of cats were smaller than those of monkeys. This suggested that the differences in OKN between cats and monkeys were mainly responsible for the direct pathway.     

Vestibular-visual conflict sickness in the squirrel monkey

When combined vestibular and visual (optokinetic) stimuli were given to squirrel monkeys in a direction and phase mismatching sinusoidal mode (in yaw plane), five of six monkeys exhibited actual emesis during three repeated exposures on separate trial days. Two animals vomited on two trial days and three animals, on one day. The incidence took place in 38.9% of the total trial days. When the identical vestibular and visual stimuli were given individually, the effect was negligible, thus confirming each stimulus magnitude was not stressful enough to produce emesis when used separately.     

Impedance audiometry in the squirrel monkey. Effect of middle ear surgery.

The two principal components of impedance audiometry, the tympanogram and acoustic reflexes, were measured preoperatively and postoperatively in seven squirrel monkeys subjected to experimental middle ear surgery. A probe-tone frequency of 1,200 Hz yielded both tympanograms and acoustic reflex thresholds roughly comparable to analogous data in humans at a probe-tone frequency of 220 Hz. Both tympanograms and acoustic reflex thresholds could be obtained reliably and with comparative ease. In general, postoperative data were modified in the expected fashion by the experimental middle ear surgery. Impedance audiometry appears to be a worthwhile and useful technique for monitoring middle ear status in the squirrel monkey.     

Horizontal and vertical optokinetic nystagmus in man

Horizontal and vertical optokinetic nystagmus (OKN) was studied in 20 healthy adults. Although the horizontal OKN showed no directional difference, a statistically significant difference was found between horizontal and vertical OKN. Upward optokinetic pursuit was better on average than downward pursuit, but more variable. The inferiority of vertical OKN seems to indicate a suppression of optokinetic pursuit due to a different direction of the rotational axis from that of gravity. Regarding the vertical OKN findings, it is speculated that manifest directionality found in quadrupeds is modified in men by the change of their visual field on forward locomotion accomplished by their upright walking posture.     

Statoconia displacement in squirrel monkey ears

When findings in squirrel monkey temporal bones after streptomycin sulfate intoxication were compared with those after application of intense linear acceleration, the dislocated utricular statoconia were more frequently found around the cupula of the crista ampullaris posterior in the latter group, whereas statoconia were rather widely disseminated within three semicircular canals (cupula, ampulla, and duct) after streptomycin intoxication. This difference is most probably due to the fact that, in the linear acceleration group, normal statoconia were instantaneously dislodged in a block (or forming a large-sized mass) and thereafter moved in the gravity direction, whereas in the streptomycin group, degenerated statoconia formed smaller groups and were more freely disseminated. In the endolymphatic sac, dislocated statoconia were readily phagocytosed by the epithelium of the intermediate portion and rugous portion (and transitory portion). The incidence of phagocytosis was found more in the streptomycin group, compared to the linear acceleration group.     

Vestibular end organ mapping in the squirrel monkey

Summary Vestibular end organ mapping (surface preparation) was performed in squirrel monkeys by using succinic dehydrogenase staining. Comparing sensory hair cell counts and surface area measurements of the squirrel monkey to those of man and the guinea pig, the values obtained from the squirrel monkey were between those of man and the guinea pig. When regional (central versus peripheral) cell density was compared, the density of the peripheral area was 1.62 × more on the average than that of the central portion.This study was supported in part by NINDS grant: NS07237, P01-NS-10940, and the McFadden Charitable Trust Fund.     

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.