ORL 39: 321-329 (1977)

Comparison of Vertical and Horizontal Optokinetic Nystagmus in the Squirrel Monkey1 Masahiro Takahashi and Makoto Igarashi Department of Otorhinolaryngology and Communicative Sciences, Baylor College of Medicine, Houston, Tex.

Key Words. Optokinetic nystagmus • Squirrel monkey Abstract. Horizontal and vertical optokinetic nystagmus (OKN) and optokinetic afternystagmus (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 opto­ kinetic 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 nystag­ mus) 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 dif­ ference 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.

Optokinetic nystagmus (OKN) has been extensively studied not only in humans, but in different animal species. Although OKN is a physiological response and can be observed even in lower vertebrates, there is great variation in pursuing ability among different animal species. Numerous investigators have reported findings of horizontal optokinetic nystagmus (HOKN). On the other hand, the studies of vertical optokinetic nystagmus (VOKN) are not many; those are in humans (Stiefel and Smith, 1962; Smith, 1962; Collins et al, 1970; Rosberg et al., 1972); in monkeys (Krieger and Bender, 1956); in cats (Collins et al, 1970; Vital-Durand and Jeannerod, 1974); and in rabbits (Collewijn and Noorduin, 1972a, b). In the rabbit, OKN can be elicited by horizontal, vertical ' Supported in part by NASA contract NAS-9-14546, NIH grants NS-10940 and NS-07237, and McFadden Trust Research Fund.

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Received: July 19, 1977; accepted: August 16, 1977.

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and torsional stimulations although its maximum pursuing speed is very low. When compared to the situation with horizontal optokinetic stimulus, the cat cannot pursue well a vertical optokinetic stimulus and particularly cannot pursue downward stimulation at all. In contrast, Krieger and Bender (1956) reported that the rhesus monkey pursued a downward stimulus better than an upward stimulus. In humans, it has been reported that VOKN is generally similar to HOKN, even though the former shows larger differences depending on the stimulus direction than the latter does. This experiment was undertaken to study the differences in pursuing abili­ ties between HOKN and VOKN in squirrel monkeys, and to compare these findings with those in other animal species which were previously reported.

Subjects and Method The subjects were randomly selected 13 healthy young adult squirrel monkeys (Saimiri sciureus). They were males and females, and their body weights ranged from 450 to 600 g. The optokinetic stimulator was a motor-driven rotatory cylinder, 60 cm in height and diameter, with 16 black stripes (1.5 cm wide, evenly distributed) on its white inside wall, ceiling and floor. This optokinetic stimulator could be placed on its side (90° tilt) when the vertical optokinetic stimulation was to be given. Squirrel monkeys were secured in a labora­ tory-built restrainer and placed in the center of the optokinetic cylinder. The monkey’s head was fixed straight upright. Eye speed was calibrated about 15 min after injecting amphetamine (0.5 mg/kg). The cylinder was rotated at a constant speed of 15°/sec for more 'than 30 sec, and the most stable part of the nystagmic recording was selected for calibra­ tion. The optokinetic stimulus was a cylinder rotation, with l°/sec2 acceleration, from 0°/sec to 200°/sec. Clockwise and counterclockwise, or upward and downward stimuli were given on the same day with an inter-trial interval of 5 min or more. HOKN or VOKN tests were done alternatively but no more than twice a week. Eye movements were recorded by a Beckmann dynograph through a DC amplifier, and the slow-phase eye velocity was manually evaluated.

All monkeys in this experimental series could pursue horizontal cylinder rotation well up to 100°/sec with a minimal inter-individual variation. Over the level of the 110°/sec stimulus, this variation became larger as the stimulus speed increased. However, the nystagmus rhythm was regular and no obvious direc­ tional preponderance (left versus right) was noticed. The vertical optokinetic pursuit was not only poorer when compared with the horizontal pursuit, but it showed a directional difference between upward and downward pursuits (fig. 1). Even at a slow cylinder rotation used for calibra­ tion (15°/sec), most of the squirrel monkeys exhibited unstable vertical pursuit.

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Results

Vertical and Horizontal Optokinetic Nystagmus in Monkey

323

This result is quite different from HOKN which showed accurate pursuit im­ mediately after the initiation of cylinder rotation. The majority of the monkeys could pursue upward stimuli (slow phase) well, up to the speed of 30°/sec, but the pursuit ability became poor over 40°/sec stimulus speed, and case to case variation increased. The comparison of slow-phase eye velocities between horizontal and vertical upward OKN showed a statistically significant difference (p

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

ORL 39: 321-329 (1977) Comparison of Vertical and Horizontal Optokinetic Nystagmus in the Squirrel Monkey1 Masahiro Takahashi and Makoto Igarashi Dep...
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