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Head and eye movements in the yaw plane were recorded during and after optokinetic stimulation in squirrel monkeys. 1) Phasic or tonic head deviations to the side of the ocular quick phase occurred in 94% of total recordings (n = 50) during the perstimulus period, and in 75% of recordings (n = 49) during the poststimulus period. Magnitude of mean head deviation was significantly different between perstimulus and poststimulus periods. 2) Head nystagmus associated with eye nystagmus was consistently observed in seven of nine squirrel monkeys during optokinetic stimulation. Squirrel monkeys are thereby less prone to display head nystagmus than either guinea pigs, pigeons or chickens. 3) Slow phase speeds of coupled head and eye nystagmus were subjected to statistical analysis. A highly significant negative correlation was found between slow phase head and eye speeds. The correlation coefficient was −0.81 at 60°/sec stimulus (n = 119) and −0.72 at 100°/sec stimulus (n = 131). The gaze speed, calculated by summing the head and eye speeds, was 59.1 ± 6.8/sec at 60°/sec and 92.2 ± 11.4 at 100°/sec stimulus. There was no significant difference between the gaze speed in a free head condition and the eye speed when the head was fixed.

At the present time, neither the etiologic factors nor the pathophysiologic mechanisms that lead to the development of endolymphatic hydrops or Ménière's disease are known; however, the anatomic-pathologic alterations in the labyrinth that are produced by the disease are well-documented. Undoubtedly, what is not known is responsible for the controversial status and questionable benefit that surgical decompression and drainage procedures have upon the endolymphatic system.

The vestibular ganglia of adult squirrel monkeys (Saimiri sciureus) were studied with the aid of light and electron microscopy. The vestibular ganglia are formed by small and large neurons (perikarya). The perikarya's outermost surface is surrounded by 2–20 lamellae of compact myelin. The compact myelin sheath varies its thickness around one perikaryon. The sheath generally thins out in the vicinity of the axon hillock. Occasionally, the compact myelin transforms completely into loose myelin for a length of 2–5 μ. Two to eight lamellae of loose myelin usually face the cytoplasm of the neurons. This myelin arrangement is constant in the majority of the cells examined regardless of shape and/or size. Large myelinated perikarya form the majority of the ganglion cells. There are, however, a few neurons that lack a true compact myelin sheath. Others are surrounded by very few lamellae of loose myelin. The general morphology of the neurons was compared to the available literature on other species. Possible variation on the myelination pattern due to fixation differences, and variation on perikaryal size due to the angle of cut and irregular shape of the cells are also discussed.

Eye and head nystagmus during horizontal optokinetic stimulation was analyzed in squirrel monkeys. Slow phase speed of coupled head and eye nystagmus showed highly significant negative correlations in normal subjects; correlation coefficients were −0.81 at 60°/sec and −0.72 at 100°/sec stimuli. Bilateral lateral semicircular canal block resulted in a reduction of the negative correlation. The postoperative reduction of the correlation coefficient value ranged from 0.17–0.28 (significant at P<0.01 level) at 60°/sec stimulus and 0.06–0.13 (not significant) at 100°/sec stimulus. Gaze speed was calculated by summing slow phase speeds of coupled head and eye nystagmus. Mean error of gaze in normal subjects was 1.5% at 60°/sec stimulus and 7.8% at 100°/sec stimulus. Mean gaze speed showed an overshoot (8% at 60°/sec stimulus and 6.5% at 100°/sec stimulus) in the three to four weeks postoperative period; however, this subsided two to three months after the operation. Measurement of head-turning performance (in yaw plane) in humans may provide a clinically useful index for the assessment of certain types of vestibular function.