OCULOMOTOR RESPONSES ON VESTIBULAR STIMULATION OF MONKEYS WITH LESIONS OF THE BRAIN-STEM

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Abstract
The The oculomotor responses to vestibular stimulation were studied in 70 monkeys with lesions in the brainstem. A variety of methods of vestibular testing were employed. Caloric (cold or hot) as well as rotational (body erect or laterally placed) stimulations were carried out before and serially after lesions were made. Electrolytic lesions (2-4 mm) above the level of the oculomotor nerve nucleus caused defects in vestibulo-ocular response in a horizontal direction which was contralateral to the side of the lesion. With the monkey in the lateral position, rotational tests also demonstrated defects in the vertical plane; there was decreased to absent downward nystagmus. Small (1-2 mm) lesions below the level of the trochlear nucleus produced defects in the vestibulo-ocular response in a horizontal direction which was ipsilateral to the side of the lesion. With the monkey in the lateral position rotation tests also demonstrated defects in the vertical plane; upward nystagmus was decreased or absent. The defects in ipsilateral eye movements were most conspicuous and found only in lesions of the paramedian zone of the tegmentum of the brainstem, situated between the levels of the trochlear and vestibular nuclei. Lesions which involved only part of the vestibular nuclei produced a loss of responses when the ear on the side of the lesion was calorically stimulated. Rotational tests showed defects in tonic deviation as well as in the quick component of nystagmus in the direction of the vestibular nucleus lesion. Comparable sized (1-4 mm) lesions in other areas of the brain-stem produced no manifest defects in the oculomotor response on vestibular stimulations.

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