Abstract
Oculomotor function was studied after midsagittal section of the brain stem in rhesus monkeys. After lesions that involved both the medulla and pons, there was permanent paralysis of adduction during conjugate eye movements and divergent strabismus. Analysis of the lesions indicated that activity responsible for ocular adduction had crossed the brain stem in a localized region at the level of the abducens nuclei. The adductive paralysis was probably due to interruption of axons of abducens internuclear neurons that ascend in the medial longitudinal fasciculus (MLF). In view of the functional relationship of this region to ocular adduction and its close anatomic relationship to the MLF, we propose that it be called the MLF decussation. Because the eyes beat synchronously after deep interruption of vestibular commissural fibers, the latter are not essential for the production of quick phases of nystagmus. After medullary lesions, animals had spontaneous downward nystagmus and perverted caloric nystagmus. Optokinetic after-nystagmus was lost. The perverted nystagmus appears due to unmasking of anterior canal responses, secondary to a loss of inhibitory activity that normally crosses the brain stem in the vestibular commissural system.
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