Irrepressible saccades from a tectal lesion in a Rhesus monkey.

We present a case of spontaneously occurring irrepressible saccades in an experimental Rhesus monkey. Though eye jerks are sometimes associated with cerebellar disease, central demyelination or brainstem lesions, there is little consensus on their neurological mechanisms. From neurological and anatomical investigation we report that these irrepressible saccades were caused by a discrete cerebrovascular accident that involved the rostral superior colliculus along with its commissure, and with minor invasion of periaqueductal gray and adjacent mesencephalic reticular formation. Other suspected structures, like the raphe interpositus, substantia nigra and the cerebellum, were unaffected.