The medial longitudinal fasciculus and disturbances of conjugate horizontal eye movements in the monkey

Abstract

Abstract In 16 monkeys attempts were made to produce localized unilateral and bilateral lesions in the medial longitudinal fasciculus (MLF) in the pons in order to study resulting disturbances of eye movements and fiber degeneration. Noteworthy physiological findings could be correlated with lesions and fiber degeneration in eight animals. Degeneration was studied by the Nauta technic. Bilateral lesions of the MLF between the abducens nuclei impair all horizontal eye movements, except convergence. Unilateral lesions of the MLF rostral to the abducens nuclei produce: (1) paresis of ipsilateral ocular adduction on attempted lateral gaze and (2) monocular horizontal nystagmus in the contralateral abducting eye. Bilateral lesions in the latter location produce these disturbances bilaterally. Paresis of ocular adduction resulting from lesions in the MLF is associated with ascending degeneration in the MLF projecting differentially upon cells of the ventral nucleus of the oculomotor complex, a cell group innervating the ipsilateral medial rectus muscle. Monocular horizontal nystagmus in the contralateral abducting eye appears to be a consequence of interruption of either: (1) descending fibers in the MLF, or (2) secondary vestibular fibers passing to the opposite abducens nucleus. Unilateral lesions of the MLF rostral to the abducens nucleus produce degeneration ipsilaterally in: (1) the nuclei of the extraocular muscles, (2) the interstitial nucleus (Cajal), and (3) parts of the ventral posterior inferior, ventral posterior medial, parafascicular, centromedian, and recticular nuclei of the thalamus.