Brain control of conjugate horizontal and vertical eye movements: a survey of the structural and functional correlates

Abstract

(1) It appears that all oculomotor pathways originating within the cerebrum and mediating stimulations and lesions, project from the two sides of the brain through the diencephalon to the brain-stem. (2) The pathways subserving horizontal movements decussate at the level of the oculomotor and trochlear nuclei, across the midsagittal plane. The direction of vector action within the brain above the 'electroanatomical' oculomotor decussation is predominately contraversive; below this levelit is ipsiversive. (3) The pontine reticular formation, the abducens and oculomotor nuclei and the median longitudinal fasciculus play an important role in the physiology of ipsilateral conjugate gaze. A 1 mm lesion within the paramedian pontine reticular formation causes paralysis of ipsilateral conjugate gaze, while a 1 mm lesion within the median longitudinal fasciculus causes impairment of contralateral (disconjugate) gaze with paralysis of adduction of the ipsilateral eye and nystagmus in the contralateral or abducting eye. (4) True binocular vertical movements occur only when both sides of the brain are activated either directly or through bilateral sensory (visual or vestibular) inputs. Vertical and oblique monocular movements can be elicited on unilateral stimulation at the level of the oculmotor nucleus. (5) Paralysis of vertical gaze is caused by bilateral lesions. Bilateral (1 to 2 mm) lesions within the region of the rostral interstitial nucleus of the median longitudinal fasciculus result in isolated paralysis of downward gaze. More caudally, bilateral (1 mm) lesions within the pretectum or midsection of the posterior commissure result in paralysis of upward gaze. (6) In different regions of the brain a theoretical transverse plane can be drawn between pathways which transmit impulses for vertical eye movements. Those which transmit impulses for binocular downward movement are situated dorsal to this plane, while those that trasmit impulses for upward movement are located ventrally to this plane. This topographical relationship can be demonstrated in the occipital lobe and to some extent in the frontal lobes. A hypothetical transverse plane separating the down and up eye movement can also be drawn at the mesodiencephalic junction. At the level of the oculmotor nucleus stimulations at the most rostral pole result in monocular downward movements, while the most caudal pole stimulations produce monocular upward movements. There is no evidence that the pathways which mediate binocular upward and binocular downward movement project across a hypothetical transverse plane. (7) Our knowledge of the synaptic connections between the cerebrum, diencephalon and the brain-stem nuclei, especially the paramedian pontine reticular formation, involved in binocular movements remains incomplete. Moreover, the anatomical location of the decussation of the right and left cerebral pathways which transmit conjugate eye movements are still unknown...