Oculomotor signals in medial longitudinal fasciculus of the monkey

Abstract

The discharge rate of fibers in the medial longitudinal fasciculus (MLF) was recorded in alert monkeys during visual and vestibular stimulation. Four types of fibers were found. Horizontal burst-tonic fibers (40%) fired at rates that increased with contralateral eye position. They burst at high rates during contralateral saccades and paused during ipsilateral saccades. An extra modulation occurred proportional to contralateral horizontal eye velocity produced by either pursuit or vestibular stimulation. Vertical tonic-pause fibers (45%) fired at rates proportional to vertical eye position (either up or down) and paused during saccades and quick phases in all directions. An extra modulation was proportional to pursuit eye velocity. Vertical vestibular stimulation was not used. Horizontal vestibular-pause fibers (8%) fired at rates roughly proportional to head velocity. Most paused during saccades and quick phases in all directions. Burst fibers (7%) discharged at high rates just before and during vertical saccades (either up or down) and were silent otherwise. Brief stimulation of the MLF produced adduction twitches in the ipsilateral eye with a latency of 6.7 ms. The vestibuloocular reflex consists of a 2-stage conversion from a pure vestibular to a pure oculomotor signal. The 1st conversion would consist of adding an eye-position signal to the vestibular signal on cells in the vestibular nuclei to create tonic-pause activity. For vertical gaze this signal ascends in the MLF to the oculomotor nucleus where the saccadic pulse and a signal that can suppress the vestibuloocular reflex would be added on to create the motoneuron (burst-tonic) signal. For horizontal gaze, it is proposed that tonic-pause activity is converted to burst-tonic activity in the region of the abducens nucleus. Burst-tonic cells there would then act as surrogate medial rectus motoneurons and project their fibers in the MLF to the oculomotor nucleus. The fact that the MLF carries 1 signal for horizontal eye movements and another for vertical comes about in a natural way.