Effects on the optokinetic system of midline lesions in the pretectum of monkeys

The nucleus of the optic tract (NOT), an important visuo-motor relay between the retina and preoculomotor structures, is responsible for mediating horizontal optokinetic nystagmus (OKN) in monkeys, cats, rabbits and rats. In addition to its projection to the vestibular nuclei, the NOT has a prominent projection to the contralateral NOT via the posterior commissure. In order to evaluate the role of the commissural fibers between the NOTs in OKN, we cut the posterior commissure in three Macaca fuscata. The animals viewed the OKN stripes under three conditions: right eye viewing, left eye viewing, and both eyes viewing. OKN was recorded in response to counter-clockwise and clockwise stimulation at stimulus velocities of 30degrees/s, 60degrees/s and 90degrees/s. After control data were gathered, the posterior commissure was transected with an operating knife. Before the animal was sacrificed, biocytin, an anterograde tracer, was injected into the left NOT in order to confirm that all of the commissural fibers had been cut. Although the midline lesions decreased the initial rapid rise and steady state OKN slow-phase velocity in all three animals, there were no directional differences observed during monocular clockwise or counter-clockwise visual stimulation to either eye, In two of the three animals, there were no significant differences in the time-constants of optokinetic after nystagmus (OKAN) after the lesion. In the remaining animal, the time-constants decreased at stimulus velocities of 30degrees/s and 60degrees/s. In conclusion, gain reduction in the rapid rise and steady state slow-phase velocity of OKN can be explained by removal of an excitatory signal mediated by commissural fibers to inhibitory interneurons in the contralateral NOT. However, interrupting the commissural fibers had no effect on the velocity storage mechanism, because the time-constants of OKAN mostly remained largely unchanged by the lesion.