Eye movement deficits after ibotenic acid lesions of the nucleus prepositus hypoglossi in monkeys. I. Saccades and fixation.

It has been suggested that the function of the nucleus prepositus hypoglossi (nph) is the mathematical integration of velocity-coded signals to produce position-coded commands that drive abducens motoneurons and generate horizontal eye movements. In early models of the saccadic system, a single integrator provided not only the signal that maintained steady gaze after a saccade but also an efference copy of eye position, which provided a feedback signal to control the dynamics of the saccade. In this study, permanent, serial ibotenic acid lesions were made in the nph of three rhesus macaques, and their effects were studied while the alert monkeys performed a visual tracking task. Localized damage to the nph was confirmed in both Nissl and immunohistochemically stained material. The lesions clearly were correlated with long-lasting deficits in eye movement. The animals' ability to fixate in the dark was compromised quickly and uniformly so that saccades to peripheral locations were followed by postsaccadic centripetal drift. The time constant of the drift decreased to approximately one-tenth of its normal values but remained 10 times longer than that attributable to the mechanics of the eye. In contrast, saccades were affected minimally. The results are more consistent with models of the neural saccade generator that use separate feedback and position integrators than with the classical models, which use a single multipurpose element. Likewise, the data contradict models that rely on feedback from the nph. In addition, they show that the oculomotor neural integrator is not a single neural entity but is most likely distributed among a number of nuclei.