Failure of the oculomotor neural integrator from a discrete midline lesion between the abducens nuclei in the monkey

Recent anatomical studies indicate that axons of neurons in the vestibular nuclei, projecting to the contralateral abducens nuclei, cross the midline at the abducens level. These axons then give off collaterals to the contralateral vestibular and prepositus nuclei that may be important for the neural integrator that converts eye-velocity to eye-position signals. We disrupted a subset of these commissural projections by making a small midline lesion between the abducens nuclei in a monkey. The vestibulo-ocular reflex and saccades were still present post-lesion, indicating that premotor drive was intact, but the lesion produced severe post-saccadic drift, indicating failure of the neural integrator. We conclude that commissural projections crossing at the abducens level may be important for oculomotor integration.