Neurophysiology of Disjunctive Saccades

ABSTRACT For more than 100 years much of oculomotor research in primates has been guided by Hering's law, which states that any eye movement generated by the brain will be sent equally to the two eyes so that they move as a yoked pair. Refixations between targets in different directions involve saccades, which are conjugate movements that have peak velocities of hundreds of degrees per second. Refixations between targets at different distances require vergence eye movements, which slowly rotate the eyes by equal amounts in opposite directions. For gaze shifts between targets that differ in both direction and distance, vergence velocity is substantially higher during the saccade. The neural basis for this effect has been the subject of intense debate for more than 35 years. According to one view, consistent with Hering's Law, the extremely high intrasaccaddic vergence velocity results from a non-linear interaction between saccades and vergence. Alternatively, the vergence angle changes rapidly during the saccade because the saccadic system programs unequal saccades. This latter view would be inconsistent with Hering's law, since it necessitates the generation of unequal saccadic commands. For this reason, virtually all of the relevant neurophysiological studies have targeted brain areas involved in the generation of saccadic eye movements. The existing literature has yielded conflicting results, which is why this problem remains controversial. The experiments in this proposal take a novel approach, primarily by targeting brain areas involved in the generation of vergence eye movements during both slow vergence and disjunctive saccades.