[Influence of vestibular input on visually guided saccades]

A common pathway in the brainstem is thought to exist for the induction of visually guided saccades and rapid phase nystagmus (Ref. Ohki et al. 1988). This common pathway is often referred to as "the saccade generator" or "the burst generator." The presence of this pathway suggests that vestibular function can be examined by applying "visual stimuli" and "vestibular stimuli" at the same time. We therefore investigated whether caloric stimulation affects the latency, amplitude and velocity of saccadic eye movements and if so, how. Nine normal volunteers participated in the experiment. Eye movements were recorded using DC electrooculography. Caloric stimulation was applied to the right ear using an air caloric stimulator (10 degrees C) during saccades. Two experiments involving visually guided saccades were performed. Task 1: The subject was asked to fixate on a central fixation point. Another spot of light then randomly appeared at one of two different locations (20 degrees to the right or left). The subject was then asked to scan towards the second light as quickly as possible. Task 2: The procedure was the same as in Task 1, except that the target appeared only briefly and then later reappeared at the same location. The saccade latency was unaffected by caloric stimulation in Task 1, while the latency of the rightward saccades increased significantly in Task 2. The saccade amplitude and velocity increased significantly in both directions in Task 1, while the amplitude and velocity of the leftward saccades increased significantly in Task 2. These results can be explained by the synergistic activation of Burster-Driving Neurons by the saccade signal and the vestibular signals. The combinations of these saccade tasks and caloric stimulation, especially Task 2, may be useful in clinically examining vestibulo-oculomotor functions.