Influence of optokinetic and vestibular stimuli on the performance of smooth pursuit eye movements: Implications for a clinical test

In real-life situations, such as during locomotion, or while driving a vehicle, it is necessary to maintain visual fixation and tracking in the presence of the visual flow of the surroundings, which represents a potentially adequate stimulus for the elicitation of optokinetic nystagmus. The present study is concerned with the influence of vestibular disorders, whether pathological or experimentally induced, on those cortically controlled fixation mechanisms, predominantly in the smooth pursuit system, which are involved in suppressing optokinetic information. The study examines the possibility of obtaining an objective measure to assist in counselling patients with unilateral vestibular loss on their vehicle driving ability. To this end, the influence of optokinetic and vestibular stimulation on the execution of smooth pursuit target tracking was measured by recording eye movements during a combination of standard pursuit tasks (0.25, 0.5 and 1 Hz sinusoidal) against standard optokinetic striped backgrounds (0, 30 and 60°/sec). The influence of vestibular imbalance, induced in healthy subjects (n = 35) by unilateral caloric irrigation, and caused by unilateral vestibular loss (in five patients), was also examined under these conditions. During induced vestibular imbalance in normal subjects, and to a greater extent in the tested patients, significant deficits in smooth pursuit gain and increases in saccade frequency were observed during target pursuit against an optokinetic background. Moreover, the findings indicate that the most sensitive parameter for the influence of vestibular/optokinetic stimuli on smooth pursuit is frequency of saccades, rather than the gain factor. The tests described here are appropriate for clinical and medico-legal assessment of the influence of vestibular disorder on vehicle driving.