Aftereffects of vestibular and optokinetic stimulation and their interaction

In humans the influence of prior vestibular stimulation (3, 6, 9, 12, and 18 degrees/second2 for 10 seconds) and subsequent whole-field optokinetic stimulation (30, 60, 90, 120, and 180 degrees/second for 1 minute) or the presentation of a stationary pattern on after-nystagmus (AN) was studied. For comparison, pure vestibular and pure optokinetic stimuli also were employed. The presentation of a stationary pattern resulted in suppression of vestibular nystagmus, which recovered after the termination of fixation. Fixation during the period of AN I did not inhibit an AN II. During the combinations of vestibular and optokinetic stimuli when the elicited vestibular (VN) and optokinetic nystagmus (OKN) had the same direction, there was a weak AN I toward the direction of the preceding VN and OKN, and a strong AN II toward the opposite side. When VN had been opposite to the subsequent OKN, there was a strong AN I toward the direction of OKN; AN II toward the opposite direction was small or mostly absent. Thus, AN was always stronger into the direction opposite to the previously elicited VN, indicating that the vestibular afference is the predominant input to the VAN II-integrator.