Age differences in oculomotor responses to step changes in body velocity and visual surround velocity

We compared the slow-phase eye velocity elicited by sudden cessation of prolonged, constant velocity, vertical z-axis rotation of the body or the visual surround in 10 healthy college-age and over-65 individuals. The step gain of vestibular post-rotary nystagmus did not differ across age groups, but the time constant of slow-phase velocity decay was longer and more asymmetrical in the older group. The slow-phase velocity of optokinetic nystagmus attained the same initial levels for both age groups; it declined significantly during 60 s of stimulation for the older but not the younger group. The decay rate of optokinetic afternystagmus was quicker for the older subjects. This pattern of results may be related to already identified structural changes in the vestibular system and suggests the existence of yet unidentified changes in central vestibular and visual processing.