Biphasic Aftereffects of Vestibular Stimuli, Optokinetic Nystagmus and Pursuit — Common Integrators?

Primary (phase I) and secondary (phase II — opposite in direction) afternystagmus after vestibular, optokinetic and pursuit stimuli of different duration and velocity were quantitatively studied in humans to determine the charge and discharge characteristics of the underlying central storage mechanisms. Three different storage mechanisms were identified: 1. A common integrator for the vestibular and the retinal periphery dependent optokinetic system gives rise to optokinetic afternystagmus I and prolongs the decay of vestibular afternystagmus I. Its vestibular charge is fast and the OKN charge slower (30 s). It discharges within 1 min and is related to storage of ego motion sensation. 2. An integrating mechanism for pursuit with a rapid charge (2 s) and a fast discharge (10 s) stores a velocity signal concerning object motion possibly playing a role in prediction of pursuit. 3. Secondary afternystagmus is a common feature of vestibular stimuli and prolonged optokinetic and pursuit stimulation. It shows rapid charge for vestibular and slower charge for OKN and pursuit stimuli, but slow discharge after vestibular and visual stimuli. Its gain is lower than the one of primary vestibular, optokinetic and pursuit afternystagmus. The secondary nystagmus integrator is independent of whether or not nystagmus occurs during stimulation. Its inputs seem to bypass the phase I integrator.