Different time constants for optokinetic and vestibular nystagmus with a single velocity-storage element

Models have been proposed for the combined optokinetic system and vestibulo-ocular reflex that share a single, common, velocity-storage element that generates both optokinetic after-nystagmus and prolongs post-rotatory nystagmus. It has been thought by many that such a model requires the time course of these two types of nystagmus to be similar. The demonstration that vestibular or optokinetic habituation can selectively and significantly reduce the duration of each type of nystagmus without affecting that of the other has been taken to imply that the concept of shared velocity storage is incorrect. This need not be the case because post-rotatory nystagmus is described by two time constants, not one. Changes in the model's parameters can create time courses of this nystagmus that, when fit by single exponentials, have apparent time constants quite different from that of optokinetic after-nystagmus. Consequently, a common velocity-storage mechanism is still a tenable hypothesis for optokinetic and vestibular eye movements.