Pursuit afternystagmus asymmetry in humans

It is known that prolonged unidirectional motion of a large field induces a reflexive drift of the eyes in the same direction when the stimulus is turned off. The phenomenon, which is called optokinetic afternystagmus, is known to be stronger after upward than downward stimulus motion. It is now reported that a similar anisotropy exists in the afternystagmus associated with the smooth pursuit system (PAN). The speed of the PAN reflexive drift was found to be greater following upward tracking at all times tested during a 15 s interval when compared to the values following downward tracking. A psychophysical measure of illusory motion, presumed to be generated by suppression of PAN in order to maintain fixation upon a stationary target, also showed a significantly greater amplitude and duration for the upward direction. If the directional asymmetry is a property of a velocity integrator that is believed to generate the afternystagmus, then the results are compatible with the existence of a common integrator for both optokinetic and pursuit systems.