Post-rotary nystagmus and optokinetic after-nystagmus in the rabbit linear rather than exponential decay.

Abstract

The decay of the slow phase velocity of post-rotatory (PRN) and optokinetic (OKAN) after-nystagmus as a function of time was measured in Dutch rabbits after stimulation with velocity steps of 30, 60, and 150 degrees/s. The decays fitted linear functions very well, but only poorly exponential ones. Typical decay rates were 2-5 degrees/s2, with apparent time constants (defined by decay to 37% of initial velocity) in the order of 10-20 s. Within one animal, the decays of OKAN and PRN with similar initial velocities were indistinguishable. With sinusoidal oscillation, the time constant of the vestibulo-ocular reflex - estimated from phase lead - was only 2-3 s, and probably similar to the cupular time constant. In general, time constants increased when eye velocities increased. This indicates that the vestibulo-ocular reflex of the rabbit behaves as a non-linear system. A velocity storage system with a constant discharge rate is postulated as a main non-linear element. This would introduce a linear decay of velocity as well as a threshold for velocity. This storage system would be common to both vestibulo-ocular and optokinetic reflexes.