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

Summary

The decay of the slow phase velocity of post-rotatory (PRN) and optokinetic (OKAN) afternystagmus as a function of time was measured in Dutch rabbits after stimulation with velocity steps of 30, 60, and 150 °/s. The decays fitted linear functions very well, but only poorly exponential ones. Typical decay rates were 2–5 °/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.