A "beat-to-beat" interval generator for optokinetic nystagmus.

An analysis of optokinetic responses was used to derive an iterative model that reproduces the duration of nystagmus slow phases and eye position control during optokinetic nystagmus. Optokinetic nystagmus was recorded with magnetic search coils from red-eared turtles (Pseudemys scripta elegans) during monocular, random dot pattern stimulation at constant velocities ranging from 0.25-63 degrees/s. The beat-to-beat behavior of slow phase durations was consistent with the existence of an underlying neural clock, termed the basic interval generator, that is based on an integrate-to-fire neuron model. This hypothetical basic interval generator produces an interval that is the product of the duration of the previous interval and a mean 1 truncated normal variate with variance sigma 2. Data analyses indicated that the initial value of the interval generator during a period of nystagmus, termed tau 0, is proportional to the inverse square root of slow phase eye velocity. Further, if the eye was deviated in the slow phase direction (re mean eye position) when the slow phase began, the slow phase duration was consistent with a single cycle of the basic interval generator. However, if the eye was deviated in the fast phase direction, the distribution of the durations of the ensuing slow phases indicated that a proportion of the slow phases were produced by more than one cycle of the basic interval generator. This phenomenon is termed "skipping a beat" and occurs with probability Ps. Finally, the amplitude of fast phases behaved as a linear function of eye position at the fast phase onset and the product of tau 0 and slow phase eye velocity. A computer simulation reproduced the observed distribution of slow phase durations, the proportion of fast phases in the fast phase and slow phase directions and the distribution of eye positions at the onset and end of fast phases. This novel model suggests that both timing and eye position information contribute to the alternation of nystagmus fast and slow phases.