End-point nystagmus and ocular drift: an experimental and theoretical study.

End-point nystagmus (EPN) and ocular drift during eccentric fixation were investigated in five normal subjects. All had EPN during prolonged eccentric fixation, two had sustained EPN within a few seconds of fixating an eccentric target. Our data show that the occurrence of EPN was determined by the velocity of slow phase ocular drift. Drift velocities greater than 1 deg/sec elicited sustained EPN or fatigue EPN while drift velocities lower than 0.3 deg/sec did not elicit EPN. Slow phase drift velocity increased with eccentricity and fixation duration and decreased with visual feedback. Computer simulations and theoretical analysis suggest that the reduction in drift eye movement velocity during fixation of a visible target, as compared to drift eye movement velocity during attempted fixation of a remembered target in darkness, is mainly due to the smooth pursuit system.