Human smooth pursuit and optokinetic nystagmus : effects of stimulus factors and instructions

Eye movements are made to improve vision. A high slip velocity of the image over the retina precludes the detection of image details. For example, the sleepers of a railway track, which are easily distinguished from one another when the train is stationary, become blurred and hard to discern when one looks down through the window of a train moving at a high speed. Visual acuity would thus be served best by fixating the eye with respect to the environment while the animal moves about. This goal is achieved approximately by the reflexive compensatory eye movements which are found in all vertebrates. These eye movements consist of a typical alternation of slow rotations of the eye opposite to the body motion and fast (saccadic) eye movements which carry the eye in the direction of the resting position. When the body motion consists of a unidirectional rotation or translation, a typical rhythmic alternation of slow and quick phases occurs which is called nystagmus. During the slow phases vision is improved, because the orientation of the eye is approximately fixed with respect to the environment, while during the brief quick phases visual sensitivity is reduced (see Matin, 1974, for a review).