Inferences about the efferent system based on a perceptual illusion produced by eye movements.

Precise measurement of the position of the eye as it follows a moving target makes possible the exact computation of retinal information about the path of movement of that target. Comparing this retinal information with the reported visual perception of the path of movement enables inferences to be made concerning what information about eye position was used by the perceptual system. On the assumption that information available to the perceptual system about eye position comes only from monitoring efferent commands, these inferences are also about the content of those commands. Our data and analysis suggest that the efferent command for smooth pursuit eye movement, at the stage where it is monitored, contains good information about the direction of movement but only crude information about speed.