Smooth pursuit along a solid line: a new variation on the theme of efference-copy-bound eye tracking

Theoretical considerations of the physiology of eye tracking have long implied that the kinematics of smooth pursuit need not parallel those of the pursued target, with pursuit in the absence of a physically moving target as an extreme case. Yet these theoretical implications have been largely ignored, with the consequence that observed kinematic discrepancies between target and pursuit have generally been presented as somewhat surprising, and this view has maintained unnecessary diversity among reported smooth-pursuit phenomena. The case of yet another stimulus-discrepant smooth-pursuit phenomenon is presented and is used to suggest a role which recognized eye-tracking theory might advantageously play in classifying smooth-pursuit phenomena. The new phenomenon, smooth pursuit along a stationary solid straight line under continuous illumination, was demonstrated in a study where eye movements were documented by continuous recordings of the electrooculogram. All twelve subjects were able to initiate smooth pursuit along the line and to maintain it in the absence of any other source of stimulation, and all reported experiencing apparent movement during smooth pursuit. It is suggested that smooth-pursuit phenomena can best be classified in terms of degree of efference-copy dependency.