THE RELATIONSHIP BETWEEN CURVATURE AND VELOCITY IN 2-DIMENSIONAL SMOOTH-PURSUIT EYE-MOVEMENTS

Curvature and tangential velocity of voluntary hand movements are cons trained by an empirical relation known as the Two-Thirds Power Law. It has been argued that the law reflects the working of central control mechanisms, but it is not known whether these mechanisms are specific to the hand or shared also by other types of movement. Three experimen ts tested whether the power law applies to the smooth pursuit movement s of the eye, which are controlled by distinct neural motor structures and a peculiar set of muscles. The first experiment showed that smoot h pursuit of elliptic targets with various curvature-velocity relation ships was most accurate when targets were compatible with the Two-Thir ds Power Law. Tracking errors in all other cases reflected the fact th at, irrespective of target kinematics, eye movements tended to comply with the law. Using only compatible targets, the second experiment dem onstrated that kinematics per se cannot account for the pattern of pur suit errors. The third experiment showed that two-dimensional performa nce cannot be fully predicted on the basis of the performance observed when the horizontal and vertical components of the targets used in th e first condition were tracked separately. We conclude that the Two-Th irds Power Law, in its various manifestations, reflects neural mechani sms common to otherwise distinct control modules.