Volume 17, Number 10,
Issue of May 15, 1997
pp. 3932-3945
Copyright ©1997 Society for Neuroscience
The Relationship between Curvature and Velocity in
Two-Dimensional Smooth Pursuit Eye Movements
Received Nov. 6, 1996; revised Feb. 18, 1997; accepted Feb. 24, 1997.
Claudio de'Sperati1 and
Paolo Viviani1, 2
1 Laboratory of Action, Perception and Cognition,
Department of Cognitive Science, San Raffaele Vita-Salute University,
20133 Milan, Italy, and 2 Department of Psychobiology,
Faculty of Psychology and Educational Science, University of Geneva,
1227 Carouge, Switzerland
Curvature and tangential velocity of voluntary hand movements are
constrained 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
experiments tested whether the power law applies to the smooth pursuit
movements of the eye, which are controlled by distinct neural motor
structures and a peculiar set of muscles. The first experiment showed
that smooth pursuit of elliptic targets with various
curvature-velocity relationships was most accurate when targets were
compatible with the Two-Thirds Power Law. Tracking errors in all other
cases reflected the fact that, irrespective of target kinematics, eye
movements tended to comply with the law. Using only compatible targets,
the second experiment demonstrated that kinematics per se cannot
account for the pattern of pursuit errors. The third experiment showed
that two-dimensional performance cannot be fully predicted on the basis
of the performance observed when the horizontal and vertical components
of the targets used in the first condition were tracked separately. We
conclude that the Two-Thirds Power Law, in its various manifestations,
reflects neural mechanisms common to otherwise distinct control
modules.
Key words:
smooth pursuit eye movements;
two-dimensional tracking;
Lissajous motion;
curvature-velocity covariation;
Two-Thirds Power
Law;
neural coding of direction
Previous Article | Next Article
