EYE-HEAD COORDINATION DURING HEAD UNRESTRAINED GAZE SHIFTS IN RHESUS
MONKEYS.
Edward G. Freedman and David L. Sparks.
Institute of Neurological Sciences, and the Department of Psychology,
University of Pennsylvania, Philadelphia, PA 19104-6196.
APStracts 4:0047, 1997.
ABSTRACT
We analyzed gaze shifts made by trained rhesus monkeys with completely
unrestrained heads during performance of a delayed gaze shift task. Subjects
made horizontal, vertical and oblique gaze shifts to visual targets. We found
that coordinated eye-head movements are characterized by a set of lawful
relationships, and that the initial position of the eyes in the orbits and the
direction of the gaze shift are two factors which influence these
relationships. Head movements did not contribute to the change in gaze
position during small gaze shifts (<20() directed along the horizontal
meridian, when the eyes were initially centered in the orbits. For larger gaze
shifts (25(-90(), the head contribution to the gaze shift increased linearly
with increasing gaze shift amplitude, and eye movement amplitude saturated at
an asymptotic amplitude of ÿ7E35(. When the eyes began deviated in the orbits
contralateral to the direction of the ensuing gaze shift, the head contributed
less and the eyes more to amplitude-matched gaze shifts. The relative timing
of eye and head movements was altered by initial eye position; head latency
relative to gaze onset increased as the eyes began in more contralateral
initial positions. The direction of the gaze shift also affected the relative
amplitudes of eye and head movements; as gaze shifts were made in
progressively more vertical directions, eye amplitude increased and head
contribution declined systematically. Eye velocity was a saturating function
of gaze amplitude for movements without a head contribution (gaze amplitude
<20(). As head contribution increased with increasing gaze amplitude (20( to
60(), peak eye velocity declined by more than 200(/s and head velocity
increased by 100(/s. For constant amplitude eye movements (ÿ7E30(),eye velocity
declined as the velocity of the concurrent head movement increased. Based on
these relationships, it is possible to accurately predict gaze amplitude, the
amplitudes of the eye and head components of the gaze shift, as well as gaze,
eye and head velocities, durations and latencies if the two dimensional
displacement of the target and the initial position of the eyes in the orbits
are known. These data indicate that signals related to the initial positions
of the eyes in the orbits and the direction of the gaze shift influence
separate eye and head movement commands. The hypothesis that this divergence
of eye and head commands occurs downstream from the superior colliculus is
supported by recent electrical stimulation and single unit recording data
Received 5 September 1996; accepted in final form 9 January 1997.
APS Manuscript Number J715-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 February 1997