Post-saccadic enhancement of the initiation of smooth pursuit eye movements
in monkeys.
Stephen G. Lisberger.
Howard Hughes Medical Institute, Department of Physiology, and W.M. Keck
Foundation Center for Integrative Neuroscience, University of California, San
Francisco, CA, 94143.
APStracts 4:359N, 1997.
ABSTRACT
Step-ramp target motion evokes a characteristic sequence of pre-saccadic
smooth eye movement in the direction of the target ramp, catch-up targets to
bring eye position close to the position of the moving target, and post-
saccadic eye velocities that nearly match target velocity. We have analyzed
this sequence of eye movements in monkeys to reveal a strong post-saccadic
enhancement of pursuit eye velocity and to document the conditions that lead
to that enhancement. Smooth eye velocity was measured in the last 10 ms
before and the first 10 ms after the first saccade evoked by step-ramp target
motion. Plots of eye velocity as a function of time after the onset of the
target ramp revealed that eye velocity at a given time was much higher if
measured after versus before the saccade. Post saccadic enhancement of
pursuit was recorded consistently when the target stepped 3o eccentric on the
horizontal axis and moved upward, downward, or away from the position of
fixation. To determine whether post-saccadic enhancement of pursuit was
invoked by smear of the visual scene during a saccade, we recorded the effect
of simulated saccades on the pre-saccadic eye velocity for step-ramp target
motion. The 3o simulated saccade, which consisted of motion of a textured
background at 150o/s for 20 ms, failed to cause any enhancement of pre-
saccadic eye velocity. By using a strategically-selected set of oblique
target steps with horizontal ramp target motion, we found clear enhancement
for saccades in all directions, even those that were orthogonal to target
motion. When the size of the target step was varied up to 15o along the
horizontal meridian, post-saccadic eye velocity did not depend strongly either
on the initial target position or on whether the target moved toward or away
from the position of fixation. In contrast, earlier studies and data in this
paper show that pre-saccadic eye velocity is much stronger when the target is
close to the center of the visual field and when the target moves toward
versus away from the position of fixation. We suggest that post-saccadic
enhancement of pursuit reflects activation, by saccades, of a switch that
regulates the strength of transmission through the visual-motor pathways for
pursuit. Targets can cause strong visual motion signals but still evoke low
pre-saccadic eye velocities if they are ineffective at activating the pursuit
Received 10 June 1997; accepted in final form 3 December 1997.
APS Manuscript Number J485-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997