SACCADES TO REMEMBERED TARGETS: The effects of smooth pursuit and illusory
Zivotofsky, Ari Z., Klaus G. Rottach, Lea Averbuch-Heller, Adriana A. Kori,
Cecil W. Thomas, Louis F. Dell'Osso, R. John Leigh.
Departments of Neurology, Biomedical Engineering, Otolaryngology and
Neuroscience, Department of Veterans Affairs Medical Center and University
Hospitals, Case Western Reserve University, Cleveland, Ohio 44106.
APStracts 3:0163N, 1996.
SUMMARY AND CONCLUSIONS
(1) Measurements were made in four normal human subjects of the accuracy of
saccades to remembered locations of targets that were flashed on a 20 X 30 deg
random dot display that was either stationary or moving horizontally and
sinusoidally at +/-9 deg at 0.3 Hz. During the interval between the target
flash and the memory-guided saccade, the "memory period" (1.4 seconds),
subjects either fixated a stationary spot or pursued a spot moving vertically
sinusoidally at +/-9 deg at 0.3 Hz. (2) When saccades were made toward the
location of targets previously flashed on a stationary background as subjects
fixated the stationary spot, median saccadic error was 0.93 deg horizontally
and 1.1 deg vertically. These errors were greater than for saccades to visible
targets, which had median values of 0.59 deg horizontally and 0.60 deg
vertically. (3) When targets were flashed as subjects smoothly pursued a spot
that moved vertically across the stationary background, median saccadic error
was 1.1 deg horizontally and 1.2 deg vertically, thus being of similar
accuracy to when targets were flashed during fixation. In addition, the
vertical component of the memory-guided saccade was much more closely
correlated with the "spatial error" than with the "retinal error"; this
indicated that, when programming the saccade, the brain had taken into account
eye movements that occurred during the memory period. (4) When saccades were
made to targets flashed during attempted fixation of a stationary spot on a
horizontally moving background, a condition that produces a weak Duncker-type
illusion of horizontal movement of the primary target, median saccadic error
increased horizontally to 3.2 deg, but was 1.1 deg vertically. (5) When
targets were flashed as subjects smoothly pursued a spot that moved vertically
on the horizontally moving background, a condition that induces a strong
illusion of diagonal target motion, median saccadic error was 4.0 deg
horizontally and 1.5 deg vertically; thus the horizontal error was greater
than under any other experimental condition. (6) In most trials, the initial
saccade to the remembered target was followed by additional saccades while the
subject was still in darkness. These secondary saccades, which were executed
in the absence of visual feedback, brought the eye closer to the target
location. During paradigms involving horizontal background movement these
corrections were more prominent horizontally than vertically. (7) Further
measurements were made in two subjects to determine whether inaccuracy of
memory-guided saccades, in the horizontal plane, was due to mislocalization at
the time that the target flashed, misrepresentation of the trajectory of the
pursuit eye movement during the memory period, or both. (8) The magnitude of
the saccadic error, both with and without corrections made in darkness, was
mislocalized by 30% of the displacement of the background at the time that the
target flashed. The magnitude of the saccadic error was also influenced by net
movement of the background during the memory period, corresponding to 25% of
net background movement for the initial saccade and 13% for the final eye
position achieved in darkness. (9) We formulated simple linear models to test
specific hypotheses about which combinations of signals best describe the
observed saccadic amplitudes. We tested the possibilities that the brain made
an accurate memory of target location and a reliable representation of the eye
movement during the memory period, or that one or both of these was corrupted
by the illusory visual stimulus. Our data were best accounted for by a model
in which both the working memory of target location and the internal
representation of the horizontal eye movements were corrupted by the illusory
visual stimulus. We conclude that extraretinal signals played only a minor
role, in comparison to visual estimates of the direction of gaze, in planning
eye movements to remembered target locations during our illusory paradigms.
Received 26 January 1996; accepted in final form 7 August 1996.
APS Manuscript Number J53-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 29 August 1996