|
Horizontal Saccade
Disconjugacy in Strabismic Monkeys
LaiNgor Fu,1,2 Ronald J.
Tusa,1,3 Michael J. Mustari,1,3
and Vallabh E. Das1,3
1 From the Division of Sensory-Motor
Systems, Yerkes National Primate Research Center, and the
3Department of Neurology, Emory University,
Atlanta, Georgia; and the 2Retina
Foundation of the Southwest, Dallas, Texas.
Subjects and Rearing Paradigms
Behavioral data were collected from four strabismic (AMO1 to AMO4)
juvenile rhesus monkeys (Macaca mulatta) and 1 monkey without
strabismus. Monkeys with strabismus were born and reared at the Yerkes
National Primate Research Center according to an alternate monocular
occlusion (AMO) method for the first few months of life designed to
induce ocular misalignment but not to affect visual acuity.9,10 In the
AMO rearing procedure, soon after birth (within the first 24 hours), an
occluding patch (either dark opaque goggle lenses or dark opaque contact
lenses) is placed in front of one eye for a period of 24 hours and then
switched to the fellow eye for the next 24 hours. The patch is
alternated daily for a period of 4 to 6 months. In this method,
binocular vision is severely disrupted during the first few months of
life, the critical period during which the monkey brain normally
develops proper eye alignment, stereovision, and binocular
sensitivity.3,11,12 During AMO rearing, contact lens wear was monitored
every 2 hours during the day to verify compliance with the rearing
protocol.
Surgical Procedures and Eye Movement Measurements
After AMO rearing, the animals were allowed to grow normally until they
were approximately 2 to 3 years of age, and then behavioral experiments
were begun. Sterile surgical procedures carried out under aseptic
conditions with isoflurane anesthesia (1.25%-2.5%) were used to
stereotaxically implant a head stabilization post. During the same
surgical procedure, a scleral search coil was implanted in one eye
according to the technique of Judge et al.17 Later, during second
surgery, a second scleral search coil was implanted in the other eye.
All procedures were performed in strict compliance with National
Institutes of Health guidelines and the ARVO Statement for the Use of
Animals in Ophthalmic and Vision Research, and the protocols were
reviewed and approved by the Institutional Animal Care and Use Committee
at Emory University.
PURPOSE. Previous studies have shown that binocular
coordination during saccadic eye movement is affected in humans with
large strabismus. The purpose of this study was to examine the conjugacy
of saccadic eye movements in monkeys with sensory strabismus.
METHODS. The authors recorded binocular eye movements
in four strabismic monkeys and one unaffected monkey. Strabismus was
induced by first occluding one eye for 24 hours, switching the occluder
to the fellow eye for the next 24 hours, and repeating this pattern of
daily alternating monocular occlusion for the first 4 to 6 months of
life. Horizontal saccades were measured during monocular viewing when
the animals were 2 to 3 years of age.
RESULTS. Horizontal saccade testing during monocular
viewing showed that the amplitude of saccades in the nonviewing eye was
usually different from that in the viewing eye (saccade disconjugacy).
The amount of saccade disconjugacy varied among animals as a function of
the degree of ocular misalignment as measured in primary gaze. Saccade
disconjugacy also increased with eccentric orbital positions of the
nonviewing eye. If the saccade disconjugacy was large, there was an
immediate postsaccadic drift for less than 200 ms. The control animal
showed none of these effects.
CONCLUSIONS. As do humans with large strabismus,
strabismic monkey display disconjugate saccadic eye movements. Saccade
disconjugacy varies with orbital position and increases as a function of
ocular misalignment as measured in primary gaze. This type of
sensory-induced strabismus serves as a useful animal model to
investigate the neural or mechanical factors responsible for saccade
disconjugacy observed in humans with strabismus |
