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Grant Number: 2 R01 EY010214-08
Project Title: Early Functional-Structural Repair of Macaque
Strabismus
PI Information: PROFESSOR LAWRENCE TYCHSEN, M.D.
tourville@vision.wustl.edu
Phone: 314-362-3743 Fax: 314-362-3131
Trans Am Ophthalmol Soc. 2007 December; 105: 564–593.
PMCID: PMC2258131
Copyright ©American Ophthalmological Society 2007
CAUSING AND CURING INFANTILE ESOTROPIA IN PRIMATES: THE ROLE OF
DECORRELATED BINOCULAR INPUT (AN AMERICAN OPHTHALMOLOGICAL SOCIETY
THESIS)
Lawrence Tychsen, MD
From the Departments of Ophthalmology and Visual Sciences,
Pediatrics, Anatomy and Neurobiology, Washington University School of
Medicine, St Louis, Missouri
ANIMALS AND GOGGLE REARING GROUPS
Monkeys (Macaca mulatta) born at the Yerkes Primate Center in
Atlanta, Georgia, were fitted with goggles on the first day of life (Figure
1). The fitting procedure was an adaptation of that
originally described by Crawford.27,28
The procedure was not stressful to the newborn macaques and did not
require anesthesia or fabrication of a head mold. Padded head straps
held the goggles firmly in place and prevented the infant from removing
the apparatus, which was custom-fabricated for each monkey from
lightweight plastic. The front piece consisted of 2 lens holders, which
unscrewed so that ultra-lightweight, 2-mm-thick Fresnel plastic prisms
could be inserted. Animals were observed several times per day in the
primate nursery and during bottle feedings to ensure that the goggles
remained clear and in proper position. The goggles did not interfere
noticeably with normal play or mingling with other infant macaques. The
goggle helmet was removed from each monkey for cleaning a minimum of
once per day. During cleaning and, if necessary, adjustment of the
goggle, the animal was placed briefly in a dark (light-tight) enclosure
to preclude normal binocular experience. Inspections of the infant
monkeys during these brief periods when the goggles were removed for
cleaning disclosed that, after several weeks, each of the animals
manifested esotropia.
A total of 8 monkeys were studied: 6 experimental and 2 controls. The
6 experimental monkeys (Table
1) were divided into 3 prism-rearing groups:
3-week (2 animals), 12-week (1 animal), and 24-week (3 animals) groups.
In each group, experimental animals wore prism goggles (for periods of 3
to 24 weeks) to impose horizontal and vertical binocular
noncorrespondence (decorrelation) of 11.4º (20 prism diopters) in each
eye; 11.4º base-in in one eye, and 11.4º base-down in the other eye. The
2 controls wore goggles with plano lenses. At 4 to 6 months of age, the
monkeys were shipped to Washington University in St Louis, Missouri,
where they were trained to perform visual fixation and tracking tasks
without goggles, using a positive-feedback reward (a small bolus of
fruit juice).29
Cycloplegic refractions revealed a refractive error ≤ +3.00 spherical
equivalent in each of the experimental and control animals.
At age 1 year, eye coils were implanted29 and ocular motor as well as
sensory testing initiated, which proceeded typically for 3 to 6 months.
Monocular visual acuity was measured using spatial sweep visual evoked
potentials (SSVEP)30,31 (without correction for refractive error).
EYE MOVEMENT RECORDINGS
Detailed descriptions of the surgical and recording methods have been
published in previous reports, and for this reason only, an abbreviated
description is provided here.29,32
Using deep general inhalation anesthesia (supplemented by local
infiltration and topical anesthesia), scleral search coils were
implanted in both eyes and a custom-built, polycarbonate head-restraint
device was attached to the skull. All procedures were performed in
compliance with the ARVO resolution on the use of animals in research
and were approved by the Washington University Animal Care and Use
Committee.
Eye movements were recorded using standard magnetic search coil
techniques.33,34
The monkey sat in a primate chair in the middle of field coils. The head
restraint was locked to preclude head movement and the room was lit with
dim background illumination. Eye position was calibrated at the start of
each recording session by using a calibration coil and by having the
animal maintain eye position within a 2º window of target position. The
target was a laser spot subtending ~ 0.05º projected onto the back of a
translucent screen located 50 cm in front of the animal. The calibration
sequence was repeated separately for each eye.
Recordings were performed under conditions of binocular and monocular
viewing. Monocular viewing was achieved by use of liquid-crystal shutter
goggles which cycled from transparent to opaque (or the reverse) in 80
microseconds (0.08 msec).35 Voltages proportional to horizontal and
vertical eye position were digitized at 500 Hz. Eye velocity signals
were obtained by passing the eye position signals through a Finite
Impulse Response filter (DC to 90 Hz) and differentiated. Angular
resolution of the system was about 0.05º. Experiments were controlled
and the data were acquired and analyzed with the aid of a computer and
interactive signal processing software (Spike2 for Macintosh, Cambridge
Electronic Design, United Kingdom, and Igor Graphics, Wave Metrics, Lake
Oswego, Oregon).
VISUAL STIMULI AND TRIAL DESIGN
Eye Alignment
In the months before coil implantation, eye alignment was assessed
using 35 mm photographs and video recordings of each monkey (Hirshberg
method36,37).
After implantation of eye coils, alignment was measured under conditions
of binocular viewing to document precisely the magnitude of any
intermittent or constant heterotropia. The fixation target was displaced
from primary position (straight ahead) to the cardinal positions of gaze
to assess concomitance of any misalignment. Alignment during periods of
binocular viewing was compared with alignment when viewing with either
eye covered, to reveal the presence of any heterophoria (horizontal or
vertical).
Stable Fixation
Viewing monocularly, each monkey was required to fixate
the laser spot at straight-ahead gaze or at eccentricities of ± 10º
horizontally and vertically (Figure
2). The target was presented in repeated
trials. In order to receive a juice reward, the animal had to maintain
eye position of the nonoccluded, fixating eye within a 2º fixation
window, surrounding the target, for a randomized interval of 2 to 5 sec.
The small target size, variability of target location, small fixation
window, and random duration of required fixation ensured a high level of
visual attention.29
Smooth Pursuit
Smooth pursuit was recorded under conditions of monocular viewing
using a modification of the “step-ramp” paradigm of Rashbass (Figure
2).38,39
At the beginning of each trial, the animal fixated on the stationary
spot at straight-ahead gaze. When the animal’s eye remained within a 2º
window continuously for an interval of 2 to 5 sec, the stationary spot
disappeared and a second spot appeared, moving rightward or leftward at
30º/sec. The moving spot started either from the point of fixation (zero
eccentricity) or from 1 of 8 other initial positions along the
horizontal meridian (5º and 10º above, below, to the right and left of
zero eccentricity). The “step-ramp” approach allowed presentation of
target motion at a precise location on the retina as determined by the
relative positions of the stationary and moving spots. When viewing with
the left eye, rightward target velocities represented nasally directed
motion, and leftward target velocities temporally directed stimulus
motion in the visual field. To receive the juice reward, the monkey had
to track the stimulus within the 2º window for a duration > 750 msec.
The onset, direction (up, down, left, right), and speed of the target
were controlled by the computer program, which selected combinations of
initial target position and direction in a pseudorandom fashion to
preclude prediction. |
