When the animals had grown sufficiently to fit comfortably into
our behavioral apparatus (approximately 18 months of age, i.e., after
at least 1 year of visual experience without the treatment lenses),
spatial contrast sensitivity functions were measured behaviorally for
each eye. The basic apparatus and operant procedures were similar to
those used in previous investigations.
18 20 27
During the daily experimental sessions, the monkeys were seated in a
primate chair inside a light-proof, sound-attenuating chamber. The
primate chair was fitted with a response lever on the waist plate and a
drink spout on the neck plate through which orange drink reinforcement
was delivered. The animal’s optimal spectacle correction, which was
determined for each eye independently using a subjective refraction
procedure,
20 was held in a face mask at approximately a
14-mm vertex distance. For monocular viewing, the lens well for one of
the eyes was occluded with an opaque disc.
The detection stimuli were vertical sinusoidal gratings that were
generated using a graphics board (VSG; Cambridge Research Systems,
McLean, VA) on a 20-inch video monitor (FlexScan 9080; EIZO
Nanao, Cypress, CA) that operated at a 100-Hz frame rate. The usable
display subtended a visual angle of 11 × 14° at the 114-cm
viewing distance and had a space-averaged luminance of 60 candelas
(cd)/m
2. The grating stimuli were presented as
Gabor patches, which consisted of a carrier grating presented in cosine
phase with the center of the display. The contrast of the grating was
attenuated by a two-dimensional Gaussian envelope and declined to a
value of 1/
e of the maximum contrast at 4° from the
Gabor’s center. The number of grating cycles within the Gabor varied
as a function of spatial frequency. As a result, at low spatial
frequencies when a small number of grating cycles were presented,
probabilistic concerns may have limited absolute sensitivity by a small
amount.
28 However, for spatial frequencies above the peak
of the monkey’s contrast sensitivity function, the number of grating
cycles exceeded the number required for optimal performance. A
photometer (Spectra; Pritchard, Photo Research Corp., Burbank, CA)
equipped with an automated scanning spot was used to calibrate the
luminance and contrast of the display. The contrast of the grating
pattern was defined as (
L max −
L min)/(
L max+
L min), where
L max and
L min represent the maximum and minimum
luminances of the grating, respectively.
The behavioral paradigm was a temporal–interval detection task that
required the monkey to press and hold down the response lever to
initiate a trial and then to release the lever within a criterion
response interval after the presentation of the grating stimulus. The
duration of the grating stimuli was 500 msec. Contrast detection
thresholds were measured as a function of spatial frequency from 0.125
or 0.25 cyc/deg to 16 cyc/deg in 0.15 log unit intervals. Data were
collected using an adaptive staircase procedure. The decision rules (a
0.05 log unit reduction in contrast after each hit and a 0.6 log unit
increase in contrast after two consecutive misses) converged to a
contrast threshold on the step increasing part of the psychometric
function that corresponded to a 25% detection rate. During a given
experimental session, the staircases for five to seven different
spatial frequencies were simultaneously interleaved.
Contrast sensitivity functions were generated from the geometric means
of a minimum of 10 threshold measurements at each spatial frequency.
For descriptive purposes and to calculate an eye’s grating visual
acuity, each contrast sensitivity function was fit with a double
exponential function
29 \[\mathrm{Contrast\ sensitivity\ {=}\ (}\mathit{k}_{\mathrm{s}}\mathrm{(}\mathit{sf}\mathrm{\ {\cdot}\ }\mathit{kf}\mathrm{){\wedge}}\mathit{al}\mathrm{)\ exp(-}\mathit{ah\ {\cdot}\ sf\ {\cdot}\ kf}\mathrm{)}\]
where
sf is spatial frequency;
al and
ah are
parameters that reflect the slopes of the low- and high-spatial
frequency portions of the function, respectively;
ks and
kf are proportional to the peak contrast sensitivity and the
optimum spatial frequency, respectively. An iterative routine was used
that minimized the sum of squared errors.