Stimuli were generated by computer (Macintosh G3; Apple,
Cupertino, CA) using software adapted from the VideoToolbox
routines (available without charge at
http://www.vision.nyu.edu/VideoToolbox, host institution, New York
University).
16 Images were displayed on a multiscan
gray-scale monitor (model 400PS, Trinitron; Sony, Tokyo, Japan) at a
frame rate of 75 Hz and a mean luminance of 50 candelas
(cd)/m
2. The luminance of the display was
linearized to a pseudo 12-bit resolution with an ISR video
attenuator
17 and calibrated with a photometer (Minolta,
Osaka, Japan). Pseudo 12-bit resolution in this case allowed the
presentation of 2
8 monochrome levels from a
possible 2
12 levels. Images were presented in
gray-scale by amplifying the monochrome signal and driving the
red-green-blue guns equally. The display was 9° horizontally (1152
pixels) by 6.8° vertically (870 pixels) and was viewed binocularly in
a dark room from a distance of 230 cm. Conventional psychophysical
procedures were used throughout. Observers were required to perform
two-alternative forced-choice (2-AFC) discriminations, with auditory
feedback provided for incorrect responses. In those subjects wearing
tinted lenses this measurement was repeated both with and without the
lenses, in random order. Stimulus levels were varied from trial to
trial according to an adaptive staircase Quality, Utilization,
Effectiveness, Statistically Tabulated (QUEST) procedure designed to
concentrate observations near threshold level.
18 The raw
data across a minimum of four runs for each condition for each observer
were combined and were fitted with cumulative normal psychometric
functions by a least χ
2 fit. From this fit, the
thresholds and 95% confidence limits were estimated at the 75%
correct point with standard methods.
19