Bright flashes of diffuse white light, produced by a photoflash
(Model 283; Vivitar, Santa Monica, CA) were used to elicit the a- and
b-waves. Stimuli were reflected to the eye by a ganzfeld hemisphere 30
cm in diameter with an interior surface coated with reflective white
paint. Flash intensity was varied over 6 log units by means of neutral
density filters (Eastman Kodak, Rochester, NY) available in steps of
0.5 log units. At 0 log units, the intensity of the flash at the cornea
was 28,057 scotopic candelas (cd)-seconds per square meter (measured
with a photometer; model 40X; UDT, Hawthorne, CA) in photopic units and
converted to scotopic units by assuming the spectral distribution of
the xenon-filled flash tube).
13 The conversion from the
intensity of the flash at the cornea to the number of
photoisomerizations per rod per flash (Φ), which was used in the
a-wave model, was performed according to Breton et al. (see Reference
12 , equation 13) with a few corrections for cat eye. The Φ is a
product of
Q, the retinal illuminance in scotopic
troland-seconds (scot td-sec) and
K, the overall conversion
factor in photoisomerizations/rod per scot td-sec. The pupil area was
113 mm
2, and retinal illuminance
Q in
our study was therefore 3.17 × 10
6 scot
td-sec (or 12.2 log q/deg
2). The conversion
factor
K was adjusted for the cat eye by considering the
following three factors: 1) the smaller loss in media and tapetal
reflection, a difference of 0.24 log units from human
14 ;
2) the smaller posterior nodal distance in cat, differing 0.26 log
units from human
14 ; and 3) the smaller area of the rod
outer segment, assumed to have an average diameter of 1.3 μm in
cat.
15 The value of
K was then 27.13
photoisomerizations/rod per scot td-sec. Our maximum unattenuated
retinal illuminance corresponded to 7.93 log photoisomerizations per
rod per flash. To test for a cone contribution, responses were obtained
to short-wavelength flashes (Wratten 47B filter; Eastman Kodak,
Rochester, NY) and long-wavelength flashes (Wratten 26 filter).