A mesopic background of 3.15 apostilbs was used for this study.
Previous studies in our laboratory have shown that a higher mesopic
background of 31.5 apostilbs (like that used in standard Humphrey
perimetry) causes the pupil to become too small in some individuals,
limiting its response range of movement.
10 We stimulated
at a centrally located fixation point and 10 other locations that were
located 3° above the horizontal meridian and separated by 6° (see
inset in
Fig. 3B ). Unlike standard threshold perimetry, which uses a
test target size of 0.4° in diameter (Goldmann target III), a larger
4° target was used in this pupil perimetry study. We have recently
found that stimuli of smaller size (e.g., 1.7° or Goldmann size V)
sometimes fail to elicit a large enough pupil response (e.g., >0.3 mm
of contraction) in the nasal visual field in normal subjects.
Therefore, the larger 4° stimulus was used to ensure that we could
obtain adequate pupillary responses over the range of stimulus
intensities used in this study. Stimulus duration was kept at 0.2
seconds (similar to standard threshold perimetry), and a stimulus
interval of 2.5 seconds was used. The units of stimulus intensities
were calculated as equivalent to the units used in Humphrey automated
perimeters so that the visual sensitivity results from patients tested
on the perimeter could be compared directly. Because the Humphrey
perimeter uses a neutral density filter wheel interposed in the light
path to attenuate the light stimulus, a higher value of decibel
attenuation is equivalent to a dimmer light. The following stimulus
light intensities were used (given in units of decibels of attenuation
above background; e.g., 0 dB attenuation = 10,000 apostilbs or
3183 cd/m
2 above background): 37 dB (0.64
cd/m
2), 30 dB (3.18 cd/m
2),
25 dB (10.07 cd/m
2), 21 dB (25.28
cd/m
2), 17 dB (63.51
cd/m
2), and 13 dB (159.53
cd/m
2). Subjects were asked to fixate on X-shaped
fixation target in the eye being tested (2° in diameter) during the
test. A background adaptation of 30 seconds was given before the test.
We stimulated perimetric locations in the nasal and temporal fields
alternately for one eye at a time. Testing time for 11 locations at six
different intensities was 2.42 minutes. Subjects were tested by this
strategy with three repetitions for each eye, and the right eye was
tested first. We averaged the three repetitions of pupil response for
the analysis of this study.