One eye per subject was first tested with natural pupils to make sure the subjects would understand the requirements of the testing and also have some measure of practice. These data were discarded. For all three experiments the subjects were then dilated with 0.5% tropicamide ophthalmic solution. Following stable dilation and refraction and visual acuity testing at the testing distance, neutral density filters were introduced at the correcting lens plane in either ascending or descending order. For Experiments 1 and 2, the descending order started at 0 ND (no filter), then each subsequent test used the next filter: 0.6, 1.2, 1.6 ND. Under this condition, the subject was asked to fixate on the testing monitor for 2 minutes before initial testing and then after each change in filter. For the ascending order, subjects began with densest filter (1.6 ND), and were asked to adapt to the low light level by fixating on the testing monitor for 10 minutes prior to testing. With each subsequent filter (1.2, 0.6, no filter), subjects were asked to adapt by fixating for 2 minutes prior to testing. For Experiment 1, CSP testing and N-30 testing were on two different visits. All the stimulus conditions for Experiments 2 and 3 were tested on a single visit. For Experiment 3, the neutral density filters were 0.6, 1.2, and 1.8 log units. The ascending and descending adaptation periods were the same as for the first two experiments.
For our custom testing stations, contrast sensitivity was measured by having the subject fixate a target in the center of the display and click a button whenever a stimulus was seen, as stimuli were presented at the different visual field locations on a uniform gray background, in a darkened room. Experiments 1 and 2 used a staircase
14 with a variable interstimulus interval averaging 1700 msec. Experiment 3 used a ZEST algorithm
16 with a variable interstimulus interval averaging 1200 msec. Further details on strategies for assessing and managing fixation loss, false positives, false negatives, and other artifacts are described elsewhere.
14,19