Contrast sensitivity across the central visual field was measured by having the subject fixate a target in the center of the CRT screen and respond to stimuli presented at a range of locations in visual space. The subject was asked to click a button whenever the stimulus was seen; the interstimulus interval averaged 1700 ms, with a variable foreperiod. A one-down, one-up staircase method with four reversals was used to determine the subject's contrast threshold at each location. For each staircase a stimulus with 25% contrast was presented first. Contrast of the stimulus was decreased by 0.3 log unit if responded to, or else increased by 0.3 log unit if not responded to, until a reversal (change in response/no-response) occurred. This was considered the first of four reversals. Then the stimulus contrast was either increased (if not responded to) or decreased (if responded to), by 0.3 log unit again for the second reversal. After the second reversal, the staircase used steps of 0.15 log unit. The average of the log contrasts for the last two reversals was taken as an estimate of contrast threshold.
In addition to the trials used in the staircases, other trials (∼15% of total trials) were used to assess rates for fixation loss, false positives, and false negatives, as described in detail in Hot et al.
13 Briefly, fixation loss trials were presentations of the stimulus in the center of the blind spot, at maximum contrast; false-positive trials were those in which no stimulus was presented at the end of an interstimulus interval, and a false positive was scored if there was a response during the following interstimulus interval before the next stimulus was presented. False-negative trials were trials at which the stimulus was set to a contrast 0.6 log unit above the ongoing threshold estimate. The rates for fixation loss and false positives were computed as fractions of responses to the two types of trials. After the experiment was finished, maximum likelihood estimation was performed to estimate threshold, slope and false-negative rate at each location, and the average of false-negative rates across all locations was used as the false-negative rate for that experiment.
When there was no response to the stimulus for a presentation at maximum contrast, it was scored as a “reversal” at that location, and the maximum stimulus was presented again the next time that location was tested. When there was never a response to a stimulus at a given location, the location was scored as “not seen” for that test. Because our subjects were free of eye disease, “not seen” was taken as an indication that: the stimulus spatial frequency was higher than the subject could detect at that location, the location was within the physiological blind spot, or the location was obscured by eyelid ptosis (eyelid artifact). Because there were four reversals and starting contrast was 25%, a score of not seen means that there was no response at that location to six stimuli with contrasts presented in the sequence 25%, 50%, 100%, 100%, 100%, and 100%. With typical psychometric slopes for this study, the likelihood function for this set of stimulus contrasts and subject responses has an asymptotic plateau starting near a log contrast sensitivity of −0.15, which was assigned for nonseeing locations. Across subjects, frequency, and locations the value −0.15 log contrast sensitivity was assigned a total of 50 times (0.9%). Nonseeing locations occurred more often at peripheral locations and with higher spatial frequencies. Percentage of locations not seen with maximum contrast were 3%, 2%, 0.5%, and 0.1% for the 2.0, 1.5, 1.0, and 0.5 cpd spatial frequency stimuli respectively. There were no nonseeing locations for the other spatial frequencies.