In both steady-pedestal and pulsed-pedestal paradigms, the measurement started with a 10-second preadaptation phase, followed by a staircase procedure for contrast discrimination threshold determination. Each staircase trial consisted of three phases, including a 1-second pretest adaptation phase, a 26.7-ms test phase, and a posttest adaptation phase waiting for the observer's response (
Fig. 1). The stimulus parameters, including presentation duration (26.7 ms) and square sizes (1° and 0.57°), were chosen to achieve a large separation between MC and PC functions based on previous findings using the pedestal paradigms.
25,26,29 During the pretest and posttest adaptation phases in the steady-pedestal paradigm, the pedestal with a predefined luminance level (i.e., pedestal luminance) was shown together with the background. Observers adapted to both the background and the pedestal luminances. By contrast, during both adaptation phases in the pulsed-pedestal paradigm, only the background was shown, not the pedestal. In other words, observers adapted to only the background luminance, not the pedestal luminance. While the adaptation phases differ in the steady- versus pulsed-pedestal paradigms, the test phases were identical in both paradigms, during which the pedestal was presented with three of the four squares having the predefined pedestal luminance and one of them (i.e., the test square) having an incremental or decremental luminance from the predefined pedestal luminance. Observers were instructed to identify the test square in a four-alternative-forced-choice (4AFC) task; that is, observers reported which square in the array differed in luminance from remaining squares. While the predefined pedestal luminance was fixed in a given block, the luminance of the test square varied according to a 2-yes-1-no double-random staircase procedure that allowed estimation of the contrast discrimination threshold from the predefined pedestal luminance. The staircase started with an easily discriminable test square luminance increment or decrement, with an initial step size of 20% contrast (i.e., test square luminance 20% higher or lower than the pedestal luminance). Whenever a reversal occurred, that is, the moving direction of the staircase changed, the step size was halved until a minimum step size of 0.3125% was reached. The staircase procedure continued until 10 reversals at the minimum step size occurred. The average value of the last six reversals was taken as the discrimination threshold for that pedestal luminance. Full details of the pedestal paradigms and the staircase procedure can be found in the original paper.
25 Thresholds for eight predefined pedestal luminances were estimated in both the steady- and pulsed-pedestal paradigms: 8.5, 9.5, 10.7, 12.0, 13.4, 15.1, 16.9, and 19.0 cd/m
2. Increment pedestals (luminance brighter than the background) and decrement pedestals (luminance darker than the background) were included to measure discrimination thresholds in the ON- and OFF-pathways, respectively.
26 For either pedestal polarity (decrement or inclement pedestals), incremental and decremental discrimination thresholds were similar
26; therefore, to be time efficient, we chose to measure the discrimination thresholds in the same polarity as the pedestals (i.e., the test square always had a luminance increment for the increment pedestals or had a luminance decrement for the decrement pedestals). The order of the eight pedestal luminance blocks was randomized in each paradigm.