The Effect of Pedestal Contrast on Choice Reaction Times to Contrast Increments

Two-alternative forced-choice reaction times (RTs) were measured and psychometric functions constructed for ten contrast increments at seven pedestal contrasts ranging from 0% to 4.8%. Two sine gratings at 2 cycles deg⁻¹ differing only in contrast were presented on a computer screen, and the subject's task was to indicate as quickly as possible whether the stimulus with higher contrast appeared to the left or to the right of a fixation point. There were 100 trials per stimulus pair from which the percentage of correct responses and the median correct RTs were calculated. As the contrast increment increased, the percentage of correct responses increased and RTs decreased reaching a minimum with large increments. However, RTs continued to decrease even when performance reached 100% correct. Contrast increment thresholds calculated at 82% correct level formed a classical dipper-shaped function when plotted as a function of pedestal contrast. Response variability, reflected in standard errors of increment thresholds, was greater at high pedestal contrasts. When RTs corresponding to threshold increments were interpolated and plotted against pedestal contrast, the functions were also dipper-shaped. Discriminative RTs were on average faster at low and slower at high pedestal contrasts compared to detection. These findings show that equalising the percentage of correct responses did not equalise RTs, and that processing time increased with pedestal contrast. The increase of both increment thresholds and RTs with pedestal contrast may be due to an increase in signal-dependent noise which increases response variability and slows down the decision process.