Contrast discrimination and choice reaction times at near-threshold pedestals

Choice reaction times (CRTs) to contrast differences were measured and compared with contrast increment thresholds obtained from concurrently measured psychometric functions at pedestal contrasts in the vicinity of detection threshold. Contrast discrimination functions had a classical dipper shape. The main finding was that CRTs were shorter at low pedestal contrasts but longer at higher pedestal contrasts compared to detection, reflecting the behaviour of increment thresholds. Even when equalized for response accuracy, CRTs varied with pedestal contrast in a similar manner to the contrast increment thresholds. The finding that CRTs and contrast increment thresholds depended on pedestal contrast in a similar manner suggests that both share a common origin. This common origin is proposed to lie in the variability of the sensory effect which determines the variability of the information accumulation process, which in turn affects the response criterion and contrast increment thresholds. At low pedestals, a decrease in variability lowers thresholds and results in a lower response criterion, thereby accelerating reaction times. At high pedestals, increasing signal-dependent noise inflates variability and thus raises thresholds and the response criterion, which results in slower CRTs.