Reaction time to differences in an objectively measurable perceptual dimension such as brightness, length, or weight. A common procedure for measuring it involves an observer comparing pairs of visual displays, presented simultaneously, and responding by pressing one of two buttons according to which display appears brighter, longer, heavier, or greater in magnitude on whatever dimension is being examined, the time elapsing between stimulus presentation and key-pressing response being the discrimination reaction time. The three main dependent variables in research using this technique are reaction time, degree of confidence, and relative frequency of each available response. Compare choice reaction time, simple reaction time. DRT abbrev.