““In many experiments, the boundary between seeing and not seeing is relatively sharp: an image is downright invisible when presented for 40 milliseconds, but is easily seen, on most trials, when the duration is increased to 60 milliseconds. This fonding justifies the use of the words subliminal (below threshhold) and supraliminal (above threshhold). Metaphorically, the gateway to consciousness is a well-defined threshhold, and a flashed image is either in or out. The length of the threshhold varies across subjects, but it always falls close to 50 milliseconds. At this duration, one perceives the flashed image about half the time. Presenting visual stimuli at threshhold therefore offers a wonderfully controlled experimental paradigm: the objective stimulus is constant, yet its subjective perception varies from trial to trial.””
