Psychophysics refers to the relation between physical stimuli and their corresponding sensations and perceptions. Psychophysics are used to measure the limits of human perception, and to observe cognitive states when those limits are approached.
Related concepts include signal detection theory and just-noticeable difference. Signal detection theory refers to an individual's ability to identify objects or patterns in the face of distractors or noise. Just-noticeable difference refers to the smallest physical difference between to stimuli that is detectable by an observer.