Psychophysics is a subfield of psychology addressing the relationship between physical stimuli and subjective responses, or percepts. The term "psychophysics" was coined by the field's founder, Gustav Theodor Fechner, in 1860. Earlier scientists, including German physiologist Ernst Heinrich Weber and medieval scientist Alhazen, conducted similar experiments, though the field was not clearly defined until Fechner's work. Experiments can focus on any sensory system: hearing, taste, touch, smell, or sight.
Objectively measurable stimuli are used in psychophysics experiments, such as lights varying in brightness, or sounds varying in loudness. A threshold, or limin, is the point at which a subject can detect a stimulus, or a change in the stimulus. Stimuli falling under the threshold are considered sub-liminal or undetectable.
An absolute threshold, or detection threshold, is the point at which a subject can detect the presence of a stimulus, while a difference threshold is the magnitude of difference perceptible between two stimuli. For example, a difference threshold may be tested by asking a subject to adjust a sound until it is the same as another, and then measuring the difference between the two sounds. The point of subjective equality (PSE) is the point at which the subject considers two stimuli to be the same, while the just noticeable difference (JND) or difference limen (DL) is a difference between stimuli perceived 50% of the time.
Classical psychophysics experiments may take a number of forms. They may use the ascending method of limits, in which stimuli are presented beginning at a very low, undetectable level, then gradually increased to note the point, at which they become perceptible. Another method is the method of constant stimuli, in which stimuli are administered in random order rather than ascending order. The method of adjustment requires the subject to manipulate stimuli until they are just barely perceptible against a background, or until they are the same as or just barely different from another stimulus.
Newer methods in psychophysics experimentation include those called staircase procedures, first used by the Hungarian biophysicist Georg von Békésy in 1960. In experiments using staircase procedures, stimuli are first presented at a high, detectable level. The intensity is decreased until the subject makes a mistake in perceiving it. After the mistake, the scale is reversed, with intensity increasing until the subject responds correctly. At that point, the intensity is decreased again. The values for the reversals are then averaged. The staircase method helps experimenters narrow in on the threshold.