Psychophysical functions

Psychophysics is concerned with measuring how external physical stimuli cause internal psychological sensations. In a typical psychophysical experiment, subjects are repeatedly confronted with two similar stimuli, such as two sounds, two weights, two smells, two lines, or two time intervals. One stimulus—the standard stimulus—always has the same intensity, whereas the other stimulus—the test stimulus—varies in intensity from trial to trial. On each trial of the experiment, the subject's task is to detect which of the two stimuli is more intense: louder, heavier, stronger-smelling, more tilted, or longer lasting. The more similar the stimuli, the more difficult it is for the subject to discriminate between them.

The relation between task difficulty and performance usually follows a sigmoid or S-shaped curve, as shown in Figure 12.1. The x-axis represents differences in stimulus intensity between the test stimulus and the standard. The y-axis represents the probability of a response indicating that the test stimulus has higher intensity. The curve linking these physical and psychological measures is known as a psychophysical function, and is used to define several values of interest. The “point of subjective equality” (PSE) represents that difference in intensity for which the participant chooses the correct response 50% of the time, which is not necessarily the point where the two stimuli are equally intense physically. The “just noticeable difference” (JND) is the intensity threshold at which the subject “just” notices a difference in intensity between two stimuli.