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The subfield of psychology that deals with the transformation from the physical to the psychological through detection, identification, discrimination, and scaling.

Psychophysics originated with the research of Gustav Fechner (1801-1887), who first studied the relationship between incoming physical stimuli and the responses to them. Psychophysicists have generally used two approaches in studying our sensitivity to stimuli around us: measuring the absolute threshold or discovering the difference threshold. In studying the absolute threshold using the method of constant stimuli, an experimenter will, for example, produce an extremely faint tone which the listener cannot hear, then gradually increase the intensity until the person can just hear it; on the next trial, the experimenter will play a sound that is clearly heard, then reduce its intensity until the listener can no longer hear it. Thresholds can also be ascertained through the method of constant stimuli. In this approach, stimuli of varying intensity are randomly presented. Although an observer's measured threshold will change depending on methodology, this technique gives an estimate of an individual's sensitivity.

A different psychophysical approach combines the concept of sensory abilities with the decisions and strategies that an observer uses to maximize performance in a difficult task. Rather than try to identify a single point for the threshold, psychophysicists who employ the signal detection theory have developed ways to measure an observer's sensitivity to stimuli in ways that go beyond the simple concept of the threshold. Some psychophysical research involves the identification of stimuli. There may be no question as to whether we can detect a stimulus, but sometimes we cannot identify it. For example, people can often detect odors but cannot identify them. Research in this area has centered on determining how much information is needed to allow a person to identify a stimulus. Identification constitutes a relatively small part of psychophysical research, although such research has important practical applications. For example, in the development of useful telephones, researchers had to assess how much "noise" or unwanted sound could accompany speech in a phone conversation so that a listener could understand what was said—that is, identify the spoken words accurately.

A third area of psychophysics involves discrimination of different stimuli, or difference thresholds. No two physical stimuli are absolutely identical, although they may seem to be. The question of interest here is how large must the difference be between two stimuli in order for us to detect it. The amount by which two stimuli must differ in order for us to detect the difference is referred to as the JND, or just noticeable difference. Research has indicated that for stimuli of low intensity, we can detect a difference that is small, as the intensity increases, we need a larger difference. Sometimes psychophysicists use reaction time as a measure of how different two stimuli are from one another. When two stimuli are very similar, it takes a longer time to decide if they are different, whereas large differences lead to fast reaction times.


Absolute threshold: as the stimulus strengthens from the undetectable, the point at which the person first detects it.

Signal detection theory: theory pertaining to the interaction of the sensory capabilities and the decision making factors in detecting a stimulus.

Difference thresholds: at which point can one differentiate between two stimuli. This point is termed just-noticeable difference.

Scaling: using rating scales to assign relative values (for example, rating on a scale of one to ten) to sensory experiences.

The final area of interest to psychophysicists is scaling, the activity of deciding how large or small something is or how much of it is present. Any sensory experience can be scaled. For instance, if the attractiveness of a painting is rated on a scale of one to ten, it is being scaled. If the painting is rated nine, it is considered more attractive than a painting rated eight. This simple example gives the concept underlying scaling, but psychologists have developed more complicated techniques and sophisticated mathematical approaches to scaling.

Additional topics

Psychology EncyclopediaBranches of Psychology