What is Psychophysics?
Psychophysics is concerned with describing how an organism uses its sensory systems to detect events in its environment. This
description is functional, because the processes of the sensory systems are of interest, rather than their structure (physiology). One
psychophysical theory, the Theory of Signal Detectability (TSD), uses a combination of statistical decision theory and the concept
of the ideal observer to model an observer's sensitivity to events in its environment. TSD is stimulus-oriented, because properties
of the stimuli are used to determine the theoretically best, or ideal, observer for a given detection task. This observer may then be
used to compare the performance of human, and other, observers. For instance, the ability of humans to detect
Methods for the quantitative study of the relations between physical stimulus magnitudes and the corresponding magnitudes of
sensation, for example, between the physical intensity of a light and its perceived brightness or the concentration of a sugar
solution and its observed sweetness. To establish these relations, measurement scales are needed, not only for physical magnitudes
but also for subjective magnitudes. Subjective scales are not obtained directly from observation but are theoretical models which
summarize observed relations between stimuli and responses.
The term psychophysical methods is sometimes extended to include certain scaling techniques which are most often used with
subjective dimensions to which there correspond no simple physical dimensions, for example, food preferences.
In 1860, G. Fechner designed psychophysical methods to measure the absolute threshold, defined as the minimum stimulus energy
that an organism can detect, and the differential threshold, defined as the minimum detectable change in a stimulus. Both
quantities had to be defined as statistical averages. To obtain reliable measurements for these averages, Fechner devised the
method of limits (also called the method of minimal changes) and the method of constant stimuli.
In the method of limits, the experimenter begins with a stimulus which is too weak for the subject to detect. In successive
presentations, the stimulus intensity is increased in small, equal steps, the subject reporting after each presentation whether the
stimulus was perceived until it has been detected. The descending series is then begun, the stimulus intensity beginning at an
above-threshold value and decreasing in steps until the subject signals the disappearance of the stimulus. Many such series are
In measuring the difference threshold, essentially the same procedure is involved, except that the subject now signals the relation
of a comparison stimulus to a standard stimulus. After a large number of such trials, the average of each of these four threshold
values is computed.
To measure the absolute threshold by the method of constant stimuli, the experimenter selects a small number of stimulus values