Psychophysics and psychophysical scaling

Psychophysics as the study of the relation between variation in physical dimensions of stimuli is symbolized as ɸ (for physical) and their associated responses, historically called “ sensations” symbolized ψ (for psychological). The physical dimension need not be intensity, and the associated responses will describe apparent intensity. A 5 pound weight obviously feels heavier than a 1 pound weight. In particular, the probability that a week event will be detected also increases as the intensity increases. Psychophysics is concerned with making more detailed statements about the relations with ɸ and ψ which, as was also noted, are usually required by the problem under study. Three particular questions are historically important yet relevant to many contemporary problems:

1.                  What is the minimal energy needed for a particular event to be perceived under particular conditions, i.e, the absolute threshold or limen? For reasons to be noted below , this normally involves determining the stimulus event that is perceptible 50 percent of the time.

2.                  How different must two stimuli be in order to detect a difference between them or to determine which is of greater intensity?  This involves what is variously called the difference threshold, difference limen, or just noticeable difference (JND) between a standard and a comparison stimuli.

3.                  How may be the relation between physical intensity and its associated sensation be described in the interval or ratio terms. This is known as the problem of psychophysical scaling.

         Psychophysics is important for its own sake as exemplified by its use in such areas as communications engineering and photography. Audiologists perform psychophysical scaling on individuals in testing for hearing loss when they compare absolute thresholds they obtain with norms. An abnormally high threshold implies hearing loss. Psychophysics is limited to the study of relationships that hold when stimuli vary along a specified physical dimension such as sound intensity.             

        Several psychophysical methods developed by Fechner are still widely used. One is called the method of constant stimuli. Assume that a tone whose physical intensity is 185 units is essentially never reported as being heard, but a tone whose physical intensity is 215 units is nearly always reported a being heard. The experimenter might choose to use intensities of 185, 190, 195, ……….., 215 units. On each trail, one level (magnitude) is chosen at random for presentation. There is no limit upon the number of levels the experimenter may use. The levels need not be equally spaced and they need not occur equally often, but it is typical to use from 5 to 10 equally spaced and equally probable levels. The results are the probabilities of an affirmative response (e.g. saying the tone was heard) for each level.