Weber-Fechner Law

a basic law of psychophysics; determines the relationship between the intensity of sensa- tion and the force of a stimulus acting on a sensory organ. It is based on the observation of the German physiologist E. Weber, who established (1830-34) that it is a relative rather than absolute increase in the force of a stimulus (light, sound, weight pressing on the skin, and so on) that is perceived: ΔI /I = const. For example, with an original weight of 75 g pressing on the skin, man can sense an increase of 2.7 g; with an original mass of 150 g, he can sense an increase of 5.4 g. The German physicist G. Fechner (1858) analyzed the results of the research mathematically and derived the formula S = alnI + b (where S is the intensity of sensation, I is the force of the stimulus, and a and b are constants). The Weber-Fechner law is valid only at moderate stimulus intensities. It is considerably distorted at threshold or very great intensities.