PSYCHOPHYSICS

study of quantitative relations between psychological events and physical events, or, more specifically, between sensations and the stimuli that produce them. Physical science permits, at least for some of the senses, accurate measurement on a physical scale of the magnitude of a stimulus. By determining the stimulus magnitude that is just sufficient to produce a sensation (or a response), it is possible to specify the minimum sensible stimulus or the absolute stimulus threshold (stimulus limen) for the various senses. It is also possible, although practically more difficult, to determine the lowest stimulus magnitude that produces maximal sensation, the terminal thresholdi.e., that point on the physical scale beyond which no increase in stimulus produces any appreciable increase in sensation. Thus are determined limiting stimulus values, between which changes in stimulus intensity are accompanied by changes in sensation. The central inquiry of psychophysics pertains to the search for a lawful, quantitative relation between stimulus and sensation for the range of stimuli between these limits. Psychophysics was established by a German scientist and philosopher, Gustav Theodor Fechner, who coined the word, invented the fundamental methods, conducted elaborate psychophysical experiments, and began a line of investigation that still persists in experimental psychology. Fechner's classic book Elemente der Psychophysik (1860) may be looked upon as the beginning not only of psychophysics but also of experimental psychology. Trained in physics, Fechner in his later life became interested in metaphysics and searched for a way of relating the spiritual to the physical world. He hit upon the notion of measuring sensation in relation to its stimulus. A German physiologist named Ernst Heinrich Weber had discovered that the amount of change in magnitude of a given stimulus necessary to produce a just-noticeable change in sensation always bore an approximately constant ratio to the total stimulus magnitude. This fact, properly speaking, is Weber's law: if two weights differ by a just-noticeable amount when separated by a given increment, then, when the weights are increased, the increment must be proportionally increased for the difference to remain noticeable. Fechner learned of Weber's law and undertook to use it for the measurement of sensation in relation to a stimulus. The resulting formula Fechner named Weber's law, although it is really Fechner's law (often called the FechnerWeber law). It expresses the simple relation that the magnitude of a stimulus must be increased geometrically if the magnitude of sensation is to increase arithmetically. For Fechner it meant that the relation between the spiritual and physical worlds is statable and that there is therefore only one world, the spiritual; but for physiologists and for many philosophers it meant the measuring of sensation in relation to a measured stimulus and thus the possibility of a scientific quantitative psychology. The data and theories on which Fechnerian psychophysics is based were challenged in the 1950s by the American theoretical psychologist S.S. Stevens, whose theory of signal detectability questioned the classical methods for assessing the psychophysical thresholds. The American theoretical psychologist Eugene Galanter wrote in 1974 that experiments generated by the theory of signal detectability have included those designed to elucidate nonperceptual features of the experiment that are important in influencing judgments about perceptual events. Another direction of modern psychophysics has been the experimental repudiation of Fechner's theory of measurement as an accurate representation of the psychic effects of stimulus magnitudes. Psychophysicists have suggested that psychic magnitudes be assessed by direct scaling experiments rather than by deriving a sensation scale based upon discrimination judgments. Psychophysical methods are used today in studies of sensation and in such practical areas as product comparisons and evaluations (e.g., tobacco, perfume, and liquor) and in psychological and personnel testing.