Psychophysics

Experiments on the sense of touch were carried out by the physiologist E. H. Weber (1795–1878), who distinguished among the feelings of pressure, temperature, and the location of stimulation on the skin. In conducting experiments in which he stimulated his own skin, Weber explored skin sensitivity and demonstrated that “on the tip of the forefinger and lips two fine compass points could be felt as two when they were less than one-twentieth of an inch apart, but if they were nearer they seemed to be one”. Not only could touch sensitivity be measured at different points on the skin, but relative sensitivity at a single point could also be measured. Placing a standard weight at a given spot on the skin and then asking for a second weight to be judged “heavier” or “lighter” showed that the amount of weight that could be judged heavier or lighter than the standard varied as a proportion of the magnitude of the standard weight. Thus, the minimal detectable difference between two weights was relative to the weights involved; for heavy weights, differences could have to be large, but smaller differences could be detected when the weights involved were light. G. T. Fechner (1801–1887), a physicist, saw in Weber’s results the possibility of relating mental events to physical events; subjective judgments about physical magnitudes could be compared to the actual physical magnitudes. Fechner had believed since his student days “that the phenomena of mind and body run in parallel” (Marshall, 1982, p. 67). His solution to the problem of relating these two aspects of the world was to make “the relative increase of bodily energy the measure of the increase of the corresponding mental intensity”. Although Fechner conceived of the possibility independently of Weber’s results, he came to realize that his speculations about arithmetic and logarithmic relations between physical and subjective magnitudes were in fact demonstrated by Weber’s observations (Adler, 1966; Marshall, 1982). Weber’s results showed that sensory judgments of magnitude formed ratios that were sufficiently regular to assume the status of a law. Fechner designated as Weber’s law the mathematical equation that stated that the increase in perceived intensity of a stimulus (the “just noticeable difference”) was, as Weber had demonstrated, a constant proportion of the intensity of the stimulus to be increased. The regularity in ratios across a wide range of intensities led Fechner to rewrite the law in terms of a logarithmic progression, with the strength ofa sensation equal to the logarithm of the intensity of a stimulus multiplied by a constant established experimentally for the sensory system under study. “Weber’s law” now typically refers to the “simple statement that the just noticeable difference in a stimulus bears a constant ratio to the stimulus”, while “Fechner’s law” typically refers to the logarithmic relationship that Fechner formulated. Fechner called the new science that he established psychophysics and developed laboratory procedures that became part of the laboratory experiments of the new psychology as well as of the physiological research on the special senses. The measurements of the smallest detectable intensity (absolute threshold) and the smallest detectable difference in intensities between stimuli (difference threshold) for the different senses were pursued by the several methods that Fechner had devised for the purpose (see, e.g., Woodworth,1938). Resolving differences in results obtained for different methods, testing psychophysical laws over a wide range of stimulus intensities, and developing scales of psychological measurement offered significant research challenges for psychological laboratories well into the twentieth century (Stevens, 1951; Woodworth, 1938).