Psychophysical Measurement of Thresholds: Differential Sensitivity
Prior to a century ago the approach to psychological problems consisted primarily of philosophical speculation. The transition of psychology from a philosophical to a scientific discipline was greatly facilitated when the German physicist G. T. Fechner introduced techniques for measuring mental events ( 1860). The attempt to measure sensations through the use of Fechner's procedures was termed psychophysics and constituted the major research activity of early experimental psychologists. Since this time psychophysics has consisted primarily of investigating the relationships between sensations (ψ) in the psychological domain and stimuli (ϕ) in the physical domain.
Central to psychophysics is the concept of a sensory threshold. The philosopher Herbart ( 1824) had conceived of the idea of a threshold by assuming that mental events had to be stronger than some critical amount in order to be consciously experienced. Although measurement is not a part of this description of the threshold, scientists eventually were able to see the implication of such a concept for psychological measurement. In the early nineteenth century, for example, German scientists such as E. H. Weber and G. T. Fechner were interested in the measurement of the sensitivity limits of the human sense organs. Using measurement techniques of physics and well-trained human observers, they were able to specify the weakest detectable sensations in terms of the stimulus energy necessary to produce them. The absolute threshold or stimulus threshold (RL for the German Reiz Limen) was defined as the smallest amount of stimulus energy necessary to produce a sensation. Since an organism's sensitivity to external stimuli tends to fluctuate somewhat from moment to moment,