Story of Psychology - Morton Hunt [78]
To express the relationship between stimulus intensity and sensation intensity, Fechner mathematically transformed Weber’s Law, integrating it and making it:
which means, in English, that stepwise increases in sensation intensity are the result of doublings of stimulus intensity (multiplied by some ratio or factor). Bending over backward to give credit to his former teacher, Fechner called this Weber’s Law—it was he who gave the name to Weber’s formula and his own—but later psychologists, giving credit where credit is due, have called the reformulation Fechner’s Law.
Fechner spent the next nine years in plodding experimentation, collecting data to confirm the law. Despite the mystical and poetic aspects of his personality, in the laboratory he was the very model of a compulsive and rigorous researcher. He tirelessly had subjects lift weights, look at lights, listen to noises and tones, look at color samples, and so on, and pronounce them either different or the same. Over those years he experimented with a wide range of intensities of each kind of stimulus, using three methods of measuring such judgments. With just one of those methods he tabulated and computed no fewer than 24,576 judgments.39He considered this first systematic exploration of the quantitative relationship between the physical and psychological realms a new scientific specialty and named it “psychophysics.”
Of the three methods of experimental measurement that he used, he had borrowed two from predecessors and perfected them, and invented the third himself. Until then, no one had ever used such careful, quantitative, and precisely controllable methods to explore psychological responses. His methods were soon widely adopted, and are in constant use today in every laboratory of psychophysical research.
One is the method of limits, which Fechner called the “method of just noticeable differences.” To determine the threshold of a stimulus, the experimenter presents stimuli one at a time, starting with the most minimal and increasing the magnitude until the subject can perceive them. To determine the j.n.d., the experimenter presents a “standard” stimulus and a “comparison” stimulus, increasing the difference by small steps until the subject says it is perceptible.
A second is the method of constant stimuli, which Fechner called the “method of right and wrong cases.” The experimenter presents identical stimuli time and again—either single ones at the threshold, or pairs of stimuli that are very similar. The subject replies “Yes” (meaning that he perceives it, or that the two are different), or “No” (he does not perceive it, or the two are not different). The subject’s responses yield averages, and these indicate how likely it is that, at any given stimulus level or difference between stimuli, the subject will perceive the stimulus or the difference between two stimuli.
The third, Fechner’s original contribution, is called the method of adjustment, which he called the “method of average error.” Either the experimenter or the subject adjusts the comparison stimulus until it seems (to the subject) identical with the standard stimulus. There is always some error, however minuscule, to one side or the other. Every error is recorded, and after many trials the average error is computed; it, too, is a measure of the j.n.d. This method established the useful principle that measuring the variability of data can be as informative as measuring the central tendency.
In 1860, Fechner published the fruits of his work in the two-volume Elements of Psychophysics. He was fifty-nine, an age at which scientists rarely produce their most original work; Elements, however, was truly original and had an immediate impact. Interest was intense and wide-spread—not in the panpsychism it espoused but in its experimental and quantitative methodology. As Boring once said of Fechner’s failure and triumph, “He attacked the ramparts of materialism and was decorated for measuring sensation.”40 Some psychologists,