Story of Psychology - Morton Hunt [70]
All of Weber’s many experiments on the sensitivity of the sensory systems were similarly simple—and important in the history of psychology. At a time when most other mechanists were working only with reflexes and nerve transmission, Weber was looking at the entire sensory system: not just organs and the consequent nerve responses but the mind’s interpretation of them. Moreover, his were among psychology’s first true experiments; that is, he altered one variable at a time—in the two-point threshold test, the area of the body being tested—and observed how much change that caused in a second variable—the critical distance between the two compass points.
To recognize how remarkable it was of Weber to conduct such experiments in the early 1830s, consider the period. James Mill, without budging from his desk, was espousing simplistic associationism; Johann Friedrich Herbart, occupying Kant’s chair at Göttingen, was maintaining, as Kant had, that psychology could never be an experimental science; Johann Christoph Spurzheim, at the peak of his popularity, was assuring crowds of enthusiasts that phrenologists could read a person’s character from the shape of his skull.
Weber (1795–1878), born in Wittenberg in Saxony, was one of three brothers, all of whom became scientists of distinction and, at times, worked together. Wilhelm, a physicist, aided Weber in his research on touch; Eduard, a physiologist, discovered along with him the paradoxical effect of the vagus nerve, which, when stimulated, slows the heartbeat.25
Like many another psychological mechanist, Weber had had medical training and specialized in physiological and anatomical research. Early in his career, he became interested in determining the minimum tactile stimulation necessary to produce a sensation of touch in different parts of the body, but soon moved on to a more complicated and interesting question about perceptual sensitivity. Many years earlier, the Swiss mathematician Daniel Bernoulli had made a psychologically shrewd observation: a poor man who gains a franc feels far more enriched by it than does a wealthy man; the perception of gain produced by any given sum of money depends on one’s economic status. This led Weber to formulate an analogous hypothesis: The smallest difference we can perceive between two stimuli—two weights, for instance—is not an objective, fixed amount but is subjective and varies with the weights of the objects.
To test the hypothesis, Weber asked volunteers to heft first one small weight and then a second, and say which was heavier. Using a graduated series of weights, he was able to ascertain the smallest difference—the “just noticeable difference” (j.n.d.)—that his subjects could perceive. As he had correctly surmised, the j.n.d. was not a specific unvarying weight. The heavier the first weight, the greater the difference had to be before his subjects could perceive it, and the lighter the first weight, the greater their perceptual sensitivity. “The smallest perceptible difference,” he later reported, was “that between two weights standing approximately in the relation of 39 to 40: that is, one of which is about a fortieth heavier than the other.”26 If the first weight was an ounce, the j.n.d. of a second weight was a fortieth of an ounce; if ten ounces, a quarter of an ounce.
Weber went on to conduct similar experiments on other sensory systems, determining the j.n.d. between, among other things, the length of two lines, the temperatures