The smallest difference between two stimuli that can be reliably detected.The external world comes to us in the form of ever-changing sensations, and we are constantly called upon to react to slight differences in sounds, colors, shapes and sizes. We could not enjoy a great painting, drive a car safely, play a good game of tennis, or do any precision work if we were not able to make fine discriminations. It is important, therefore, to find out just how sensitive we are to differences— that is, what is the least difference in length, or loudness, or brightness that we can detect?This was probably the first question ever to be answered experimentally in the history of psychology. Early in the nineteenth century the German physicist, Ernst Weber, developed a technique for measuring the JND, or differential threshold, for different senses. In determining the IND for weight, for example, he would ask a subject to heft an unmarked weight of, say, 300 grams repeatedly, and then heft other weights until he found one that was just noticeably heavier than the given weight on 75 per cent of the trials. The result was then stated in fractional form: if the new weight was 306 grams, the “Weber fraction,” as it was later called, would be %()0, or %0. The experiment was continued with other weights, and he found that if the base weight was 600, the JND weight was 612; and if the base weight was 200, the JND weight was 204. When he determined the fraction in each case, he made the startling discovery that it was exactly the same in every instance—that is,Weber performed tests on different senses and made the even more astonishing discovery that there was a constant fraction for each of them. He found, for instance, that if he started with 60 lighted candles, it took one additional candle to make a noticeable difference. If he started with 120 candles; it took two, and so on. Here the fraction was %0 in every case. This finding led to Weber’s Law, first proposed in 1834: the smallest noticeable difference in perceived intensity is a constant fraction of the original stimulus.Weber’s Law has been found to hold, with some variation, for all senses— but only in the middle range of intensity. This limitation does not greatly diminish its value, since most of our experiences involve stimuli of medium intensity. The most important Weber fractions (or Weber constants) are: vision (for brightness of white light): %0; for kinesthesis (lifted weights): y50; for pain (heat on skin): %0; for hearing (middle pitch, moderate loudness): for pressure (on skin spots): y7; for smell (odor of India rubber): %; for taste (table salt): %. Many other fractions have been determined—for example, for visual detection of differences in length (%0o)—but fractions for each modality always remain about the same.These results shed interesting light on our sense experience. They indicate that vision is our most sensitive, and smell and taste our least sensitive modality. Moreover, these sensitivities seem to be roughly proportionate to the importance of the different sense organs, since we depend far more on vision than on taste or smell for survival. The only result that is surprising is our extremely high kinesthetic sensitivity. We use our muscle sense in learning the proper “reaches” on the typewriter or piano, and in acrobatics or dancing, but we seldom use this sense to its fullest advantage.Animal studies lend support to the evolutionary hypothesis. Weber fractions obtained from discrimination experiments show that fish are extremely sensitive to tastes, dogs and cats to smell, and bats to high-pitched sounds. Each of these senses is of high survival value for the particular animal.The Weber fractions given above are, of course, only averages, for there are wide individual differences in sensitivity. They would be far smaller for tea and wine tasters or perfume specialists than for the average person. Such individuals can detect, within an incredibly small margin of error whether a batch of a certain product meets a certain standard. Sensitivity of this kind is undoubtedly increased through training, but it may also be due in part to constitutional factors. At any rate it has considerable survival value of the economic kind, since these experts are usually in great demand.Weber’s Law has been applied in a variety of fields, including esthetics, consumer attitudes, and stock-market analysis. Artists can usually perceive differences in color values, shape and size that are imperceptible to the untrained eye, and the layman may actually perceive differences without being fully aware of them. Experiments show that if extremely slight changes are made in a work of art they are likely to make a noticeable difference in our reaction. (An equivalent change in a publicity poster would be completely overlooked.) In practical matters, a five- cent increase in the price of a newspaper will cause a violent reaction, while a one hundred or two hundred dollar increase in the price of a $40,000 house would cause little concern.Sometimes, however, a series of changes which are close to the differential threshold will go unnoticed until they produce a major shift—for example, fractional increases in the cost of living index may “sneak up” on us until we suddenly have an inflation on our hands. The same idea lies behind warnings against “creeping socialism.” It also applies to progressive disorders, such as deafness, schizophrenia and brain tumor, which grow worse at an extremely slow rate. In terms of the JND, each change is so small that it does not reach the threshold of perceptibility, and therefore the condition often goes undetected until it has reached an advanced state. The term “insidious onset” is used to describe this process.