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Weber's law


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In psychophysics, the proposition that the smallest detectable difference in the magnitude of a stimulus (the Weber fraction) is proportional to the magnitude of the lesser stimulus. Thus, for example, the smallest difference in weight that can be detected is proportional to the original weight, so that most people can just barely feel the difference in weight between an object of 530 grams and one of 540 grams, or between 1,060 grams and 1,080 grams, the Weber fraction being 1/53 for weight discrimination according to experimental tests. The law is usually expressed by the equation (ΔI)/I = k, where ΔI is the difference threshold in physical magnitude, I is the magnitude of the lesser stimulus, and k is the Weber fraction, which is a constant for any type of sensation but that varies from one type of sensation to another. The law is accurate over most of the usable ranges for most sensory modalities (across 99.9 per cent of the range of visual brightness intensities that can be tested without damaging people's eyes, across 999,999-millionths of the usable range of auditory loudness intensities, and so on) but it breaks down at the extremes, and for auditory pitch perception the Weber fraction remains constant above 500 hertz but is slightly larger for lower pitches. Compare Fullerton-Cattell law. [Named after the German psychophysiologist Ernst Heinrich Weber (1795–1878) who formulated it in 1834]

Subjects: Psychology.


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