E.H. Weber, a German psychologist, proposed the measure of JND (Just Noticeable Difference) within the year 1934. Weber gave the observation that the dimensions of the differential threshold are proportional to the intensity of the quality stimulus. This ratio is constant. The size of the differential threshold, a continuing ratio of the quality stimulus, is usually mentioned as Weber Fraction.

The Difference Threshold (also known as the “Just Noticeable Difference”) is the smallest change in stimulus intensity required to generate a noticeable change in sensory perception.

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 Weber’s Law, more simply stated, says that the size of the just noticeable difference (i.e., delta I) is a constant proportion of the original stimulus value.

This example will make this law more clear. Suppose that you can just tell the difference between 100 and 104 grams then you will be able to just distinguish between 200 and 208 grams, 400 and 416 grams and so forth.

Many studies were conducted within the past to ascertain whether Weber’s law holds for all of the sensory modalities. It was verified in most of the cases except a few where the nervous system geared to notice relative differences rather than absolute ones.

This law allows us to match the sensitivities of various sensory modalities. Suppose you would like to understand, whether the attention is more sensitive than the ear. This can be seen using Weber’s law. If Weber’s ratio is small, the discriminative power of the sense modality is great and vice-versa

This law helps in understanding the salient features of various sensory modalities. It has been found out, using this law, that humans are keen on discriminating brightness over loudness, Weber’s fraction being 1/62 and 1/11 respectively

A modified version of Weber’s law is as follows:

where a is a constant, usually small that represents a baseline level of activity that must be surpassed.

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