Just-Noticeable Difference (JND)

1834

Ernst Weber (1795–1878), Gustav Fechner (1801–1878)

Many scholars in the eighteenth and nineteenth centuries believed that it was not possible to scientifically study the mind in part because mental processes could not described in mathematical terms. German physiologist Ernst Weber’s study of touch, first published in 1834, led to research on the human ability to perceive differences in sensation. Research on detecting variations in weight and temperature led him to conclude that such distinctions could be described lawfully, or mathematically; he found that the second, comparative weight must be heavier by a proportion of the original, not by an absolute amount, in order to make an accurate discrimination. In the case of each sensory modality—touch, light, sound, and so on—the amount necessary to detect a difference was proportional to the original stimulus. Weber calculated the proportions and established what he called the just-noticeable difference, or JND, for each sensory discrimination. For example, the just-noticeable difference for weight discrimination was always an amount equal to one-thirtieth of the heavier weight.

In 1850, German philosopher and psychologist Gustav Fechner empirically demonstrated a lawful relationship between the physical and psychological worlds. His experimental approach to this problem came to be known as psychophysics. Building on Weber’s work on the JND, Fechner reasoned that if the JND was a constant fraction for each of the senses, then it could stand as a unit of measurement representing the subjectively experienced intensity of a stimulus. By starting with the lowest intensity of a stimulus that could be perceived and then plotting each successive JND, Fechner discovered a mathematical law that allowed him to describe and predict the relationship between the physical world and our subjective experience of that world. Weber and Fechner proved that psychological phenomena could be described quantitatively.

SEE ALSO Sensory Physiology (1867), Experimental Psychology (1874), Mental Chronometry (1879)

The research of Gustav Fechner

Conclusively showed that the JND is represented by a logarithmic rather than a linear function, as shown in this image from Gustav Theodor Fechner by Johannes Emil Kuntze, 1892.