jND

Just noticeable difference (JND), also called the differential threshold, is the minimum magnitude of change in a stimulus that a person can detect under specified conditions. It is typically defined as the smallest change that can be observed in at least a given proportion of trials, often 50%.

In many sensory modalities, JND scales with the baseline stimulus. This relationship is described by Weber's law: the ratio of the JND to the initial stimulus magnitude is approximately constant for a given sense and range. The constant of proportionality is known as the Weber fraction, and it varies by modality (for example, vision, hearing, or touch) and by the range of intensities tested.

JND is measured in psychophysics experiments using methods such as limits, constant stimuli, and staircase procedures. These methods estimate the point at which a change in stimulus is reliably detected, producing a psychometric function that characterizes detection probability across stimulus differences.

Several factors influence JND. It is not fixed and can depend on the sensory modality, baseline intensity, adaptation to stimuli, attention, task demands, and fatigue. Context and previous experience can also modify perceptual thresholds.

Applications of the JND concept include sensory testing for products and interfaces, calibration of audio and visual displays, and research in perception and neuroscience. Historically, the idea arose from 19th-century work by Weber and was later formalized by Fechner in the field of psychophysics.