justnoticeable

Justnoticeable is a term used in psychophysics to describe the threshold at which a change in a stimulus becomes detectable. It most often refers to the just noticeable difference (JND), also known as the difference threshold—the smallest change in a stimulus that a given observer can reliably perceive. The concept applies across sensory modalities and is central to the study of perception.

In practice, the JND is defined as the change in stimulus intensity that yields an observer's detection at a specified criterion, typically 50 percent. For a given baseline stimulus, the required change is proportional to the baseline level according to Weber's law: ΔI / I = k, where ΔI is the JND, I is the initial stimulus, and k is a constant that depends on the sensory modality. Consequently, stronger baseline stimuli require larger changes to be noticed; for example, heavier weights require a larger increase to be perceived as different, while small brightness changes at low light levels may be easier to detect than the same absolute change at higher brightness.

Researchers measure JNDs using psychophysical methods such as staircase procedures and adaptive testing, constructing psychometric curves that relate stimulus intensity to detection probability. The concept informs the design of consumer products, displays, and audio systems, where perceptible changes must be calibrated to be detectable but not intrusive. It also relates to absolute thresholds, memory, and the decision processes involved in perception.