JNDs

JNDs, or just noticeable differences, refer to the smallest detectable change in a stimulus that a person can notice under specified conditions. In psychophysics this value is a differential threshold, often defined as the difference between two stimuli that can be detected about half of the time. The JND depends on the sensory modality and context and can vary with factors such as adaptation, attention, and task design.

Weber's law describes a common relationship between stimulus intensity and the size of the JND: the ratio of the JND to the baseline intensity is approximately constant for many senses, especially over moderate ranges. This implies that larger baseline intensities require larger absolute changes to be noticed. However, Weber's law is not universal; at very low or very high intensities, or with different tasks, the relationship can deviate. Other models, such as Stevens' power law, offer alternative descriptions of perceived magnitude and tell a related story about how changes are judged.

Methods to measure JNDs include the method of limits, the method of constant stimuli, and staircase procedures, often using two-alternative forced-choice or yes-no tasks. Researchers report JNDs for modalities such as vision (contrast or luminance), audition (sound level), touch (pressure or weight), and sometimes taste or smell. These values help characterize sensory systems and guide design.

Applications of JND data span product design, calibration of displays and audio equipment, clinical assessment of sensory function, and basic perception research. Limitations include context effects, learning, fatigue, and individual differences that can affect JND values across conditions.