 in the branch of experimental psychology focused on sensation and perception, psychophysics, a just-noticeable difference or jnd is the amount something must be changed in order for a difference to be noticeable, detectable at least half the time. weber's law of just noticeable difference, university of south dakota: http://apps.usd.edu/coglab/weberslaw.htmlwhat does jnd.org mean?, http://jnd.org/about.html#jnd it is also known as the difference limen, differential threshold, or least perceptible difference. ==explanation== for many sensory modalities, over a wide range of stimulus magnitudes sufficiently far from the upper and lower limits of perception, the 'jnd' is a fixed proportion of the reference sensory level, and so the ratio of the jnd/reference is roughly constant (that is the jnd is a constant proportion/percentage of the reference level). measured in physical units, we have: : \frac {\delta i} {i} = k, where i\! is the original intensity of the particular stimulation, \delta i\! is the addition to it required for the change to be perceived (the jnd), and k is a constant. this rule was first discovered by ernst heinrich weber (1795â1878), an anatomist and physiologist, in experiments on the thresholds of perception of lifted weights. a theoretical rationale (not universally accepted) was subsequently provided by gustav fechner, so the rule is therefore known either as the weber law or as the weberâfechner law; the constant k is called the weber constant. it is true, at least to a good approximation, of many but not all sensory dimensions, for example the brightness of lights, and the intensity and the pitch of sounds. it is not true, however, of the wavelength of light. stanley smith stevens argued that it would hold only for what he called prothetic sensory continua, where change of input takes the form of increase in intensity or something obviously analogous; it would not hold for metathetic continua, where change of input produces a qualitative rather than a quantitative change of the percept. stevens developed his own law, called stevens' power law, that raises the stimulus to a constant power while, like weber, also multiplying it by a constant factor in order to achieve the perceived stimulus. the jnd is a statistical, rather than an exact quantity: from trial to trial, the difference that a given person notices will vary somewhat, and it is therefore necessary to conduct many trials in order to determine the threshold. the jnd usually reported is
Perception Psychophysics