In
psychophysicsPsychophysics is a discipline within psychology that investigates the relationship between physical stimuli and their subjective correlates, or percepts...
, a
just noticeable difference, customarily abbreviated with lowercase letters as
jnd, is the smallest detectable difference between a starting and secondary level of a particular sensory stimulus. It is also known as the
difference limen or the
differential threshold.
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).
In
psychophysicsPsychophysics is a discipline within psychology that investigates the relationship between physical stimuli and their subjective correlates, or percepts...
, a
just noticeable difference, customarily abbreviated with lowercase letters as
jnd, is the smallest detectable difference between a starting and secondary level of a particular sensory stimulus. It is also known as the
difference limen or the
differential threshold.
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
where is the original intensity of stimulation, is the addition to it required for the difference to be perceived (the
jnd), and
k is a constant. This rule was first discovered by
Ernst Heinrich WeberErnst Heinrich Weber was a German physician who is considered a founder of experimental psychology.Weber studied medicine at Wittenberg University...
, in experiments on the thresholds of perception of lifted weights. A theoretical rationale (not universally accepted) was subsequently provided by
Gustav FechnerGustav Theodor Fechner , was a German experimental psychologist. An early pioneer in experimental psychology and founder of psychophysics, he inspired many 20th century scientists and philosophers...
, so the rule is therefore known either as the Weber Law or as the
Weber–Fechner lawThe Weber–Fechner law attempts to describe the relationship between the physical magnitudes of stimuli and the perceived intensity of the stimuli. Ernst Heinrich Weber was one of the first people to approach the study of the human response to a physical stimulus in a quantitative fashion...
; 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
pitchPitch represents the perceived fundamental frequency of a sound. It is one of the three major auditory attributes of sounds along with loudness and timbre. When the actual fundamental frequency can be precisely determined through physical measurement, it may differ from the perceived pitch because...
of sounds. It is not true, however, of the wavelength of light.
Stanley Smith StevensStanley Smith Stevens was an American psychologist who founded Harvard's Psycho-Acoustic Laboratory and is credited with the introduction of Stevens' power law. Stevens authored a milestone textbook, the 1400+ page "Handbook of Experimental Psychology" . He was also one of the founding organizers...
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.
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 the difference that a person notices on 50% of trials. If a different proportion is used, this should be included in the description—for example one might report the value of the "75% jnd".
Modern approaches to psychophysics, for example signal detection theory, imply that the observed jnd, even in this statistical sense, is not an absolute quantity, but will depend on situational and motivational as well as perceptual factors.
The jnd formula has an objective interpretation (implied at the start of this entry) as the disparity between levels of the presented stimulus that is detected on 50% of occasions by a particular observed response (Torgerson, 1958), rather than what is subjectively "noticed" or as a difference in magnitudes of consciously experienced 'sensations.' This 50%-discriminated disparity can be used as a universal unit of measurement of the psychological distance of the level of a feature in an object or situation and an internal standard of comparison in memory, such as the 'template' for a category or the 'norm' of recognition (Booth & Freeman, 1993). The jnd-scaled distances from norm can be combined among observed and inferred psychophysical functions to generate diagnostics among hypothesised information-transforming (mental) processes mediating observed quantitative judgments (Richardson & Booth, 1993).
See also
- Absolute threshold
In neuroscience and psychophysics, an absolute threshold is the smallest detectable level of a stimulus. For example, in an experiment on sound detection, researchers may present a sound with varying levels of volume...
- Limen
In physiology, psychology, or psychophysics, a limen or a liminal point is a threshold of a physiological or psychological response.Liminal, as an adjective, means situated at a sensory threshold, hence barely perceptible.-See also:...
- Minimal negation operator
In logic and mathematics, the minimal negation operator is a multigrade operator where each is a k-ary boolean function defined in such a way that if and only if exactly one of the arguments is 0....
- Psychometric function
A psychometric function describes the relationship between a parameter of a physical stimulus and the responses of a person who has to decide about a certain aspect of that stimulus. The psychometric function usually resembles a sigmoid function with the percentage of correct responses displayed...
- Sensor resolution
- Visual perception
Visual perception is the ability to interpret information and surroundings from visible light reaching the eye. The resulting perception is also known as eyesight, sight or vision...