The difference threshold often referred to as just noticeable difference (JND), is the minimum amount of change required to be detected in a stimulus. The concept was first proposed by German psychologist Ernst Heinrich Weber (1795-1878).
- The difference threshold is the minimum difference in the intensity of two stimuli necessary to detect they are different. For example, two lights may be illuminated at the same time. The difference threshold is reached when an observer can tell that one is brighter than the other.
- Often defined as the difference for which the percentage of correct discrimination is 75%, though other percentages are sometimes used.
- The difference threshold allows experimental psychologists to understand the relationship between the physical intensity of a stimulus and people’s or animals’ perception of that stimulus. The smaller the difference threshold, the more sensitive someone is to changes in the quantity described by it.
- Difference thresholds are differentiated from absolute thresholds in that the former refers to the smallest difference that can be detected between two stimuli, while the latter refers to the minimum amount of change required to detect a stimulus.
In This Article
Development of the Concept
The difference threshold, otherwise known as the just notable difference or the difference limen, is the smallest difference between two stimuli that can be consistently and accurately detected in experimental trials 50% of the time.
This concept describes the minimum amount by which a stimulus” intensity must be changed in order to produce a noticeable variation in the sensory experience of a participant in an experiment.
Ernst Heinrich Weber (1795-1878), a notable 19th-century experimental psychologist, observed that the difference threshold was related to the strength of the stimulus being used in a mathematical way.
The relationship between these two is called Weber’s law.
In other words, Weber’s law says that the size of the difference threshold — delta I — is in direct proportion to the original value of the stimulus.
For example, consider a scenario where a researcher gives an observer two spots of light, each with an intensity of 100 units.
Then the researcher asked the observer to increase the intensity of one of these spots until it was just noticeably brighter than the others.
If the brightness needed to yield this difference threshold was 110, then the observer’s difference threshold would be ten units.
Thus, the change in I needed to spot the difference over the original brightness is 0.1.
Using Weber’s law, the person conducting the experiment could now predict the size of the observer’s difference threshold for a light spot of any other intensity, so long as it were neither extremely dim nor extremely bright.
That is to say, if the Weber fraction for the change in brightness needed to be able to detect a change is a constant proportion equal to 0.1, then the observer would notice a change in brightness when a 1000-unit bright light became an 1100-unit light, or a 10000 unit bright light became an 11000 unit bright light (Ross, 1995).
This law can be applied to any of the senses. The difference threshold can apply across, say, brightness, loudness, mass, line length, and so on.
Although the size of the weber fraction varies across the medium that is being measured and groom observer to observer, the relationship tends to stay linear (Ross, 1995).
This mathematical relationship between what people perceive and some physical quantity is a feature of a field called psychophysics.
Psychophysics is a branch of psychology concerned with the relationships between physical stimuli and mental phenomena (Gescheider, 2013).
Say that someone held two weights in their hands and could make them heavier and heavier until they noticed that the mass of one was greater than another.
For example, someone may notice that a 100g and 125g mass differed at least 50% of the time.
According to Weber’s law, then, the difference threshold of this observer sensing the mass of these weights is 125g/100g or .25.
This means if the observer were holding a 1 or 10kg object in one hand, the object in the other hand would need to have a mass of 1.25 or 12.5kg, respectively, to feel heavier.
Similarly, the object would need a mass of .75 or 7.5kg to feel lighter than the 1kg object for that observer.
Similarly, consider a participant in an experiment who is asked to change the volume of a piece of music until they are able to hear that it is at a different volume.
If the original volume of the piece were 200 units, and the person needed to increase or reduce the volume by 100 units to hear a difference, then the difference threshold for that person hearing that piece would be 0.5.
So, if the music were playing at 30 or 500 units, they would need to change the volume to 45 or 750 units to hear a difference, respectively.
Taste and Smell
The difference threshold can also apply to senses that are not as commonly quantified as mass and sound, such as taste and perfume.
To imagine the first of these experiments, consider a scenario where someone is asked to eat saltier and saltier bowls of soup until they can perceive a difference in taste.
In a similar vein to before, the mass of salt needed to create a perceived difference in taste would be linear to the amount of salt originally in the dish.
In the case of smell, someone could be asked to discern the minimum difference in the quantity of perfume needed to perceive a difference in something’s smell in a similar manner, with a similar relationship.
To further this idea of things that normally do not seem quantifiable, being quantified through the concept of the difference threshold, consider the differences between colors.
A scientist may be able to measure the differences between colored paint by the amount of each primary color it is composed of.
When testing a participant on their color perception, they may ask them to mix increasing, small increments of blue dye into a tub of red water until they notice a color difference.
As with the preceding examples, this difference threshold is constant.
So, if it took five drops of blue dye to perceptively change the color of a tub of water with 30 drops of red dye, the difference threshold for this color change would be 1/6.
Thus, if someone were to replicate the same experiment on a tub containing 90 drops of red dye, it would take about 18 drops of blue dye to perceptively change the color of the water in the eyes of the observer about 50% of the time.
Absolute vs. Difference Threshold
One concept that is often confused with the difference threshold is the absolute threshold.
While the difference threshold involves an observer’s ability to detect a difference in stimulation levels, the absolute threshold refers to the smallest detectable level of stimulation.
Hence the word absolute.
For example, the absolute threshold for sound would be the absolutely lowest volume that someone can hear or detect.
Meanwhile, the absolute difference for sound would refer to the smallest difference in volume that that person could sense.
Another way that the absolute threshold is different from the difference threshold is that the absolute threshold is specific to each type of stimulus being measured. In contrast, the difference threshold can vary depending on the starting point of the stimulus.
For example, the absolute threshold for light might be the lowest level of brightness that someone could detect in a room, while the absolute threshold for sound might refer to the faintest noise that someone could hear.
However, the difference threshold for light would be the smallest increase or decrease in brightness that someone could notice, regardless of how initially bright or dim the room is.
Similarly, the difference threshold for sound would remain the same for a person, regardless of how loud or soft the noise they heard originally is.
The difference threshold has many implications for experimental psychology.
Namely, the concept of the difference threshold helps psychologists to understand why people do or do not sense the progression they are making as they move through an experiment.
For example, say that a researcher is conducting a study on people’s ability to taste different flavors of ice cream. In their experiment, the researcher might give each participant cups of ice cream with varying amounts of sugar.
The researcher would then have each participant rate the sweetness of each cup of ice cream.
If, after eating the ice cream, the participants discerned that one cup of ice cream was sweeter than another, the researcher could conclude that the difference in the amount of sugar in these cups was above the just noticeable difference.
However, if there was no significant, consistent difference between the ratings for the two flavors, the researchers would need to consider the possibility that the difference in the amount of sugar was too small to be noticed by the participants — that is, to surpass the difference threshold.
The difference threshold can also explain, for example, why someone may not be able to notice a gradual change in their weight, even though the change is happening over time.
The concept of a difference threshold can also be useful in marketing and advertising. For example, companies may take into consideration the difference threshold for brightness when choosing how to design their product packaging.
A company might want its products to stand out on store shelves, but they also do not want its products to be so bright that they become annoying or overwhelming.
By choosing a minimal discernable amount of brightness for their packaging, the company’s product can stand out from the display while minimizing the effects of visual irritation (Vojtko, 2014).
Gescheider, G. A. (2013). Psychophysics: the fundamentals . Psychology Press.
Helen, E. R., David, J. M., Ross, H. E., & Murray, D. J. (2018). EH Weber on the tactile senses. Psychology Press.
Miiller J, 1833-1840/1838-1842 Handbuch der Physiologie des Menschen (1833-1840,
Coblenz: Holscher), translated in English by W Baly Elements of Physiology Vols 1 and 2
(1838 -1842, London: Taylor and Walton)
Ross, H. E. (1995). Weber then and now. Perception, 24 (6), 599-602.
Vojtko, V. (2014). Rethinking the concept of just noticeable difference in online marketing. Acta Informatica Pragensia, 3 (2), 204-218.