Key Takeaways
- The just noticeable difference (JND) is the smallest change in a stimulus that a person can detect 50% of the time.
- JND applies to all five senses and involves detecting differences in things like brightness and sound.
- Ernst Weber first described the JND, and it is important in understanding how we perceive changes in our environment.
Sometimes the difference between two things is glaring, like with a red car and a blue sedan. Other times it is barely noticeable, such as whether a toothbrush has 2,500 or 10,000 bristles. The amount of difference required for us to recognize that two items aren’t the same is known as the just noticeable difference (JND).
Here we do a deeper dive into what the just noticeable difference is, why it’s important, and how it was discovered. We also share how JND is measured, a few examples in everyday life, and how it is used to impact our buying decisions.
What is the Just Noticeable Difference?
The just noticeable difference, also known as the difference threshold, is the minimum level of stimulation that a person can detect 50% of the time. For example, if we were asked to hold two objects of different weights, the just noticeable difference would be the minimum weight difference between the two that we could sense half of the time.
Just noticeable difference is part of a field of study known as psychophysics. Psychophysics examines how physical stimuli in the environment affect and interact with mental processes. While scientists used to focus on measuring objective data, psychophysics allows them to measure subjective experiences as data.
The just noticeable difference applies to all five senses: touch, taste, smell, hearing, and sight. It can also apply to things such as brightness, sweetness, weight, pressure, and noisiness, among other factors.
Difference Threshold vs. Absolute Threshold
The just noticeable difference is not the same as the absolute threshold. While the difference threshold between two stimuli involves detecting differences in stimulation levels, the absolute threshold refers to the smallest detectable level of stimulation.
The absolute threshold for sound, for example, would be the lowest volume level that a person could detect. The just noticeable difference would be the smallest change in volume that a person could sense.
Importance of JND
Difference thresholds can help researchers understand how we respond to our environment. It also gives insight into where our senses are subject to error, such as in not being able to detect the differences between two things.
The just noticeable difference also impacts our preferences. For example, it can affect whether we prefer to drink skim, low-fat, or full-fat milk as each is slightly different in taste and visual appearance.
History of the JND Concept
The difference threshold was first described by Ernst Weber, a 19th-century physiologist and experimental psychologist, and later expanded upon by his student, psychologist Gustav Fechner. Weber’s Law, also sometimes known as the Weber-Fechner Law, suggests that the just noticeable difference is proportionate to a variable’s magnitude.
Weber studied the just noticeable difference using weights to see when subjects could detect when the weight was different. He discovered that we’re better at detecting relative differences. In other words, when the weights got heavier, subjects needed a bigger difference between them to notice they were different.
How Just Noticeable Difference Is Measured
The JND is usually determined by conducting multiple trials and then using the smallest levels that participants could detect at least 50% of the time. To measure sensory information and difference thresholds, scientists rely on responses from study participants.
The measurements scientists take depend on the sensory information being received. For instance, when measuring heat, researchers may use joules. When measuring sound, they may use decibels instead.
Imagine that you present a sound to a subject and then slowly increase the decibel levels. You increase the sound level by 7 decibels before the participant notices that the volume is louder. In this case, the just noticeable difference is 7 decibels. Using this information, you can use Weber’s law to predict the just noticeable difference for other sound levels.
Neural Activity
Our brains receive sensory input (such as light, sound, or taste), and our sensory organs convert this input into electrical signals. Therefore, measuring neural activity is another way in which researchers may determine how much (and what type of) sensory input a person is receiving.
Looking at a person’s brain activation patterns seems to be a good way of measuring the just noticeable difference. For example, in one study, researchers found they could distinguish whether or not someone was touching something sticky based on their neural activity.
Stimulus Intensity
The intensity level of the stimulus can also play a role in how much people notice any changes. If a light is very dim, people may be more likely to notice smaller changes in intensity than they would if those same changes were made to brighter light.
For example, imagine being in a dark movie theater. When the house lights slowly start to turn on, this small change in the light intensity is immediately noticeable. However, upon leaving the theater and heading outside where the sun is shining brightly, these same changes in light intensity might be less noticeable since the stimulus level is much higher.
Does Just Noticeable Difference Change?
The just noticeable difference does change throughout the day. The amount and intensity of other stimuli experienced beforehand also affect how we perceive additional stimuli.
Examples of the Just Noticeable Difference
The following are everyday examples in which the just noticeable difference can be observed.
Touch
Imagine that during a psychology experiment, researchers ask all participants to hold two small amounts of sand in each hand. They slowly add tiny amounts of sand to one hand and ask when the subjects notice that one hand feels heavier than the other. The smallest weight difference that can be detected at least half the time is the just noticeable difference.
Sound
Another example of the just noticeable difference is watching television with someone else, but the volume is too low for us to hear. We ask them to turn it up and they press the volume button twice, but we still can’t tell a difference. They then press the button two more times before we can notice an increase in volume.
Taste
Maybe we drink our coffee with sugar in it. When staying at a friend’s house, our friend makes the coffee for us but we don’t taste the sweetness. They likely didn’t add enough sugar for us to meet our threshold and, thereby, notice the difference.
Sight
We dye our hair but afterward, the color still looks the same as it did before. This may be because we dyed it a similar shade to what we already had, and the color isn’t above the difference threshold.
Uses for the Just Noticeable Difference
The JND is often studied in product development. For instance, when manufacturing cell phones, companies measure the JND of sound so there are noticeable increases when the volume is raised on the device. Companies that create food products can also study the JND to understand consumer preferences.
Companies may take advantage of the JND in several ways. For instance, a company may determine the JND of a price point. This enables it to increase the price of a product so slightly that consumers likely won’t notice it.
Companies can also use the just noticeable difference to reduce the size of their packages, such as boxes of pasta or cans of corn. They’ll knowingly decrease the size by an amount that is below the difference threshold, so people don’t realize that their favorite products have gotten smaller.
Downsizing packaging saves companies money, but it’s often seen as an unethical practice—”tricking” the consumer into buying their favorite products at the same (or higher) prices, so they’re getting less without knowing it.
