Weber's law and thresholds | Processing the Environment | MCAT | Khan Academy

Main Ideas

Just Noticeable Difference

The just noticeable difference (JND) is the smallest change in a stimulus that a person can reliably notice.

For example, adding 0.05 pounds to a 2-pound weight may be too subtle to detect, while adding 0.2 pounds may be noticeable. The JND depends on the starting weight: a change noticeable for a lighter weight may not be noticeable for a heavier one.

Weber’s Law

The detectable change tends to grow in proportion to the stimulus’s starting intensity.

Let I represent the background intensity, or original stimulus, and ΔI represent the smallest detectable increase. Weber’s law is expressed as:

ΔI / I = K

Here, K, the Weber fraction, is approximately constant for a given person and type of stimulus.

In the weight example, the illustrative ratios are 0.2 / 2 = 0.1 and 0.5 / 5 = 0.1. These are examples; actual thresholds may vary.

Applications Beyond Weight

The same general principle can apply to sound. In a quiet room, a whisper may be heard because the background sound level is low. At a loud concert, a person must speak much louder to be heard because the background intensity is higher.

Graphical Implication

Rearranging Weber’s law gives ΔI = K × I. This predicts a linear relationship: as background intensity increases, the difference threshold—the change needed to notice a difference—increases.

Limitations

Weber’s law is a useful general rule, not an exact rule for every situation or stimulus. Real-world versions may include an extra constant to account for baseline sensory activity.

Example of the Concept

1. Start with a stimulus, such as a 2-pound weight.
2. Make a small change and determine whether it is noticed.
3. Increase the change until it becomes noticeable; this is the JND.
4. Repeat with a stronger starting stimulus, such as a 5-pound weight.
5. Compare the JNDs relative to their starting intensities. Weber’s law predicts that this ratio will be roughly constant.

Speakers and Sources

• Speaker: One unnamed voiceover narrator.
• Source or figure discussed: Weber, whose observation is dated to 1834 in the subtitles.