just noticeable difference
Summary
The just noticeable difference (difference threshold) is the smallest change in a stimulus that a person detects half the time. By Weber's law, it is roughly a constant fraction of the starting intensity, so heavier or brighter baselines need bigger changes.
Detail
Meaning: The just noticeable difference (JND), or difference threshold, is the smallest change in a stimulus that a person detects 50% of the time. It differs from the absolute threshold, the minimum intensity needed to detect a stimulus at all. By Weber's law, the JND grows in proportion to the starting intensity.
Equation: ΔI / I = k, where ΔI is the JND (in the stimulus's own unit, such as kg for weight), I is the baseline intensity (same unit), and k is the Weber fraction for that sense (dimensionless).
On the MCAT: A passage gives k and a baseline and asks for the JND. If k = 0.02 for lifted weight, a 100 g baseline needs about a 2 g change to be noticed, and a 500 g baseline about 10 g. Other questions ask which comparison is easier to notice.
Common confusion: The JND is not a fixed amount. Adding one candle to a room with two is obvious; adding one to a room with 100 is not, because the ratio matters. Detection is also probabilistic, so the JND is a 50% point, not a sharp cutoff. Weber's law holds best at moderate intensities.
Sources
- Psychology (Myers)
- Sensation and Perception (Wolfe et al.)
- Sensation and Perception (Goldstein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.