Weber's Law

Background

Weber's Law (Ernst Heinrich Weber, 1834) states that the just-noticeable difference (JND) between two stimuli is proportional to the magnitude of the stimuli:

ΔI / I = k

where ΔI is the JND, I is the standard intensity, and the constant k is the Weber fraction.

This means larger stimuli need larger differences to be discriminable. A 100 px line needs a larger pixel difference to tell apart than a 50 px line.

Method of Constant Stimuli

Comparison stimuli at fixed increments around the standard are presented repeatedly. The JND is estimated from the psychometric function — the S-shaped curve relating stimulus difference to proportion of “comparison longer” responses.

References

Weber, E.H. (1834). De pulsu, resorptione, auditu et tactu. Leipzig: Koehler.
Gescheider, G.A. (1997). Psychophysics: The Fundamentals (3rd ed.). Lawrence Erlbaum.

Two horizontal lines will appear side by side. Judge which line is LONGER.

Press ← if the LEFT line is longer, → if the RIGHT line is longer.

The differences are subtle — respond with your best judgment.

There are 84 trials total (~6 minutes).

Weber's Law

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