Difference Threshold

What is a difference threshold?

A difference threshold, or just noticeable difference (JND), is the stimulus difference corresponding to a specified level of discrimination performance. It is an estimate made with a defined task and criterion.

Measuring a just noticeable difference

Levitt (1971) shows how adaptive procedures can target different points on a psychometric function. A threshold is not universally “50% accuracy”: with two equally likely alternatives, 50% correct is chance performance.

Examples of difference thresholds

A threshold experiment can compare brightness, sound level, pressure, or sweetness. The question is how much the stimulus must change before the observer can discriminate the alternatives at the chosen performance level.

Brightness comparison

For example, a brightness-discrimination experiment must specify which stimuli were compared, how observers responded and which performance level counted as threshold. Without these details, threshold values from different procedures may not be comparable. A just noticeable difference is not a fixed quantity that applies to every observer or stimulus.

Sound comparison

In a hypothetical sound task, a listener hears two tones of the same pitch and duration, then identifies which was louder and which quieter. The experiment varies the difference in sound level across trials.

Taste comparison

A taste task could instead present two otherwise matched drinks with different sugar concentrations and ask which is sweeter. Neither example supplies a universal decibel change or amount of sugar that everyone will detect.

Interpreting the comparisons

Threshold results need to be reported with the conditions under which they were measured. Background sound, how samples are presented, and whether the observer is attending to the comparison can affect performance. The estimate concerns that task under those conditions; a missed difference during a distraction is not enough to establish a change in sensory sensitivity.

Difference threshold and Weber’s law

Weber’s law describes the difference threshold as a proportion of the starting stimulus intensity. As an OpenStax introduction explains, the same physical increment can be more noticeable against a weaker background. It is a useful relationship, not a guarantee that one fixed ratio describes every stimulus and condition.

Difference threshold vs. absolute threshold

An absolute threshold concerns detecting that a stimulus is present. A difference threshold concerns detecting a change or distinguishing stimuli. Both require a stated procedure and performance criterion.

Weber’s relationship can be written as ΔI/I ≈ k: the detectable change (ΔI) is approximately a constant fraction (k) of the starting intensity (I), within conditions where the relationship holds.