Absolute Threshold vs Difference Threshold (JND) with Examples

The absolute threshold is the minimum stimulus intensity you can detect. The difference threshold, also called the just noticeable difference (JND), is the minimum change between two stimuli you can detect. Both are defined as the value detected 50% of the time.

Comparison

Weber's law

The JND is not a fixed amount. It is a constant fraction of the starting intensity:

ΔI / I = k

where I is the starting intensity, ΔI is the JND and k is the Weber fraction for that sense.

JND example

Suppose the Weber fraction for lifted weight is 0.02 (an illustrative value).

• Starting at 100 g, the JND is 0.02 × 100 = 2 g. You notice 102 g.
• Starting at 500 g, the JND is 0.02 × 500 = 10 g. Adding 2 g goes unnoticed.

The same logic explains why one extra candle changes a dark room but not a stadium.

Why thresholds are not fixed

Detection also depends on the observer's expectations and motivation. Signal detection theory separates sensitivity from response bias, so the 50% point shifts with context. Sensory adaptation raises effective thresholds when a stimulus stays constant.

Common traps

• Treating the JND as constant in absolute units. It scales with the starting intensity.
• Calling the absolute threshold a hard cutoff. It is a probability point.
• Mixing up the two thresholds. Absolute is stimulus vs nothing; difference is stimulus vs stimulus.

One-minute recap

• Absolute threshold: weakest detectable stimulus.
• Difference threshold (JND): smallest detectable change.
• Weber's law: JND = k × starting intensity.