Absolute threshold
Also known as: absolute threshold of sensation, detection threshold
The absolute threshold is the smallest amount of a stimulus that a person can detect 50% of the time. It marks the lower boundary of sensation for a given sense — anything weaker generally goes unnoticed.
Psychophysics studies the relationship between physical stimuli and the sensations they produce, and the absolute threshold is one of its foundational measures. It is defined statistically rather than absolutely: because detection varies from trial to trial, the threshold is set at the intensity a person reports detecting on half of the presentations.
Each sense has its own threshold, and the classic textbook illustrations convey the scale involved — a candle flame visible from many miles away on a clear dark night, the tick of a watch in an otherwise silent room, a drop of perfume diffused through a large space. In a laboratory, the value is found by presenting stimuli at gradually increasing or decreasing intensities and recording when the participant reports detection.
The absolute threshold is easily confused with the difference threshold, or just-noticeable difference, which is the smallest change between two stimuli that a person can detect. Weber's law describes that second quantity, holding that the just-noticeable difference is a constant proportion of the original stimulus rather than a fixed amount. Signal detection theory adds a further refinement: detection is not purely sensory but also depends on expectation, motivation, fatigue, and the cost of a false alarm, so the same physical stimulus may be detected in one context and missed in another. Stimuli below the absolute threshold are described as subliminal.
Both the MCAT and AP Psychology test the absolute threshold. The MCAT covers it in the Psychological, Social, and Biological Foundations section under sensory perception, usually alongside signal detection theory and Weber's law. AP Psychology covers it in the biological bases of behavior unit, where questions commonly ask you to distinguish absolute threshold from difference threshold.
Key takeaways
- The absolute threshold is the minimum stimulus intensity detectable 50% of the time.
- Each sensory modality has its own absolute threshold.
- It differs from the difference threshold, which is the smallest detectable change between two stimuli.
- Signal detection theory shows that expectation, motivation, and context shift detection independently of stimulus intensity.