threshold
Summary
A threshold is the minimum stimulus intensity needed to produce a response. In sensation, the absolute threshold is the weakest stimulus detected about 50% of the time, and the difference threshold is the smallest change noticed about 50% of the time. Thresholds are statistical, so they shift with the person and context.
Detail
Meaning: In sensation and perception, a threshold marks where detection begins. The absolute threshold is the lowest stimulus intensity a person detects on 50% of trials. The difference threshold, or just noticeable difference (JND), is the smallest change between two stimuli that is noticed 50% of the time. Because detection rises gradually with intensity, the data form an S-shaped curve, not a sharp step. Stimuli below the absolute threshold are called subliminal.
Equation: Weber's law: ΔI/I = k, where ΔI is the JND (in the units of the stimulus), I is the original stimulus intensity (same units), and k is the Weber constant for that sense (dimensionless).
On the MCAT: Passages describe an experiment in which participants report whether they sensed a stimulus. You may be asked to locate the 50% point on a curve, or to predict that a heavier baseline weight needs a larger change to be noticed. Signal detection theory is also tested: expectation, motivation and fatigue change response bias, so the measured threshold is not purely sensory.
Common confusion: Threshold does not mean a fixed cutoff below which nothing is ever detected. It is a probability point. In neuroscience, threshold potential is a different idea: the membrane potential at which a neuron fires an action potential.
Sources
- Psychology (Myers)
- Psychology (Schacter, Gilbert, Wegner)
- Sensation and Perception (Goldstein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.