1 Psychology and human development1.1 Sensation and perception
1.1.1 Sensory thresholds and coding
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Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
Detecting a stimulus
Absolute threshold: the stimulus intensity that a person detects on half of repeated presentations, under specified conditions.
Difference threshold: the smallest difference between two stimuli that a person can detect, also called the just noticeable difference.
Signal detection theory separates sensory sensitivity (how well the person picks up the signal) from the decision criterion (how ready they are to say a signal is there). The criterion shifts with expectation, motivation and the cost of errors.
Signal | Person reports present | Person reports absent |
|---|---|---|
Present | Hit | Miss |
Absent | False alarm | Correct rejection |
Coding and adaptation
Transduction: receptors convert stimulus energy into receptor activity. Which pathway is activated contributes to the modality the person perceives.
Intensity coding: a stronger stimulus makes neurones fire faster and recruits more responsive neurones. The action potentials do not get larger.
Sensory adaptation: responses fall during sustained stimulation. This differs from habituation, which is learning to respond less to repeated stimulation.
Weber's law
Weber's law says that the just noticeable difference is roughly a constant fraction of the starting intensity. This only holds within a limited operating range.
Illustrative weights, not patient data:
Quantity | Value |
|---|---|
Starting weight | 100 g |
Smallest detectable increase | 2 g |
New starting weight | 300 g |
Work out the Weber fraction: 2 / 100 = 0.02.
Apply the same fraction to the new weight: 300 × 0.02 = 6 g.
The predicted detectable change is 6 g. The law is an approximation, and fits least well near the limits of the senses.