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Behavioural Science & Sociocultural Psychiatry · Paper A

Psychophysics and perceptual thresholds

The idea

Psychophysics matters because it turns 'how strong does a stimulus need to be to notice it' into something precisely measurable, rather than just a vague impression. The absolute threshold is the smallest intensity of a stimulus a person can detect at all, e.g. the quietest sound someone can just barely hear. For example, imagine an audiologist testing hearing by playing tones at gradually increasing volume until the patient first reports hearing a sound, that exact volume is their absolute threshold.

How it works

The difference threshold, or just-noticeable difference (JND), is the smallest detectable difference between two stimuli, e.g. the smallest weight increase a person can feel when an object is made heavier. Weber's law states the JND is a constant PROPORTION of the original stimulus, not a fixed amount, so a much heavier starting weight needs a much bigger absolute increase before the difference is noticeable. Signal detection theory separates two things that get confused in simple threshold testing: sensitivity (a person's actual ability to detect a stimulus) and response bias (how willing they are to say 'yes, I detected it' when unsure). Take two radiologists with identical actual visual sensitivity to a subtle scan finding: the one who is cautious and rarely says 'abnormal' unless certain will miss more true abnormalities than one who says 'abnormal' more readily. In practice, signal detection theory means a test's raw hit rate cannot be interpreted alone, since it always reflects a mix of true sensitivity and personal response bias together.

How to spot it

A psychophysicist extends Weber's work by proposing that sensation grows in proportion to the logarithm of stimulus intensity. Whose law is this?
In signal detection theory, a measure separates a person's true sensitivity from their willingness to say a signal is present. Which measure captures sensitivity?
A radiologist under pressure to avoid missing any cancer begins reporting more scans as abnormal. In signal detection terms, what has changed?
A researcher terms the smallest detectable change in a stimulus the just noticeable difference. According to Weber's law, how does this quantity behave as the baseline stimulus grows?

Tell it apart

Absolute thresholdthe smallest detectable intensity of a single stimulus, unlike the difference threshold (JND), which is the smallest detectable CHANGE between two stimuli.
Weber's lawthe JND is a constant proportion of the original stimulus, rather than a fixed absolute amount regardless of starting intensity.
Signal detection theoryseparates true sensitivity from response bias, whereas a simple threshold test conflates the two into one single detection score.
Psychophysicsmeasuring detection of a single stimulus change, unlike Gestalt principles, which describe how whole perceptual patterns are organised.

Watch for

That law proposes an exponential rather than logarithmic relation.
That law relates arousal to performance rather than sensation.
That law concerns spatial summation in vision.
That law concerns temporal summation in vision.
That rate alone confounds sensitivity with response bias.

Use it again

Name the learning, social or research principle being tested before reading the options.
Identify the antecedent, behaviour, consequence or cognitive process in the vignette.
Compare the direction of effect with the nearest competing principle.
Apply the same rule to a new example to confirm that the answer transfers.