Sensation, perception and psychophysics
Introduced| English |
|---|
| difference threshold/ˈdɪfrəns ˈθreʃəʊld/ |
| transduction/trænsˈdʌkʃn/ |
A decision before an answer
- A just-noticeable difference is not a fixed amount of stimulus; it is a fixed fraction.
- Your goal: Distinguish thresholds, Weber's law and signal-detection measures.
Measure the thresholds
- Psychophysics relates physical intensity to experience. An absolute threshold is a stimulus level meeting a specified detection criterion under specified conditions, often 50% detection; a difference threshold is a detectable change under a stated criterion.
- Weber's law states that the difference threshold is a constant fraction of the baseline: ΔI/I=k. With k=.02, a 100 g weight needs a 2 g change, while 200 g needs 4 g. Fechner proposed that perceived intensity grows logarithmically with physical intensity.
With Weber fraction k=.02, the just noticeable difference for a 250 g baseline is:
ΔI=k×I=.02×250=5 g.
Separate sensitivity from bias
- Real detection is noisy, so signal detection theory separates sensitivity from bias. Presenting signal or noise on each trial gives four outcomes: hits, misses, false alarms and correct rejections.
- Sensitivity d′ measures how separated the signal and noise distributions are; the criterion measures the observer's willingness to say yes. A payoff change can move the criterion without changing d′, so hit rate alone is not sensitivity.
After a payoff change, an observer says yes more often while the stimulus is unchanged. In signal detection theory, what moved?
Payoffs move the criterion (bias); sensitivity depends on the signal and noise distributions.
Convert energy to signals
- Sensation begins with transduction: receptors convert physical energy into neural signals, as photoreceptors convert light and hair cells convert sound pressure.
- Sensory adaptation reduces response to a constant stimulus. Dark adaptation is the recovering sensitivity of the visual system in low light.
With Weber fraction .02, detecting a change from 50 g needs 1 g, and from 500 g needs 10 g. A radiologist who says yes more often after a pay-per-hit scheme shifts criterion, not d′. A listener who cannot distinguish two within-category speech sounds shows categorical perception of a physically continuous dimension.
Weber's law states that the difference threshold equals k times the ____.
ΔI/I=k: the just noticeable change scales with the starting intensity.
Organise the percept
- Perception organises input rather than copying it. Gestalt principles group by proximity, similarity, closure and good continuation.
- Bottom-up processing builds percepts from features; top-down processing applies context and expectation. Categorical perception treats continuous acoustic variation as discrete phonemes: within a category, differences are hard to hear even when physically present.
Treating the difference threshold as a fixed amount rather than a fraction of the baseline, or reading a higher hit rate as better sensitivity when the observer simply says yes more.
Which answer fits this case?
Distinguish thresholds, Weber's law and signal-detection measures
A higher hit rate by itself demonstrates higher sensitivity.
Hits rise when the criterion loosens, even with unchanged d′; sensitivity and bias are separate.
Keep the distinctions
- difference threshold 差别阈限 — The minimum detectable change in a stimulus, a constant fraction of the baseline by Weber's law.
- transduction 换能 — Conversion of physical stimulus energy into neural signals by receptors.
- Distinguish thresholds, Weber's law and signal-detection measures.
- Trace transduction from physical stimulus to neural signal.
- Apply perceptual organisation and categorical perception principles.
Match each term with its precise meaning in this lesson.
Keep the distinctions stated in the teaching example.
Put this lesson’s reasoning or event sequence in order.
The order follows the stated process; check each stage before the next.