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Classical Psychophysical Methods: Understanding Perception Through Sensory Thresholds

Explore the study of sensation and perception through classical psychophysical methods to understand how the brain processes and reconstructs physical reality. Dive into historical research, theories, and empirical techniques to uncover the intricate relationship between stimuli and sensory capabilities.

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Classical Psychophysical Methods: Understanding Perception Through Sensory Thresholds

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  1. Sensation & Perception Lecture 2: Psychophysical Methods Andy Clark September 29, 2004

  2. Administrative • E-mail: • See the website for emailing details if you are a person • Go away if you are a spam robot

  3. Review… • Large percentage of brain (subcortical/cortical) devoted to perceptual processing • Perception is (re) construction of physical reality • Instances in which perception does not correspond to reality (illusions) provide useful insight into structure of system

  4. How to Study?

  5. Phenomenology • Philosophical Movement • Study of structures of experience (consciousness) • OBSERVATIONAL • SUBJECTIVE • LACKS FIRM MEASURE

  6. How to Study Empirically?

  7. Psychophysics • Study of the relationships between physical energy and sensory capabilities • Weber (1834) • Fechner (1860) After Descartes, 1644

  8. C A B Perception Processing recognition Action Transduction Stimulus on receptors Environmental Stimulus Attended Stimulus

  9. Levels of Analysis • A (stimulus-perception) • Psychophysics • Classical methods, Magnitude Estimation, TSD • B (stimulus-processing) • Physiology (extra- & intracellular recordings) • C (physiology-perception) • fMRI, awake-behaving monkeys

  10. Classical Psychophysical Methods • Grew out of research into Absolute Detection • Absolute Detection-description of sensory events in terms of the minimum amount of stimulus energy required to elicit them • i.e. dimmest visible light • ‘sensory threshold’

  11. Classical Psychophysical Theory • Sensory Threshold • Herbart (1824) critical boundary, above which neural activity signals the presence of a sensory event –ABSOLUTE

  12. Psychophysical Theory • Threshold is a statistical concept, affected by: • Chance variation in the nervous system • Intensity of signal • Observer’s criterion

  13. Psychophysical Theory

  14. Method of Limits • Example: Visual Detection Thresholds • Begin experiment by showing subject a dim light • Slowly raise luminance until subject indicates detection • Start over by initially displaying bright light • Slowly lower luminance until subjects can’t detect

  15. Start Y Y Y Y N N N N Start Method of Limits * * Etc. etc…

  16. Method of Limits • Threshold=Mean of ‘crossover’ values

  17. Method of Adjustment • Subject able to manipulate value of stimulus parameter of interest (brightness in our example) • Lower/Raise until just barely detectable • Repeat # of times, average values to estimate threshold

  18. Problems ? • Yes Hysteresis • “path dependence” • Ex. Method of Limits • Threshold value will differ dependent upon whether experimenter started with: dimbright brightdim

  19. Method of Constant Stimuli • Create stimulus set a priori • Value of parameter of interest varies slightly throughout set

  20. Method of Constant Stimuli • Display stimuli in random order • For each trial subject indicates their perception (i.e. yes/no for detection case) • Plot subjects responses as probability of detection versus stimulus value • Threshold = value for which subject detects stimulus on 50% of the trials

  21. Method of Constant Stimuli 1 .5 % “Yes” responses 0 luminance

  22. Method of Constant Stimuli 1 Psychometric function Extract 2 measures: % “Yes” responses • Absolute Threshold 0 luminance

  23. Method of Constant Stimuli 1 Psychometric function Extract 2 measures: % “Yes” responses 2. “Sensitivity” (slope) 0 luminance

  24. Sensitivity • Weber (1834) • Measure the smallest detectable change in stimulus energy • Just Noticeable Difference (JND) • Worked with discrimination of lifted weights • Studied relationship between JND for intensity and base intensity level

  25. Sensitivity • Weber’s Law • JND=S*K or JND/S=K • Where S=value of standard, K=Weber’s constant, JND=just noticeable difference • Increases in intensity that are just noticeably different to an observer are constant fraction of stimulus intensity • Holds for suprathreshold stimuli

  26. Sensitivity • Ex. Brightness % Yes Responses (Q=Stimuli Different?)  Brightness

  27. Magnitude Estimation • Observer rates stimuli in relation to some stimulus standard • Ex. Brightness • Observer rates standard light with Brightness of 10 • Successive estimations proportional to standard  light twice as bright – 20 half –5 etc. etc.

  28. Magnitude Estimation Shock Line length • Response expansion Brightness • Response compression

  29. Magnitude Estimation • Steven’s Power Law P=perceived magnitude, K=constant, S=stimulus intensity When estimates plotted on log/log scale, functions become linear, exponent determines the slope

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