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Sensation & Perception

Sensation & Perception. Lecture 2: Psychophysical Methods Andy Clark September 29, 2004. Administrative. E-mail: See the website for emailing details if you are a person Go away if you are a spam robot. Review….

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Sensation & Perception

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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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