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

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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

6. How to Study?

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

8. How to Study Empirically?

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

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

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

12. 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’

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

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

15. Psychophysical Theory

16. 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

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

18. Method of Limits Threshold=Mean of ‘crossover’ values 96 Y 95 94 93 N92 91 90

19. 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

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

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

22. 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

23. Method of Constant Stimuli1 .5
% “Yes” responses
luminance

24. Method of Constant Stimuli1
Psychometric function
Extract 2 measures:
% “Yes” responses
Absolute Threshold
luminance

25. Method of Constant Stimuli1
Psychometric function
Extract 2 measures:
% “Yes” responses
2. “Sensitivity” (slope)
luminance

26. 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

27. 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

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

29. 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.

30. Magnitude Estimation Shock Line length Brightness Response expansion
Response compression

31. 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