PSYCHOPHYSICS

1. PSYCHOPHYSICS • What is Psychophysics? • Classical Psychophysics • Thresholds • Signal Detection Theory • Psychophysical Laws

2. What is Psychophysics? • Study of the relationship between physical properties and psychological experience

3. Classical Psychophysics • Assumption is that there is an absolute threshold - point above which a stimulus can always be detected and below which it can never be detected

4. Method of Limits • Increase the intensity until the observer reports detection (ascending series) • Decrease the intensity until the observer no longer reports detection (descending series) • Look for an average over a number of series

5. Method of Adjustment • Observer adjusts intensity until the stimulus is just detectable • Can be done quickly, but results tend to be variable

6. Method of Constant Stimuli • A range of intensities is selected and presented in random order; observer responds “Yes” or “No” • Include “catch trials” on which no stimulus is presented

7. Do Absolute Thresholds exist? • Gradual change from no detection to detection • By convention, the threshold is the stimulus intensity at which the stimulus is detected 50% of the time

8. What Classical Psychophysics predicts:

9. What usually happens:

10. Difference Threshold • Smallest difference between stimuli that can be detected • Standard stimulus is compared to comparison stimulus

11. Signal Detection Theory • How can we do psychophysics without absolute thresholds? • Assumes that responses are a product of making decisions about signals embedded in noise

12. What Determines Responses in SDT? • sensitivity - ability to detect stimuli • criterion - strategy for deciding how to respond

13. What Affects Sensitivity? • how good an observer you are • signal to noise ratio

14. What Affects the Criterion? • payoff for responses • probability that the stimulus will occur

15. Psychophysics the SDT Way • Present trials with the stimulus present and with the stimulus absent • Observer can respond yes or no on each trial • Sensitivity and criterion can be measured

16. Psychophysics the SDT Way Stimulus Present? Yes No Response No Yes

17. Measuring Sensitivity • When sensitivity is higher, there should be more hits and fewer false alarms • d-prime (d’) is the standardized difference between the signal-present and signal-absent distributions

18. Signal Present Distribution probability Sensory Activity

19. Signal Absent Distribution probability Sensory Activity

20. Absent Present probability Sensory Activity

21. probability d’ Sensory Activity

22. small d’ probability Sensory Activity

23. large d’ probability Sensory Activity

24. Measuring the Criterion • The criterion (B) is the level of intensity at which the observer chooses to switch from “No” to “Yes” responses

25. Criterion Respond “Yes” Respond “No” probability Sensory Activity

26. Lax Criterion probability Sensory Activity

27. Strict Criterion probability Sensory Activity

28. Weber’s Law • Describes how much of a physical difference is needed between two stimuli before people can just tell the difference (difference threshold or “just noticeable difference”) • Works very well except at extremely low or high intensities

29. Weber’s Law

30. Example of Weber’s Law • If a standard stimulus weighs 10 pounds, how much different would another stimulus have to be for the difference to be noticeable?

32. Stevens’ Power Law • Describes relationship between physical and psychological intensity • Sensory magnitude is an exponential function of physical intensity • Exponent >1: Response Expansion • Exponent <1: Response Compression