Presentation Transcript

1. Psychophysics

   PSY 295 - Fall 2012 - Grinnell College
2. Studying perception Want to understand perception and how it arises in the brain In the 1960s, Vernon Mountcastle suggested an experimental strategy to accomplish this goal PSY 295 - Fall 2012 - Grinnell College
3. Perceptual neuroscience Study relationship between physical stimulus and perception Using the same stimuli, study relationship between stimulus parameters and neural responses Determine a model for how neural responses explain the perceptual data PSY 295 - Fall 2012 - Grinnell College
4. Perceptual neuroscience The first step is to do psychophysics– today’s lecture PSY 295 - Fall 2012 - Grinnell College
5. Classical Psychophysics PSY 295 - Fall 2012 - Grinnell College
6. Psychophysics Science which studies the relationship between physical stimulus parameters and perception Psychophysics Psycho = mind, physics = matter PSY 295 - Fall 2012 - Grinnell College
7. The father of psychophysics Gustav Fechner wanted to describe perception using the language of mathematics PSY 295 - Fall 2012 - Grinnell College
8. Thresholds One way to describe relationship between mind and matter was to measure sensory thresholds Thresholds describe the smallest stimulus intensity or difference between two stimuli which can be detected PSY 295 - Fall 2012 - Grinnell College
9. Two-point touch discrimination What is the minimum distance at which two stimuli can be distinguished? PSY 295 - Fall 2012 - Grinnell College
10. Just noticeable difference Smallest detectable difference between two stimuli JND = Just Noticeable Difference Difference threshold PSY 295 - Fall 2012 - Grinnell College
11. Absolute thresholds What is the minimum stimulus intensity that can be detected? PSY 295 - Fall 2012 - Grinnell College
12. Weight discrimination Ernst Weber found the JND for weight discrimination to be proportional to the reference weight JND = 1g for 40g reference JND = 10g for 400g reference PSY 295 - Fall 2012 - Grinnell College
13. Weber’s Law DI a I JND (DI) is proportional to reference stimulus intensity (I) Ratio (DI/I) known as Weber fraction PSY 295 - Fall 2012 - Grinnell College
14. Fechner’s law Total sensation S is simply sum of all changes DS From Weber’s law: DS aDI/I  DS/DI = k/I Simple calculus yields Fechner’s law: S a log(I/I0) PSY 295 - Fall 2012 - Grinnell College
15. Fechner’s law PSY 295 - Fall 2012 - Grinnell College
16. Hearing Many sensory modalities follow Fechner’s law Loudness in decibels: dB a log10(P/P0) Need to increase sound pressure level by a factor of 10 to increase the perceived loudness by 1 dB PSY 295 - Fall 2012 - Grinnell College
17. Psychophysical thresholds Weber and Fechner laws depend on measuring thresholds How do we measure absolute and JND thresholds? PSY 295 - Fall 2012 - Grinnell College
18. Method of constant stimuli Series of stimuli from imperceptible to easily perceptible Find the smallest stimulus which can be perceived PSY 295 - Fall 2012 - Grinnell College
19. In a perfect world PSY 295 - Fall 2012 - Grinnell College
20. In reality PSY 295 - Fall 2012 - Grinnell College
21. Method of limits Start with a perceptible stimulus and slowly decrease its magnitude until it cannot be perceived Start with imperceptible stimulus and slowly increase magnitude until it is perceived Take average of thresholds PSY 295 - Fall 2012 - Grinnell College
22. Method of limits PSY 295 - Fall 2012 - Grinnell College
23. Other methods Method of adjustment: Subject adjusts stimulus until he/she can just barely perceive it Magnitude estimation: Subject assigns values according to perceived magnitudes of the stimuli PSY 295 - Fall 2012 - Grinnell College
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25. Repealing Fechner’s Law SS Stevens discovered that magnitude estimates often better described by a power law S = k Im Different exponents (m) for different modalities PSY 295 - Fall 2012 - Grinnell College
26. Power law PSY 295 - Fall 2012 - Grinnell College
27. Signal Detection Theory PSY 295 - Fall 2012 - Grinnell College
28. Signal Detection Theory Is the blob in this mammogram a tumor or not? Radiologists face the problem of detecting a signal in the presence of background noise Sensory perception faces the same challenge since sensory signals and neural processing can be noisy PSY 295 - Fall 2012 - Grinnell College
29. Signal Detection Theory Theoretical framework for studying detection and discrimination of sensory stimuli in noisy environments Used in a wide variety of fields like engineering, communications and medicine PSY 295 - Fall 2012 - Grinnell College
30. Phone in the shower We have all experienced the problem of listening for a phone call while in the shower The shower is the noise, phone ring is the signal PSY 295 - Fall 2012 - Grinnell College
31. The problem Sometimes by chance, the shower water will make noises sounding somewhat like a phone ringing For repeated samples of shower noise, we can plot a histogram how phone-like they sound PSY 295 - Fall 2012 - Grinnell College
32. Overlapping distributions When the phone is ringing with the shower noise present, we can also plot a distribution of how phone-like the sounds is We see that there is substantial overlap in the distributions Sometimes the shower sounds phone-like, sometimes the shower noise masks the phone signal PSY 295 - Fall 2012 - Grinnell College
33. Criterion The observer needs to choose a criterion to determine what degree of phone-like sound they will respond to PSY 295 - Fall 2012 - Grinnell College
34. Possible outcomes Good outcomes! Bad outcomes! PSY 295 - Fall 2012 - Grinnell College
35. Measuring sensitivity Ease of task depends on how well separated the signal and noise distributions are Quantified by a measure called d-prime How is probability of correct response related to separation? PSY 295 - Fall 2012 - Grinnell College
36. How to set the criterion Optimal way to set criterion depends on the rewards and penalties associated with hits and misses PSY 295 - Fall 2012 - Grinnell College
37. Thought experiment What does it mean when you set your criterion to negative infinity? What is your hit rate? What is your false positive rate? PSY 295 - Fall 2012 - Grinnell College
38. Thought experiment What happens when you set your criterion to infinity? What is your hit rate? What is your false positive rate? PSY 295 - Fall 2012 - Grinnell College
39. Receiver Operating Characteristics Graphical plot of the hit rate as a function of the false alarm rate as criterion varies Better sensitivity of observer, more `bowed’ the curve looks PSY 295 - Fall 2012 - Grinnell College
40. Receiver Operating Characteristics What would the ROC curve look like for perfect performance? What would be the area under the ROC? PSY 295 - Fall 2012 - Grinnell College
41. Application to neuroscience Experiment to study neural basis of motion perception Subject needs to determine direction of array of random dots having varying degrees of correlation PSY 295 - Fall 2012 - Grinnell College
42. Overlapping neural responses PSY 295 - Fall 2012 - Grinnell College
43. Neuron task performance PSY 295 - Fall 2012 - Grinnell College
44. Conclusions Psychophysics is an important tool for understanding perception Combined with physiology to study neural basis of perception PSY 295 - Fall 2012 - Grinnell College