1. Methods in Brain Research: psychophysics
Shaul Hochstein

2. What is psychophysics? term coined by Gustav Fechner in 1860
relationship between:
physical stimuli and subjective percepts; world events and personal experiences;
physics and mind.

3. What is psychophysics? relationship between:
physical stimuli and subjective percepts; world events and personal experiences;
physics and mind.
We know how to measure physical stimuli,, world events, physics.
How do we measure subjective percepts, personal experiences and mind?

4. What is psychophysics?
How do we measure subjective percepts, personal experiences and mind?
Just ask!
Experimenter=observer introspection.
Measure behavior.
- 3 ways leading to 3 different results

5. Psychophysics
stimulus 
task 
method 
analysis 
measure

6. Example 1:
Determination of contrast detection threshold.
What is the minimum amount of contrast needed for a stimulus to be just detectable?

7. Psychophysics
stimulus 
task  detect circle
method 
analysis 
measure

8. Psychophysics
Methods:
Method of limits:
start with very invisible contrast, gradually increase until observer says, “yes”.
OR: start with very visible contrast, gradually decrease until observer says, “no”.

9. Psychophysics
Methods:
Method of limits:
start with very invisible contrast, gradually increase until observer says, “yes”.
OR: start with very visible contrast, gradually decrease until observer says, “no”.
Analysis: average point of switch from No to Yes and from Yes to No over many repetitions.
Average is Measure of Detection Threshold

10. Method of Limits Adaptive Testing - Staircase Procedure
Determination of Absolute Threshold. Response (Stimulus Perceived): yes (Y), no (N).
Stimulus Alternating Ascending and Descending Series
Intensity
0 N N N
1 N N N
2 N N N N
3 N N N Y N N
4 N Y N Y N Y
5 N Y Y Y Y Y
6 Y Y Y Y
7 Y Y Y
Transition
Points
Threshold = Average Transition Points = ( )/6 = 24/6 = 4
Adaptive Testing - Staircase Procedure
Staircase – increase each error, decrease each 2 correct

11. Method of constant stimuli:
Psychophysics
Methods:
Method of constant stimuli:
Used fixed set of contrasts in random order, plot % “yes” responses as function of contrast.
Psychometric function
% yes
responses
Stimulus intensity

12. Psychophysics
Forced Choice: respond yes/no.
Two Alternative Forced Choice:
Present two alternative stimulation periods, one of which contains the stimulus, the other not. Observer responds which period contains the stimulus: the first or the second.
Measure fraction of correct responses, assuming fraction will be 0.5 far below threshold, define 0.75 as threshold.

13. Psychophysics
Methods:
Method of adjustment:
Observer adjusts stimulus intensity to point where he or she feels this is “just” detected, determining his or her subjective threshold.

14. Psychophysics
Threshold Methods:
Besides determining threshold itself,
useful to compare thresholds for different conditions (e.g. wavelengths of light) to determine absorption/action spectra, compare spectra with/without drugs to determine effect, etc.

15. Psychophysics
Methods:
“detection” versus “discrimination”
Is there a face in this picture? versus
Is the face in this picture of a man or a woman?
“threshold” versus “supra-threshold”
j.n.d. or “just noticeable difference”

16. Psychophysics Methods
1AFC //// left/right oblique
2AFC \\\\ - //// 1st/2nd left?
Same/Different \\\\ - //// same/different?
3AFC oddity \\\\ - \\\\ - //// which is odd?
2AFC sample: ////
match to sample \\\\ - //// which matches?
2AFC //// - //// which pair has
same/different //// - \\\\ difference?

17. Psychophysics
Methods:
Nulling:

18. Psychophysics
Methods:
Nulling:

19. Psychophysics
Supra-threshold methods:
paired comparisons: order/% order/jnd
triads: is central stimulus closer to left or right?
Magnitude estimation
magic number 7

20. Psychophysics
Other methods:
Response timing: generally, observers respond faster when the task is easier.
Magnitude estimation
magic number 7

21. Psychophysics
Noise:
In world
In Brain
In mind

23. Signal Detection Theory
Internal response = firing rate of neuron
If have large range of uncertainty, how can we determine how good is the system?

24. Signal Detection Theory
Probability
Internal response

25. Signal Detection Theory
d' = z (hit rate) – z (false alarm rate)
Probability
Internal response

26. Receiver Operating Characteristic
Signal Detection Theory
Receiver Operating Characteristic A B C
Effect of shifting the criterion

27. Receiver Operating Characteristic
Signal Detection Theory
2-Aletrnative Forced Choice
Receiver Operating Characteristic A B C
Effect of shifting the criterion
2 alternative forced choice method