1. How do we make decisions about uncertain events?
Psychophysics
Absolute and Difference Thresholds
Weber’s Law and Fechner’s Law
The concept of the jnd (just-noticeable difference)
Signal detection and Decision Theory
Response bias: payoffs and expectations
Liberal vs. conservative strategies

2. Physical
Stimulus
Perception

3. The absolute threshold of detection: idealized and actual
100 90 80 70 60 50 40 30 20 10 6 8 12 14
Intensity of stimulus
% presentations detected
Step function
Ogive
Actual
Idealized

4. How to obtain empirically an individual’s absolute detection threshold:
100 90 80 70 60 50 40 30 20 10 6 8 12 14
Intensity of stimulus
% presentations detected

5. Finding the Absolute threshold:
“Is the stimulus there?”
(yes, no)
Finding the Difference threshold:
standard
stimulus
comparison
“Is the comparison stimulus “stronger” than the standard stimulus?”
(yes, no)

6. How to obtain empirically an individual’s difference threshold:
100 90 80 70 60 50 40 30 20 10 11 12 13 14 15
Intensity of comparison stimulus
% differences detected
(standard
stimulus)
Difference threshold = I = 12.7 – 10 = 2.7 stimulus units

7. I (Intensity of Stimulus) I (difference threshold)
Weber’s Law: or I
c I
I (Intensity of Stimulus)
I (difference threshold)

8. Fechner’s Law: S = k log I
I (Intensity of the stimulus) S8 S7 S6 S5 S4 S3 S2 S1 S0
S (Sensation Units or JND’s) I I I

9. A single Signal Detection trial
Reality:
Stimulus
present
absent
Hit
False
Alarm
Responds
“yes”
“no”
Decision:
Miss
Correct
rejection

10. Session 17: Payoff matrix is $2 for hit and -$2 for false alarm
Reality:
Stimulus
present
absent $2 $0
-$2 $0
Hit
False
Alarm
Responds
“yes”
“no”
70%
30%
60%
40%
Decision:
Miss
Correct
rejection

11. Session 22: Payoff matrix is $2 for hit and $0 for false alarm
Reality:
Stimulus
present
absent $2 $0 $0
Hit
False
Alarm
Responds
“yes”
“no”
100% 0% 0%
100%
Decision:
Miss
Correct
rejection

12. Reality: Decision: Session 18: Payoff matrix is
0 for hit and -$2 for false alarm
Reality:
Stimulus
present
absent $0
-$2 $0
Hit
False
Alarm
Responds
“yes”
“no”
Decision:
100%
100%
Miss
Correct
rejection

13. Summary of sample results:
Experimental Conditions
(Payoff schedule)
Hypothetical results
(Hit, F.A.)
% hits %F.A.
1. ($0, -$2 )
2. ($1, -$2)
3. ($2, -$2)
4. ($2, -$1)
5. ($2, $0)
0% 0%
40% 10%
70% 40%
90% 70%
100% 100%

14. Summary of sample results:
Experimental Conditions
(Expectation of signal)
Hypothetical Results
% hits %F.A.
% of trials observer thinks signal is present % % % %
5. 100%
0% 0%
40% 10%
70% 40%
90% 70%
100% 100%