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

Physical Stimulus Perception

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

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

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)

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

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

Fechner’s Law: S = k log I I (Intensity of the stimulus) 0 20 40 60 80 100 S8 S7 S6 S5 S4 S3 S2 S1 S0 S (Sensation Units or JND’s) I I I

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

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

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

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

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%

Summary of sample results: Experimental Conditions (Expectation of signal) Hypothetical Results % hits %F.A. % of trials observer thinks signal is present 1. 0% 2. 25% 3. 50% 4. 75% 5. 100% 0% 0% 40% 10% 70% 40% 90% 70% 100% 100%