Modern Psychophysics

1
Modern Psychophysics

• Signal Detection Theory and the analysis of the
  ROC curve

2
Modern Psychophysics

• Basic assumption
• The human observer cannot report experienced
  sensation without bias
• Measurement of sensitivity
• Direct, through measurement of Hits (H) and False
  Alarms (FA)
• Sensitivity (S) separation of H from FA
• S d (d-prime)

3
Representation of Experience of Observer in
Signal Detection
4
The Response Criterion
Respond Signal Present
Respond No Signal
5
The Response Criterion
Respond Signal Present
Respond No Signal
6
High Criterion No FAs
7
Low Criterion No Misses
8
Matrix of Conditions and Responses
9
Methods

• Yes-No Detection
• Hits, Misses
• False Alarms, Correct Rejections
• One ROC point/condition
• Two-alternative forced choice
• Bias-free
• Confidence rating scales
• Hits, Misses
• False Alarms, Correct Rejections
• One ROC curve/condition

10
Calculating d using p-to-z table

• Convert HR and FA to z-scores
• Look up HR and FA in p-to-z table
• Change the sign of the z-score in the table
• HR .85, z-score for .85 is 1.04, change sign
  to make z 1.04
• FA .25, z-score for .25 is -.67, change sign to
  make z .67
• Calculate d using z-scores
• d zFA zHR
• .67 (-1.04)
• 1.71