Psychophysics
• Mathematical formula for relationship between
changes in the physical stimulus and changes in
conscious experience of the stimulus
• Classical methods
© 2001 Dr. Laura Snodgrass, Ph.D.
1
Method of Constant Stimuli
• Choose ~ 9 stimuli in equal steps covering a range
above and below threshold
• Present in random order
• Each stimulus is presented many times and % yes
(different) is calculated
• 50% yes (different) = threshold
• Problems
– choosing, gaps, trials that waste time
© 2001 Dr. Laura Snodgrass, Ph.D.
2
Method of Limits
• Similar to constant stimuli, but present in
ascending and descending order until response
changes
• Average point of change in response
– from yes to no or same to different
• Problems
– errors of habituation and anticipation
© 2001 Dr. Laura Snodgrass, Ph.D.
3
Method of Adjustment
• Ascending and descending trials with continuously
adjustable stimuli
• Same calculations and same problems as method of
limits, but more precise estimates
© 2001 Dr. Laura Snodgrass, Ph.D.
4
Magnitude Estimation
• Assign a value to a standard
• Let participant assign values to comparison
stimuli
– if this is 10 then if that is half as intense it is 5,
twice as intense would be 20
• Produces a ratio scale
© 2001 Dr. Laura Snodgrass, Ph.D.
5
Signal detection theory
• Calculate d’ (detectability)
• Must have trials with a stimulus and trials with no
stimulus
• % hits = say yes when stim is present
• % false alarms = say yes when stim is absent
• Use % hits and % false alarms to look up d’ on chart
• Shows degree of brain response
© 2001 Dr. Laura Snodgrass, Ph.D.
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