Psychophysics

1
Psychophysics

• Measuring the sensitivity of an observer to
  physical stimulation

2
Psychophysical Tasks

• Measuring sensitivity to physical stimulation
• Sensitivity refers to the ability of an
  individual to be reactive to a sensory input
• Sensitivity is a relatively stable characteristic
• Developing psychological scales of sensory inputs

3
Classical Psychophysics

• Basic assumption
• The human observer can report experienced
  sensation without bias
• Measurement of sensitivity
• Indirect, through measurement of threshold
• Sensitivity (S) inverse of Threshold (T)
• S 1/ T

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Thresholds

• Absolute threshold
• The minimal amount of stimulation necessary to
  produce a response
• Difference threshold
• The minimal amount of stimulation necessary to
  distinguish one stimulus (the comparison) from
  another (the standard)

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Methods

• Method of Limits
• Approach the threshold gradually, with variation
  controlled by experimenter
• Ascending method
• Descending method
• Method of Adjustment
• Approach point of subjective equality gradually,
  with variation controlled by observer
• Method of Constant Stimuli
• Determine threshold from psychometric function

6
History

• Ernst Weber (1795-1878)
• measuring sensitivity to touch
• Webers Law
• k jnd/Std
• Gustav Fechner (1801 - 1887)
• scaling sensitivity
• Fechners Law
• S a log (P)

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Webers Law

• Applicability
• Measurement of Sensitivity
• Basis of Law
• The just noticeable difference (jnd) depends on
  the magnitude of the standard
• Form of Law
• The jnd is a constant proportion of the magnitude
  of the standard
• jnd k Std

8
Weber fraction

• The constant in Webers Law, k, is called the
  Weber fraction
• k jnd / Std
• Quantitative index of sensitivity
• small k good sensitivity
• large k poor sensitivity

9
Sample Weber Fractions
Sensory Dimension k Auditory
frequency .003 Visual brightness .016 Lifted
weight .019 Taste of salt .200
10
What does Webers k mean?

• If you put a 1/2 tsp of salt in a recipe, how
  much salt will you have to put in to make it
  taste saltier?
• If you are tuning a piano to a 1000 Hz tone, how
  far off can it be for you to hear a false note?

11
Fechners Law

• Applicability
• Psychological Scaling
• Basis of Law
• If the jnd is treated as a psychological
  constant, then Webers Law implies Fechners Law
• Form of Law
• Psychological magnitude (?) is a log function of
  physical magnitude (P)
• ? a log (P)

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Fechners Law