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Chapter 3 Presentation Transcript

  • 1. The Classical Psychophysical Methods CHAPTER 3, Psychophysics the Fundamentals
  • 2. Outline
    • Method of constant stimuli Absolute Thresholds
    • Difference Thresholds
    • Method of limits
    • Absolute Thresholds
    • Difference Thresholds
    • Variation of the Method of Limits
    • Method of adjustment
    • Absolute Thresholds
    • Difference Thresholds
    • Application of classical psychophysical methods to problems of stimulus matching
  • 3. Introduction
    • The threshold has been defined as the stimulus value which is perceptible in 50% of the trails.
    • Three methods of threshold measurement:
    •   1.Method of constant stimuli
    •   2.Method of limits
    •   3.Method of adjustment
  • 4. Method of constant stimuli - Absolute Thresholds
    • Procedure: using the same set of stimuli(5~9 different values in the set) , each stimulus is present repeatedly, usually 100 times or more, but in a random order.
    • The percentage of detections as a function of stimulus intensity, φ is determined.
    • Yes or No response: the proportion( p ) of yes response is computed, and a graph called a psychometric function is constructed.
  • 5.  
  • 6. Method of constant stimuli - Absolute Thresholds
    • Absolute threshold: the stimulus intensity for which the proportion of trials resulting in yes response is 0.5
    • Ogive function : S-shaped, is a cumulative form of normal distribution.
    • Fitting ogive function to threshold data range:
    • z-score
    • method of least squares
  • 7.  
  • 8.  
  • 9. Method of constant stimuli - Difference Thresholds
    • Standard stimulus(St):
    • fixed value
    • Comparison stimulus(Co):
    • the value is changed from trial to trial
    • Space error
    • Time error
  • 10.  
  • 11. Method of constant stimuli - Difference Thresholds
    • Point of subjective equality(PSE):
    • The 0.5 point on the psychometric function , is perceived subjectively as equal to standard stimulus
    • Constant error(CE)
    • CE = PSE - St
  • 12. Method of constant stimuli - Difference Thresholds
    • Upper difference threshold(DL u )
    • the stimulus range from PSE to the .75 point
    • Lower difference threshold(DL l )
    • the stimulus range from PSE to the .25 point
    •  DL = (Dl u + DL l) / 2
    • The method of least squares
  • 13. Method of limits - Absolute Thresholds
    • Ascending series : begin with very weak subthreshold stimulus, then increased by a small amount until the observer report the presence of the sensation
    • Descending series : the value of stimulus is decreased until the observer report the absence of the sensation
    • Threshold : the average of the transition points
  • 14.  
  • 15. Method of limits - Absolute Thresholds
    • Error of habituation: the tendency for an observer to develop a habit of repeating the same response
    • Error of expectation: an observer may falsely anticipate the arrival of the stimulus at his threshold and prematurely report that the change has occurred before it really has
    • Minimize the errors:avoid using long trial series, vary the starting point
  • 16. Method of limits - Difference Thresholds
    • Upper limen(L u ): the point on the physical dimension where “greater” response change to “equal” response
    • Lower limen(L u ): the point where “less” response change to “equal” response
    • Interval of uncertainty(IU): the range on the stimulus dimension over which an observer cannot perceive a difference between the Co and St
  • 17. Method of limits - Difference Thresholds
    • IU = L u – L l
    • DL =1/2 IU = ½(L u – L l )
    • PSE = ½(L u + L l )
    • Error of habituation
    • Error of expectation
    • Space error
    • Time error
  • 18. Variation of the Method of Limits
    • Up-and-down/ staircase method (Cornsweet, 1962)
    • - efficient
    • - step size : must chosen with care
    • Threshold tracking method (Bekesy, 1947)
    • Forced choice method(Blackwell, 1953)
    • - temporal-forced choice
    • - spatial-forced choice
    • - threshold: two correct responses in succession
    • - response bias: the tendency to report the
    • presence of subthreshold or absent stimuli
  • 19. Method of adjustment - Absolute Thresholds
    • Procedure: set the stimulus intensity level either far below or far above threshold and ask the observer either to increase the intensity level until it is just perceptible or vice versa.
    • Can prevent boredom
    • Error of habituation
    • Error of expectation
  • 20. Method of adjustment - Difference Thresholds
    • Method of average error: the observer is instructed to adjust a Co until it seems equal to some St
    • Mean of X = PSE
    • CE = PSE – St
    • Standard deviation:
    • high degree of central
    • tendency when
    • discrimination is good
  • 21. Method of adjustment - Absolute Thresholds
    • Shortcomings:
    • - difficult to apply when stimulus are not
    • continuously variable
    • - difficult to apply when pairs of stimulus
    • cannot be presented simultaneously
    • - difficult to maintain constant conditions
  • 22. Application of classical psychophysical methods to problems of stimulus matching
    • Equal sensation contour: a stimulus critical value function, in which absolute threshold is plotted against some property of stimulus
  • 23. Application of classical psychophysical methods to problems of stimulus matching
    • Loudness enhancing effect : the increment in loudness of a sound caused by the presentation of another sound
  • 24.  
  • 25. Conclusion
    • Sensation is treated as a concept which must be defined in terms of stimulus-response relationships.