The Classical Psychophysical Methods CHAPTER 3, Psychophysics the Fundamentals
Outline <ul><li>Method of constant stimuli   Absolute Thresholds </li></ul><ul><li>Difference  Thresholds </li></ul><ul><l...
Introduction <ul><li>The threshold has been defined as the stimulus value which is perceptible in 50% of the trails. </li>...
Method of constant stimuli   - Absolute Thresholds <ul><li>Procedure: using the same set of stimuli(5~9 different values i...
 
Method of constant stimuli   - Absolute Thresholds <ul><li>Absolute threshold: the stimulus intensity for which the propor...
 
 
Method of constant stimuli   - Difference  Thresholds <ul><li>Standard stimulus(St):  </li></ul><ul><li>fixed value </li><...
 
Method of constant stimuli   - Difference  Thresholds <ul><li>Point of subjective equality(PSE):  </li></ul><ul><li>The 0....
Method of constant stimuli   - Difference  Thresholds <ul><li>Upper difference  threshold(DL u ) </li></ul><ul><li>the sti...
Method of limits   - Absolute Thresholds <ul><li>Ascending series : begin with very weak subthreshold stimulus, then incre...
 
Method of limits   - Absolute Thresholds <ul><li>Error of habituation: the tendency for an observer to develop a habit of ...
Method of limits   - Difference  Thresholds <ul><li>Upper limen(L u ): the point on the physical dimension where “greater”...
Method of limits   - Difference  Thresholds <ul><li>IU = L u  – L l  </li></ul><ul><li>DL =1/2 IU = ½(L u  – L l  ) </li><...
Variation of the Method of Limits <ul><li>Up-and-down/ staircase method  (Cornsweet, 1962) </li></ul><ul><li>- efficient  ...
Method of adjustment   - Absolute Thresholds <ul><li>Procedure: set the stimulus intensity level either far below or far a...
Method of adjustment   - Difference Thresholds <ul><li>Method of average error: the observer is instructed to adjust a Co ...
Method of adjustment   - Absolute Thresholds <ul><li>Shortcomings: </li></ul><ul><li>- difficult to apply when stimulus ar...
Application of classical psychophysical methods to problems of stimulus matching <ul><li>Equal sensation contour: a stimul...
Application of classical psychophysical methods to problems of stimulus matching <ul><li>Loudness enhancing effect : the i...
 
Conclusion <ul><li>Sensation is treated as a concept which must be defined in terms of stimulus-response relationships.  <...
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Chapter 3

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Chapter 3

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

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