CHAPTER 4

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CHAPTER 4

  1. 1. Classical Psychophysical Theory CHAPTER 4, Psychophysics the Fundamentals
  2. 2. Ψ Φ f f -1
  3. 3. Outline <ul><li>Part A </li></ul><ul><ul><li>Classical threshold theory </li></ul></ul><ul><ul><li>Quantal fluctuations </li></ul></ul><ul><ul><li>Neural quantum theory </li></ul></ul><ul><li>Part B </li></ul><ul><ul><li>ROC curve </li></ul></ul>Threshold Stimulus Linear Expectation
  4. 4. Classical threshold theory <ul><li>The measurement obtained in a psychophysical experiments were estimates of a neural threshold. </li></ul>
  5. 5. Empirical psychometric function What’s worng?
  6. 6. Explanation 1 <ul><li>Momentary threshold Thresholds vary randomly over time. </li></ul>
  7. 7. Phi-gamma hypothesis <ul><li>The psychometric function in which response probability is plotted against stimulus magnitude should have the ogival form of the cumulative normal distribution. </li></ul><ul><ul><li>Momentary thresholds are normally distributed over time. </li></ul></ul>
  8. 9. Variance Mean Why?
  9. 11. Remark <ul><li>In some cases the stimulus intensity can be measured by different physical scales that are not linearly related to each other. </li></ul><ul><ul><li>E. g. sound pressure & decibel (dB). </li></ul></ul><ul><li>Phi-log-gamma hypothesis </li></ul><ul><ul><li>Thurstone, 1928. </li></ul></ul><ul><ul><li>Assume Fechner’s law is correct. </li></ul></ul><ul><ul><ul><li>Stimuli can be expressed in psychological units. </li></ul></ul></ul>
  10. 12. Neural quantum theory <ul><li>Stevens, Morgan, & Volkmann, 1941. </li></ul><ul><li>A sensory dimension is made up of small discrete steps. </li></ul>
  11. 13. <ul><li>ΔΦ: Stimulus increment. </li></ul><ul><li>Q: The smallest increment that will always activate an additional neural unit. </li></ul><ul><li>p: Stimulus surplus. </li></ul>ΔΦ = Q - p r1 = ΔΦ/Q r2 = (ΔΦ - Q)/Q
  12. 14. Inferences <ul><li>Stimulus increments below a critical size produce no response. </li></ul><ul><li>Above the critical value, the number of increments detected is a linear function of the size of the increment. </li></ul><ul><li>Increments are always detected when an increment reaches a second critical size. </li></ul>
  13. 16. Remark <ul><li>Stevens, 1972: </li></ul><ul><ul><li>The operation of the neural quantum may be a central neural mechanism of sufficient generality to process information from all sensory modalities. </li></ul></ul><ul><li>Necessary conditions of linear psycho-metric functions: </li></ul><ul><ul><li>Carefully controlled stimulus. </li></ul></ul><ul><ul><li>Constant criterion during experiment. </li></ul></ul><ul><ul><li>Rapid stimulus presentation. </li></ul></ul>
  14. 17. Comments <ul><li>Wright, 1974: </li></ul><ul><ul><li>The procedures employed by neural quantum theorists bias the observer to be very conservative in reporting weak stimuli. </li></ul></ul><ul><li>In most situations, an observer's judgement of weak stimuli are determined by an adjustable judgement criterion. </li></ul>
  15. 18. <ul><li>There are nonsensory factors that can influence the performance of subjects. </li></ul><ul><ul><li>Probability of the presentation of target. </li></ul></ul><ul><ul><li>Subjective cost and value. </li></ul></ul>
  16. 20. Effect of probability on threshold
  17. 21. Classical threshold theory's point of view.
  18. 23. However...
  19. 24. Receiver operating characteristic (ROC) curve

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