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Psychophysical Measurement of Thresholds: Differential Sensitivity

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  1. 1. Psychophysical Measurement of Thresholds: Differential Sensitivity CHAPTER 1, Psychophysics the Fundamentals
  2. 2. Introduction <ul><li>The transition of psychology from a philosophical to a scientific discipline </li></ul><ul><ul><li>German physicist G. T. Fechner introduced techniques for measuring mental events (1860) </li></ul></ul><ul><li>Psychophysics has consisted primarily of investigating the relationship between </li></ul><ul><ul><li>sensations( ψ ) in psychological domain and stimuli( φ ) in the physical domain </li></ul></ul>
  3. 3. Sensory threshold <ul><li>Assuming that mental events had to be stronger than some critical amount in order to be consciously experienced. (Herbart, 1824) </li></ul><ul><li>E. H. Weber & G. T. Fechner were interested in the measurement of the sensitivity limits of the human sense organs. </li></ul>
  4. 4. <ul><li>Absolute Threshold (RL) </li></ul><ul><ul><li>Was defined as the smallest amount of stimulus energy necessary to produce a sensation. </li></ul></ul><ul><li>Difference Threshold (DL) </li></ul><ul><ul><li>Was defined as the amount of change in a stimulus( Δφ ) required to produce a just noticeable difference (jnd) in the sensation. </li></ul></ul>
  5. 5. <ul><li>Sensations can differ on at least four basic dimensions </li></ul><ul><ul><li>intensity, quality, extension, and duration </li></ul></ul>
  6. 6. Differential Sensitivity <ul><li>The relation between the difference threshold for intensity and the intensity level of a stimulus </li></ul><ul><li>Heavier weights are harder to discriminate and are associated with larger difference threshold. (Weber, 1834) </li></ul><ul><li>The size of the difference threshold was a linear function of stimulus intensity </li></ul>
  7. 7. Weber’s Law <ul><li>The change in stimulus intensity that can just be discriminated( Δφ ) is a constant fraction(c) of the starting intensity of the stimulus( φ ) </li></ul><ul><li>Δ φ= c φ or Δφ/φ= c </li></ul>
  8. 9. <ul><li>If Weber’s law is valid, we would expect, Δφ/φ to be constant as intensity is varied ( Δφ/φ= c) </li></ul><ul><ul><li>Δφ/φ tends to increase greatly at extremely low intensities </li></ul></ul>
  9. 11. <ul><li>König and Brodhun (1889) </li></ul><ul><li>Miller (1947) </li></ul>
  10. 12. <ul><li>Modification of Weber’s law, more closely corresponding to empirical data states </li></ul><ul><li>a -represent the amount of sensory noise that exists when the value of φ is zero </li></ul>
  11. 13. <ul><li>The actual stimulus intensity which really determines Δφ may not be φ , but φ+ a </li></ul><ul><ul><li>The continuing back ground noise </li></ul></ul><ul><ul><li>Thus, its level may greatly influence the value of Δφ for very low intensity falues </li></ul></ul><ul><li>When we consider the level of sensory noise, Weber’s law may be essentially correct. </li></ul>
  12. 15. <ul><li>The concept of sensory noise provides a unifying principle </li></ul><ul><li>Absolute threshold </li></ul><ul><ul><li>The value of φ needed to increase the neural activity level above the sensory noise level </li></ul></ul><ul><li>Difference threshold </li></ul><ul><ul><li>The change in φ needed to produce a critical difference </li></ul></ul><ul><li>Both involve the discrimination of differences in levels of neural activity </li></ul>
  13. 16. <ul><li>Noise- outside/ inside the observer </li></ul><ul><li>External noise </li></ul><ul><ul><li>Uncontrolled fluctuations in the stimulus </li></ul></ul><ul><ul><li>e.g. sense of smell (Cain, 1977) </li></ul></ul><ul><ul><li>Odorant at the nose / substance in the apparatus </li></ul></ul><ul><li>Cain’s research illustrate the importance of precise stimulus control in psychopysics </li></ul>
  14. 17. Notable Exceptions <ul><li>Auditory discrimination of pure tones </li></ul><ul><ul><li>Riesz, 1928 </li></ul></ul><ul><ul><li>Frequency of 4000 Hz yielded the lowest values of Δφ </li></ul></ul><ul><ul><li>Instead of becoming constant (as it did with white noise) , Δφ/φ continues to decrease gradually as φ increases </li></ul></ul><ul><ul><li>Because it is so slight, known as “near miss” to Weber’s law </li></ul></ul>
  15. 19. Notable Exceptions <ul><li>Tactile discrimination of vibration </li></ul><ul><ul><li>Vibratory stimulation of the palm near the base of the thumb </li></ul></ul><ul><ul><li>Random noise stimulation as well as sinusoidal stimuli are presented </li></ul></ul><ul><li>As yet, there are no widely agreed on explanations for this phenomenon </li></ul>
  16. 21. Fechner’s Investigation <ul><li>1860, Elements of psychophysics </li></ul><ul><li>Mind and matter are equal </li></ul><ul><li>Describe the relationship between physical events and conscious experience </li></ul><ul><li>Same as what Weber’s result seems to imply </li></ul>
  17. 22. <ul><li>Relating the values of Δφ on the physical scale to the corresponding values of the just noticeable difference (jnd) on the psychological scale </li></ul><ul><li>Central assumption </li></ul><ul><ul><li>All jnd’s were equal psychological scale, regardless of the size of Δφ </li></ul></ul>
  18. 23. <ul><li>The stimulus dimension φ and the sensation dimension ψ </li></ul>
  19. 24. <ul><li>Jnd is a standard unit of sensation magnitude because it is the smallest detectable increment in a sensation </li></ul><ul><li> always psychologically the same size </li></ul><ul><li>Absolute threshold-sensation/no sensation </li></ul><ul><ul><li>Correspond to the zero point on the psychological scale of sensation magnitude( ψ ) </li></ul></ul>
  20. 25. <ul><li>The first, second, third…. Δφ would be record </li></ul><ul><li> specified in terms of a graph or an equation </li></ul><ul><li>Assuming the validity of Weber’s law </li></ul><ul><ul><li>Calculate the number of jnd’s above absolute threshold for specific values of the stimulus </li></ul></ul><ul><li>Equal increments in sensation correspond to larger and larger increases in stimulus intensity as it increases </li></ul>
  21. 28. <ul><li>Fechner derived a general formula from Weber’s, known as Fechner’s law </li></ul><ul><li>k – is a constant multiplier, the value of which depends upon the particular sensory dimension and modality </li></ul>
  22. 30. Fechner’s Law <ul><li>Two main assumptions </li></ul><ul><li>1. Fechner’s law is valid only to the extent of Weber’s law is correct </li></ul><ul><ul><li>Weber’s fraction is not a constant at the low end of the stimulus </li></ul></ul><ul><li>2. The jnd is an equal increment in sensation at all levels of stimulus intensity </li></ul>
  23. 31. <ul><li>Experimental test have shown that jnd’s along the intensive dimension are psychologically unequal (S. S. Stevens, 1936) </li></ul><ul><li>Durup and Piéron, 1933 </li></ul><ul><li>The jnd’s are not subjectively equal thus cannot be used as the basic unit for measurement of sensation magnitude </li></ul>
  24. 32. <ul><li>The importance of his accomplishments lies in the direction he took while trying to deal with problems of mental event </li></ul>