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Perceptual Issues

Perceptual Issues. Humans can discriminate about ½ a minute of arc At fovea, so only in center of view, 20/20 vision At 1m, about 0.2mm (“Dot Pitch” of monitors) Limits the required number of pixels Humans can discriminate about 8 bits of intensity “Just Noticeable Difference” experiments

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Perceptual Issues

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  1. Perceptual Issues • Humans can discriminate about ½ a minute of arc • At fovea, so only in center of view, 20/20 vision • At 1m, about 0.2mm (“Dot Pitch” of monitors) • Limits the required number of pixels • Humans can discriminate about 8 bits of intensity • “Just Noticeable Difference” experiments • Limits the required depth for typical dynamic ranges • Actually, it’s 9 bits, but 8 is far more convenient • BUT, while perception can guide resolution requirements for display, when manipulating images much higher resolution may be required 129 128 125

  2. Origins in Philosophy • Mind-body problem – are the mind and body the same or different? • If they are different substances, how do they interact or communicate? • Dualism – mind (soul) is not governed by physical laws but possesses free will. • Descartes – mutual interaction. • Animals do not possess souls and can be studied because they are physical.

  3. Rene Descartes (1596-1650)

  4. British Empiricism • Locke, Berkeley, Hume, Hartley • Mind may follow laws and thus be modeled just as the physical world is. • Elements (ideas) • Forces (associations between ideas) • Tabula rasa – mind is a blank slate written upon by experience. • Mental activity may be mechanical: • Mind as a machine

  5. Helmholtz (1821-1894) • Used experimental methods to study vision and audition. • Reaction times were used to determine the speed of neural impulses. • Test response-times for stimuli from the shoulder and from the ankle. • Nerve impulses are slow – 50 meters per sec. • Reaction times vary considerably across individuals and across trials – how is precise measurement possible?

  6. Weber (1795-1878) • Weber studied perceptions of weight and tried to relate these to actual physical weight. • Weight is an objective physical property of objects. • The greater the weight, the greater the difference between a standard and comparison must be to be detectable. • Weber’s Law -- Just-noticeable difference (jnd) is a constant across sensory modalities.

  7. Fechner (1801-1887) • Tried to relate physical properties to psychological sensations: • Related the objective to the subjective. • Fechner’s Law – each JND corresponds to one subjective unit of measure, with the relationship described mathematically. • Credited with founding psychophysics.

  8. Wundt & Ebbinghaus • Wundt (1832-1920) organized psychology and helped to establish it as an independent discipline. • Wrote “Principles of Physiological Psychology” • Did not believe higher mental processes (memory, thought, creativity) could be studied experimentally. • Ebbinghaus (1850-1909) demonstrated that memory could be studied experimentally.

  9. Stucturalism vs Functionalism • Structuralism – focused on the contents of mind. • Sensations, images (ideas), affections • Used introspection to identify basic elements. • Introspection proved to be an unreliable method. • Functionalism – focused on the adaptive function of psychological processes within a context. • Not much experimental work done.

  10. The Same Color?

  11. The Same Color?

  12. Webber’s Law

  13. Sensing the World Around Us • Absolute threshold • The smallest intensity of a stimulus that must be present for it to be detected

  14. Contrast Sensitivity 0% 1% 2% 3% 4% Circle constant Background constant Just noticeable difference (JND) at 2%

  15. Contrast Sensitivity 0% 1% 2% 3% 4% Circle constant Background constant Just noticeable difference (JND) at 2%

  16. Contrast Sensitivity 0% 1% 2% 3% 4% Backgrounddifferent thenboth halves Backgroundsame asright half Just noticeable difference (JND): 4% (top) and 2% (bottom)

  17. Contrast Sensitivity 0% 1% 2% 3% 4% Backgrounddifferent thenboth halves Backgroundsame asright half Just noticeable difference (JND): 4% (top) and 2% (bottom)

  18. Brightness versus intensity • standard light at fixed intensity • test light with adjustable intensity • adjust power of test until just begins to differ • just noticeable difference: JND

  19. Brightness versus intensity Standard Test A just noticeable difference (JND) at 11W 1 W above standard

  20. Forced-choice Response • A bit more rigorous • Q: brighter light on left or right?

  21. Forced-choice Response • A bit more rigorous • Q: brighter light on left or right?

  22. Forced-choice Response • A bit more rigorous • Q: brighter light on left or right?

  23. Forced-choice Response • A bit more rigorous • Q: brighter light on left or right?

  24. Forced-choice Response • A bit more rigorous • Q: brighter light on left or right?

  25. Forced-choice Response • A bit more rigorous • Q: brighter light on left or right?

  26. Forced-choice Response • A bit more rigorous • Q: brighter light on left or right? • Analyse accuracy of response versus intensity of lights

  27. Brightness depends on wavelength • Light 1: at one wavelength • Light 2: at different wavelengthAdjust power of second light until its brightness is the same as the first

  28. Brightness depends on wavelength • Light 1: at one wavelength • Light 2: at different wavelengthAdjust power of second light until its brightness is the same as the first

  29. Brightness depends on wavelength • Light 1: at one wavelength • Light 2: at different wavelengthAdjust power of second light until its brightness is the same as the first

  30. Brightness depends on wavelength • Light 1: at one wavelength • Light 2: at different wavelengthAdjust power of second light until its brightness is the same as the first

  31. Simultaneous brightness contrast:two squares of the same intensity

  32. Simultaneous brightness contrast:left one looks brighter

  33. Simultaneous brightness contrast:pattern increases difference

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