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PSYC 330: Perception

Seeing i n Color. PSYC 330: Perception . SEEING IN COLOR. PSYC 330: Perception. Color. Some more terminology….. scotopic – low light levels, appropriate for rods photopic – higher light levels, appropriate for cones. The Stimulus. Color dimensions HUE wavelength

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PSYC 330: Perception

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  1. Seeing in Color PSYC 330: Perception SEEING IN COLOR PSYC 330: Perception

  2. Color Some more terminology….. scotopic – low light levels, appropriate for rods photopic – higher light levels, appropriate for cones

  3. The Stimulus • Color dimensions • HUE wavelength • BRIGHTNESS intensity/luminance • SATURATION mix of wavelengths for intensity

  4. Cones • Why three kinds? • The problem of univariancehttp://sites.sinauer.com/wolfe3e/chap5/univarianceF.htm • A single light gives us two dimensions (wavelength and brightness) • A single photoreceptor can vary in only one way (firing rate) • SO…. Different combinations of wavelength and intensities can yield identical firing rates in an individual cell…. therefore we can’t properly interpret color

  5. The solution? • Trichromacy….. Three different cones for the detection of color • Photopigments maximally responsive to given wavelength http://sites.sinauer.com/wolfe3e/chap5/trichromF.htm

  6. BUT we are usually seeing mixtures of lightMetamers

  7. http://www.youtube.com/watch?v=w6ccBwnc5KU&feature=player_embeddedhttp://www.youtube.com/watch?v=w6ccBwnc5KU&feature=player_embedded WTH????

  8. Opponent Process Theory • How the heck can trichromacy explain that? • Cone opponent cells • Retina L-M and [L+M]-SM-L and [M+L]-S Center/surround • LGN

  9. Color opponent cells and double opponents…. cortex

  10. But that’s NOT all, folks! Culture effects – from the Dani to the Russians…. and beyond? Color is a psychological experience!

  11. What is the problem? Contrast and assimilation effects (effects of nearby colors) Related and unrelated colors (e.g. gray and brown only experienced when compared to lighter regions) Variations in the illuminant Cone output NOT equal, yet we still see color appropriately (usually)

  12. Color constancy http://sites.sinauer.com/wolfe3e/chap5/illusionsF.htm

  13. How do we do it? • Look for and learn predictable features of the world • e.g. sharp divisions (edges) between colors are usually due to surfaces, not illuminants • e.g. shades of a given color, on the other hand, are an exception (shadows)

  14. Bloj et al (1999) • Took card folded in half (red and white halves) • When folded to inward (like a greeting card), red reflects onto white and makes it pink. But observer subtracts that out and sees it as white. • When folded outward (like a roof) pink half looks pink to observer • Shows role of INFERENCES • In other words, we’re not done with vision yet….

  15. Tracing the signal Retina  ganglion cells (M and P) M – large receptive field, fast responding, sensitive at low illumination P – small receptive field, slow responding, require more illumination LGN  magno and parvocellular Magno – peripheral vision, motion, broad form Parvo – foveal vision, color, detail

  16. LGN  Striate cortex (PVC)

  17. Columns

  18. What can we actually “see” at this point? • Topographical mapping? • LGN and PVC • Simple, complex, hypercomplex (end-stopping) • Separate channels for color, motion, location

  19. Where? What?

  20. A Quick Review Psychophysics- Methodology: what are the methods for collecting data? • Results: what are the findings? • Thresholds (definition, kinds) • Psychophysical laws (Weber, Fechner, Stevens) • What do these tell us? • Signal detection theory (d’, B, ROC curves)

  21. A Quick Review • Philosophy of perception • Sense data theories and naïve realism • Adverbial theories • Intentionalist theories (representation) • Alt theories

  22. A Quick Review • Physiology • Doctrine of Specific Nerve Energies • Sense receptors and transduction • Neuron (parts) • Resting and action potential • Synapse • Sensory cortex (lobes)

  23. A Quick Review • Physiology of vision • Properties of light • Structures in the eye • How the structures focus light • Rods and cones • How they transduce light • Distribution on the retina • Functions/specializations • Other layers of cells and their functions

  24. A Quick Review • Brain or bust • Optic chiasm • LGN • PVC • Visual fields and where the info goes • Magnocellular and parvocellular • Visual stimuli (spatial frequency) more to come……

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