Chapter 10: Color perceptionLec. 20, Thursday, March 18• Trichromacy• Metamers• Psychological primaries• Channels • Color deficiency• Animal vision• Temporal processing1We are here1BAReview• Cones: S, I and L • Metamers• Channels: r-g, y-b, w-bl• Color constancy• Simultaneous color contrast• Color deficiency• Animal vision 2Review: primary colors3Channels (again)Here is a review of the way the red minus green channel worksHigh center signal = red. Low center signal = green. (No negative signals.)Green surround turns up red at the center. Strongest signal (interpreted as red)Weakest signal (interpreted as green)No change in signal (color not noticed)No change in signal (color not noticed)Note, you would still "see" red if the center were gray!Note, you would still "see" green if the center were gray!4Strongest signal (interpreted as yellow)Weakest signal (interpreted as blue)No change in signal (color not noticed)No change in signal (color not noticed)Note, you would still "see" yellow if the center were grey!Note, you would still "see" blue if the center were grey!Channels (again)Here is a review of the way the yellow minus blue channel worksHigh center signal = yellow. Low center signal = blue. (No negative signals.)Blue surround turns up yellow at the center. 5Temporal processingNegative afterimage:Look at a strong color, then white paper. Fatigue of nerves causes a negative afterimage. Example: Stare at red, then white, You will see cyan (white – red)Positive afterimage:Strongly stimulated nerves keep firing667788910Afterimage illusion11http://www.youtube.com/watch?v=7knnO4vKgXg11Addition in timeColors flashed rapidly will appear constant and will add (persistence of vision). What will this disk look like if rotated?1212Demo: thisBidwell’s disk13A rotating white disk covers and uncovers a red light. What will you see?13Demo: thisBenham’s disk14What do you see when the disk is rotated?14Benham’s disk Java Applet: http://lite.bu.edu/vision-flash10/applets/Color/Benham/Benham.htmlFechner’s colorshttp://dogfeathers.com/java/fechner.htmlhttp://dogfeathers.com/java/fechner2.htmlhttp://www.michaelbach.de/ot/col_benham/index.htmlBenham’s colorsWhile nobody knows for certain why the colours appear, lateral inhibition and the different rates of stimulation for the color-specific retinal ganglion cells clearly are involved. They code the pattern of light in space and time into patterns of nerve firings in space and time.Fechner (1838) was the first to describe this phenomenon. In 1894, toy maker CE Benhamdiscovered that a spinning disk with a particular pattern of black and white marks (more intricate than Fechner’s) could cause people to see colours. He called his disk an “Artificial Spectrum Top” ---Michael Bach’s web site15Lilac chaserhttp://www.michaelbach.de/ot/col_lilacChaser/index.htmlNegative after image. The succession of negative afterimages gives the appearance that the after-image is moving. 1617Stare at the cross, please. Concept QuestionWhat is an additive mixture of red and yellow?A. Green;B. Red;C. White;D. Black;E. Orange;18Concept QuestionWhat is an additive mixture of blue and yellow?A. Green;B. Red;C. White;D. Black;E. Orange;19Concept Question:What is the effect of blue light falling on surround of receptive field only?a) No colorb) Sensation of bluec) Sensation of greend) Sensation of yellowe) Sensation of red20Concept Question:What is the effect of green light falling on surround and red light falling on the center of the receptive field?a) No colorb) Sensation of redc) Sensation of greend) Sensation of yellow21Concept Question:What is effect of green light falling on surround only?a) No colorb) Sensation of redc) Sensation of greend) Sensation of yellow22Concept Question:What is effect of red light falling on both the center AND surround?a) No colorb) Sensation of redc) Sensation of greend) Sensation of yellow23Skipped this one24http://www.skidmore.edu/~hfoley/Perc7.htmAdditional web sites: Great detail on retinal cells firing: http://webvision.med.utah.edu/Color.htmlhttp://www.purveslab.net/seeforyourself/Cool applets, particularly the subtractive color mixing: http://www.cs.brown.edu/exploratories/freeSoftware/catalogs/repositoryApplets.htmlMore “snake” illusions:http://michaelbach.de/ot/mot_snakeAdLib/index.htmlhttp://michaelbach.de/ot/mot_enigma/index.htmlIlusiones Ópticas Volumen 1 de Loquendohttp://www.youtube.com/watch?v=1kg-pnrpD74&feature=relatedhttp://www.youtube.com/watch?v=05UOiiL1GO4&feature=related25Ames window:http://www.youtube.com/watch?v=bLDweJWGkZkAmes’