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Lecture #4

Lecture #4. Visual pigments I 2/5/13. Atmosphere scatters short wavelengths best. Lizzie’s question. Today ’ s topics. Rod and cone visual pigments Absorption Types in humans Seeing color Seeing less color. What happens with a single photo receptor?. LIGHT. Electrical output.

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Lecture #4

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  1. Lecture #4 Visual pigments I 2/5/13

  2. Atmosphere scatters short wavelengths best Lizzie’s question

  3. Today’s topics • Rod and cone visual pigments • Absorption • Types in humans • Seeing color • Seeing less color

  4. What happens with a single photo receptor? LIGHT Electrical output What information can we get?

  5. Three layers of the eye Sclera and cornea protect the eye 2) Iris and ciliary body in front and choroid in the back 3) Retina - senses light Webvision.med.utah.edu

  6. Photoreceptors at the very back of the retina (face away from light) Pigment epithelium http://webvision.med.utah.edu/

  7. Looking at an object forms an image of the object on the retina

  8. Human retina: rods and cones Pigment epithelium at back of retina

  9. Looking directly at retina

  10. Fovea has only cones. Rods more prevalent outside fovea cones Wolfe et al Sensory Perception fig 2.9

  11. Photoreceptors: Rods and cones • Rods Use at low light levels Very sensitive to light All rods have same visual pigment

  12. Photoreceptors: Rods and cones • Cones Use during day Not as sensitive 3 types each with different visual pigments which detect different parts of spectrum

  13. Parts of photoreceptors • Outer segment • Lots of membrane • Where light gets detected • Inner segment • Mitochondria to power cell • Nucleus - DNA • Synapse • Sends signal to next neuron

  14. Inner and outer segment Outer segment is composed of lots and lots of membrane Either in discs or layers

  15. Visual pigment held in membrane • 80% of the protein in the outer segment is the visual pigment • Pigment absorbs light

  16. Visual pigments absorb light – measure their absorbance spectra I0

  17. Just like spectrometer last time: I0 I Measure fraction of light transmitted through cell fT= I / I0 = exp(-ε C l) ελ= extinction coefficient of pigment C = pigment concentration l = cell diameter

  18. Just like spectrometer last time: I0 I Fraction of light absorbed (neglect reflectance) fT + fR + fA= 1 fA= 1 - fT= 1 - I / I0 = 1 - exp(-ε C l)

  19. Measure light from light source, I0

  20. Compare that to light with cone cell in beam, I Light after cone Calculate fT= I/I0

  21. Fraction of light transmitted and absorbed fT Fraction of light transmitted, T or absorbed, A fA= 1-fT fA

  22. λmax Measure how visual pigment in rods and cones absorb light Absorbs After expose to light Fish cones

  23. Bowmaker and Dartnall 1980

  24. Human photoreceptor absorbances Green Rod Red Blue Bowmaker and Dartnall 1980

  25. Summary of human rods and cones Rod 498 nm (n=11) Green 534 nm (n=11) Blue 420 nm (n=3) Red 564 nm (n=19) Bowmaker and Dartnall 1980

  26. Rod pigment - Visual purple

  27. Rods only have this one visual pigment B=G>Y>R

  28. Rods - can only detect light and dark

  29. Three human cones : short, medium, and long S M L

  30. Which cone types does this color stimulate? Short Medium Long Short>>medium Medium>>long Long>>medium

  31. Which cone types does this color stimulate? Short Medium Long Short>>medium Medium>>long Long>>medium

  32. Intermediate colors excite multiple cones

  33. How can we use multiple receptors to create an image? What’s the intensity of the Red light here? The Green light?

  34. The retina and then your brain processes output from three cones to determine “color”

  35. Can take picture of mosaic of photoreceptors in a live eye S, M, L cones all show up Expose retina to white light– then take picture All visual pigment excited by white light so don’t absorb any more light and look “bright”

  36. 550 nm will only be absorbed by M and L cones 550 nm

  37. Can take picture of mosaic of photoreceptors in a live eye at different λ S, M, L cones all show up Dark adapt, excite @ 550 nm then take picture L + M cones excited by 550 nm so look bright S cones are dark spots

  38. Excitation wavelengths to preferentially excite one cone type M>L M=L L>M 470 550 650 nm

  39. Cone distributions from photos of live retinas - excite at different λ Excite at 470 nm = M cones so see M as bright Bleach at 650 nm – so see L as bright S, M, L cells all show up Dark adapt, excite @ 550 nm so S cones are dark

  40. Overlay and false colorize Human retinal mosaic in fovea Roorda and Williams 1999

  41. Human retinal mosaic in fovea JW temporal nasal AN nasal Huge variation from person to person in distribution of cones and in M/L cone ratios! Roorda and Williams 1999

  42. Cone ratios L:M ratio can vary between 0.8 and 9.7

  43. Psychophysical test of color matching - mix red + green to make yellow + =

  44. Color sliders in real time! http://www.chriscassell.net/projects/flash/color_slider.html

  45. Where does the light come from?

  46. Direct light is additive = R + G + B Long cone Short cone Medium cone

  47. Colors on computer screens Three colors which can vary in intensity : R G and B 8 bit colors 28 = 256 11111111 = 255 Each pixel varies between 0 and 255 for each of three color channels: 2563 = 16.77 million colors

  48. Which color is R,G,B = 255, 255, 0 1 2 3 4 5 6 7 8

  49. Which color is R,G,B = 255, 255, 255 1 2 3 4 5 6 7 8

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