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Lecture #9

Lecture #9. L&N ch 4 : Grand challenges in aquatic vision 2/21/13. Topics for today. You are now experts in the physical principles that underlie visual system design We’re going to present you with several challenges and see what sorts of solutions you can come up with

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Lecture #9

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  1. Lecture #9 L&N ch 4 : Grand challenges in aquatic vision 2/21/13

  2. Topics for today • You are now experts in the physical principles that underlie visual system design • We’re going to present you with several challenges and see what sorts of solutions you can come up with • We’ll discuss some examples and key points as we go

  3. Challenge 1 • How do you design an aquatic eye when the index of refraction of tissue is nearly the same as water?

  4. Eye designs a pit c lens g single chamber mirror b compound e apposition f superposition h compound mirror Single chamber Compound Fig 1.9

  5. Solution 1a) Pinhole eye • Acceptance angle is about 10° • Limits amount of light coming in • Can form an image Abolone

  6. Leonardo Da Vinci - camera obscura • Image passes through pinhole • Keeps all in focus • Inverts the image

  7. Nautilus • Largest pinhole eye, 1 cm diameter • Pinhole size can vary • 0.4 to 2.8 mm • Pinhole is actually oval

  8. Build model of Nautilus eye to test resolution • Image falling on Nautilus retina shows blurring as pupil gets larger • Incident on eye • 1 x 0.4 mm pupil • 2 x 1 mm • 2.8 x 1.7 mm 1 2 3 4 Acceptance angle is 2.3°for smallest pupil

  9. Nautilus vision • Low light collection efficiency of Nautilus eye makes the world a darker place • 400 times dimmer than fish eye

  10. Optomotor response to measure visual angular resolution • Rotate grating • Nautilus swims to keep image of world constant • Decrease grating - determine when nautilus can’t resolve it - stops swimming Angles < 11°

  11. Solution 1b) Add a lens - Molluscs • Helix has a blob of jelly • Increases light collection – originally no image! • But eventually evolved to focus light onto retina

  12. Single chambered aquatic eye – if life evolved on land, the lens might not have been invented! Molluscs Vertebrates

  13. What kind of lens do you want? • Lens focal length determined by curvature of lens surface

  14. Shortest radius of curvature is a sphere • Sphere defining the radius of curvature is the spherical lens itself r Shortest possible focal length

  15. Lens focal length • Lens maker’s equation

  16. Lens focal length • If n2=1.53 (lens crystallin) and n1=1.34 (water) • Focal length is 4*radius or 2*lens diameter

  17. How does focal length depend on lens material, n2 Many fish and cephalopods have lens focal length of 2.5r This is Matthiessen’s ratio

  18. 1c) Flat cornea – no contribution to focusing

  19. 1d) Compound eye with individual lens for each receptor set

  20. 1e) Collect light with mirror

  21. Challenge 2 • Water limits the transmission of light

  22. Animals came out of the water onto land with evolution of amphibians about 350 Mya Vertebrate eyes evolved >500 MYa 2a) Spectral properties of water shape the evolution of life

  23. Aquatic eye sets the course of the vertebrate eye

  24. Several families of opsin genes RH1 - rods RH2 SWS2 cones SWS1 LWS

  25. Light spectrum into clear shallow water

  26. Visual pigment diversity in vertebrates matches light transmitted by water

  27. 2b) Deep sea tubular eyesDeep sea fish - Scopelarchus • No light from below • Only look up and to side • Tubular eyes • Large binocular overlap Fig 4.9

  28. Fish of the deep… http://www.youtube.com/watch?v=RM9o4VnfHJU

  29. 2c) Compound eyes • Can make large lenses to collect lots of light • Crab – surface vs deep sea isopod

  30. 2d) Phosphorescence and detection Malacosteusniger • Deep sea dragonfish • Red emitting photophore • Private communication • Or Head light

  31. Malacosteusniger • Deep sea dragonfish • To detect red light put chlorophyll into photoreceptors • Chlorophyll absorbs red light and transfers energy to visual pigment

  32. Challenge #3 • Eyes are complicated to make. If you can only afford to have one receptor for a lens, how can you learn about your environment?

  33. Dual element lens – scan receptor across image • Copepod Saphirina • Large lens on top • Small lens above retina with 6-7 receptors • Acceptance angle about 3° • Scan small lens and retina about 15° at 0.5-10 Hz Fig 4.11

  34. Sea urchin tube feet • Early photoreceptors were single detectors which collect light in many directions

  35. Sea urchin tube foot is a photoreceptor

  36. 3c) Compound eye: Take lots of single receptors and put them together

  37. Challenge 4 • Eyes have to trade off sensitivity and resolution. Is there another way to get around this trade off?

  38. Lens eye - cnidarians Neither lens eye focuses an image

  39. Challenge 5 • A single chambered eye has high resolution. Is it possible to evolve this eye more than once? If two organisms had a single chambered eye, how would you know if they were from a common ancestor?

  40. Fish and cephalopod eyes Fig 4.6

  41. Similarities

  42. Adaptation for aquatic lens Homogeneous lens Actual fish lens does not show aberration Spherical homogeneous lens - focuses too much on edges: spherical aberration Fig 4.3

  43. Graded index lens - refractive index decreases at edge so longer focal length on edges Fig 4.3

  44. Fish and cephalopod eyes Fig 4.6

  45. Differences

  46. Giant squid eyes http://www.youtube.com/watch?v=JSBDoCoJTZg

  47. Squid lens is in two halves

  48. Why do squid have such large eyes?

  49. Detect predators which induce phosphorescence – low photon vision

  50. Summary • Lens eyes evolved from pigmented eyes multiple times • Evidence for convergent evolution • Most aquatic lenses have graded index of refraction • Retina can be specialized • Can have multi-element lenses

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