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Eye and Early Vision

Eye and Early Vision. NRS 495 – Neuroscience Seminar Christopher DiMattina , PhD. Light. Reflected light is a useful source of information Organisms have evolved means of detecting light. Eyespots. The simplest `eyes’ are the eyespots of simple organisms

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Eye and Early Vision

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  1. Eye and Early Vision NRS 495 – Neuroscience Seminar Christopher DiMattina, PhD

  2. Light • Reflected light is a useful source of information • Organisms have evolved means of detecting light NRS 495 - Grinnell College - Fall 2012

  3. Eyespots • The simplest `eyes’ are the eyespots of simple organisms • Euglena uses its eyespot to move towards light NRS 495 - Grinnell College - Fall 2012

  4. Cup eyes • Cup eyes give directionality from Land & Ferdnald 1992 NRS 495 - Grinnell College - Fall 2012

  5. Pinhole eyes • Pinhole opening helps to focus light • Reduces amount of light entering eye from Land & Ferdnald 1992 from Palmer 1999 NRS 495 - Grinnell College - Fall 2012

  6. Lens • A lens focuses the image while permitting lots of light into eye from Palmer 1999 from Land & Ferdnald 1992 NRS 495 - Grinnell College - Fall 2012

  7. Eyes • A wide variety of eyes are found in the animal kingdom • Organisms living in dark environments often lack eyes! Texas blind salamander NRS 495 - Grinnell College - Fall 2012

  8. Light NRS 495 - Grinnell College - Fall 2012

  9. What is light? • Light is made up of photons - both particle and wave-like • Occupies a small part of the electro-magnetic spectrum NRS 495 - Grinnell College - Fall 2012

  10. White light • White light is a mixture of different colors • Different colors have different wavelengths • Can separate by bending with a prism NRS 495 - Grinnell College - Fall 2012

  11. Illumination • Light can be emitted from a point source like the sun on a clear day to give rise to clear shadows • On a cloudy day light sources are more diffuse NRS 495 - Grinnell College - Fall 2012

  12. Light and surfaces Light reflecting off surfaces can provide useful information NRS 495 - Grinnell College - Fall 2012

  13. Refraction • When light travels between different media, it is bent • Important in optics of the eye for focusing light NRS 495 - Grinnell College - Fall 2012

  14. The goal of vision • The goal of vision is to reconstruct the distal stimulus in the world given the proximal image on the retina • Non-trivial since there is a confound between size and distance for the proximal stimulus NRS 495 - Grinnell College - Fall 2012

  15. Indeterminacy • In general, an infinity of 3-D distal images can give rise to the same proximal 2-D pattern of stimulation on the retina NRS 495 - Grinnell College - Fall 2012

  16. The Eye NRS 495 - Grinnell College - Fall 2012

  17. The eye • The eye has been compared to a camera • Both focus light on a surface (retina, film) NRS 495 - Grinnell College - Fall 2012

  18. A super-fancy camera • TV camera with automatically tracking tripod • Self cleaning lens • Adjusts for light intensity • Self-focusing • Feeds into a sophisticated super-computer NRS 495 - Grinnell College - Fall 2012

  19. The eye • Light first encounters the cornea, which focuses the light • Travels through aqueous humor to lens which focuses more NRS 495 - Grinnell College - Fall 2012

  20. Iris • Amount of light entering pupil of the eye determined by iris • Pupillary light reflex automatically adjusts size of pupil NRS 495 - Grinnell College - Fall 2012

  21. Retina • Light travels through the vitreous humor and is ultimately focused on the retina, which contains light-sensitive cells NRS 495 - Grinnell College - Fall 2012

  22. Focusing light on the retina • The point of the eye is to focus light on the retina • Cornea and lens refract light • Lens thickness is adjustable (accomodation) NRS 495 - Grinnell College - Fall 2012

  23. Emmetropia • When optics of the eye focus light in the retinal plane (good!) NRS 495 - Grinnell College - Fall 2012

  24. Myopia • Eye is too long for its optics (near-sightedness) NRS 495 - Grinnell College - Fall 2012

  25. Correcting myopia • Negative lenses which diverge the light rays correct myopia NRS 495 - Grinnell College - Fall 2012

  26. Hyperopia • Eye is too short for optics (far-sightedness) • What sort of lens corrects this? NRS 495 - Grinnell College - Fall 2012

  27. The lens • Hardens with age, so less able to accommodate to bring nearby objects into focus (presbyopia = “old sight”) • Usually clear, but sometimes can become opaque due to irregularities in crystallins – known as cataracts NRS 495 - Grinnell College - Fall 2012

  28. The Retina NRS 495 - Grinnell College - Fall 2012

  29. Retina • The retina is a neural structure located at the back of the eye • Contains light-sensitive photoreceptors and neural circuitry NRS 495 - Grinnell College - Fall 2012

  30. Backwards organization • Photoreceptors are behind the neural circuitry • Layer of pigmented light-absorbing cells behind photoreceptors (why might this be useful?) NRS 495 - Grinnell College - Fall 2012

  31. A closer view NRS 495 - Grinnell College - Fall 2012

  32. The blind spot NRS 495 - Grinnell College - Fall 2012

  33. Blind spot demonstration NRS 495 - Grinnell College - Fall 2012

  34. The fovea NRS 495 - Grinnell College - Fall 2012

  35. Eye movements • Fixation focuses image in fovea where resolution is highest NRS 495 - Grinnell College - Fall 2012

  36. Photoreceptors NRS 495 - Grinnell College - Fall 2012

  37. Rhodopsin • Found in the photoreceptor outer segment membrane • Contains a molecule which absorbs light • Initiates a complex process which results in hyperpolarization of the photoreceptor NRS 495 - Grinnell College - Fall 2012

  38. Rod and cone systems NRS 495 - Grinnell College - Fall 2012

  39. Light level adaptation NRS 495 - Grinnell College - Fall 2012

  40. Distribution of rods and cones NRS 495 - Grinnell College - Fall 2012

  41. Cones NRS 495 - Grinnell College - Fall 2012

  42. Retinal Circuitry • Photoreceptors (input) • Interneurons (processing) • Retinal ganglion cells (output) NRS 495 - Grinnell College - Fall 2012

  43. Bipolar cells • Receive input from photoreceptors • Diffuse bipolar cellsintegrate from many cones • Midget bipolar cells receive input from single cone NRS 495 - Grinnell College - Fall 2012

  44. Discuss • Where in the retina might we expect to find the midget bipolar cells? How about diffuse bipolars? • What is the trade-off between acuity and sensitivityin the two different kinds of bipolar cells? NRS 495 - Grinnell College - Fall 2012

  45. Horizontal and amacrine cells • Horizontal and amacrine cells implement lateral inhibition • Makes bipolar cells and ganglion cells sensitive to contrast NRS 495 - Grinnell College - Fall 2012

  46. Center-surround circuit • Direct pathway from photoreceptors causes activation or inhibition from center region (on/off) • Indirect pathway from horizontal cells causes opposite effect in surround (on/off) NRS 495 - Grinnell College - Fall 2012

  47. Retinal Ganglion Cells NRS 495 - Grinnell College - Fall 2012

  48. Retinal ganglion cells • Retinal ganglion cells are the output of the retina • First retinal cells which fire action potentials NRS 495 - Grinnell College - Fall 2012

  49. P and M cells • Midget bipolar cells project to parvocellular (P) ganglion cells • Diffuse bipolar cells project to magnocellular (M) ganglion cells NRS 495 - Grinnell College - Fall 2012

  50. Receptive field • What is the definition of receptive field?? • This is a central concept in sensory neuroscience NRS 495 - Grinnell College - Fall 2012

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