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Eye and Early Vision – Part II

Eye and Early Vision – Part II. PSY 295 – Sensation & Perception Christopher DiMattina , PhD. Photoreceptors. Rhodopsin. Found in the photoreceptor outer segment membrane Contains a molecule which absorbs light

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Eye and Early Vision – Part II

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  1. Eye and Early Vision – Part II PSY 295 – Sensation & Perception Christopher DiMattina, PhD

  2. Photoreceptors

  3. 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

  4. Rod and cone systems

  5. Light level adaptation

  6. Distribution of rods and cones

  7. Cones

  8. Retinal Circuitry • Photoreceptors (input) • Interneurons (processing) • Retinal ganglion cells (output)

  9. Bipolar cells • Receive input from photoreceptors • Diffuse bipolar cellsintegrate from many cones • Midget bipolar cells receive input from single cone

  10. 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?

  11. Horizontal and amacrine cells • Horizontal and amacrine cells implement lateral inhibition • Makes bipolar cells and ganglion cells sensitive to contrast

  12. 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)

  13. Web activity: Retinal structure • http://sites.sinauer.com/wolfe3e/chap2/retstructureF.htm

  14. Retinal Ganglion Cells

  15. Retinal ganglion cells • Retinal ganglion cells are the output of the retina • First retinal cells which fire action potentials

  16. P and M cells • Midget bipolar cells project to parvocellular (P) ganglion cells • Diffuse bipolar cells project to magnocellular (M) ganglion cells

  17. Receptive field • What is the definition of receptive field?? • This is a central concept in sensory neuroscience

  18. Ganglion cell receptive fields • Have a center-surround organization (Kuffler 1953)

  19. Off-center cell

  20. Web: Receptive field mapping • http://sites.sinauer.com/wolfe3e/chap2/ganglionF.htm

  21. Retinal Information Processing

  22. Spatial frequency filtering • Center-surround organization makes retinal ganglion cells into spatial frequency filters

  23. Contrast coding • Retinal ganglion cells are sensitive to contrast, or difference in luminance between adjacent regions

  24. Mach bands

  25. Mach bands

  26. Detecting edges • Center-surround receptive fields can be used to detect edges

  27. Disorders of early vision

  28. AMD • age-related macular degeneration • leading cause of visual impairment among elderly

  29. AMD • Dry AMD – macula cones die (more common) • Wet AMD – abnormal blood vessels leak blood and fluid, displacing the macula • result is a large scotoma

  30. Retinitis pigmentosa • Hereditary disease caused by death of photoreceptors and degeneration of pigment epithelium • First affects rods in the periphery – patient may report problems with peripheral night vision (stargazing)

  31. Ring scotomas

  32. Summary • The eye is a fancy camera which focuses light on the retina • The retina is the first stage of vision in the brain • The retina performs sophisticated computations • The eye and retina are subject to various medical disorders

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