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Understanding Early Vision: Photoreceptors, Retinal Circuitry, and Visual Disorders

This course segment delves into the complexities of early vision, focusing on the role of photoreceptors like rhodopsin in light absorption and the processes of phototransduction. It explores the distribution and functionality of rods and cones, as well as the retinal circuitry including bipolar, amacrine, and ganglion cells. Key concepts include lateral inhibition, receptive field organization, and the implications for visual acuity and sensitivity. The course also addresses disorders such as age-related macular degeneration and retinitis pigmentosa, highlighting their impact on visual perception.

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Understanding Early Vision: Photoreceptors, Retinal Circuitry, and Visual Disorders

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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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