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The Visual System: Retinal Mechanisms

This lecture explores the intricate mechanisms of the retina, detailing how light stimuli are transformed into visual information. Covering the electromagnetic spectrum, we delve into the roles of photoreceptors—rods and cones—and their functions in color vision and low-light sensitivity. The lecture outlines the retinal organization, signal transduction processes, and the journey of visual signals through the optic nerve to the primary visual cortex. Key concepts include brightness, wavelength, center-surround organization, and lateral inhibition mechanisms that enhance contrast in visual perception.

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The Visual System: Retinal Mechanisms

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  1. The Visual System:Retinal Mechanisms Lecture 14

  2. The light stimulus • Electromagnetic energy • visible light • 350 – 700 nm (violet – red) • Photon • Wave • Color (hue) - wavelength • Brightness (intensity) – amplitude ~

  3. Color Wavelength Amplitude Brightness

  4. Visual Pathway Optic Nerve Optic Chiasm Optic Tract LGN LGN Lateral Geniculate Nucleus (Thalamus) Optic Radiations V1 V1 Primary Visual Cortex LVF RVF Retina

  5. Retinal Organization • Photoreceptors (PR) • transduction • Bipolar cells (BP) • no APs • Retinal ganglion cells (RGC) • 1st APs in pathway • Amacrine & Horizontal cells • communication b/n retinal cells • parallel to retina ~

  6. Retinal Organization Light Which direction does the light enter? PR BP RGC Optic Nerve

  7. Photoreceptors (PR) • Transduction • Rods • achromatic • Cones • color ~

  8. Photoreceptors - PR • Dark current • Depolarized in dark • Na+ influx • NT is released • BP • inhibited ~

  9. Photoreceptors - PR • Light strikes PR • Receptor potential • hyperpolarization •  NT release • BP depolarizes •  NT release • Excites RGC •  APs ~

  10. Rods • Around edges of retina • 120 million • Pigment = rhodopsin • Convergence • high sensitivity • low acuity ~

  11. Cones • Color vision - 3 types • red = long wavelength • green = medium • blue = short • 6 million • Concentrated in fovea • Little convergence • low sensitivity • high acuity ~

  12. Receptive Field (RF) • Region of the retina where... • changes in illumination... • will change the activity of a particular neuron • Center-Surround Organization • BP • RGC • LGN neurons • convergence ~

  13. Center-Surround Organization BP RGC PR

  14. Center-Surround Organization 2 arrangements ON OFF OFF ON

  15. Center-Surround Organization • Light strikes  On area of RF • RGC excited   APs • Light strikes  Off • RGC inhibited   APs • Contrast  maximal effects on RGC • center & surround lighted cancels out ~

  16. Dark Light PR PR - - H - BP BP + + G G Lateral Inhibition • Center-surround RF • Horizontal cells • no APs • PR stimulates HC • inhibit adjacent BPs • Enhances contrast ~ + Surround Center

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