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The Visual System: Retinal Mechanisms

This lesson explores the retinal mechanisms involved in processing light stimuli. It covers electromagnetic energy, visible light wavelengths, and the role of photons. We detail the structures involved, including photoreceptors (rods and cones), bipolar cells, and retinal ganglion cells, explaining the transduction process from light reception to action potentials. Key concepts such as receptive fields, center-surround organization, lateral inhibition, and the visual pathway to the primary visual cortex (V1) are highlighted, emphasizing the integration of sensory information for visual perception.

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The Visual System: Retinal Mechanisms

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  1. The Visual System:Retinal Mechanisms Lesson 16

  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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