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Neuroscience: Exploring the Brain, 3e

Explore the significance of vision, the eye's structure, and how it processes light information. Learn about photoreceptors, retinal anatomy, visual field, and phototransduction in rods and cones. Delve into the relationship between the human eye and a camera, as well as the properties and adaptation of light. Gain insights into retinal processing, regional differences in structure, and how the brain interprets visual data. Discover the intricate pathways and mechanisms that allow us to perceive and appreciate the world through our eyes.

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Neuroscience: Exploring the Brain, 3e

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  1. Neuroscience: Exploring the Brain, 3e Chapter 9: The Eye

  2. Introduction • Significance of vision • Relationship between human eye & camera • Retina • Photoreceptors: Converts light energy into neural activity • Detects differences in intensity of light • Lateral geniculate nucleus (LGN) • First synaptic relay in the primary visual pathway • Visual information ascends to cortex interpreted and remembered

  3. Properties of Light • Light • Electromagnetic radiation • Wavelength, frequency, amplitude

  4. Properties of Light • Light • Energy is proportional to frequency • e.g., gamma radiation and cool colors - high energy • e.g., radio waves and hot colors - low energy

  5. Properties of Light • Optics • Study of light rays and their interactions • Reflection • Bouncing of light rays off a surface • Absorption • Transfer of light energy to a particle or surface • Refraction • Bending of light rays from one medium to another

  6. The Structure of the Eye • Gross Anatomy of the Eye • Pupil: Opening where light enters the eye • Sclera: White of the eye • Iris: Gives color to eyes • Cornea: Glassy transparent external surface of the eye • Optic nerve: Bundle of axons from the retina

  7. The Structure of the Eye • Ophthalmoscopic Appearance of the Eye

  8. The Structure of the Eye • Cross-Sectional Anatomy of the Eye

  9. Image Formation by the Eye • Refraction of light by the cornea • Eye collects light, focuses on retina, forms images

  10. Image Formation by the Eye • Accommodation by the Lens • Changing shape of lens allows extra focusing power

  11. Image Formation by the Eye • The Pupillary Light Reflex • Connections between retina and brain stem neurons that control muscle around pupil • Continuously adjusting to different ambient light levels • Consensual • Pupil similar to the aperture of a camera

  12. Image Formation by the Eye • The Visual Field • Amount of space viewed by the retina when the eye is fixated straight ahead

  13. Image Formation by the Eye • Visual Acuity • Ability to distinguish two nearby points • Visual Angle: Distances across the retina described in degrees

  14. Microscopic Anatomy of the Retina • Direct (vertical) pathway: • Ganglion cells  • Bipolar cells  • Photoreceptors

  15. Microscopic Anatomy of the Retina • Retinal processing also influenced lateral connections: • Horizontal cells • Receive input from photoreceptors and project to other photoreceptors and bipolar cells • Amacrine cells • Receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells

  16. Microscopic Anatomy of the Retina • The Laminar Organization • Inside-out • Light passes through ganglion and bipolar cells before reaching photoreceptors

  17. Microscopic Anatomy of the Retina • Photoreceptor Structure • Converts electromagnetic radiation to neural signals • Four main regions • Outer segment • Inner segment • Cell body • Synaptic terminal • Types of photoreceptors • Rods and cones

  18. Microscopic Anatomy of the Retina • Regional Differences in Retinal Structure • Varies from fovea to retinal periphery • Peripheral retina • Higher ratio of rods to cones • Higher ratio of photoreceptors to ganglion cells • More sensitive to light

  19. Microscopic Anatomy of the Retina • Regional Differences in Retinal Structure (Cont’d) • Cross-section of fovea: Pit in retina where outer layers are pushed aside • Maximizes visual acuity • Central fovea: All cones (no rods) • 1:1 ratio with ganglion cells • Area of highest visual acuity

  20. Phototransduction • Phototransduction in Rods • Light energy interacts with photopigment to produce a change in membrane potential • Analogous to activity at G-protein coupled neurotransmitter receptor - but causes a decrease in second messenger

  21. Phototransduction • Phototransduction in Rods • Dark current: Rod outer segments are depolarized in the dark because of steady influx of Na+ • Photoreceptors hyperpolarize in response to light

  22. Phototransduction • Phototransduction in Rods • Light activated biochemical cascade in a photoreceptor • The consequence of this biochemical cascade is signal amplification

  23. Phototransduction • Phototransduction in Cones • Similar to rod phototransduction • Different opsins • Red, green, blue • Color detection • Contributions of blue, green, and red cones to retinal signal • Spectral sensitivity • Young-Helmholtz trichromacy theory of color vision

  24. Phototransduction • Dark and Light Adaptation • Dark adaptation—factors • Dilation of pupils • Regeneration of unbleached rhodopsin • Adjustment of functional circuitry 20–25 minutes All-cone daytime vision All-rod nighttime vision

  25. Phototransduction • Dark and Light Adaptation • Calcium’s Role in Light Adaptation • Calcium concentration changes in photoreceptors • Indirectly regulates levels of cGMP channels

  26. Retinal Processing • Transformations in the Outer Plexiform Layer • Photoreceptors release less neurotransmitter when stimulated by light • Influence horizontal cells and bipolar cells

  27. Retinal Processing • Receptive Field: “On” and “Off” Bipolar Cells • Receptive field: Stimulation in a small part of the visual field changes a cell’s membrane potential • Antagonistic center-surround receptive fields

  28. Retinal Processing • On-center Bipolar Cell • Light on (less glutamate); Light off -> more glutamate ‘Inverting’ synapse (inhib)

  29. Retinal Output • Ganglion Cell Receptive Fields • On-Center and Off-Center ganglion cells • Responsive to differences in illumination

  30. Retinal Output • Types of Ganglion Cells • Appearance, connectivity, and electrophysiological properties • M-type (Magno) and P-type (Parvo)ganglion cells in monkey and human retina

  31. Retinal Output • Color-Opponent Ganglion Cells

  32. Retinal Output • Parallel Processing • Simultaneous input from two eyes • Information from compared in cortex • Depth and the distance of object • Information about light and dark: ON-center and OFF-center ganglion cells • Different receptive fields and response properties of retinal ganglion cells: M- and P- cells, and nonM-nonP cells

  33. Concluding Remarks • Light emitted by or reflected off objects in space  imaged onto the retina • Transduction • Light energy converted into membrane potentials • Phototransduction parallels olfactory transduction • Electrical-to-chemical-electrical signal • Mapping of visual space onto retina cells not uniform

  34. End of Presentation

  35. Retinal Processing • Research in ganglion cell output by • Keffer Hartline, Stephen Kuffler, and Horace Barlow • Only ganglion cells produce action potentials • Research in how ganglion cell properties are generated by synaptic interactions in the retina • John Dowling and Frank Werblin • Other retinal neurons produce graded changes in membrane potential

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