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Vision Lucky

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Vision Lucky

  1. 1. Vision
  2. 2. History <ul><li>Extromissive theory </li></ul><ul><ul><li>Plato--light flows from our eye </li></ul></ul><ul><ul><li>Ptolemy </li></ul></ul><ul><ul><li>Challenged later through experiments by Huygens, Newton </li></ul></ul><ul><li>Intromissive Theory </li></ul><ul><ul><li>Aristotle--object creates “material images” that enter our eyes </li></ul></ul>
  3. 3. History <ul><li>Pointillist theory of vision </li></ul><ul><ul><li>Abu Ali al-Hassan ibn al-Hasan ibn al-Haytham (Alhazen) </li></ul></ul><ul><ul><li>Optical Scientist of Middle Ages </li></ul></ul><ul><ul><li>We see a collection of points on surfaces of objects (1:1 ratio) </li></ul></ul>
  4. 4. Optics <ul><li>Four basic behaviors of light </li></ul><ul><ul><li>Reflection </li></ul></ul><ul><ul><li>Refraction </li></ul></ul><ul><ul><li>Diffraction </li></ul></ul><ul><ul><li>Absorption </li></ul></ul>
  5. 5. Reflection <ul><li>Light waves bounce off objects </li></ul>
  6. 6. Diffraction <ul><li>The bending of waves around small particles </li></ul><ul><li>The spreading out of waves past small openings </li></ul>
  7. 7. Diffraction <ul><li>Huygens </li></ul><ul><li>Thomas Young “Double Slit” Experiment </li></ul>
  8. 8. Absorption <ul><li>Transfer of energy from light to object </li></ul><ul><li>Frequency of light is at or near energy levels of electrons of matter </li></ul><ul><li>Electrons absorb energy of light wave and change their energy state </li></ul>
  9. 9. Refraction <ul><li>Light bends as it travels from one medium to another </li></ul>
  10. 10. Our Eyes <ul><li>The most sensitive and delicate organ we have </li></ul><ul><li>We are able to see the world and our brains receive the information from the world </li></ul><ul><li>The images we see are made up of light reflected from the objects we look at </li></ul>
  11. 11. Components of Eye <ul><li>Retina </li></ul><ul><li>Membranous, sensitive nerve tissue in the eye </li></ul><ul><li>Converts images from the eye’s optical system into electrical impulse </li></ul><ul><li>These impulses are sent along the optic nerve to the brain </li></ul><ul><li>More than 125 Million Rods and 6 Million Cones </li></ul>
  12. 12. Parts of the Eye <ul><li>Cornea </li></ul><ul><li>Transparent portion of the outer eye </li></ul><ul><li>Outward curvature </li></ul><ul><li>Primary refractor of light to eye </li></ul><ul><li>Cleaned and nourished from aqueous humor and tears </li></ul><ul><li>Iris </li></ul><ul><li>Muscular diaphragm </li></ul><ul><li>Controls the size of the pupil </li></ul><ul><li>Controls the amount of light that enters </li></ul><ul><li>Colored portion </li></ul>
  13. 13. Parts of the Eye <ul><li>A hole in the center of the iris </li></ul><ul><li>Changes size in response to changes in lighting </li></ul><ul><li>Larger in dim lighting conditions </li></ul><ul><li>Smaller in brighter lighting conditions </li></ul>
  14. 14. Parts of the Eye <ul><li>Macula- highly sensitive center of retina </li></ul><ul><ul><li>Detail </li></ul></ul><ul><li>Fovea </li></ul><ul><li>A tiny spot in the center of the retina </li></ul><ul><li>Very center of Macula </li></ul><ul><li>Contains only cone cells </li></ul><ul><li>Responsible for our sharpness of vision </li></ul>
  15. 15. Parts of the Eye <ul><li>Optic Disk </li></ul><ul><li>Small area of the retina where the optic nerve leaves the eye: any image falling here will not be seen </li></ul><ul><li>Brain covers it up </li></ul>
  16. 16. Parts of the Eye <ul><li>Choroid </li></ul><ul><li>Thin tissue layer containing blood vessels, sandwiched between the sclera and retina; also, because of the high melanocytes content, the choroid acts as a light-absorbing layer. </li></ul><ul><li>Sclera </li></ul><ul><li>tough, white outer covering of the eyeball; extraocular muscles attach here to move the eye </li></ul>
  17. 17. Parts of the Eye <ul><li>Aqueous humor </li></ul><ul><li>Clear watery fluid found in the anterior chamber of the eye; maintains pressure and nourishes the cornea and lens </li></ul><ul><li>Vitreous humor </li></ul><ul><li>Clear, jelly-like fluid found in the back portion of the eye: maintains shape of the eye and attaches to the retina </li></ul>
  18. 18. Parts of the Eye <ul><li>Lens </li></ul><ul><li>Transparent, biconvex structure </li></ul><ul><li>Refracts light to be focused on the retina </li></ul><ul><li>More spherical when focusing on close objects </li></ul><ul><li>Flatter when focusing on faraway objects </li></ul><ul><li>Along with the cornea, light rays are focused back together on the retina </li></ul><ul><li>Forms image of object on the back of retina </li></ul>
  19. 20. How We See <ul><li>Light Passes through </li></ul><ul><ul><li>Cornea </li></ul></ul><ul><ul><li>Aqueous Humor </li></ul></ul><ul><ul><li>Pupil </li></ul></ul><ul><ul><li>Lens </li></ul></ul><ul><ul><li>Vitreous Humor </li></ul></ul><ul><ul><li>Retina-->Phtoreceptors </li></ul></ul><ul><ul><li>Optic Nerve-->Brain </li></ul></ul><ul><li>Cornea refracts 70-80% </li></ul><ul><ul><li>Change from air to cornea is largest change in index of refraction </li></ul></ul><ul><li>Lens--20% </li></ul><ul><li>Ciliary Muscles/Zonule Fibers </li></ul>
  20. 21. Focal Length <ul><li>Distance to bring parallel rays into convergence </li></ul><ul><li>Diopters --(1/f) measure of lens power </li></ul>
  21. 23. How We See <ul><li>1:1 </li></ul><ul><li>Light reflected from different points are converged in the eye </li></ul><ul><li>Convex lenses--image forms upside down </li></ul>
  22. 24. Accommodation <ul><li>Lens changes shape to accomade vision from far and close distances </li></ul><ul><li>Ciliary/Zonule Fibers </li></ul><ul><li>Resting state: </li></ul><ul><ul><li>Lens fat </li></ul></ul><ul><li>At a distance--ciliary muslce relaxed, zonule fibers are tensioned->lens is pulled flat </li></ul><ul><li>Close Distance--ciliary muscle is constricted, zonule fibers relaxed-->Lens rounds </li></ul>
  23. 25. Cells <ul><li>Five different cells in retina </li></ul><ul><li>Photoreceptors </li></ul><ul><li>Bipolar Cells </li></ul><ul><li>Ganglion Cells </li></ul><ul><li>Horizontal Cells </li></ul><ul><li>Amacrine cells </li></ul>
  24. 27. Phototransduction <ul><li>Electrical changes in rods and cones cause electrical responses in other cells in the retina </li></ul><ul><li>Lead to production of action potentials in neurons </li></ul><ul><li>These neurons form optical nerve </li></ul><ul><li>The place on the retina where the axons of neurons converge is called the blind spot </li></ul>
  25. 28. Phototransduction <ul><li>To convert light energy to a change in membrane potential--G Protein coupled receptor </li></ul><ul><li>Rhodopsin has molecule bound--photon of light releases </li></ul><ul><ul><li>Activates G-protein (Transducin) </li></ul></ul><ul><ul><li>2nd Messenger Cascade (Phosphodiesterase) </li></ul></ul><ul><ul><ul><li>Turns cGMP to GMP </li></ul></ul></ul><ul><ul><ul><li>cGMP gated Na+ channels close </li></ul></ul></ul><ul><ul><li>Ion Channel closed </li></ul></ul><ul><ul><ul><li>Darkness--Rods are depolarized (release glu) </li></ul></ul></ul><ul><ul><ul><li>Light hyperpolarizes </li></ul></ul></ul>
  26. 30. Center/Surround Receptive Fields <ul><li>Center is from direct connections from photoreceptors </li></ul><ul><li>Surround--mediated by horizontal cell connections </li></ul><ul><li>on-center cell--stimulated when the center of receptive field exposed to light </li></ul><ul><ul><li>inhibited when the surround is exposed to light </li></ul></ul><ul><li>Off-center cell--opposite </li></ul><ul><li>Emphasize contrast at borders (edges) </li></ul>
  27. 32. Retinofugal Projection <ul><li>Visual Pathway from Optic Nerve to Brain </li></ul><ul><li>The optic nerves from the left and right eyes partially decussate in the optic chiasma </li></ul><ul><ul><li>Fibers from nasal retina cross over </li></ul></ul><ul><ul><li>Leads to binocular vision </li></ul></ul><ul><ul><ul><li>Left visual field viewed through right hemisphere </li></ul></ul></ul><ul><ul><ul><li>Overlap in visual fields-->Depth </li></ul></ul></ul><ul><li>Travel through optic tracts to the Lateral Geniculate Nucleus (LGN) of the Thalamus </li></ul>
  28. 34. Some Visual Projections <ul><li>Hypothalamus </li></ul><ul><ul><li>A small number of axons connect here </li></ul></ul><ul><ul><li>control of sleep wake cycles </li></ul></ul><ul><li>Midbrain </li></ul><ul><ul><li>Pretectum </li></ul></ul><ul><ul><ul><li>controls pupillary light reflex, certain eye movements </li></ul></ul></ul><ul><ul><li>Superior Colliculus </li></ul></ul><ul><ul><ul><li>voluntary and involuntary eye movements </li></ul></ul></ul><ul><li>Most go to LGN </li></ul>
  29. 35. Lateral Geniculate Nucleus <ul><li>Relays vision to visual cortex </li></ul><ul><li>Form optic radiations that terminate in the Primary Visual Cortex (Occiptal Lobe) </li></ul><ul><li>Layered </li></ul><ul><ul><li>1,2--magnocellular LGN layers, receive inputs from M-type Ganglion cells (color insensitive, respond transiently, center/surround) </li></ul></ul><ul><ul><li>3,4,5,6--parvocellular LGN layers, receive inputs from P-type ganglion cells (sustained response, center/surround, color sensitive) </li></ul></ul><ul><ul><li>Intralayers ventral to numbered layers--koniocellular layers, inputs from non M/P-type ganglion cells </li></ul></ul><ul><li>Combines inputs from two eyes </li></ul><ul><li>Receptive fields similar to retinal </li></ul>
  30. 36. LGN-Retinotopic Map <ul><li>Visual field is mapped so two points adjacent in visual field and on retina are connected to two adjacent points on neural surface </li></ul><ul><ul><li>Retinotopic map is magnified relative to the fovea b/c more photoreceptors </li></ul></ul>
  31. 37. Other Inputs <ul><li>80% of inputs from primary visual cortex </li></ul><ul><li>Brainstem </li></ul><ul><li>Modulate responses of LGN neurons </li></ul><ul><ul><li>Ex. Responsiveness can be modulated by feeling </li></ul></ul>
  32. 39. Visual Cortex <ul><li>Also known as Striate Cortex </li></ul><ul><li>Area 17--primary visual cortex </li></ul><ul><li>Layers </li></ul><ul><ul><li>I-Mostly Fibers </li></ul></ul><ul><ul><li>II, III, IVA, IVB,V, VI--pyrimdal cells, outputs from cortex </li></ul></ul><ul><ul><ul><li>III IVB--other cortical areas </li></ul></ul></ul><ul><ul><ul><li>V--Pons and superior colliculus </li></ul></ul></ul><ul><ul><ul><li>VI--LGN </li></ul></ul></ul><ul><ul><li>IVCa and IVCb--stellate cells, receive inputs from LGN </li></ul></ul><ul><ul><ul><li>IVCa--magnocellular LGN pathway </li></ul></ul></ul><ul><ul><ul><li>IVCb--parvocellular LGN pathway </li></ul></ul></ul><ul><ul><ul><li>Vertical connections from IVC to III </li></ul></ul></ul>
  33. 40. Ocular Dominance Columns <ul><li>Zebra stripes in layer IV (~0.5mm wide) </li></ul><ul><li>Adjacent areas are right or left eye inputs </li></ul><ul><li>Wiesel and Hubel </li></ul><ul><ul><li>Microelectrodes & cats </li></ul></ul>
  34. 42. Blobs <ul><li>Layers II and II </li></ul><ul><ul><li>Cytochrome oxidase staining (mitochondrial activity) </li></ul></ul><ul><li>Blobs and ocular dominance columbs overlayed in layers II, III and IV </li></ul><ul><li>Blobs and interblobs receive parvocellular LGN input (no color) </li></ul><ul><li>Blobs also receive koniocellular input (color) </li></ul>
  35. 44. Pathways <ul><li>Magnocellular Pathway (M-Channel) </li></ul><ul><ul><li>LGN to IVCa </li></ul></ul><ul><ul><li>IVCa to IVB </li></ul></ul><ul><ul><li>IVB cells have </li></ul></ul><ul><ul><ul><li>Orientation selectivity </li></ul></ul></ul><ul><ul><ul><li>Direction selectivity for moving stimuli </li></ul></ul></ul><ul><ul><ul><li>Binocular selectivity </li></ul></ul></ul><ul><ul><ul><li>No Color sensitivity </li></ul></ul></ul><ul><li>Specialized for analysis of object motion </li></ul>
  36. 45. Pathways <ul><li>Parvocellular Interblob Pathway (P-IB) </li></ul><ul><ul><li>LGN to IVCb </li></ul></ul><ul><ul><li>IVCB to layers II and II to interblobs </li></ul></ul><ul><ul><li>Have </li></ul></ul><ul><ul><ul><li>Very selective to orientation </li></ul></ul></ul><ul><ul><ul><li>No color sensitivity </li></ul></ul></ul><ul><ul><ul><li>Binocular sensitivity </li></ul></ul></ul><ul><ul><ul><li>Small rf </li></ul></ul></ul><ul><li>Specialized for analysis of object shape </li></ul>
  37. 46. Pathways <ul><li>Koniocellular Pathway--Blob Channel </li></ul><ul><ul><li>LGN inputs koniocellular layers and parvocellular layers via layer IVCb </li></ul></ul><ul><ul><li>Have: </li></ul></ul><ul><ul><ul><li>Monocular sensitivity </li></ul></ul></ul><ul><ul><ul><li>Color </li></ul></ul></ul><ul><ul><ul><li>No orientation selectivity </li></ul></ul></ul><ul><li>Analysis of Object color </li></ul>
  38. 47. Synthesis <ul><li>Cortical Module--basic unit for processing visual stimuli in visual cortex </li></ul><ul><li>Hubel and Wiesel-- </li></ul><ul><ul><li>2mm X 2mm </li></ul></ul><ul><ul><li>Cortical image of a point in space </li></ul></ul><ul><li>Two complete sets of ocular dominance columns (layer IV) </li></ul><ul><li>Sixteen blobs (III) </li></ul><ul><li>Two complete orientation columns(interblob layer III) </li></ul><ul><li>Striate cortex has about 1000 that act in parallel </li></ul>

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