Vision  Lucky
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Vision Lucky

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Vision  Lucky Vision Lucky Presentation Transcript

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