Vision- CNS.txt

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Vision- CNS.txt
2010-12-05 01:08:18

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  1. Vision: CNS
    Pathway from the eye to the cortex
  2. Themes of this lecture
    • Visual information is analyzed in more complicated ways than in the retina
    • One major pathway from the eye leads to the striate cortex and from there to many other visual centers
    • Cortical lesions can lead to difficulty in determining what an object is or where it is
  3. Optic nerve
    • one of the major targets of MS
    • is an outgrowth of the diencephalon and is surrounded by meninges
    • Optic axons become myelinated by oligodendrocytes after they leave the retina
    • The myelin of optic nerve is one of the commonest locations of damage in multiple sclerosis
  4. Optic nerve and tract
    • Inversion of the visual field; the left half of each eye sees the right half of the visual field
    • the right half of the visual field is "seen" by the left half of the brain
    • Nasal vs. temporal hemiretina: the fovea is the dividing point
    • Axons leave the eye at the optic disc (optic papilla)
    • Half of the optic axons cross to the opposite side of the brain at the chasm
  5. The lateral geniculate nucleus (LGN)
    • About 90% of optic axons project to the LGN (but optic axons account for only about 20% of the input to the LGN)
    • The receptive field properties of cells in the geniculate are almost the same as the properties of retinal ganglion cells
  6. The layers of the LGN
    • most textbooks say there are 6 layers, but there are really 12!
    • Information from the two eyes is kept separate: 6 layers get input from the left eye; 6 layers get input from the right eye
    • Retinotropic maps: each layer has a "copy" of half of the retina, with axons coming from one type of ganglion cell in one eye
    • What's different about these layers?: different kinds of ganglion cells project to different layers
    • For example, a cell with non-color selective responses will project to a different layer than a cell that is color-selective
  7. Lateral geniculate nucleus activity
    • Recent imaging studies indicate that the lateral geniculate plays a role in selective attention.
    • The activation comes from the cortex
    • During sleep and periods of inattentiveness, transmission from the eye through the lateral geniculate is reduced and cells fire spontaneous bursts
  8. The optic radiations
    • Fibers from cells in the LGN enter the optic radiations, sweep around the lateral side of the lateral ventricle and pass to the occipital lobe
    • The visual fibers are also called the geniculocalcarine tract
  9. Primary Visual Cortex
    • Area 17, Striate cortex, V1
    • Located in the pole of the occipital lobe above and below the CALCARINE SULCUS on the medial aspect of the hemisphere
  10. Cortical circuitry
    • Axons from lateral geniculate terminate in layer 4
    • Information is then passed to upper layers and deeper layers
  11. Receptive fields in striate cortex
    • Most cells in ayer 4 have circular receptive fields with center-surround organization
    • The vast majority of cells in other layers respond best to straight bars or edges: and prefer different orientations
    • Some respond best to dark bars or lines
    • Some respond best to light bars or lines
    • Typical cells outside of layer 4
  12. Output of striate cortex: more than 40 other regions!
    or why you should know that there's more than one way to be blind
  13. The "where" stream
    • Cells in some of these regions respond almost exclusively to moving objects
    • Some respond to circular or spiraling movement
    • Some respond to "visual flow"
    • Some respond best to approaching or receding objects
  14. Lesions of the "where" stream
    • loss of speed and motion perception
    • loss of ability to use visual information information to grasp objects
    • visual neglect in peripersonal space
  15. Balint's syndrome
    • Due to bilateral parietal sesion
    • Simultanagnosia: inability to use visual information to grasp objects
    • Optic apraxia: difficulty voluntarily shifting gaze
    • Optic ataxia: inability to reach correctly toward an object
  16. The "what" stream
    • some areas have many cells that are color-sensitive
    • some areas have cells that respond to complex shapes
    • some areas are particularly important for face perception
  17. Lesions of the "what" stream
    • achromatopsia: inability to percieve colors
    • Prosopagnosia: inability to recognize familiar faces