Cellular Neuroscience (PowerPoint) by yurtgc548

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									    Seeing Things 1
      Eye and Brain

   How Your Brain Works - Week 3


           Dr. Jan Schnupp
jan.schnupp@dpag.ox.ac.uk
           HowYourBrainWorks.net
Light Wavelength
            A   B

Optics of
the Eye
Eye and Retina
The Blind Spot
Retinotopy




  •   Adapted from drawings by Ramon y Cajal
The Optic
Pathway:
eye,
optic nerve,
optic chiasm,
optic tract,
thalamus,
optic
radiation,
visual cortex
Rene Descartes, Retinotopy and
     the Seat of the Soul
              Photoreceptors




• The human eye has ca. 10 million rods and ca 120
  million cones
            Phototransduction




• Light activated rhodopsin (R) activates G-protein (G)
  which in turn activates phosphodiesterase (PDE) which
  cleaves cGMP which closes cGMP-gated Na+ channel.
• What does any of this have to do with carrots?
Absorption
Spectra




• The three different types of cones and the rods have
  slightly different opsins which are sensitive to different
  wavelengths.
• “Trichromacy” theory.
Sensitivity of Receptors
Rod and Cone Distribution
   Retinal Wiring

• Photoreceptors



• Horizontal cells
• Bipolar cells
• Amacrine cells



• Retinal ganglion cells
Centre –Surround Receptive Fields

             Photo-      Horizontal
             receptors   Cell
     -
    --
      -
    ++ + +




-
    -
- -
 - -
  -                             Bipolar   Retinal
                                Cell      Ganglion
                                          Cell
On-centre and
off-centre
Receptive
Fields
• Lateral inhibition
  provided by
  photoreceptor
  ribbon synapses,
  horizontal cell
  synapses and
  amacrine cells
Lateral inhibition for contrast (edge)
               detection
     RGC
 receptive
  fields as
   “spatial
frequency
    filters”
Difference of Gaussians Model of Retinal
Ganglion Cells




 • The centre-surround structure of Retinal Ganglion Cells
   turns them into “spatial frequency filters”. Larger RGC
   receptive fields are tuned to “coarsely grained” structure in
   the visual scene, while smaller RFs are tuned to fine grain
   structure.
 Convolving a Penny with
          DoGs




• The picture of an American cent (left)
  seen through large (middle) or small
  (right) difference of Gaussian receptive
  fields.
The Fovea
Eye muscles
                    Eye movements




•   Eye-movement traces while a subject explores a picture of the bust of Nefertiti.
•   From "Eye Movements and Vision" by A. L. Yarbus; Plenum Press, New York; 1967
Dan Simmons’ visual attention task




• Count the number of passes of the white team
Colour opponency
Colour Opponency
The Colour Wheel
                   Yellow-Blue




              Red Green
Why Colour
Vision Does    surround
 Not Work
                  centre
Well in Poor
    Light
Cone mosaics


• Cone
  mosaics for
  four different
  individuals
Colour blindness
      Red-green channel broken




                                 Blue-yellow
                                 channel
                                 broken
M cells and
  P cells


• 90% P cells
• 5% M cells
• 5% non-M non-P
Projections to
 the Lateral
 Geniculate
  Nucleus
    The Stepping Feet Illusion
• http://www.michaelbach.de/ot/mot_feet_lin
  /index.html
• What is going on here?
• M cells are colour blind, but very sensitive
  to brightness (luminance) contrast.
• P cells are R-G opponent
• Non-M non-P cells are Y-B opponent
• Only M cells project to the motion
  processing streams in the brain.

								
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