Visual Sensation

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Visual Sensation Perception

• How do we see?

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Structure of the eye
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The Retina
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Visual Receptors

• Rods
• Slowly adapting
• Black White vision
• 120 million None in fovea
• Cones
• Rapidly adapting
• Color vision
• 6 million 50,000 in fovea

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Chemistry of the photoreceptor

• Light-sensitive protein retinal (a pigment)
  absorbs a photon, causing the retinal to change
  shape.
• Through a series of enzyme-based chemical
  reactions, this causes a closing of the Na
  channels in the receptor.
• This has the effect of hyperpolarizing the cell,
  since sodium can no longer enter.
• This inhibits the latent activity of the cell,
  releasing less glutamate into the next layer of
  the retina - the bipolar cells.
• This causes one set of bipolar cells to be
  hyperpolarized (those that had excitatory
  connections with the receptor) and one set to be
  depolarized (those that had inhibitory
  connections).

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Retinal Ganglion cells

• Gather information from many rods and cones
  across an area of the retina.
• How many rods and cones depends on the size of
  the ganglion cells receptive field
• The closer to the fovea, the smaller the
  receptive field.
• Project out of the eye through the optic nerve,
  creating a blind spot.
• 1 million retinal ganglion cells (receiving
  signals from 125 million receptors).

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Receptive fields of retinal ganglion cells

• Center-surround
• Most are excitatory center, inhibitory surround.
• Some are the opposite

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Out of the retina

• Signals from the two eyes cross over to the
  opposite brain hemisphere at the optic chiasm.
• Not all signals from an eye go to contra-lateral
  hemisphere.
• Which hemisphere the signal goes to is based on
  which visual hemifield the ganglion cell receives
  information from.

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Into the brain

• Ganglion cells synapse in the lateral geniculate
  nucleus (LGN) of the thalamus.
• For comparison, the auditory nerve synapses in
  the medial geniculate nucleus.
• The LGN divides the signals into layers depending
  on which eye they come from, and whether they
  come from the fovea or not.
• 1, 4, 6 from the contralateral eye 2, 3, and 5
  from the ipsilateral eye.
• 1 and 2 (magnocellular levels) from the fovea.
• Other layers (parvocellular) from extra-foveal
  regions.
• 400,000 cells leave the LGN

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V1

• From the LGN, the signals are sent to area V1 in
  the very back of the occipital lobe.
• Signals are organized into a retinotopic map
  based on where on the retina they come from, and
  which eye they come from.
• All cells project to layer IV of striate cortex,
  but M and P cells project to slightly different
  parts.
• Furthermore, P cells project from layer IV into
  layers II and III, creating blob and interblob
  areas.

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Feature detectors in V1

• The retinotopic map is not simply a light/dark
  detector. Signals are beginning to be combined
  into simple feature detectors that can detect
  lines at various orientations.
• All of the feature detectors for a particular
  area of the retina are anatomically organized
  into a column.
• A hypercolumn is two columns from corresponding
  parts of both retinas.

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Beyond V1

• From V1, signals go to area V2 where the combine
  into more complex features (corners and simple
  shapes).
• After V2, the signal splits into two streams of
  information.
• The what stream passes through V3 (which does
  color detection) into the temporal lobe.
• The where stream passes through V4 (which aids
  with motion detection) into the parietal lobe.

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