Psychology 4051

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Psychology 4051

• The Retina and LGN

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Retino-Geniculate-Cortical Pathway
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The Retina

• Located at the back of the posterior chamber,
  forms the inner tunic of the eye.
• Surface on which the visual image is focused

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The Retina

• A laminar tissue with multiple layers.
• Pigment epithelium, photoreceptor layer, external
  limiting membrane, outer nuclear layer, outer
  plexiform layer, inner nuclear layer, inner
  plexiform layer, ganglion cell layer, optic nerve
  layer, internal limiting membrane.
• Transduction takes place in the photoreceptors.
• Rods and cones

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• Light hits the outer segment of rods and cones
  which contain photosensitive chemicals
  (photopigments).
• The light changes the molecular properties of the
  photopigments, which in turn changes the
  electrical state of these cells this is called
  transduction.

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The Retina
The design of the retina is unusual. Light must
pass through 8 layers Before it hits the
Photoreceptors.
Light
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The Retina

• The retina contains a central pit, above which
  cell layers are pushed away.
• The fovea 1mm in diameter
• Packed with cones

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fovea

• Fovea
• At the center of the retina, high acuity
• Reduced light distortion

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The Retina

• Rods and cones connect with horizontal and
  bipolar cells (collector cells).
• Lateral interaction takes place at horizontal
  cells.
• Bipolar cells are connected to retinal ganglion
  cells.
• Amacrine cells connect adjacent bipolar cells.

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Rods

• 120 million rods in the human retina
• Concentrated in the periphery of the retina

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Rods

• Extremely sensitive to light.
• Many rods converge onto a single retinal ganglion
  cell (RGC)

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Rods good light sensitivity

• As a result, low levels of light can be detected
• Responsible for night vision
• But, spatial resolution is very poor
• RGCs can not determine which rods were stimulated

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Cones

• Approximately 6 million cones
• Cones are packed densely in the fovea

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Cones

• Cones are connected to bipolar cells, then to
  retinal ganglion cells (RGCs)
• Few cones converge onto a single retinal ganglion
  cell (RGC)
• As a result, resolution is excellent

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Cones

• Light sensitivity is poor
• Day vision

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Cones

• Humans have 3 different cone types, each with a
  different photopigment.
• Photopigments are maximally sensitive to specific
  wavelengths.
• Short wavelength sensitive
• Mid-wavelength sensitive
• Long wavelength sensitive

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Retinal Ganglion Cells

• Retinal ganglion cells possess receptive fields
  that are responsive to light stimulation.
• Receptive field that area of the retina over
  which a ganglion cell is sensitive to light
  stimulation (area of the retina that the RGC
  monitors).

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Receptive Fields

• Their receptive fields are designed in an
  antagonistic fashion
• A single receptive field has an on-center/off-
  surround arrangement or an off-center/on-surround
  arrangement.

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On-Center Cells

• On-center cell Light stimulation of the center
  of the receptive field produces depolarization
  and an increase in the firing rate of the
  ganglion cell.
• Stimulation of the surround produces a
  hyperpolarization and a decrease in the firing
  rate of the cell.

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Off-Center Cells

• Off-center cell Light stimulation of the
  surround of the receptive field produces
  depolarization and an increase in the firing rate
  of the ganglion cell.
• Stimulation of the center produces a
  hyperpolarization and a decrease in the firing
  rate of the cell.

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Retinal Ganglion Cells

• The axons of the RGCs converge and leave the eye
  through as the optic nerve.
• They connect to the lateral geniculate nucleus
  (LGN).

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Retinal Ganglion Cells

• Most RGCs fall into two functional classes, M and
  P cells
• M cells project to the magnocellular layers of
  the lateral geniculate nucleus (LGN)
• P cells project to the parvocellular layers of
  the LGN

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M Cells

• M cells receive input from a relatively large
  number of photoreceptors (mostly input from
  rods)
• Good light sensitivity, good temporal resolution
  (sensitive to motion)
• Poor spatial resolution
• Large with broad axons and consequently faster
  nerve conduction velocities
• Not color-sensitive

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P Cells

• P cells receive input from a relatively small
  number of photoreceptors (mostly input from
  cones)
• Good spatial resolution
• Poor temporal resolution
• Colour sensitivity

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LGN

• A six-layered thalamic relay station

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LGN

• Cell in the magnocellular layer have large cell
  bodies and long, straight axons.
• Cells in the parvocellular layer possess smaller
  cell bodies and short, curved axons.
• Layers 1, 4, and 6 receive input from the
  contralateral eye.
• Layers 2, 3, and 5 receive input from the
  ipsilateral eye.

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LGN

• Most parvocellular cells appear to be responsive
  to color and have center/surround receptive
  fields.