The Eye

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• Chapter 9
• The Eye

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Introduction

• Significance of vision
• Relationship between human eye camera
• Retina
• Photoreceptors Converts light energy into neural
  activity
• Detects differences in intensity of light
• Lateral geniculate nucleus (LGN)
• First synaptic relay in the primary visual
  pathway
• Visual information ascends to cortex? interpreted
  and remembered

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Properties of Light

• Light
• Electromagnetic radiation
• Wavelength, frequency, amplitude

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Properties of Light

• Light
• Energy is proportional to frequency
• e.g., gamma radiation and cool colors - high
  energy
• e.g., radio waves and hot colors - low energy

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Properties of Light

• Optics
• Study of light rays and their interactions
• Reflection
• Bouncing of light rays off a surface
• Absorption
• Transfer of light energy to a particle or surface
• Refraction
• Bending of light rays from one medium to another

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The Structure of the Eye

• Gross Anatomy of the Eye
• Pupil Opening where light enters the eye
• Sclera White of the eye
• Iris Gives color to eyes
• Cornea Glassy transparent external surface of
  the eye
• Optic nerve Bundle of axons from the retina

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The Structure of the Eye

• Ophthalmoscopic Appearance of the Eye

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• Cross-Sectional Anatomy of the Eye

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George Wald 1906 1997 received the Nobel Prize
in 1967 for discoveries concerning the primary
physiological and chemical visual processes in
the eye
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Image Formation by the Eye

• Refraction of light by the cornea
• Eye collects light, focuses on retina, forms
  images

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Image Formation by the Eye

• Accommodation by the Lens
• Changing shape of lens allows extra focusing power

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Image Formation by the Eye

• The Pupillary Light Reflex
• Connections between retina and brain stem neurons
  that control muscle around pupil
• Continuously adjusting to different ambient light
  levels
• Consensual
• Pupil similar to the aperture of a camera

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Image Formation by the Eye

• The Visual Field
• Amount of space viewed by the retina when the eye
  is fixated straight ahead

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• Visual Acuity
• Ability to distinguish two nearby points
• Visual Angle Distances across the retina
  described in degrees

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Microscopic Anatomy of the Retina

• Direct (vertical) pathway
• Ganglion cells
• ?
• Bipolar cells
• ?
• Photoreceptors

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Microscopic Anatomy of the Retina

• Retinal processing also influenced lateral
  connections
• Horizontal cells
• Receive input from photoreceptors and project to
  other photoreceptors and bipolar cells
• Amacrine cells
• Receive input from bipolar cells and project to
  ganglion cells, bipolar cells, and other amacrine
  cells

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Microscopic Anatomy of the Retina

• The Laminar Organization
• Inside-out
• Light passes through ganglion and bipolar cells
  before reaching photoreceptors

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• Photoreceptor Structure
• Converts electromagnetic radiation to neural
  signals
• Four main regions
• Outer segment
• Inner segment
• Cell body
• Synaptic terminal
• Types of photoreceptors
• Rods and cones

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• Regional Differences in Retinal Structure
• Varies from fovea to retinal periphery
• Peripheral retina
• Higher ratio of rods to cones
• Higher ratio of photoreceptors to ganglion cells
• More sensitive to light

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• Regional Differences in Retinal Structure
  (Contd)
• Cross-section of fovea Pit in retina where outer
  layers are pushed aside
• Maximizes visual acuity
• Central fovea All cones (no rods)
• 11 ratio with ganglion cells
• Area of highest visual acuity

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Phototransduction

• Phototransduction in Rods
• Light energy interacts with photopigment to
  produce a change in membrane potential
• Analogous to activity at G-protein coupled
  neurotransmitter receptor - but causes a decrease
  in second messenger

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• Phototransduction in Rods
• Dark current Rod outer segments are depolarized
  in the dark because of steady influx of Na
• Photoreceptors hyperpolarize in response to light

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• Phototransduction in Rods
• Light activated biochemical cascade in a
  photoreceptor
• The consequence of this biochemical cascade is
  signal amplification

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Color Blindness - a genetic disorder where
individuals lose the ability to distinguish some
or all variations in color vision
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• Phototransduction in Cones
• Similar to rod phototransduction
• Different opsins
• Red, green, blue
• Color detection
• Contributions of blue, green, and red cones to
  retinal signal
• Spectral sensitivity
• Young-Helmholtz trichromacy theory of color vision

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Phototransduction

• Dark and Light Adaptation
• Dark adaptationfactors
• Dilation of pupils
• Regeneration of unbleached rhodopsin
• Adjustment of functional circuitry

2025 minutes
All-cone daytime vision
All-rod nighttime vision
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Deuteranopia Deuteranomaly (forms of GREEN
deficiency) Deuteranopia and deuteranomaly are
the most common forms of color-blindness. People
with these conditions have cones that are
insensitive to medium wavelengths (greens), but
the end result is similar to protonopia, with the
exception that reds do not look as dark.
Deuteranomaly is the less severe of the two
conditions. Individuals with deuteranomaly cannot
see reds and greens in the same way that people
with full color vision can, they are able to
distinguish between shades of reds and greens
relatively accurately.
full color vision
green deficiencies
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Protanopia Protanomaly (forms of RED
deficiency) Color receptors in the eyes of
people with protanopia are not sensitive to long
wavelengths (the reds). Reds look more like
beiges and appear to be somewhat darker than they
actually are. The greens tend to look similar to
the reds. Protanomaly is milder than
protanopia, but the end result is similar.
Although many people with protanomaly can
distinguish some reds and greens, they cannot do
so as easily as someone with color-normal vision,
and, as with protanopia, reds tend to look darker
as well.
red deficiencies
full color vision
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Tritanopia (a form of BLUE deficiency) Note
Tritanopia is much less common than the other
categories mentioned above. Tritanopia is
insensitivity to short wavelengths (the blues).
Blues and greens can be confused, but yellows are
also affected in that they can blend with blues
or appear as lighter shades of red.
full color vision
blue deficiencies
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Rod Monochromacy or Achromacy (NO color
vision) This group constitutes an extremely
small minority among people who are color-blind.
All cones of the eye are non-functional, so the
rods (receptors which can only differentiate
between light and dark) are the only available
source of visual information. Individuals with
achromacy see no color at all. People with
achromacy often have poor visual acuity and have
an aversion to bright light.
full color vision
complete cone deficiency
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Phototransduction

• Dark and Light Adaptation
• Calciums Role in Light Adaptation
• Calcium concentration changes in photoreceptors
• Indirectly regulates levels of cGMP? channels

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Retinal Processing

• Research in ganglion cell output by
• Keffer Hartline, Stephen Kuffler, and Horace
  Barlow
• Only ganglion cells produce action potentials
• Research in how ganglion cell properties are
  generated by synaptic interactions in the retina
• John Dowling and Frank Werblin
• Other retinal neurons produce graded changes in
  membrane potential

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Retinal Processing

• Transformations in the Outer Plexiform Layer
• Photoreceptors release less neurotransmitter when
  stimulated by light
• Influence horizontal cells and bipolar cells

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Retinal Processing

• Receptive Field On and Off Bipolar Cells
• Receptive field Stimulation in a small part of
  the visual field changes a cells membrane
  potential
• Antagonistic center-surround receptive fields

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Retinal Processing

• On-center Bipolar Cell
• Light on (less glutamate) Light off -gt more
  glutamate

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• Ganglion Cell Receptive Fields
• On-Center and Off-Center ganglion cells
• Responsive to differences in illumination

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• Types of Ganglion Cells
• Appearance, connectivity, and electrophysiological
  properties
• M-type (Magno) and P-type (Parvo)ganglion cells
  in monkey and human retina

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Retinal Output

• Color-Opponent Ganglion Cells

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Retinal Output

• Parallel Processing
• Simultaneous input from two eyes
• Information from compared in cortex
• Depth and the distance of object
• Information about light and dark ON-center and
  OFF-center ganglion cells
• Different receptive fields and response
  properties of retinal ganglion cells M- and P-
  cells, and nonM-nonP cells

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Concluding Remarks

• Light emitted by or reflected off objects in
  space ? imaged onto the retina
• Transduction
• Light energy converted into membrane potentials
• Phototransduction parallels olfactory
  transduction
• Electrical-to-chemical-electrical signal
• Mapping of visual space onto retina cells not
  uniform