Neuroscience: Exploring the
Brain, 3e
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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The Structure of the Eye
• Cross-Sectional Anatomy of 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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Image Formation by the Eye
• 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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Microscopic Anatomy of the
Retina
• 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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Microscopic Anatomy of the
Retina
• 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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Microscopic Anatomy of the Retina
• Regional Differences in Retinal Structure (Cont’d)
– Cross-section of fovea: Pit in retina where outer
layers are pushed aside
• Maximizes visual acuity
– Central fovea: All cones (no rods)
• 1:1 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
• 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
• Phototransduction in Rods
–
Light activated biochemical cascade in a photoreceptor
–
The consequence of this biochemical cascade is signal
amplification
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Phototransduction
• 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
20–25 minutes
All-cone daytime vision
All-rod nighttime vision
– Dark adaptation—factors
• Dilation of pupils
• Regeneration of unbleached rhodopsin
• Adjustment of functional circuitry
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Phototransduction
• Dark and Light Adaptation
– Calcium’s Role in Light Adaptation
• Calcium concentration changes in photoreceptors
• Indirectly regulates levels of cGMP channels
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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 cell’s membrane potential
– Antagonistic center-surround receptive fields
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Retinal Processing
• On-center Bipolar Cell
– Light on (less glutamate); Light off -> more
glutamate
‘Inverting’
synapse
(inhib)
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Retinal Output
• Ganglion Cell Receptive Fields
– On-Center and Off-Center ganglion cells
– Responsive to differences in illumination
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Retinal Output
• 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 nonMnonP 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
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End of Presentation
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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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