Vision: Retina

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Vision Retina Lateral Geniculate Nucleus

• Lecture 17

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The light stimulus

• Electromagnetic energy
• visible light
• small part of spectrum
• Photon
• Wave
• Color (hue) - wavelength
• Brightness (intensity) - amplitude

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Color
Wavelength
Amplitude
Brightness
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Retinal Organization
Bipolar cell BP
Retinal Ganglion Cell RGC
Axons form Optic Nerve
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Retinal Organization
Which direction does the light enter?
PR BP RGC
Optic Nerve
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Photoreceptors (PR)

• Transduction
• Rods
• achromatic
• Cones
• color

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Photoreceptors - PR

• Do not generate action potentials
• Receptor potential
• Depolarized in dark
• Dark Current
• excited
• NT is released
• Light stimulation hyperpolarizes PR
• inhibited
• Less NT released from PR

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Receptor Potential

• Light absorbed by pigment
• Activates G-protein (Tranducin)
• Activates cGMP phosphodiesterase
• cGMP ---gt 5-GMP
• Na channels close
• Receptor hyperpolarizes

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Dark current
Rhod.
Na
cGMP PD
cGMP
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Tranduction
Rhod.
Na
cGMP PD
cGMP
5-GMP
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Tranduction
PR hyperpolarizes ---gt NT release
Rhod.
Na
cGMP PD
5-GMP
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Rods

• Pigment rhodopsin
• Around edges of retina
• 120 million rods
• Convergence
• high sensitivity
• low acuity

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Cones

• Color vision - 3 types
• red long wavelength
• green medium
• blue short
• Concentrated in fovea
• Little convergence
• low sensitivity
• high acuity

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Receptive Field

• Region of the retina ...
• where changes in illumination...
• will change the activity of a particular neuron

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Center-Surround Organization

• Receptive Fields
• Retinal Ganglion cells
• LGN cells
• 2 types

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Photoreceptors
Center
Surround
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2 arrangements
ON
OFF
OFF
ON
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Color Vision

• Retinal Mechanisms

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The Trichromatic Theory

• Young-Helmholtz (1802)
• 3 types of color receptors
• Cones
• Differential sensitivity to light wavelengths
• red (long), green (medium), blue (short)

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• Color overall pattern of stimulation
• ROY G BIV
• Negative After-image?

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Hering Opponent Process Theory

• Competing theory
• Center-Surrond organization
• BP, RGC, LGN
• Antagonistic
• Red-Green
• Blue-Yellow
• Black-White

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• Antagonistic
• Each color can be or -

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Which Theory?

• Both are correct
• Photoreceptors trichromatic
• Higher levels Opponent Process
• BP, RGC, LGN
• More complex at cortical level

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Hemiretinas Visual Fields
Retino-Geniculate-Cortical Pathway

• Each retina ---gt 2 hemiretinas
• nasal
• axons project contralaterally
• decussate at optic chiasm
• temporal
• axons project ipsilaterally

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Hemiretinas Visual Fields

• Visual field
• area in space seen by cortex
• Left Visual Field (LVF)
• right hemiretina of each eye
• left nasal right temporal
• Right Visual Field (LVF)
• left hemiretina of each eye
• left temporal right nasal

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Right Visual Field RVF
Left Visual Field LVF
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Superior visual field
Superior retina
Inferior retina
Inferior visual field
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Retino-Geniculate-Cortical Pathway

• Anatomy

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RVF
LVF
Retina
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Lateral Geniculate Nucleus

• 6 layers
• dorsal ---gt ventral
• 6 -------gt 1
• RFs center-surround
• Input from each eye
• monocular
• 2, 3, 5 from the ipsilateral eye

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Lateral Geniculate Nucleus

• Parallel channels
• Magnocellular system
• Large cells - layers 1 2
• Rods only - no color
• form movement
• Parvocellular system
• small cells - layers 3-6
• color form info

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Lesions of Visual Pathway

• Locus determines deficit

• Optic Nerve
• blind in ipsilateral eye
• monocular vision
• deficits in depth perception stereopsis

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Lesions of Visual Pathway

• Optic Chiasm
• bitemporal hemianopsia
• blind in nasal hemiretina in each eye
• no peripheral vision

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Lesions of Visual Pathway

• Optic Tract
• homonymous hemianopsia
• blind in contralateral visual field