Physics of the Eye and Vision

1
Physics of the Eye and Vision

• Lesson 2 Light Absorption in the Retina
• Crystal Sigulinsky
• crystal.cornett_at_utah.edu

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Physics in Visual Processes

• Imaging in the eye
• Optics
• Absorption of light in the eye
• Quantum mechanics
• Nerve conduction
• Visual Information Processing

http//en.wikipedia.org/wiki/FileGray722.png Gray
's Anatomy of the Human Body, 1918
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Objectives

• Optics of Imaging in the eye (Monday, June 22)
• Properties of Light
• Image Formation
• Accomodation
• The -Opias
• Glasses
• Absorption of light in the eye (Friday, June 26)
• Photoreceptor Biology
• Light absorption
• Eye Retinal Structure
• Effects on light absorption
• Limits of Visual Spectrum
• Limits of Visual Acuity
• Review Questions
• Nerve conduction Information Processing
  (Neuroscience Lecture,
  Monday, June 29)

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Eye Anatomy
http//en.wikipedia.org/wiki/Eye
Designed to focus light onto the retina
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Retina Laminar Organization
http//webvision.med.utah.edu/imageswv/husect.jpeg
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7 Retinal Cell Types
Photoreceptors
1-3
http//webvision.med.utah.edu/imageswv/schem.jpeg
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Photoreceptor Cells

• Primary sensory neurons of the eye
• Photosensitive light sensors
• Responsible for image formation
• Akin to film in a camera
• Perform phototransduction
• Convert electromagnetic radiation into electrical
  signals
• Visual pigments
• Two types
• Rods
• Cones

http//webvision.med.utah.edu/imageswv/hucsect.jpe
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Photoreceptor Cell Structure
Rod and Cone Photoreceptor Figure
http//webvision.med.utah.edu/imageswv/rodcoEM.jpe
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http//thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02
_m_vis/d_02_m_vis_1a.jpg
http//en.wikipedia.org/wiki/FileRod_cone_cells.j
pg
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Rod and Cone Distribution
http//webvision.med.utah.edu/imageswv/Ostergr.jpe
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Photoreceptor Absorption Spectra
http//en.wikipedia.org/wiki/FileCone-response.sv
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Photoreceptor Functions
Rods
Cones

• Monochromatic vision
• Single visual pigment
• Scotopic vision (low light conditions)
• Night vision
• High Sensitivity
• Often respond to single photon
• Slow response stimuli added
• Peripheral vision
• Warning vision
• Wide distribution
• Covers large visual angle
• None in fovea

• Chromatic vision
• 3 visual pigments
• Trichromatic vision
• Photopic vision (high light conditions)
• Low sensitivity
• 1000x less than rods
• Often misconstrued as Color vision
• Detail vision
• Foveal location
• High spatial acuity (resolution)
• High density
• Less escaped light
• Fast response to stimuli

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Visual Phototransduction

• Conversion of electromagnetic radiation into
  electrical signals
• Absorption of electromagnetic radiation
• Triggering of a signaling cascade
• Change in electrical properties of the cell

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Quantum Mechanics

• Classical Mechanics
• Description of large populations of particles
• An approximation of quantum mechanics
• Quantum Mechanics
• Arose from the inability to explain certain
  behaviors of electromagnetic radiation and
  electrons in atoms using classical mechanics
• Newton, Planck, Einstein, Bohr, and others
• Description of physical systems at the atomic
  level
• Light
• Electrons
• Molecules

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Quantization of energy levels

• Evidence Entities detected as individual
  quanta or discrete units
• Atomic systems exist in quantized or
  stationary states, each corresponding to a
  discrete energy level of the system
• States described by the Schrödinger Wave Equation
• Transition from one quantized state to another is
  accompanied by
• Transfer of energy to/from another system
• Emission or absorption of radiation

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Properties of Light

• Wave model
• Classical sinusoidal wave
• Unique in that can travel
• through a vacuum
• Describes reflection,
• refraction, diffraction,
• interference, and Doppler Effect phenomena, etc.
• Particle model
• photon
• Describes absorption and emission phenomena

Image from http//en.wikipedia.org/wiki/ImageWave
.png
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Absorption of Light

• Absorption of a photon transfers energy (E)
• E h? hc/?
• h Plancks constant 6.626 x 10-34 J/s
• c speed of light 3.0 x 108 m/s
• ? wavelength
• Excites the molecule to a higher energy state
• A molecule can only exist at discrete energy
  levels.
• Absorption only occurs if energy of the photon
  equals the energy difference between the
  molecules energy levels.

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Visual Pigments

• Photosensitive molecules mediating visual
  phototransduction
• Chromophore
• Chemical group that absorbs light
• Retinal
• Auxochrome
• Chemical group that modifies a chromophores
  light absorption (tuning)
• Wavelength
• Intensity
• Opsins
• Retinylidene proteins
• Protein family that uses retinal as a chromophore

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Visual Pigments Chromophore

• Retinal
• (aldehyde derivative of Vitamin A)
• Aka retinaldehyde
• Absorption in near ultraviolet (330-365 nm)
• Induces photoisomerization
• h? energy required to promote retinal to an
  excited state
• Rotation around the double bond more
  energetically favored

h?

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Visual Pigments Auxochrome

• Opsin
• Promote electron delocalization and charge
  perturbation
• Lowers energy required to excite electrons in
  retinal
• Shifts energy requirement into visual spectrum
• G protein coupled transmembrane receptor
• Covalently bonded to retinal
• Links photon absorption to signal transduction
  cascade

http//webvision.med.utah.edu/imageswv/rhodopH.jpe
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http//en.wikipedia.org/wiki/FileRhodopsin_3D.jpe
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4 Human Opsins

• Different absorbance maxima accomplished by
  differences in amino acid sequence
• Slight differences in 3D conformation
• Red vs green
• 98 identical
• Blue vs Rhodopsin
• 40 identical
• Red or Green vs Blue or Rhodopsin
• 40 identical

http//en.wikipedia.org/wiki/FileCone-response.sv
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Amino acid variants in protein structure
Nathans, Cell Press, 1999
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Vision Deficiencies

• Absence of visual pigment components
• Retinal ? complete vision deficiency
• Opsins ? color vision deficiency
• Monochromacy
• Lack 2 or all 3 cone pigments
• Dichromacy
• Lack one cone pigment
• Anomalous trichromacy
• Altered spectral sensitivity of one cone pigment
• Most common

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Visual Phototransduction

• Light ? electrical signal

http//en.wikipedia.org/wiki/FilePhototransductio
n.png
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Visual Phototransduction

• Retinal undergoes a photoisomerization
• Single photon required
• Converts 11-cis retinal to all-trans retinal
• Induces a conformational change in the opsin
  molecule
• Triggers an intracellular signal transduction
  cascade
• Closes ion channels
• Changes the electrical state of the cell

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Replenishment of 11-cis retinal
http//en.wikipedia.org/wiki/FileVisual_cycle_v2.
png
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Distance-Size Relationship
Image distance

• Image size is limited by short image distance
• Most vision restricted to small region of the
  retina
• Fovea centralis
• 200 um (0.2 mm) region 1 retinal size
• Detailed vision

Image Size
Object Size
Object distance
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Fovea Centralis

• Fovea is designed for maximum resolution
• High spatial density of photoreceptors (cones)
• Virtually no signal convergence
• 1 photoreceptor ? 1 ganglion cell
• Outside fovea high signal convergence
• 100 photoreceptors ? 1 ganglion cell
• Foveal information maps to gt50 of the visual
  cortex
• Minimal optical interference
• Pit structure

http//webvision.med.utah.edu/imageswv/fovmoswv.jp
eg
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Retinal organization with respect to light path
through eye
http//webvision.med.utah.edu/imageswv/schem.jpeg
http//en.wikipedia.org/wiki/Eye
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Fovea Centralis
http//webvision.med.utah.edu/imageswv/hufovea.jpe
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Visible Spectrum

• Small fraction of electromagnetic spectrum

http//en.wikipedia.org/wiki/ImageEM_spectrum.svg
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Aspects Determining the Visual Spectrum

• Solar emmission spectrum
• Sun temp, atomic composition
• Atmospheric transmission
• Scattering, absorption by greenhouse gases
• Absorption by optical elements
• Macula lutea (yellow spot)
• Contains the fovea
• Protective yellow pigments xanthophyll and
  carotenoids
• Short wavelength filter
• Visual pigments
• Retinal Opsins

http//en.wikipedia.org/wiki/FileMODIS_ATM_solar_
irradiance.jpg
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Aspects Determining the Visual Spectrum

• Solar emmission spectrum
• Sun temp, atomic composition
• Atmospheric transmission
• Scattering, absorption by greenhouse gases
• Absorption by optical elements
• Macula lutea (yellow spot)
• Contains the fovea
• Protective yellow pigments xanthophyll and
  carotenoids
• Short wavelength filter
• Visual pigments
• Retinal Opsins

http//en.wikipedia.org/wiki/FileAtmosphC3A4ris
che_Absorption.png
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Aspects Determining the Visual Spectrum

• Solar emmission spectrum
• Sun temp, atomic composition
• Atmospheric transmission
• Scattering, absorption by greenhouse gases
• Absorption by optical elements
• Macula lutea (yellow spot)
• Contains the fovea
• Protective yellow pigments xanthophyll and
  carotenoids
• Short wavelength filter
• Visual pigments
• Retinal Opsins

Transmission of light reaching each optical
element
Herman, I.P., Physics of the Human Body, 2007
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Aspects Determining the Visual Spectrum

• Solar emmission spectrum
• Sun temp, atomic composition
• Atmospheric transmission
• Scattering, absorption by greenhouse gases
• Absorption by optical elements
• Macula lutea (yellow spot)
• Contains the fovea
• Protective yellow pigments xanthophyll and
  carotenoids
• Short wavelength filter
• Visual pigments
• Retinal Opsins

http//webvision.med.utah.edu/imageswv/retina.jpeg
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Aspects Determining the Visual Spectrum

• Solar emmission spectrum
• Sun temp, atomic composition
• Atmospheric transmission
• Scattering, absorption by greenhouse gases
• Absorption by optical elements
• Macula lutea (yellow spot)
• Contains the fovea
• Protective yellow pigments xanthophyll and
  carotenoids
• Short wavelength filter
• Visual pigments
• Retinal Opsins

http//en.wikipedia.org/wiki/FileCone-response.sv
g
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Aspects Determining Visual Acuity

• Density of photoreceptor cells ( pixel size)
• Connectivity of photoreceptor cells
• Degree of convergence at ganglion cells
• Light levels
• Diffraction
• Significant at small apertures (when pupil lt 3
  mm)
• Spherical aberration
• Imperfect imaging by spherical surface
• Significant at larger apertures
• Chromatic aberration
• Different colors come into focus at different
  distances
• Optical scattering
• Reduced by Retinal Pigment Epithelium
• Absorbs excess photons
• Cats eyes reflective layer
• 2nd chance to absorb photons, but decreases
  resolution

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Intrinsically-Photosensitive Retinal Ganglion
Cells (ipRGCs)

• Perform phototransduction
• Visual pigment Melanopsin
• Still receive synaptic rod cone input
• Function
• Detect changes in ambient light levels
• Visual pimgent Melanopsin
• Low light sensitivity
• Slow response kinetics
• Sustained response to light over long period
• Low spatial resolution
• 1-3 Ganglion Cells
• Broad distribution
• Active in non-image forming visual functions
• Unconscious visual reflexes
• Pupillary reflexes
• Photoentrainment adjusting circadian rhythms to
  environment
• Projection neurons of retina link to brain
  regions

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ipRGC Connections
http//webvision.med.utah.edu/imageswv/DustinFig13
.jpg
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Rod Photoreceptors (Rods)

• Named for rod-shaped outer segments
• 120 million per eye
• Monochromatic vision
• Single visual pigment
• Rhodopsin
• Scotopic vision (low light conditions)
• Night vision
• High Sensitivity
• Often respond to single photon
• Slow response stimuli added
• Require 0.1 of light required by cones to
  function

• Peripheral vision
• Warning vision
• Wide distribution
• Covers large visual angle
• None in fovea
• Low spatial acuity (resolution)
• High convergence of visual information
• 1 rod ? gt100 ganglion cells
• Low internal reflection
• Lose light to surrounding area

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Cone Photoreceptors (Cones)

• Named for conical-shaped outer segments
• 6.5 million per eye
• Chromatic vision
• 3 visual pigments
• 3 cone types, each with a single visual pigment
• S cones
• Short wavelength sensitive
• Blue cones
• M cones
• Middle wavelength sensitive
• Green cones
• L cones
• Long wavelength sensitive
• Red cones

• Photopic vision (high light conditions)
• Low sensitivity
• 1000x less than rods
• Often misconstrued as Color vision
• Detail vision
• High spatial acuity (resolution)
• High density in fovea where image is restricted
  due to optics
• Little to no convergence of information 1 cone
  ? 1 ganglion cell in fovea
• High internal reflection
• Less escaped light
• Outer segment surrounded by pigment epithelium,
  which increases relative difference in indices of
  refraction that ultimately increases internal
  reflection
• Fast response to stimuli

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More info or clarification

• crystal.cornett_at_utah.edu
• Webvision by the University of Utah
• http//webvision.med.utah.edu

41
Review Questions

• Lecture 2 Light absorption in the Retina

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Question 1

• Question What are the light-sensing cells of the
  retina that mediate image formation?
• A. Bipolar cells
• B. Cones
• C. Ganglion cells
• D. Amacrine cells
• E. Rods
• Answers B and E. Ganglion cells are
  photosensitive but do not participate in image
  formation. They control such non-image forming
  functions as the pupillary reflex

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Question 2

• Question What determines whether a photon will
  be absorbed by a visual pigment?
• A. The wavelength of the photon
• B. The type of photoreceptor
• C. Principle of energy quantization
• D. The type of opsin bonded to retinal
• Answers A, C and D. The energy of the photon
  must match the difference in energy between the
  visual pigment molecules ground and excited
  states. This is described by the quantum
  mechanical principle of quantized energy levels.
  All photoreceptors have retinal, but they differ
  in their opsins.

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Question 3

• Question Which of the following improve image
  resolution at the fovea?
• A. Increased object size
• B. Increased light intensity
• C. Decreased convergence from photoreceptors to
  ganglion cells
• D. Increased levels of xanthophyll
• E. Increased ganglion cells density
• Answers A and C.

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Question 4

• Question A novel form of the Red opsin protein
  could be genetically engineered to shift the
  absorbance maximum of retinal into the yellow
  region of the electromagnetic spectrum (hereto
  referred to as OpsinY). Which of the following
  statements would be true?
• A. Less energy is required to excite retinal
  when bound to OpsinY than Red opsin.
• B. More energy is required to excite retinal
  when bound to OpsinY than Red opsin.
• C. If OpsinY is expressed in place of Red opsin,
  the resulting human vision would be a form of
  dichromacy.
• D. If OpsinY is expressed in place of Red opsin,
  the resulting human vision would be more
  sensitive to long wavelength light.
• Answers B.
• E h? hc/?.
• -Where h Plancks constant 6.626 x 10-34
  J/s, c speed of light 3.0 x 108 m/s, and ?
  wavelength.
• -Red 700 nm, Yellow 600
• -shorter wavelength higher energy
• This would be an example of anamolous
  trichromacy in humans (still 3 cone pigments) and
  results in a type of red-green color blindness.

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Four-Color Women

• Tetrachromats
• 4 color channels
• 4 types of cone cells, each containing a pigment
  responding to a unique wavelength set
• Normal variation in cone pigment genes
• Orange cone pigment
• Only women
• Red and green cone genes located on X chromosome
• Normally X-inactivation occurs

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Diffraction
http//webvision.med.utah.edu/imageswv/KallSpat12.
jpg
http//webvision.med.utah.edu/imageswv/KallSpat9.j
pg