1. Chapter 2Chapter 2
 The Human Eye: Anatomy (structure) and physiology (function)
 Visual system
◦ 3 major components:
 Eyes
 visual pathways
 visual centers of brain
 Types of senses:
◦ Far sense: detect things without needing to come into contact with
them
 Vision is a far sense
◦ Near sense: need to come into contact with stimuli to sense it
 This chapter addresses…
◦ Problem: How does the brain take papers of light energy and deduce
meaningful descriptions of objects in the world?
◦ Solution: Inverse Optics - brain makes assumptions based on
experience with the environment

2. Physics ReviewPhysics Review
Light
◦ Electromagnetic radiation
 Only a small range is considered visible light
◦ Wavelength determines electromagnetic quality
 Frequency measured in nanometers
◦ Why light?
 Reliable: Travels very fast and in straight lines
 Prevalent: Lots of it
 Informative: Reflected/Absorbed by surface molecules as it
interacts with them and tells us information about them

3. The HeadThe Head
 Eye placement:
◦ Lateral vs. Frontal
 Prey animals have laterally placed eyes
 Predator animals have frontally placed eyes
 Skull:
◦ Orbit: bony depression filled with a cushion of fat and connective tissue
 Eye Movement
◦ 6 extra-ocular muscles:
 Are held in constant tension
 Connect to orbit
 Conjugate: move in the same direction
 Vergence: move in opposite directions (crossing or outward)
 Convergence: turning inward to focus on near objects
 Eyelids:
◦ Eyelids do more than just protect the eye, they also move moisture from the tear ducts
across the cornea
◦ Blinking is spontaneous and occurs about every four seconds but varies based on if you are
speaking (especially arguing) and if you have a higher interest in what you are talking about
◦ Distance: you also have a larger distance between eyelids if you are more interested
 Tears:
◦ Convey social information, hydrate the eye, contain an antimicrobial agent, drain into mucous
membrane of nose

4. Eye FunctionEye Function
Image: light distribution that preserves
the spatial ordering of locations in space
Image formation – reversing light
divergence
◦ Optical power of:
 1) cornea: 2/3rds of eye’s optical power
 2) lens: flexing called accommodation
◦ Opening your eyes under water is a good
example of when the optical power is
changed, you can see but it is blurry

5. Eye StructureEye Structure
Three layers:
◦ Protecting: Fibrous Tunic
◦ Nourishing: Vascular Tunic
◦ Detecting: Retina

6. Fibrous TunicFibrous Tunic
Sclera – “white” of eye, dense material,
gives eye it’s shape (eye is at a pressure
2x atmosphere)
Cornea – transparent “bulge” at front of
eye that allows light into eye

7. Out of FocusOut of Focus
Myopic (nearsighted): eye is too long and
light focuses in front of retina, need a
concave lens, cannot be fixed by
accomodation
◦ Must have glasses
Hyperopic (farsighted): eye is too short, can
be corrected by accomodation unless too
extreme or image too close but this
requires constant muscle effort resulting in
eyestrain headaches and nausea
Emmetropic (normal vision)

8. Problems with LensProblems with Lens
Presbyopia:
◦ Loss of sight with age due to weakening of
accomodation
◦ Become hyperopic
◦ Note: presby = “old”
Astigmatism:
◦ Distortion of cornea
◦ Tested with a “target” that appears wavy

9. Vascular TunicVascular Tunic
 Choroid: spongy layer that is heavily pigmented
◦ Pigment epithelium (very thin layer inside of choroid)
 Blood passes through it to get to photoreceptors
 Degrades with age (macular degeneration and diabetes)
◦ Contains blood vessels and capillaries that dispose of waste
◦ Pigmentation reduces light scatter
 Anterior Chamber (between fibrous and vascular tunic)
◦ Cilliary body manufactures aqueous humor
 Aqueous humor maintains eye shape, transports nutrients to cornea, is cycled, too
much increases pressure and causes glaucoma
◦ Contains:
 Iris/Pupil: colored portion, controls amount of light entering eye, depth of field
(range of vision) where a larger aperture results in a smaller range
 Responds to arousal level, this response is the reason why poker players wear sunglasses
 In albinism, lack of melanin in iris can cause it to appear red
 Lens: layered and springy (like an onion), thins/thickens to focus light
(accomodation), constantly adding layers leading to sclerosis (hardening) of lens, can
become opaque (cataract; esp. with diabetes)
 It is important to correct vision early, especially in the case of congenital cataracts
 Vitreous chamber (2/3 total eye volume): filled with a material like eye white and can
contain floaters
 Can bleed into vitreous chamber (usually because of an accident or illness)
 If vision ever literally “goes red” go to emergency room immediately

10. RetinaRetina
 Retina very thin, delicate and fragile
 Contains photoreceptors,
◦ Backwards organization to maintain nutrient flow to "outer
segment“ of photoreceptors
 Retina has the highest metabolic rate of body
 Landmarks:
◦ Usually observed through an opthalmoscope which was
developed by Hermann von Helmholtz
◦ Macula (1.5mm diameter):
◦ Focus of eye  contains fovea
 Vision is most accurate here and degrades in macular degeneration
 Fovea is a thinned out area to speed the passage of light to the most
important parts of the retina (the photoreceptors)
◦ Optic Disk: “blind spot” where nerves exit the eye that is
filled in by the brain so that we do not notice it

11. PhotoreceptorsPhotoreceptors
Order of stimulation:
◦ 1) Photoreceptors
 Rods: about 100 million, operate in grayscale, are
mostly around the periphery
 Cones: about 5 million, see color, mostly around fovea
 Distribution of rods/cones is not always this way. It takes
about 4 years for photoreceptors to organize in infants as
cones “migrate” towards the fovea. With age, cell death can
lead to a loss of vision around the fovea leading to a decrease
in the number of cones.
◦ 2) Bipolar cells
◦ 3) Amacrine cells
◦ 4) Horizontal cells
◦ 5) Retinal ganglion cells

12. PhotopigmentsPhotopigments
Two components
◦ Opsin: large protein, 3 types that lead to tri-
chromatic (3 color) vision
◦ Retinal: from vitamin A
Phototransduction (takes less than one
thousandth of a second)
◦ 1) Light
◦ 2) Isomerize opsin and retinal
◦ 3) **Reduction** of current
◦ 4) **Reduction** of transmitter substance
(glutamate) release

13. Wavelength SensitivitiesWavelength Sensitivities
Rods: most sensitive to 500nm which appears
“bluish-green”
◦ Rods stop responding when light passes out of their
visual range
Three types of cones:
◦ Type 1 is most sensitive to 440nm (“violet”)
◦ Type 2 is most sensitive to 530nm (“green”)
◦ Type 3 is most sensitive to 560nm (“yellow”)
◦ Cones will work over a range of light and continue to
respond until “cooked”
Interesting fact: Aphakic humans (individuals who
have had their lenses removed) can sometimes see
UV light