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

Physics ReviewPhysics ReviewLight

◦ 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

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

Eye FunctionEye FunctionImage: 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

Eye StructureEye StructureThree layers:

◦Protecting: Fibrous Tunic◦Nourishing: Vascular Tunic◦Detecting: Retina

Fibrous TunicFibrous TunicSclera – “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

Out of FocusOut of FocusMyopic (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)

Problems with LensProblems with LensPresbyopia:

◦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

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

RetinaRetinaRetina very thin, delicate and fragileContains 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

PhotoreceptorsPhotoreceptorsOrder 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

PhotopigmentsPhotopigmentsTwo 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

Wavelength SensitivitiesWavelength SensitivitiesRods: most sensitive to 500nm which

appears “bluish-green”◦ Rods stop responding when light passes out

of their visual rangeThree 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