Eye Notes

STEPS

Sun produces photons. Eight minutes later, photon passes thru:

cornea = clear dome cover. It contains no blood vessels; it’s nourished by tears on the outside and the aqueous humor on the inside. The curve of the cornea accounts for 2/3 of the eye’s ability to focus, the lens provides the other third. An inherited condition (astigmatism) produces an irregularly shaped cornea; symptoms include headaches, eye strain, squinting and vision that is blurred or distorted.

aqueous humor = water-like substance continuously produced by the ciliary body. Although it’s mostly water, it also contains an antioxidant to protect the eye from UV rays. It’s presence inflates the eye. To balance the inoccurlar pressure and to carry away waste products, fluid is drained into Schlemm’s canal. Glaucoma is caused by blockage of the drainage or damage to the iris; a condition that can lead to blindness.

pupil = the opening in the middle of the iris.

lens = The crystalline lens can be irregularly shaped but is a less likely cause of astigmatism. The lens bends to focus the light on the fovea. With age, the lens becomes less flexible. It might also become clouded (cataract).

vitrious humor = jelly-like (like raw egg whites), becomes more liquid with age and separates from the retina, causing floaters (dark specks in vision). As the vitrious separates, sometimes the retina can become detached (posterior vitreous detachmentor PVD).

blood vessels = central retinal artery; 4 main branches. Supplies nourishment to non-receptor structures (ganglion, horizontal cells, etc.)

5 major layers of the retina (3 layers of nerve cells; 2 layers of connections)

1. cell bodies of ganglion cells (some amacrine cells)
2. neuropils layer of connections
3. horizontal and amacrine cells
4. neuropils layer of connections

            40 rods and cones

choroidal blood vessels = supply rods and cones. Receive about 75% of the retina’s blood flow. Macular degeneration occurs when abnormal blood vessels grow between retina and choroid. These blood vessels can also be damaged as a side effect of diabetes. This condition (diabetic retinopathy) includes leakage of blood and fluid into the eye (capillaries easily burst). The third major cause of blindness (retinitis pigmentosa) is a hereditary disease which causes rods (starting in the periphery) to degenerate. The result is the gradual onset of night blindness, followed by tunnel vision (only cones are working).

macula = about 7mm (1/4 inch), yellow spot in the eye, contains the fovea.

fovea = about 1.5 mm (1/16^th of an inch).

6 million cones

rods 500 times more sensitive than cones; 120 million

lens is held in place by strings (zonules); suspended

sclera = white of the eye; strong fibers to resist the internal pressure of the eye (twice the atmosphere). Covers entire ball, expect cornea, exit for optic nerve.

iris = color part of eye; can be translucent (albinos); two sets of muscles to open and close pupil

Visual System

Sensation & Perception

Sensation:

Hardware wiring; similar among all human beings; no meaning.

Stage 1 (sense): blotches of red, yellow, brown, pink & white

Perception:

Based on individual experiences; individual differences.

Stage 2 (perceive)

Identification; depends on past experience (apple, plum, pluot)

Characterization: delicious, hard, etc.

Perception is so fast that we don’t notice the sensation stage

Except when sensory data is vague

3 summary principles

1. Correspondence between physical and psychological reality is NOT one-one

2. Sensation and perception are adaptive

Color Context = how color behaves in relation to other colors and shapes

Effects of different color backgrounds for the same red square

Different readings of same color

3. Sensation & perception are active processes

Top-down processing

Stroop effect

Canonical perspective (prototypical perspective)

Rat-Man

Perceptual Rules

Mental sets

Illusions

Color Physics

Nanometer (nm) = billionth of a meter

Any given light is some collection of photons

The spectrum of a light tells how many photons it contains at each wavelength

Human vision detects photons from 400 nm to 700 nm in size

Color History

Isaac Newton

Prism

White sunlight is not “pure” light; a mixture of colored lights

Those colored lights ARE pure and cannot be decomposed by the prism

When all the pure colors are remixed back together, we see the mixture as white.

Mixture just two pure colors (Y+B) can also produce a light that looks white metameric light

Color Experience

Hue – the “color” of a light (red, orange, yellow, green, etc.)

Brightness – the “intensity” of a light (dark red vs. bright red, etc)

Saturation – color “strength”, how much white does the light contain? (pink is desaturated red)

Color Mixture

Additive color mixing: mixture of light

Subtractive color mixing:  mixture of paint

Optical color mixing: colors add in the eye when small spots with different colors are viewed from a distance (pointillism)

Pointillism

tiny dots of primary-colors are used to create secondary colors

post-impressionism form of painting

Rod vision

Pure rod vision is completely COLOR BLIND

Two lights of different wavelengths can always be made visually identical by adjusting their relative intensities

Principal of UNIVARIANCE

Photoreceptor can signal the NUMBER of PHOTONS it absorbs, but NOT the wavelengths of those photons

Consequently, it requires at least two types of photo pigments to perceive color

Color Theory

1. Trichromatic Theory

Thomas Young & Hermann von Helmholtz

Any color can be made by mixing 3 lights

red, green, and blue lights; adjustable intensity

3 types of photoreceptors with peak sensitivity in different regions of the visible spectrum

S, M, L conesy

2. Opponent Theory

Proposed by Ewald Hering

Trichromatic theory does not explain

afterimages

simultaneous contrast

color blindness

Hue cancellation experiment

Must add red light to green light to get white light

Yellow light is the only color that cancels blue

Two basic types of opponent-color cells

+R-G

+B-Y

Ganglion cells

Spontaneously fire

a base rate while resting

increased firing when excited

decreased firing when inhibited

Unmylinated inside eye; mylinated outside eye

Each gets input from 100+ rods & cones

In fovea = 1 cone to 5 ganglions

In periphery, multiple rods to 1 ganglion

2 major types in retina:

Midget retinal ganglion cells

their dendrite trees are small

their cell bodies are small

80% of all retinal ganglion cells

receive input from relatively few rods & cones

project to parvocellular layers of LGN

also called parvocellular cells

respond to changes in color

slow conduction velocity

respond weakly to changes in contrast; unless the change is substantial

simple center-surround receptive fields

center may be either ON or OFF to one cone

surround is the opposite to another cone

Parasol retinal ganglion cells

Project to the magnocellular layers of LGN

Dendritic trees & cell bodies are large

10% of retinal ganglion cells

Not very sensitive to changes in color

Can respond to low-contrast stimuli

Inputs from lots of rods & cones

Fast conduction velocity

Center-surround receptive fields

Larger receptive fields

Giant ganglion cells

Only about 3000 in each retina

Respond directly to light; contain a visual pigment (melanesian)

Also receive connections from rods & cones

Encode color & spatial info

COLOR Deficiency

1. Monochromatism

Rod vision only, no functioning cones

Very rare (10 out of a million)

See everything in shades of gray

True definition of color-blind

Usually poor visual acuity

Very sensitive to light

2. Dichromatism

Dichromacy means “two-chromacy”

One of the three cone photopigments is missing

Three types

Protanopia

missing L pigment (long)

1% of men, .01% of women

Deuteranopia

missing M pigment (middle)

1% of men, .01% of women

Tritanopia

missing S pigment (short)

less than .01% of both M and F

3. Anomalous Trichromats

All three cone types are present

But one has a photopigment with an abnormal absorption spectrum

3 types

Protanomalous

abnormal L pigment

1% of men, .021% of women

Deuteranomalous

abnormal M pigment

5% of men, .04% of women

Tritanomalous

abnormal S pigment

very rare in men and women

4. Cortical Color Blindness

Cerebral achromatiopsia

brain function is normal but color vision lost because of brain injury

Color Constancy

Perception of lightness remains relatively constant; even when objects are viewed under different intensities of light

What causes color constancy?

Photoreceptor level: chromatic adaptation

Higher level: a lateral spatial comparison that “discount” the illuminant

  Light Constancy

Achromatic version of Color Constancy

How the Eye Sees Color

1. All the colors of sunlight shine on the apple

2. Red apple absorbs all the colored light rays except red, and reflects only red to the eye

3. The eye receives the reflected red light and sends a message to the brain.

How the Brain Interprets Color

Can color influence others

Teal shirt

Red dress

Yellow is cheerful or cause aggression;

Blue commands authority

Green was the sacred color of the Zhou Dynasty

Brown is a trusted color

Soothing pink

Alexander G. Schauss, Ph.D.

Experimented with hundreds of shades of pink

Identified a shade of pink that had maximum effect on reducing hyperexcitability

Painted jail cell pink: prisoners calm, stayed calm 30 min. after

Called it P-618

“Baker-Miller Pink“

Pepto-Bismol pink

Bubble gum pink

Approx. RGB formula:

R:255

G:145

B:175

Color wheel

Sir Isaac Newton

the first circular diagram of colors in 1666l

Primary Colors

All other colors are derived from these 3 hues

red

yellow

blue

Secondary colors

Formed by mixing the primary colors

green

orange

purple

Tertiary Colors

Formed by mixing one primary and one secondary color

yellow-orange

red-orange

red-purple

blue-purple

blue-green

yellow-green.

Color Harmony

Harmony = pleasing arrangement of parts

not too bleak

not too cluttered

Analogous Colors

Any 3 colors which are side by side

yellow-green

yellow

yellow-orange

Usually one of the 3 colors predominates

Complementary Colors

Any 2 colors, directly opposite

red and green

red-purple and yellow-green

Nature’s Colors

Anything in nature

6 Functions of Color

Facilitate perceptual organization

Form

Depth

Motion

Perceptual Constancy

Separate figure from ground

Survival (camouflage)

The “walking stick”

M-113 APC

Wild Turkey

American Dipper

Help attention shift (airport signs)

Add beauty to life

Makes life easier

Choose color to stand out or fit in

sensation

Stage I: sensation

Stage 2: perception

Principles

All-or-None Law

Either a neuron fires or it does not

It fires if it passes a certain threshold

If it fires, it does so at full strength

Neurons cannot change intensity of an impulse

Neurons  change the rate (# of impulses per sec)

Refractory Period

Can’t fire until it recovers

1 millisecond

Absolute refractory period

Can’t fire no matter how much stimulation

Relative refractory period

Can fire with lots of stimulation

Thresholds

Super-threshold stimulus

1 neuron releases enough neurotransmitter to activate (depolarize) the post-synaptic neuron in 1 shot

Summation

Multiple sub-threshold releases of neurotransmitter builds up

Temporal Summation

1 neuron, several impulses in rapid succession. So rapidly neurotransmitter doesn’t dissipate

Spatial Summation

Many neurons give 1+ impulses

1 cell can code 2+ perceptual experiences

excitation signals one quality; inhibition another

For example:

excitation = blue, inhibition = yellow

excitation = left, inhibition = right

Efficiency

Separate systems

Separate subsystems for specific functions

Ignore steady state information

Pre-code for critical features

All of our senses are data reduction systems

Light

Travels in straight lines

(assuming unchanging optical density)

Reflects off of surfaces

Absorption of light by atmosphere

Refracts when traveling into new medium

Has various frequencies (colors)

Has various amplitudes (intensities)

Electromagnetic wave

Carrier of information

Radiance

Amount of energy from a light source

Measured in lumens

Illumination

Amount of light falling on a surface

 Seeing

Definition

The physical recording of the pattern of light energy received from the outside world

Process

Selective gathering of light

Projection or focusing of the light on a photosensitive surface

Conversion of the light into a pattern of chemical or electrical activity

Doesn’t matter if an object is a source or a reflector of light

Strength of reflection is a function of:

color of object

smoothness of object

relative orientation between light rays, surface and observer

Eye

Photoreceptor = receptor of photons

Transforms light into nerve impulses

more light = higher frequency of impulses

Directional Sensitivity

If photoreceptor responds to light from any direction

cannot determine direction of light

can only determine overall amount of light

Eye Cups

Need to exclude all light, except rays which come from a particular direction

pigmented cells behind eye cups:

Cambrian period, 570–500 million years ago

Directional Sensitivity  (parasitic worm)

Mermis nigrescens

Directional Sensitivity  (ommatidium)

pigmented cells around

extend eye cup into a tube

Compound Eyes

Insects

trilobites (300 million years ago)

very near sighted but wide field microscopic vision

very poor far vision

Average human vision; would require array 1m diameter

 Pinhole Eye

Array of photo receptors along cavity

Pinhole selects light traveling in the direction between it and photoreceptor

Result is a complete image on photoreceptor array

Image represented by set of outputs

Nautilus: squid that lives in a shell

Advantages

Sharp images (like a pinhole camera)

Drawbacks

Requires high intensity of light or long exposures

Can’t change focus

Lens

Vertebrates

Octopus

Instead of a single ray thru a narrow pinhole

Lens focuses and selects light rays

beam of light focused onto each photoreceptor

Human Eye

Retina covered with light-sensitive receptors:

rods

primarily for night vision & perceiving movement

sensitive to broad spectrum of light

can’t discriminate between colors

sense intensity or shades of gray

cones

used to sense color

Center of retina has most of the cones

allows for high acuity of objects focused at center

cones packed very tightly in fovea “depression”

Edge of retina is dominated by rods

allows detecting motion of threats in periphery

Photoreceptor array “copies” incoming light

lens forms an image on the retina

photoreceptors fire at a rate “proportional” to intensity of light

But, absolute intensity is not that useful

changes when light level changes

Better to represent objects via changes of intensity over space: edges

For example:

The pattern of light from an object with a patch of black

If incoming light is twice as strong, twice as much gets reflected…

Very efficient compression

100 million axons from photo receptors per eye

1 million axons from ganglion cells per eye

Reduction by a factor of 100!!

 

 

Eyes

Structure cornea
iris
pupil crystalline lens
Made up of transparent proteins = crystallins Very concentrated collection of proteins
20,000 concentric layers
retina
wired backwards
rods
target detection
spread across eye
wired together (summed)
cones
target identification
concentrated in fovea
wired one by one
intensity = amount of light needed to fire