The Eye

hole that allows

light to enter eye

transparent outer

covering of eye

muscle that controls

(pupil) how much

light enters eye

focuses light

on the retina

contains visual

sensory receptors

contains most of

the eye’s cones

Where optic nerve

leaves eye…no

receptors here

Wavelength (Hue)

Hue (color):

dimension of color

determined by

wavelength of

light.

Wavelength the

distance from the

peak of one wave

to the peak of the

next.

Wavelength (Hue)

Different wavelengths of light result

in different colors.

400 nm 700 nmLong wavelengthsShort wavelengths

Violet Indigo Blu

e

Gree

n

Yellow Orang

e

Re

d

Intensity (Brightness)

Intensity

Amount of

energy in a

wave

determined by

amplitude;

related to

perceived

brightness.

Intensity (Brightness)

Blue color with varying levels of intensity.

As intensity increases or decreases, blue color

looks more “washed out” or “darkened.”

The Lens

Nearsightedness: A condition in

which nearby objects are seen

more clearly than distant objects

b/c eye elongated & image

focus before it hits the retina

Farsightedness: A condition in

which faraway objects are seen

more clearly than near objects

b/c eye shortened & image

would focus after it hit the retina

Retina

The light-sensitive

inner surface of the

eye, containing

receptor rods and

cones plus layers of

other neurons

(bipolar, ganglion

cells) that process

visual information.

Fovea

http://www.bergen.org

Central point in the retina, around which the eye’s

cones cluster…because of this there is little color

vision in the farthest periphery of our vision

Let’s test this fact…

Photoreceptors

E.R. Lewis, Y.Y. Zeevi, F.S Werblin,

1969

RODS CONES

CONES RODS

Number 6 million 120 million

Location Center (fovea) Periphery

Color sensitive? Yes No

Sensitivity in

dim light?Low High

Ability to detect

detail?High Low

Number of

bipolar cells?Each has own

Shares one

with other

rods

Bipolar & Ganglion Cells

Bipolar cells receive messages from

photoreceptors and transmit them to ganglion

cells, which form the optic nerve (axons).

Remember…

Cones each have their

own bipolar cell.

Multiple rods share one

bipolar cell.

Test your Blind SpotBlind spot – no visual receptors here because of all the

information leaving the eye

To find your blind spot, close your right eye and

look at the cookie monster at eye level. Maintain

your focus on the cookie monster and slowly

move away from the screen (or get closer) until

the cookie disappears.

Under normal circumstances, you are unaware of your blind spot because one eye

sees what the other does not. WHY DO YOU SEE WHITE WHERE THE COOKIE

SHOULD BE? Even with one eye closed, you're not aware of the gap because your

brain "fills in" the missing information by making an assumption about what belongs in

the void.

Feature DetectionNerve cells in the visual cortex that respond to

specific features, like edges, angle, length and

movement.

There are even some feature detectors that are

specifically sensitive to the human face!

Visual Information Processing

Processing of several aspects of the stimulus

simultaneously is called parallel processing. The

brain divides a visual scene into subdivisions such

as color, depth, form and movement etc.

Tim Bieber/ The Image

Bank

info sent

to visual cortex

from visual cortex

to assoc. areas

from assoc.

areas to memory

Theories of Color Vision

Trichromatic theory: Based on behavioral

experiments, Helmholtz suggested that retina

contains three receptors sensitive to red, blue

and green colors.

Blue Green Red

Medium Lo

w

Max

Standard stimulus

Comparison stimulus

PhotoreceptorsRed

Cone

s

Gree

n

Cone

s

Long

wave

Medium

wave

Short

wave

MacNichol, Wald

and Brown (1967)

measured directly

the absorption

spectra of visual

pigments of single

cones obtained from

the retinas of

humans.

Blue

Cone

s

Color Deficiency(blindness)

Genetic disorder which prevents individuals from

discriminating between certain colors due to a

weakness in or lack of one of the cones

Most common form of color “blindness” is difficulty

distinguishing between red and green

Complete color deficiency does exist but is very

rare…it would be like watching a black and white

movie.

Ishihara (color blindness)Test

25 6

5

42

56 8

Normal vision red/green deficiency

Normal vision red/green deficiency

Photos from: http://critiquewall.com

Above represent the art in “normal vision”…below what it would look like to a person

with red/green deficiency

http://www.vischeck.com/examples/

Opponent Process Theory

Hering, proposed that we process four primary

colors opposed in pairs of red-green, blue-

yellow, and black-white.

Cone

s

Retinal

Ganglion

Cells

Afterimages…

Gaze at the middle of the flag for about 30

seconds, when it disappears, stare at the dot and

report if you see America’s flag.

Afterimages…

Gaze at the middle of the light bulb for about 30

seconds, when it disappears, stare at the white space

and report what you see.

Afterimages…

Spanish castle afterimage….

Color Constancy

the color of an object will remain roughly constant

even if the lighting and wavelengths change