Office Hours• Office hours are posted on the website.

– Molly: Tuesdays 2-4pm– Dr. Keister: Wednesdays 10am-12– Prof. Goldman: Wednesdays 2-3:30pm

• All office hours are in the help room downstairs.

Color Perception and Processing• Review Color Perception

• Three Channels of Perception

• Double Opponent Receptive Fields

Neural Wiring for Color Perception

Recall the three color perception channelsYellow-blueRed-greenAchromatic (white-black)

The different cone cells increase or decrease the signals from each channel

s-cone i-cone L-cone

neural cellfor y-blue channel

neural cellred-green channel

neural cellachromaticchannel

Electrical signal to brain

+++ ++ ++

The r-g Receptive Field and Cell

• The color receptive fields consist of photoreceptors in a center-surround geometry, pooled in a special way to one final neural cell (ganglion cell)

• The response of the ganglion cell depends on what color light is hitting the different sections of the receptive field

The r-g Receptive Field and Cell• Red light has two effects on the signal the ganglion

cell sends to the brain:

• Red light on the cones in the center of the receptive field increase the signal the ganglion cell sends

• Red light on the cones in the surround of the receptive field decreases the signal

R+R-

Concept Question• What is effect of red light falling on both the

center AND surround?a) No colorb) Sensation of redc) Sensation of greend) Sensation of yellow

R+R-

The r-g Receptive Field and Cell• Green light has two effects on the signal the ganglion

cell sends to the brain:

• Green light on the cones in the center of the receptive field decrease the signal the ganglion cell sends

• Green light on the cones in the surround of the receptive field increases the signal

G-

G+

The r-g Receptive Field and Cell• Electrical signal to brain from ganglion cell is at

ambient level when no light is on center or surround• When the signal to brain is increased we interpret

that as redred• When the signal to brain is decreased we interpret

that as greengreen

R+ G-

R- G+Summary of effect on ganglion cell signal for different light colors:

The r-g Receptive Field and Cell• There are two ways to increase the signal from the

ganglion cell (signal red):– Red light falling on the center– Green light falling on the surround

• And two ways to decrease the signal (signal green):– Red light falling on the surround– Green light falling on the center

R+ G-

R- G+

Summary of effect on ganglion cell signal for different light colors:

Double Opponent r-g Cell

Note, you would still "see" red if the center were grey!

Note, you would still "see" green if the center were grey!

Strongest signal (interpreted as red)

Weakest signal (interpreted as green)

No change in signal (color not noticed)

No change in signal (color not noticed)

R+ G-

R- G+

Double Opponent y-b Receptive Fields and Cells

Strongest signal (interpreted as yellow)

Weakest signal (interpreted as blue)

No change in signal (color not noticed)

Y+ B-

Y- B+

No change in signal (color not noticed)

Note, you would still "see" yellow if the center were grey!

Note, you would still "see" blue if the center were grey!

Concept Question

• Does the cell associated with this receptive field send a large or small signal?a) Largeb) Small

• What color is that interpreted as?a) Yellowb) Blue

Y+ B-

Y- B+

Color Constancy

• Color constancy means we see the proper colors of a picture or scene or object relatively correctly even though the overall illumination may change its color

• Color constancy depends on double-opponent processing the same way lightness constancy depends on lateral inhibition

Color Constancy

• This is because our double-opponent receptive fields compare spatially adjacent colors and are not very sensitive to an overall change in color of a scene

• Color constancy might have developed in the evolution of mankind so that cavemen could hunt in broad daylight, late afternoon, and early evening and still recognize their prey

No change in signal (color not noticed)

Perception of Edges

• Here are the enhanced edges resulting from your y-b chromatic channel• Note the edges that separate a

yellowish from a bluish color are enhanced the most

• The y-b receptive field in the top position enhances your perception of yellow

• The y-b receptive field in the lower position enhances your perception of blue

Color Edge Perception

• Here are the enhanced edges resulting from your r-g chromatic channel• Note the edges that

separate a reddish from a greenish color are enhanced the most

• Here are the enhanced edges resulting from your wt-blk achromatic channel• Compare with the way a

photocopy machine would see the design

Concept QuestionWhich illumination pattern will cause the strongest signal from the receptive field cell? (signal the strongest yellow)

Y+ B-

Y- B+

A B C

Optical Illusions

• Look at the grey squares in your peripheral vision (look at the center of the picture where the black "lines" cross)

• Does the grey square surrounded by yellow appear to take on a tint?• What color is it?

Optical Illusions

The receptive field shown has no color on the center of the field, but has a yellow signal on the surround, which decreases the signal from the associated ganglion cell, signaling blue

Y+ B-

Y- B+

Digital Color• Digital Information

• Bitmapped Images

• Digital Displays

• Indexed Color and Pixel Depth

Digital Information: Bits

• A bit is the smallest unit of information used by computers

• It can have two values, you can think of it as– An on/off switch (values are “on” or “off”)– A yes/no instruction (values are “yes” or “no”)– A 0 or 1 digit (values are the digits ‘0’ or ‘1’)

• Streams and collections of bits are used to carry and store complex information and instructions

Binary Number System• A collection of bits represents a “binary” number

(binary means two values)

• For standard counting and numbers, we use the decimal number system, also called “base 10”

• The binary system is called “base 2”

Binary Number SystemThe location of each digit in a number tells you what to multiply that digit by to add it to the totalIn base 10, the “places” are powers of 10, in base 2, the “places” are powers of 2

Binary Number System

1,543

1x1,000+5x100+4x10+3x1=1,543

thousands hundreds tens ones

1011

1x8+0x4+1x2+1x1=11 (base 10)

eights fours twos ones

Concept Question

What is the base 10 (decimal) value of the binary number shown at right?

A.24B.25C.26D.27E.28

11010eights

fours twos

onessixteens

16+8+2

=26

Bytes• One byte is an ordered set of 8 bits

• It can have any one of 256 (256=28) values

• This value is given by the values (0 or 1) of each of its 8 bits

• A megabyte (MB) is one million bytes• A gigabyte (GB) is one billion bytes• A terabyte (TB) is one trillion bytes

Discrete vs. Continuous • A byte can have any one of 256 values. It can have the

value 154, but can never have the value 36.25• A quantity that can only have specific values is called

discrete (this is not the same as discreet)

• The opposite of discrete is continuous, or a quantity that can have any value (usually within some range)

• Ex: speedometer on your car, the weight of a bag of apples

Pixels• Pixel is short for “picture element”

• A pixel is the smallest discrete unit of information in an image or picture

• It is often convenient to think of pixels as small dots of color

Digital vs. “Analog” Images• Photographic film has randomly arranged, randomly

sized particles • A digital photograph has regularly spaced, same-sized

pixels

film grain pixels of a CCD sensor

Bitmapped Images

• A “bitmap” is a map of the location and value of the pixels making up an image

• A digital photo is an example of a bitmapped image

Pixel Depth• The “depth” of a pixel is determined by the

number of bits of information in contains

• A one bit pixel can have two values, say black and white:

• A two bit pixel can have four values:

0 1

00 01 10 11

Digital Displays• A computer monitor or tv screen consists of a

large array of pixels

• The colors of a displayed image are produced by partitive mixing of red green and blue dots, one colored pixel is actually composed of one red dot one green dot and one blue dot

• The relative brightnesses of the red, green and blue dots gives the color of that specific pixel

Display Pixels: Plasma Display Panels• Each pixel in a plasma display is like a tiny

fluorescent or neon light bulb.

• UV light is generated by applying a voltage to the cell

• Different phosphors illuminated by this UV light in turn generate red, green, and blue light

Display Pixels: Liquid Crystal Display• LCD pixels do not generate their own light, but

are illuminated from behind (backlit) and act as variable filters

• The amount of red, green, and blue light allowed through is controlled by voltage on liquid crystal cells.