1 Chapter 10: Color Perception How we see color Three types of cones - each with different responses at all wavelengths Color matching Opponent processing.

1

• Chapter 10: Color Perception

• How we see color• Three types of cones - each

with different responses at all wavelengths

• Color matching• Opponent processing• Color blindnesses

(deficiencies in seeing color)

• Spatial color processing in your retina- • Double opponent with

receptive fields

Physics 1230: Light and ColorChapter 10

Clicker grades updated!!!

We have three different kinds of cones — whose responses are mainly at short, intermediate and long wavelengths

• s-cones absorb short wavelength light best, with peak response at 450 nm (blue)

• L-cones absorb long wavelength light best, with peak response at 580 nm (red)

• i-cones absorb intermediate wavelengths best, with peak response at 540 nm (green)

• Light at any wavelength in the visual spectrum from 400 to 700 nm will excite these 3 types of cones to a degree depending on the intensity at each wavelength.

• Our perception of which color we are seeing (color sensation) is determined by how much S, i and L resonse occurs to light of a particular intensity distribution.

Rule: To get the overall response of each type of cone, multiply the intensity of the light at each wavelength by the response of the cone at that wavelength and then add together all of the products for all of the wavenumbers in the intensity distribution

i-conesL-cones

s-cones

Spectral Spectral response of conesresponse of cones in typical human eye in typical human eye

rela

tive

res

pons

ere

lati

ve r

espo

nse

Concept Question

• What is an additive mixture of blue and yellow?

• A. Green;• B. Red;• C. White;• D. Black;• E. Orange;

• We learned this in Chapter 9… Why do we still need this?

Examples of two different ways we see whiteLight color Brightness S-cone r esponse I-cone r esponse L-cone response 460 nm blue 1 60 5 2 575 nm yellow 1.66 0 1.66 x 33 1.66 x 35 Mixtur e ( perceived as white) 60 + 0 = 60 5+1.66 x 33 = 60 2+1.66 x 35 = 60

Spectral Spectral response of conesresponse of cones in typical human eye in typical human eye

rela

tive

resp

onse

rela

tive

resp

onse

• Our sensation of color depends on how much total s, i & L cone response occurs due to a light intensity-distribution

• Multiply the intensity distribution curve by each response curve to determine how much total S, i, and L response occurs

• We experience the sensation white when we have equal total s, i & L responses

• There are many ways this can occur!!• E.g., when broadband light enters our eye• Another way to experience white is by viewing a

mixture of blue and yellow• E.g., 460 nm blue of intensity 1 and 575 nm

yellow of intensity 1.66• The blue excites mainly s-cones but also a

bit of i-cones and a bit of L-cones• The yellow excites i-cones and (slightly

more) L-cones but no s-cones• The result is an equal response of s-cones,

i-cones and L-cones (details)

575 nm yellowof intensity 1.66460 nm blue of

intensity 11

1.66

0

How does a normal person see yellow when only red and green lights are superimposed?

Spectral Spectral response of conesresponse of cones in typical human eye in typical human eye

rela

tive

resp

onse

rela

tive

resp

onse

• Our sensation of yellow depends on a special s, i & L cone response

• We experience the sensation yellow when 575 nm light reaches our eyes

• What really gives us the sensation of yellow is the almost equal response of i and L cones together with no s-cones!!

• Another way to experience yellow is by seeing overlapping red & green lights

• E.g., 530 nm green of intensity 1 and 650 nm red of intensity 2.15

• The green excites mainly i-cones but also L-cones, while the red excites mainly L-cones but also i-cones

• The total respone of s & i-cones due to the spectral green and red is the same as the total response due to spectral yellow

• In general need 3 wavelength lights to mix to any color

575 nm yellowof intensity 1.35

530 nm green of intensity 11

2.15

0

650 nm red of intensity

2.15

Light color

Brightness S-cone response I-cone response L-cone response

530 nm green 1 negligible 41 28

650 nm red 2.15 negligible 2.15 x 2 2.15 x 9

Mixture (perceived as yellowyellow ) negligible 41 +2.15 x 2 =45 28 +2.15 x 9 =47

575 nm yellow 1.35 negligible 1.35 x 33 = 45 1.35 x 35 = 47

What happens if a person is missing one type of cone?

• Missing one type of cone results in one type of color-blindness

• E.g., someone whose L-cone is missing will not see colors correctly

• They will see white or grey when a single wavelength 495 nm is present because light at 495 nm excites S & i cones equally no matter what its intensity

• They will also be able to see white by mixing any 2 wavelength lights with the correct intensities so that the S and i cones respond equally

• All colors they see can be obtained by mixing only 2 different wavelength lights

• This type of color-blindedness is called protanopia (a kind of dichromacy)

• Dichromats can match any light color by mixing only 2 wavelength lights

Spectral Spectral response of conesresponse of cones in protanopic eye in protanopic eye

rela

tive

res

pons

ere

lati

ve r

espo

nse

What happens if a person is missing 2 (or all 3) types of cones?

• Missing 2 or all 3 type of cones results in a different (rare) type of color-blindness called monochromacy

• Cone monochromats have only one type of cone (s, i or L).

• Rod monochromats have no cones and have difficulty seeing with their rods under bright light (photopic) conditions

• Monochromats can match a light of any color by varying the intensity of only one spectral (wavelength) light

• They are truly color-blind because they cannot distinguish any wavelength color from any other

• They see in blacks, whites and greys

• Trichromats (those with trichromacy) possess all 3 types cones, but either have shifted response curves for one or more of those cones or else have a problem with opponent processing (to be discussed next)

Concept question

• Can rod monochromats distinguish red color from green color?

• A. Yes• B. No• C. Only during a bright

day;• D. Only during gthe

night;

The four psychological primaries

• In addition to the additive primaries (RGB) and the subtractive primaries (CMY) there is another set of (4) primary colors, called the psychological primaries Blue Green Yellow Red (really closer to magenta)

• These hues can be used to describe all other hues.

• All hues can be verbally described as combinations of these colors. For example,• Yellowish red• Greenish yellow• Bluish green• Bluish red

• BUT we don't recognize hues such as• Reddish green• Yellowish blue

• Red and green are opponent hues• Yellow and blue are opponent hues

We can verify color naming of hues in terms of the psychological primaries on the chromaticity diagram

• All of the hues can be named qualitatively by how much green, red, blue or yellow is "in" them

• We don't need orange, purple or pink: • orange can be thought of as yellow-red• purple can be thought of as red-blue• pink has the same hue as red but differs

only in lightness• We can break up the diagram into 4

different regions by drawing two lines whose endpoints are the psychological primary hues

• The endpoints of the yellow line are 580 nm "unique" yellow and 475 nm "unique" blue

• One endpoint of the red line is 500 nm "unique" green and the other is "red" (not unique or spectral - really more like magentamagenta)

Greenness &Greenness &yellownessyellowness

Redness &Redness &yellownessyellowness

Redness &Redness &

bluenessblueness

Greenness &

Greenness &blueness

blueness

What is meant by the opponent nature of red vs green (r-g) perception and of yellow vs blue (y-b) perception.

• Viewing a progression of colors in the direction of the yellow line from 475 nm blue towards 580 nm yellow, we see more yellowness of each color and less blueness.

• We call this perception our y-b channel

• Yellow & blue are opponents • Moving parallel to the red line from

500 nm green towards nonspectral red we see more redness in each color and less greenness.

• We call this perception our r-g channel

• Red and green are opponents• The lines cross at white, where both

y-b & r-g are neutralized

Greenness &Greenness &yellownessyellowness

Redness &Redness &yellownessyellowness

Redness &Redness &

bluenessblueness

Greenness &

Greenness &blueness

blueness

y-b

r-g

How might the three types of cones be "wired" to neural cells to account for our perception of hues in terms of two

opponent pairs of psychological primaries r-g and y-b?• The 3 kinds of cones are related to r-g and y-b by

the way they are connected to neural cells (such as ganglion cells)

• Cones of each kind are attached to 3 different neural cells which control the two chromatic channels, y-b and r-g, and the white vs black channel called the achromatic channel (lightness)

• "wiring" is the following:• When light falls on the L-cones they tell all 3

neural cells to increase the electrical signal they send to the brain

• When light falls on the i-cones they tell the r-g channel cell to decrease (inhibit) its signal but tell the other cells to increase their signal

• When light falls on the s-cones they tell the y-b channel cell to decrease (inhibit) its signal but tell the other cells to increse their signal

s-cone i-cone L-cone

neural cellfor y-b

chromaticchannel

neural cellfor r-g

chromaticchannel

neural cellfor w-blk

achromaticchannel

Electrical signal to brain

+++ ++ ++

s-cone i-cone L-cone

neural cellfor y-b

chromaticchannel

neural cellfor r-g

chromaticchannel

neural cellfor w-blk

achromaticchannel

Electrical signal to brain

++++ +++

How can this "wiring" work to produce the chromatic channels?

• The neural cell for the y-b chromatic channel has its signal

• inhibited when (bluE) light excites the s-cone INTERPRETED AS BLUE

• enhanced when light excites the i & L cones INTERPRETED AS YELLOW

• The neural cell for the r-g chromatic channel has its signal

• inhibited when (green) light falls on the i-cone INTERPRETED AS GREEN

• enhanced when light excites the s and L coneINTERPRETED AS MAGENTA (Psychological red)

• The neural cell for the achromatic channel has its signal enhanced when light excites any of the cones

s-cone i-cone L-cone

neural cellfor y-b

chromaticchannel

neural cellfor r-g

chromaticchannel

neural cellfor w-blk

achromaticchannel

Electrical signal to brain

++++ +++

We learned: how cone-neural cell "wiring" works to produce the chromatic channels

• The neural cell for the y-b chromatic channel has its signal

• inhibited when (bluE) light excites the s-cone INTERPRETED AS BLUE

• enhanced when light excites the i & L cones INTERPRETED AS YELLOW

• The neural cell for the r-g chromatic channel has its signal

• inhibited when (green) light falls on the i-cone INTERPRETED AS GREEN

• enhanced when light excites the s and L coneINTERPRETED AS MAGENTA (Psychological red)

• The neural cell for the achromatic channel has its signal enhanced when light excites any of the cones

More systematic descriptions of color-blindedness (no need

to memorize terminology)

• Monochromacy (can match any colored light with any 1 spectral light by adjusting intensity)

• Either has no cones (rod monochromat) or has only 1 of the 3 types of cones working (cone monochromat).

• Sees ony whites, greys, blacks, no hues• Dichromacy (can match any colored light

with 2 spectral lights of different intensities of (rather than the normal 3)

• L-cone function lacking = protanopia• i-cone function lacking = deuteranopia• s-cone function lacking = tritanopia• no y-b channel but all 3 cones OK =

tetartanopia

• Anomalous trichromacy (can match any colored light with 3 spectral lights of different intensities as in normal vision, but still have color perception problems)• Protanomaly

• Shifted L-cone response curve• Deuteranomaly (most common)

• Shifted i-cone response curve• Confusion between red and green.

• Tritanomaly• Yellow-blue problems: probably

defective s-cones• Neuteranomaly

• ineffective r-g channel

y-b

Visualizing dichromacy: protanopia

r-gSpectral Spectral response of conesresponse of cones in protanopic eye in protanopic eye

rela

tive

res

pons

ere

lati

ve r

espo

nse

• No L-cone function• See yellows & blues instead of reds & greens

• Neutral hue pts. below 500 nm & non-spectral magenta; neutral line close to r-g line in chromaticity diagram (effectively missing)

• As move in direction of black arrows all colors aligned with white arrows only have different yellowness and blueness, not different greenness or redness

Visualizing dichromacy: deuteranopia

• i-cone function lacking• Like protanopes, they see yellows & blues

instead of reds & greens• Neutral hue points near 500 nm and non-

spectral purple• Neutral hue line close to the line joining the

neutral points in the chromaticity diagram• Hence, like protonopes, deuteranopes don't

distinguish green from red very well

y-b

Visualizing dichromacy: tritanopia

• s-cone function lacking• They see reds and greens instead of blues

and yellows• Neutral hue points at 570 nm and blue

purple; neutral hue line between neutral hue points in the chromaticity diagram which is effectively missing

• Hence, they don't distinguish blues and yellows very well

• Tetartanopes lack the y-b channel• See similarly to tritanopes

r-g

Take the color blindness test

• The color blindness test consists of a set of five charts. Each chart shows a number in one color on a different backgound color.

• People with normal color vision will have no problem seeing the numbers on the charts, but people with color blindness will see only random colored dots.

• Seventy-five percent of color blind people have poor green perception. Of the remaining, 24% have poor red perception, and one percent are affected by a rare tritan type.

The opponency of red and green and of yellow and blue can be understood in terms of special receptive fields in

our retina called double-opponent receptive fields

• Double opponent receptive fields in our retina

• are responsible for lateral inhibition, just like light-dark receptive fields we have studied

• enable us to notice sharp color boundaries in the same way that light-dark receptive fields allowed us to notice sharp light-dark boundaries

• exaggerate colors on either side of an opponent color boundary in the same way that light-dark receptive fields exaggerated the lightness or darkness on either side of the boundary

• are responsible for color constancy in the same way that light-dark receptive fields were responsible for lightness constancy

• consist of photoreceptors in a center-surround geometry, all pooled to one final neural cell (ganglion cell)

• There are two types of double-opponent receptive fields (each paired with its own neural cell)• The r-g receptive field and cell

• The y-b receptive field and cell

Receptive field of a double-opponent cell of the r-g type

• 2 different ways to2 different ways to INCREASEINCREASE the the signal the ganglion cell sends to brainsignal the ganglion cell sends to brain• RedRed light falling on cones in centercenter

of receptive field attached to ganglion cell

• GreenGreen light on surroundsurround• 2 different ways to2 different ways to decrease the the

signal the ganglion cell sends to the signal the ganglion cell sends to the brainbrain• RedRed light on surroundsurround• Green Green light on light on centercenter

• Electrical signal to brain from ganglion cell is at ambient level when no light is on center or surround

• When signal to brain is INCREASEDwe interpret that as redred

• When signal to brain is decreased we interpret that as greengreen

signal to brain

We can summarize this by just showing the center & surround of the receptive field and indicating the effect of

red (R) and green (G) on each

• A double-opponent cell differs from a single opponent cell

• In both of them R in the center increases the signal

• In a single-opponent cell G in surround would inhibit signal, whereas in double-opponent cell G enhances

• In a double-opponent cell• R in center enhances signal

(ganglion cell signals red)• G in surround enhances signal

(ganglion cell signals red)• R in surround inhibits signal

(ganglion cell signals green)• G in center inhibits signal

(ganglion cell signals green)

Fictional cell real cell

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

Concept Question:

• What is effect of green light falling on surround only?a) No colorb) Sensation of redc) Sensation of greend) Sensation of

yellow

Concept Question:

• What is the effect of green light falling on surround and red light falling on the center of the receptive field?a) No colorb) Sensation of redc) Sensation of greend) Sensation of

yellow

Here is an illustration of the effect of red or green light falling in various combinations on the center or

surround of a double-opponent r-g cell

Strongest signal (interpreted as red)

Weakest signal (interpreted as green)

No change in signal (color not noticed)

No change in signal (color not noticed)

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

grey!

Note, you would still "see" green

if the center were grey!

y-b double-opponent receptive fields and cells work the same way

Strongest signal (interpreted as yellow)

Weakest signal (interpreted as blue)

No change in signal (color not noticed)

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!

y+b-

b+y-

Concept Question:

• What is the effect of blue light falling on surround of receptive field only?a) No colorb) Sensation of bluec) Sensation of greend) Sensation of

yellowe) Sensation of red

Here is an optical illusion which can be explained by double-opponent retinal fields and cells

• Look at the grey squares in your peripheral vision

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

• Repeat for the grey squares surrounded by• Blue

• Green

• Red (pink)

Color constancy depends on double-opponent processing

• 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• This is because our double-opponent

receptiive fields compare neighboring colors and are not very sensitive to an overall change in color

• Color constancy developed in the evolution of mankind so that we could recognize colorful things in broad daylight, late afternoon, and early evening

No change in signal (color not noticed)

No change in signal (color not noticed)

Illustration of how the three opponency channels work in your perception of the design below

• 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

• 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

The artist Van Gogh knew how to use the opponency of yellow and blue to enhance each of them

Note also that we use yellow letters against a blue background in these notes for emphasis, although we prefer white in general. Red wouldbe less effective than yellow because it is not an opponent to blue

Negative afterimages occur when you stare at an image for a long time without moving your eyes

1 Conditions for negative afterimageso Prolonged stimulation by an image on

the retina adapts or desensitizes part of retina.

o That part of retina has a weaker response to subsequent to stimulation.

o Demo Fig. 7.16

2 Negative afterimages are a temporal version of lateral inhibition.o In simultaneous lightness contrast, a

signal received at a different place in your receptive field inhibits response.

• In successive lightness contrast, a signal received at a later time inhibits response in the receptive field.

• Try it in home;