VSP 3 Lecture 3 Color Vision Part 2

Trichromatic Theory of Color Vision

3 types of photoreceptors

3 different photopigments

Each sensitive to a different wavelength

Yong-Helmholtz Trichromatic Theory

states that within your eye are tiny cells that can receive waves of light and translate them into one of three colors: (S) blue, (M) green, and (L) red. These three colors can then be combined to create the entire visible spectrum of light as we see it.

cyanolabe

426 nm

Short

SWS

S cones

Chlorolabe

530 nm

medium

MWS

M cones

Erythrolabe

552 or 557 nm

long

LWS

L cones

Univariance

photoreceptors cannot register the wavelength of the photoms they catch

output depends on quantum catch

have higher likilihood of respondong to light that correspond to its peak wavelength than of a different wavelength

LWS absorb maximally at

yellow

so its a misnomer to call them red cones!!

The MWS and LWS cones respond to light

over almost nthe entire spectrum

both have the same reduced sensitivity to shorter wavelengths

SWS cones make up ____% of the cone populaiton

6-7%

L and M cones make up _____ of cone populatino

90-95%

ratio varies from person to person

Range of L:M cones in normal trichromatic vision

1:1 to 16:1

why we may view colors different!

LWS and MWS are desnsest in

the fovea

SWS are absent in

central .3 to .4 degrees of foveal pit

Normal color vision with very small, centrally fixated stimuli is

tritanopic bc SWS are absent

tritanopic

lack of blue cones!

THEREFORE SWS are less sensitive overall

Trichromat: Changing the wavelength

changes the response of each type of cone

each wavelength has

a unique ratio of the 3 cone types

Grassman's LAw

- additive property

- scalar porperty

-associative propery

Additive Proptery

if the same radiation L(x) is added to 2 metamers, they remain metamers

if: L(a) + L(b) = L(c) + L(d)

then: L(a) + L(b) + L(x) = L(c) + L(d) + L(x)

Scalar Property

if the intensities of 2 metamers is increaed or decreaed by the smae amount, they remain metamers

Associative Property

a match will be maintained if one metamer is subsituted for another metamer

if: L(a) + L(b) = L(c) + L(d)

and: L(f) + L(g) + L(h) = L9b)

then: (a) + L(f) + L(g) + L(h) = L(c) + L(d)

Color labes are based on 3 perceptual atrtibutes

hue

saturation

brightness

hue

perception of color based on wavelength

510 appears

green

400 appears

violet

hue is lay persons term for color

but color is mcuh broader

Saturation (and desaturation)

colorimetric purity or fullness

how much white light is in the sample

based on wavelength and purity

Saturation P =

p = Ly/(Ly +Lw)

(luminance of the test wavelength)/(luminance of the test wavelength + luminance of the white light that is combined with the test wavelength)

colorimetric purity

the proportional amount of the spectral light in a mixture of the spectral light and a white light

When there is no light added the colorimetric purity is

1

p = Ly/(Ly +Lw)

the closer to 1

the more its made up of only the spectral light

a sample of 100% white light would have a colormetric purity of

0

a 540 nm stimulus with a colormetric purity of .4 is a combination of

60% white light and 40% 540 nm light

ON EXAM

Presit and Brickwedde and Kaiser found that monochromatic light at 570 nm

appears less saturated than a monochromatic wavelength at any other wavelength (for trichromats)

This is not true for other ppl w color def

Brightness

the subjective correlate of photometric luminance or luminous intensity

follows the luminance function which is the addition of M and L cones (dominate cone poopulation)

S cones less becuase frewer and lower luminance profile

therfore 555 will appear

brighter than other monochromatic stimuli of equal energy in photopic conditinos

Wavelength discrimination

How much does a stimulus of wavelength need to be changed to appear different

W curve

W curve of color discrimination: We need the least amount of change to distinguish between two colors

495 nm (cyan blue) and 590nm (yellowish orange)

our best discrimination!

less than 2 nm

we can see the difference between 495 and 498nm!

Which two hues will appear the most similar to an observer

445 and 450

500 and 505

595 and 600

445 and 450

wavelength discrimination is poor

need to be within 6 nm to disciminate at this point on the W curve

Bezold Brucke Phenomenon

the change in hue of most spectral colors that accompanies a change in their intensity or luminance

for most wavelength hue changes slightly as intensity is adjusted

Bezold Brucke Effect example

as increase luminance the light appears more yellow therefore must decrease wavelength to keep the color the same as intensity increases

Bezold Brucke: Hue contour line

all stimulu that fall on this line (have the same hue)

Bezold Brucke: Invariant wavelengths or invariant points

most lines are tilted - as intensity increases hue changes and adjustments are made to keep the color the same

some are not called Invariant Wavelengths

Invariant Wavelengths

478nm (475)

503 nm (510)

578 nm (570)

Unique hues

hues associated with these invariant wavelengths

appear pure

not mixing with other wavelengths

--- are the neural elements that begin the processes of color vision, asife from sepctral composition

Cone

Other factors affect how we see colors

in the retina, after PR

though the visual cortex an on to

higher cortical processing

color constancy

is the stability of our perception of the color of objects even though there may be changes in the composition of incident light

Land's Mondrian Experiment

we tend to perceptive colors as stable regardless of changes in illumination

Retinex theory

the brain and eye work together through comparison

color depends on context!

Opponent Color Theory

Interpreted the results of color mixing

- certain pairs of colors that are never seen together in the same place at the same time

Opponent Color Theory Consisting bipolar hue channels

Red Green

Blue Yellow

black White (codes brighntess)

no such thing as reddish green

when mixed together = white

Herring Primary Colors

Yellow

Blue

red

Responses to one color of an opponents channel are ----- to those to the other color

antagonists

opponenent process stats that the human visual system inteprests informaiton about color by processing signals in an antagonsiic manner

Hering After Images

after images with chromatic stimuli of complement color

Red --> Green (Cyan) after image

Blue -> Yellow after image

Young Helmholtz Theory was correct

at the level of the PR

Hering Opponent Theory was correct for

subsequent levels of visual pathwayT

Today

Three cones and three oponent pathways

Leo Hurvich and Dorothea jameson

provided the first quantitative color opponent model based on psychophsyical experimentation

Hue Cancellation

Hue Cancellation

If a light appeared red, the redness could be canceled by adding green

the amount of green required to neutralize would reflect the strength of the red

'chromatic valence functions'

perception of color can be explained by relative activity of these red-green, blue-yellow, and brightess channesl

Crossover points

only one of the two channels os active at these points resulting in the perception of a true hue

unique blue

478nm

unique green

503 nm

Unique yellow wavelength

578 nm

Unique red?

falls outside the spectrum

(contains yellow in spectrum)

Bezold-Brucke Effect Unique Hues

appear pure and are not mixed

478 nm

503 nm

578 nm

The opponents processing curves and Hue cancellation predict the correct location of the

unique hues from the Bezold-brucke effect

Gunnar Svaetichin founded the field of intracellular recording of light responsed from retinal neurons in fish

Gunnar Svaetichin discovered

S potentials or Retinal Horizontal ell responses of two types

Color opponents

Non color opponent

discovered objective neural color opponency !!

Non color opponent celsl

- no inhibition

- excitation only at all wavelengths

- monochromatic

- most peak at 550nm

- Input from M and L cones

- based on spectral sensitivity

Trichromatic vision

expains color mathcing

begins at the pothotrecpots

Color opponency

helps code color inforamtion

ebgins at bipolar cells

Diffse bipolar cells are

non color opponent

midget bipolar cells are

color opponent

bipolar cells receive input from

more than one cone

directly from PR

or indirectly from horizontal cells

____ begin the process of color opponency

Bipolar cells

for Midget Bipolar cells

single M or L cone connects to a Midget Bipolar Cell centrally and in mid periphery

forming the receptive field

H1 Horizontal contribute to the bipolar cell surround

for Midget Bipolar cells M or L cones

oppose each other to produce L-M opponent cells

For diffuse bipolar cells

center has input from 5-10 cones

more than 1 cone type may contribute to center

H1 contribute to surround (also input from M and L)

spectrak sensitivity in the central and surround may be similar

NO color opponency

spectral similar to non color opponent cells

S cones Bipolar Cells

synapse on bistratified GC

the center respond to only 1 cones in the fovea

absent on central .3 to .4 degrees fovea

Yellow on, Blue off

Blue on, yellow off

S cones are opposed by

Addition of L and M cones

Yellow on, blue off bp cell

What cells contribute to the center

M cones and L cones

On center Midget BP synpase with

on center midget ganglion cells (retinal parvo) cells

On center diffuse BP cells cynapse with

on center parasol (retinal magno) ganglion cells

Off center midget BP cells synpase with

off center midget ganglion cells (retinal parvo) cells

OFF center diffuse BP cells synapse with

off center parasol (retinal magno) ganglion cellsM

Midget Ganglion Cells (retinal parvo cells) allow for

- high spatial resolution (poor temporal resolution)

- Are color sensitive (color opponency)

- Project to the parvocellular layers of the LGN

Parasol ganglion cells (retinal magno cells) allow for

- good temporal resolution

(sensitive to motion, poor spatial resolution)

- no color sensitive (non color opponent)

- project to the magnocellular layers of the LGN

Smalll Bistratided Ganglion Cells

S cones synpase with these

color sensitive

prokect to konio layers of LGN