CNS L5 - Color Vision

wavelength

the distance between 2 wave peaks

• different wavelengths correspond to different colors

visible spectrum

400 nm - 700 nm (violet, red)

Why did evolution give us eyes that see 400-700 nm?

• The power in sunlight peaks there

• Earth’s atmosphere is most transparent to these wavelengths.

• And sea water, where eyes first evolved is most transparent < 500 nm

3 kinds of cones for sensing color

• red cones (63%)

• green cones (31%)

• blue cones (6%)

Cone characteristics

• Each type of cone has its own type of visual pigment

• All these pigments are similar to rhodopsin but not identical to it, and so all prefer different wavelengths of light

Trichromats

ones who sense color with 3 types of cone

Different visual pigments prefer ________ _________

different wavelengths

The brain infers color by comparing data from the ____ ____ __ _____

3 types of cones

yellow light affects…

red and green cones, but NOT blue ones — brain perceives yellow

You can be fooled:

a red and a green light, with no yellow, can produce the same cone activities as a yellow light would, and so the brain sees yellow either way.

We can produce any color perception by mixing 3 wavelengths

Any color we can experience corresponds to a pattern of activity in our 3 types of cone. It is possible to produce any such pattern by mixing just 3 colors of light

The Primary Colors

Red, green, blue

Not all colors are in the …

rainbow

Spectral colors

those that can be evoked by light of a single wavelength

• rainbow colors, from violet through blue, green, yellow and orange to red

Extraspectral colors

colors (such as purple or white) that are evoked only by a mix of wavelengths

• e.g. we see purple when 2 or more wavelengths affect red and blue cones more than green cones.

Ganglion cell color signals are …

Combinations of cone signals

R + G cells (the yellow channel)

ganglion cells that are excited by red light and by green light

R - G channels

ganglion cells that are excited by red light and inhibited by green

G-R channels

ganglion cells are inhibited by red and excited by green

R-G , G-R

the red-green opponent channel

B - R - G or B - (R + G)

• i.e. blue minus yellow

• Others are yellow minus blue. These 2 types form the blue-yellow opponent channel.

ganglion cells that are excited by blue light and inhibited by red and green

Opponent channels explain…

afterimages

• As you stare at something green, your G – R cells gradually fatigue. When you look away, those fatigued cells are less active than your R – G’s, and so you see red. And similarly for blue versus yellow.

• We aren’t sure the responsible cells are in the retina, as there are color-opponent cells in LGN and visual cortex as well

red-green color blindness (Daltonism)

where people have trouble distinguishing those colors.

Inheritance pattern of Daltonism

color blind fathers have color-normal daughters who have color blind sons

Location of blue cone pigment gene

chromosome 7

Women are seldom color blind because …

if one X-chromosome codes a faulty pigment then the other X-chromosome compensates

tetrachromat

One having 2 different red cone pigments

• common in women

• richer color perception

reflectance

the intrinsic color of a surface — its tendency to reflect certain wavelengths of light and absorb others

• eg: yellow banana reflects yellow light more than other wavelengths

• reflectance of an object carries information about it, e.g. about ripeness

The light an object sends to our eyes depends on…

reflectance and illumination

illumination

the color and intensity of the light source

color constancy

our brains can usually infer the reflectance, so we see the ripe banana as yellow even in green light.

A demonstration of color constancy

■ The 2 pictures show the same cube in yellow and in blue light. You correctly see the blue facets in the left picture as blue, and the yellow facets in the right picture as yellow.

■ But the light these facets send to your eyes is the same, as you can see in the next slide, where I hide everything except one blue facet on the left and one yellow facet on the right ...

These comparisons which underlie constancy give rise to illusions

In each vertical pair, the center squares are identical in color, but their surroundings affect our perception of brightness, hue, and saturation