Exam next week

Exam next week

• Covers everything about all sensory modalities except hearing

• This includes:visionbalance/touch/taste/smell/

proprioception/theroception

Color Vision• Primary colors

Perceiving Color

Red Green Blue

Color VisionTrichromatic Theory of Color Vision

“Blue”

“Green”

“Red”

Blue

Wavelength Input Cone Signal to Brain

Color VisionTrichromatic Theory of Color Vision

“Blue”

“Green”

“Red”

Green

Wavelength Input Cone Signal to Brain

Color VisionTrichromatic Theory of Color Vision

“Blue”

“Green”

“Red” Red

Wavelength Input Cone Signal to Brain

Color VisionTrichromatic Theory of Color Vision

“Blue”

“Green”

“Red”

Yellow

Equal Parts Red and Green =

Wavelength Input Cone Signal to Brain

Color VisionTrichromatic Theory of Color Vision

“Blue”

“Green”

“Red”

Yellow

Equal Parts Red and Green =

Wavelength Input Cone Signal to Brain

Color VisionTrichromatic Theory of Color Vision

“Blue”

“Green”

“Red”

Yellow

Equal Parts Red and Green =

Wavelength Input Cone Signal to Brain

• Problem with Trichromatic Theory:

YELLOW

Theories of Color Vision: Trichromatic Theory

• Opponent-Process Theory– color is determined by outputs of two

different continuously variable channels:• red - green opponent channel• blue - yellow opponent channel

Theories of Color Vision: Opponent-Process Theory

• Opponent-Process Theory– Red opposes Green– (Red + Green) opposes Blue

• Opponent-Process Theory explains color afterimages – because the “opposite” of blue is

yellow, the “opposite” of green is red, etc.

Theories of Color Vision: Opponent-Process Theory

• Everything you’ve learned so far is wrong.

• Everything you’ve learned so far is wrong.

• Well, not really wrong, just far from complete.

What Newton Found (and everyone believed)

• White light can be split into all wavelengths by a prism

• According to previous theories: two wavelengths combine to yield intermediate color and no others

Red LightGreen Light

Red + Green = YELLOW

What Newton Found (and everyone believed)

• White light can be split into all wavelengths by a prism

• According to previous theories: two wavelengths combine to yield intermediate color and no others

• Red + Green light can never yield blue

• Blue + Green light can never yield red

What twist did Land do to this paradigm that confounds the

conventional understanding of color mixing?

What Land found:

• Two bands (colors) of the spectrum recombine to produce all the possible colors– provided the appropriate relative amount of

each wavelength is projected

transparency slides

Red LightGreen Light

How did Land project the “appropriate” ratio of

wavelengths?

Short- and Long- “record”

• Capture two grey-scale images of the scene using filters that allow only the wavelengths you will project

Camera

“short” filter

“Long” filter

film Projector

Object

Image“Long” filter

“short” filter

medium filter

longfilter

Camera splits image intomaps of “longer” and “shorter” wavelengths

medium/“green” light

long/“red”light

Projector combines “longer” and “shorter” wavelengths using the maps to get the appropriate amounts of each

Viewer perceivesdesaturated huesincluding blues

What is Land’s interpretation? How do we perceive color?

Land’s interpretation:

• perception of color is a weighing of the ratio of shorter and longer wavelengths

Land’s interpretation:

• perception of color is a weighing of the ratio of shorter and longer wavelengths

Why would the visual system have evolved this way?

Why would the visual system have evolved this way?

• Hint: “Within broad limits, the actual values of the wavelengths make no difference, nor does the over-all available brightness of each”

What is color for?• What is color vision used for?

– Identification - what is this thing?– Discrimination - what other things is this

thing like?– Communication - indicates this thing to

others• But in each case color refers not to the

illuminating light, but to the surface of the object itself

What is color for?

Does the color of an object remain constant under different lighting conditions?

Color Constancy• The “color” of objects is independent of the ambient

light – even though light can vary dramatically

Rel

ativ

e In

tens

ity

Wavelength

Rel

ativ

e In

tens

itySunlight Incandescent Light

Color Constancy• Because of our

mechanism of color constancy we can even use completely artificial spectra

Color Constancy

• The “color” of objects is independent of the ambient light

Next Time

• ATTENTION!