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What are the functions of color vision ?

1. Signaling function
ex. traffic light red = stop

2. Perceptual organization

3. Object segregation
ex.: finding fruit in forest (crucial to survival)

4. Recognizing objects
ex.: Banana = yellow


Basic colors

- red

- yellow

- green

- blue


Extraspectral colors

Do not appear in the spectrum

--> brown / purple



A colors saturation is changed by adding white
--> this decreases it

ex.: red + white = pink ( less saturated )


How do you create a variety of colors ?

By changing the

a) wavelength
b) intensity
c) saturation


Chromatic color/

Light reflection is a-similar across the full spectrum

--> some wavelenghts are reflected more than others

ex.: red, green , yellow, blue ( basic colors )


Achromatic color

Light reflection is similar across the full spectrum

--> contains no hue

ex.: black, grey, white


Selective transmission

Only some wavelengths pass through the object or substance of transparency to create chromatic color

ex.: cranberry juice selectively transmits LW light


Additive color mixture

Mixing lights, which involves adding up the wavelengths if each light in the mixture

ex.: yellow light + blue light = white light
--> reflects the sum of the wavelengths


Subtractive color mixture

Mixing paints, which causes fewer wavelengths to be reflected

--> when mixed, both paints absorb the same wavelengths as when they are alone

--> wavelenghts reflected are those that are reflected by both paints in common

ex.: yellow + blue = green, because both reflect M wavelengths


What are Wavelengths ?

WL are energy

--> colors are created by our perceptual system


Trichromatic theory of color vision

Color vision depends on the activity of 3 different receptor mechanisms

--> describes what is happening at the beginning of the visual system


Young - Helmholtz theory of color vision

Light of a particular WL stimulates the 3 receptor mechanisms to different degrees and the pattern of activity in the 3 mechanisms results in the perception of color

--> each WL is represented in the NS by its own pattern of activity in the 3 receptors



2 physically different stimuli are perceptually identical


Are 3 receptor mechanisms necessary for color vision ?

1. color vision is possible with 2 receptor types, but not one

2. a person with only one visual pigment can match any WL in the spectrum by adjusting the intensity of any of the WL

--> a second pigment is although needed to distinguish between the WLs independent of light intensity


Color deficiency

Partial loss of color perception
--> associated with problems with the receptors in the retina


(Color Blindness)

Type of color deficiency in which only one WL is needed to match any color in the spectrum

--> no functioning cones
--> sees only in shades of grey



Needs 2 WLs to match all other WLs in the spectrum

--> experience only some colors
--> inherited through a gene located on X chromosome (more men are affected, because they don't have a second X chromosome like women)


Opponent process theory

Color vision is caused by opposing responses

--> happens for blue + yellow ; green + red ; black + white
(some receptor cells might be stimulated by red, but inhibited by green, while others do the opposite)

--> this theory describes events happening later in the visual system


Behavioral evidence for the opponent process theory

Viewing a green (blue) field results in a red (yellow) afterimage and vice versa


Simultaneous color contrast

Describes an effect that occurs when surrounding an area with a color that changes the appearance of the surrounded area

--> this explains afterimages


Opponent neurons

Respond with an excitatory response to light from one part and an inhibitory response to light from another part of the spectrum

--> they are responsible for perceptual experiences like

a) afterimages
b) simultaneous contrast


Why are opponent neurons necessary ?

The firing of opponent cells transmits information about wavelength more efficiently than the information contained in the receptor response


Color constancy

We perceive the colors of objects as being relatively constant even under changing illumination

--> without color constancy the color of an object would depend on how it was illuminated


Tungsten light

WL distribution of a light bulb

--> contains much more energy at long WLs (light appears yellow)

--> decreases your eyes to sensitivity to long WLs due to adaptation to it



contains equal amounts of energy at all WLs (white light)


Chromatic adaptation

Prolonged exposure to chromatic color

ex.: adaptation to red light will selectively bleach LW cone pigments - decreasing sensitivity to it


Memory color

Knowledge about the usual colors of objects in the environment helps us achieve color constancy


Lightness constancy

We see whites/ grey/ blacks/ as staying about the same shade under different illuminations

--> when it occurs it is determined by the objects reflectance


Object reflectance

The proportion of the light that the object reflects into our eyes