What are the functions of color vision and how is it perceived?

primary functions of color vision: detection and discrimination of objects

Grouping of objects

Recognition of familiar objects

all colors can discriminate using just four words: red, yellow, green and blue.

Saturation more white is added as you move to the center of the circle.

Hue is chromatic color; it changes as you move around the circle

What processes cause objects to appear colored?

Reflectance and transmission

Selective Reflection/Transmission 

The color of an object is determined by the wavelengths that are reflected from it or through it. 

Reflectance (reflection)

Is the percentage of light falling on an object that is reflected from it 

Selective reflection

 is the reflection of different amounts of different wavelengths.

Selective Transmission

Is the transmission of different amounts of different wavelengths

What types of color are they? How is color mixed?

Two types of color: chromatic and achromatic

Chromatic - when an object selectively reflects or transmits some wavelength

Achromatic - These are seen when all wavelengths are reflected or transmitted equally

Color mixing is either done additively or subtractively

Additive mixing  – Mixing lights of different wavelengths

Subtractive - Mixing paints with different pigments

Additional pigments reflect fewer wavelengths

What are the two theories of color vision? What support is there for each?

Two different theories: The Trichromatic Theory and the Opponent-process Theory

Trichromatic Theory (Young & Helmholtz)

3 different receptor mechanisms responsible for color vision.

Proposal based upon Behavioral Evidence:  of Color matching experiments

Support Trichromatic Theory Identification of three types of cone pigments/absorption spectra

 Short (419nm) 2. Medium (531 nm) 3. Long (558nm)

Opponent-Process Theory (Hering

color vision is caused by opposing responses generated by blue & yellow and by green & red

Based on phenomenological observations of color pairings 

Color Afterimages

Simultaneous color contrast

Color descriptions/ color deficits

Three mechanisms, each responds in opposite ways to different wavelengths or intensities of light. 

Support for Hering

DeValois found color opponent cells in LGN These cells were organized: R+G- / R-G+, Y+B- / B+Y

Two main types of opponent neurons have been identified:

 Single-opponent and Double-opponent neurons

Single opponent cells

have a center surround organization

Important for perceiving color within a region

Double opponent cells

have an elongated, side-by-side receptive field arrangement 

Seem to be important for perceiving the boundaries between colored regions

Can color be perceived with any amount of the three receptors and what deficiencies are there and what is their effect?

Color vision is possible with only 2 types of receptors

One receptor type couldn’t signal difference in wavelength, only in intensity.  This is called the principle of univariance

Monochromat - A person who needs only one wavelength to match any color just see variance in luminance

Dichromat –

A  person who needs only two wavelengths to match any color

Anomalous trichromat –

A person who needs three wavelengths in different proportions than normal trichromat 

unilateral dichromat

 where only the cones in one eye are deficient.

Monocromatism - can match any color using one other   , usually lacking any cones. are truly colorblind

How many types of dichromatism is there? What are their names and effects?

three main types of dichromatism:  Protanopia, Deuteranopia and Tritanopia

Protanopia Long wavelength cones missing   perceive blues and yellows,  neutral point (grey) around 492nm

Deuteranopia neutral point is at 498 nm Missing medium wavelength pigment

Tritanopia can perceive green and red neutral point around 570nm

What is color constancy? How is it achieved?

color constancy is when Colors retain their general appearance, in spite of changes in lighting. 

color constancy is achieved is by taking into account the illumination through  Chromatic adaptation can be achieved through prolonged exposure/adaptation to a particular color

What is lightness constancy? How is it achieved?

Lightness constancy Occurs when our perception of achromatic color is not influenced by illumination

Achieved by taking into account the ratio of lightness across regions of an image and through the use of reflectance edges and illumination edges. 

Reflectance edges occur when the surface material changes (for example painted areas) causing changes in the amount of reflected light.  

Illumination edges occur when the lighting changes, for example in the case of shadows.  

What are Metamers?

Metamers: stimuli that are physically different but perceptually identical

What does Kirschmann's color contrast law state?

color contrast occurs even if there is a spatial gap between the two fields. The larger the gap, the smaller the effect.