Low saturation in chromatic colours; when white is added to a colour. Ex. Pink is less saturated then red
Chromatic colours (hues)
Colours that reflect wavelengths more than others
Property of chromatic colours to reflect some wavelengths more than others
Plots of the percentage of light reflected vs wavelength
Colours that reflect similar amounts of wavelength = no hue (white, black, gray)
Property of transparent objects to allow only some wavelengths to pass through.
The 2 ways of mixing colour
1) mixing lights 2) mixing paints
Colours of light are related to...
Colours of objects are related to...
Wavelengths reflected off of the them = reflectance curve
Colours that don't appear on the visible light spectrum
Additive colour mixture
When the wavelength of each light colour in a mixture is added up making the mixture brighter; involves mixing colours of light
Subtractive colour mixture
When the wavelength that two mixed paints have in common only gets reflected, The two different paints mixed together absorb the same wavelength when alone.ex. Blue and yellow make green. The green absorbs long and short wavelengths and reflect the medium wavelength (green).
Trichromatic theory of colour vision
Colour vision depends on the activity of three different receptor mechanisms. Based on colour matching
According to Newton colours are
Created by our perceptual field
Trichromatic theory of colour vision
Colour vision depends on the activity of three different receptor mechanisms. Based on the colour matching experiment
Colour matching experiment
Observers adjust the amount of three different wavelengths of light mixed together in a "comparison field" until the colour of the mixture matches the colour of a single wavelength in a "test field". In other words they are trying to match the colour of the comparison field with the colour of the "test field".
Young-Helmholtz theory of colour vision
A refinement of
What phenomenon does the colour matching experiment display?
How can we tell which colour is being perceived?
By looking at the pattern of firing of the three cones.
Situation in which two physically different stimuli are perceptually identical. The one wavelength in the test field is matched with the 3 wavelegnths in the comparison field.
Two lights that have different wavelength distributions
but are perceptually identical. The two fields in the ciolour-matching experiment receive different amounts of wavelengths but are perceived as the same colour on both fields.
Colour deficieny where people can only use 2 wavelengths to match all other wavelengths in the spectrum; have only 2 visual pigments
People with 3 visual pigments
Partial loss of colour perception associated with problems with the receptors in the retina
Described his experience with colour deficiency. This lead to the term Daltonism
Colour vision tests that determine if someone has a colour deficiency. People with normal colour vision will see a symbol under standardized illumination
Needs only one wavelength to match any colour in the spectrum; sees everything in shades of lightness (white, gray, black) = colour blind
Use three wavelengths like a normal trichomat but they mix the wavelengths in different proportions. They're bad at discriminating between wavelengths that are close together
Person with trichromatic vision in one eye and dichromatic vision in the other
What are unilateral dichromats used for?
To study what a dichromat perceives
What type of vision do monochromats depend on? what is the downside to this?
Rod viision. This creates a perception of glare and they have poor visual acuity.
What 3 tyoes if colour deficiencies has colour-matching revealed?
3) Anomalous trichromatism
What are the 3 forms of dichromatism?
The two most common are protanopia and deuteranopia.
-Which chromosome are protanopia and deuteranopia inherited from?
-How many genes does normal colour vision require
-X chromosome. Males are more likely to inherit them.
-Normal colour vision requires only one normal gene
Missing the long-wavelength pigment. Perceive short wavelength light as blue. Their neutral point (perceiving gray) is 492nm. Above the neutral point they perceive wavlengths as yellow.
Missing the medium-wavelength pigment. Perceive blue at short-wavelengths and yellow at long-wavelengths. The neutral point is 498nm
Probably missing short-wavelength pigment. Perceive blue at short-wavelengths and red at long-wavelengths. Neutral point is 570nm.
Opponent-process theory of colour vision
Our perception of color is determined by the activity of two opponent mechanisms: a blue–yellow mechanism and a red–green mechanism. The responses to the two colors in each mechanism oppose each other, one being an excitatory response and the other an inhibitory response.
Ewald Hering (1834-1918)
Proposed the opponent-process theory of colour vision. Conducted the after-image experiments.
What did Ewald Hering observe in his experiments with after-images?
Viewing a green fiels creates a red afterimage, and a yellow field creates a blue afterimage. Therefore, red and green are paired & blue and yellow are paired.
What causes the afterimages
Afterimages are due to simultaneous colour contrast
Simultaneous colour contrast
An effect that occurs when surrounding one colour with another changes the apperance of the surrounded colour. In the middle of a red afterimage appears a green square.
Neuron that has an excitatory response to a wavelength in one part of the spectrum and an inhibitory response to another wavelength in the spectrum
Where are opponent neurons located?
Found in the retina and lateral geniculate nucleus.
Why are opponent neurons necssary?
They transmit the information about the wavelengths more efficently then the cone receptors
How is our perception of colour created in terms of the trichromatic theory + opponent-process theory?
1st step: Trichomatic theory describes how the receptors of retinas receive light and each wavelength produces a different response in the three cone receptors (colour matching).
2nd step: Opponent-process theory describes how opponent neurons produce the perceptual experiences such as afterimages and simultaneous contrast
Colour blindness due to damage to the cortex.
What supports the idea of a specialized "colour center" in the cortex?
Cerebral achromatopsia since these people are colour blind but all their other visual functions are normal
Colour vision represents an example of what type of processing? and why?
Distributed processing becuase there are a number of areas in the cortex that are involved in processing wavelength info and creating colour perception.
The relatively constant perception of an objects hue under changing illumination.
Ex. Green sweater illuminated underh longer wavelength looks very similar under shorter wavelength light.
What is Dorthea Jameson refering to when she says "a blue bird would not be mistaken for a goldfinch if it were brought indoors"?
She's refering to colour constancy because under different illumination the bird still looks blue.
What is responsible for colour constancy?
1) chromatic adaption
2) Objects surroundings
3) Memory colou
Prolonged exposure to chromatic colour, which adapts receptors that fire to these wavelengths by selectively bleaching a specific visual pigment.
Ex. Adaption to long-wavelengths selectively bleaches long-wavelength pigments.
Keiki Uchikawa and coworkers (1989)
studied colour constancy. Made observers view isolated patches of coloured paper under three different conditions:
2) observer not adapted
3) observer adapted to colour
In Keiki Uchikawa's experiment what happens when the observer is adapted?
The observers chromatic adaption to the red light shinning on both the green paper and the observers eye creates the effect of "partial colour constancy"
Partial colour constancy
When the perception of the objects hue is shifted (changes) after adaption, but not as much as when there was no adaption.
How does the surrounding environment of an object affect colour constancy?
When the surroundings are masked colour constancy works less well. The colour of the surrounding objects provide the visual system with information about their illumination which allows it to estimate the characteristics of the illumination and to make corrections.
Idea that an object's characteristic colour (our background knowledge about a colour) infulences our perception of that objects colour.
How does memory colour affect colour constancy?
knowing the colours of familiar objects, like a red stop sign, causes people to judge them as having richer, more saturated colours than unfamiliar objects that reflect the same wavelengths.
Thorsten Hansen and coworkers (2006)
Studied colour memory. Showed that when fruits are given the same physical characteristic as their gray background (gray coloured fruit), they still appear to be slightly coloured.
What did Thorsten Hansen concluded about his colour memory experiment?
He concluded that the observers knowledge of the fruit's characteristic colours changes the colours they were experiencing. The background knowldge of the colours of familiar objects affects how people perceive those colours under different illuminations.
The perception of the shade of achromatic colour (white, gray, and black). In other words, the perception of reflectance.
Low reflectance = gray, black
High reflectance = white
Seeing achromatic colours as staying the same shade under different illuminations. In other words, the object's lightness is perceived the same under different illuminations.
What 2 things does the visual system system depend on in order for light from object to reach the eye?
-The total amount of light striking the onject's surface
-The proportion of this light that the object reflects into out eyes.
What determines an objects lightness?
The objects reflectance. Objects that look black reflect 5% of light. Objects that look gray reflect 10-70% of light. Objects that look white reflect 80-95%.
What is the difference between the "amount" of light reflected and the "percentage" of light reflected?
The "amount" of light reflected refers to how many units of light is reflected under ONE intensity of light.
The "percentage" of light refers to how many units of light an object is able to relfect under DIFFERENT light intensities. Ex. On a checkboard, white squares reflect 90%. Black squares 9%. Under 100 units of light intensity the white relfects 90 units and the black 9 units. Under 10,000 the white reflects 9,000 and the black 900.
Two areas that reflect different amounts of light will look the same if the ratios of reflectance for both the object and surroundings stay the same at different light intensities. Ex. ratio of black(9%) to surrounding white(90%) is 9/90 under low illumination. Under high illumination ratio is 900/9,000
What objects does the ratio principle work best with
Flat, evenly illuminated objects like a checkerboard.
How does the perceptual system take uneven illumination of objects into account?
By distinguishing between "reflectance edges" and "illumination edges".
An edge where the reflectance of two surfaces changes. A reflectance edge can be created by two different materials that reflect different amounts of light
Edge where the lighting changes. One area receives more light than the other.
The fuzzy border at the edge of a shadow