Colour part 2 Flashcards
What is another way we perceive colour?
Opponent process theory
What is the opponent-process theory based on?
Colour perception is based on 4 primary colours
Color perception is based on 3 opponent mechanisms
Colour perception is a 3-dimensional construct
What are the three colour dimensions according to the opponent-process theory
Red/green
Blue/yellow
white/black
How were the three colour dimensions determined for the opponent-process theory?
something is never redish and greenish at the same time
What are the 3 things that support the opponent process theory?
Complementary after image
Hue cancellation experiments
Opponent cells in monkey LGN
How do complementary after images work?
After staring at colours they look like the opposite colour –> brain interprets it as being the opposite
How do hue cancellation experiments work?
Figure out how much yellow you need to cancel our blue or vise versa –> they are opponent mechanisms
What is the strength of blue/yellow opponent mechanisms? What does the diagram show?
It takes a lot of yellow to cancel out the blue
They don’t overlap–> not blue and yellow at same time –> mutually exclusive
What is the strength of red/green opponent mechanism? what does the diagram show
Takes a lot of red to cancel out green
Red is located both at the high and short wavelengths
They don’t overlap
What does the graph look like for the strength of blue/yellow and red/green mechanisms?
Combo of both opponent mechanisms.
Where they switch there is neither colour. Draw a line to determine the colour at that wavelength
Where are opponent cells found?
in LGN
What are opponent cells
cells that are excited by blue and inhibited by yellow
or cells that are excited by red and inhibited by blue
only represent colour along 2 axis
If you have a +B and -Y opponent cell and you shine a blue light on it what happens
faster firing
If you have a +B and -Y opponent cell and you shine a yellow light on it what happens
inhibited
How are receptors wired together to create red/green opponent cells? Both cases
+R -G (+L, - M) = excitatory L cone and a inhibitory M cone
-R +G (+M, -L) = excitatory M cone and inhibitory L cone
How are receptors wired together to create blue/yellow opponent cells? Both cases
+S-ML (+B, -L) = Excitatory S cone and combination of inhibitory M and L cone on another cell
-S, +ML (+L, -B) = Inhibitory S and excitatory combination of M and L
What is the colour equivalent of +S, -LM and +L -M
+B, -Y
+R, -G
How do we reconcile the trichromatic and opponent process theories?
Treat it as 2 different steps of colour perception
Takes output of receptors and combines them in certain ways
light goes in and triggers receptors (trichromatic theory), then the opponent cells combines the actavation of receptors (opponent-process theory) then info goes to the brain
What are the limitations of the trichromatic and opponent process theories?
There theories propose a direct link between proximal stimulus and colour percept
Colour is determined by activation of the L, M, and S photoreceptors
Colour should just be based on light hitting eye then –> not true
What are the examples of the limitations of trichromatic and opponent process theories?
Squares of colours look different if they are surrounded by different colours
Or line looks like different colours –> doesn’t tell us this
Scintillating grid illusion –> black circles in center
Rubix cube
Explain the rubix cube llusion that is a limit of the trichromatic and opponent process theories
The squares are actually grey –> same cone activation for both but we percive it as blue in yellow light and yellow in blue light
colour processing depends on more than just light hitting eye and 3 cone types
What does the light that reflects off an object depend on?
Reflectance - how the object reflects light
Illumination - how light falls on the object
Explain what happens if we shine 100 units of light vs 10000 units of light on a checker board?
90 units of light reflected off white and 10 units reflected off black
9000 units reflected off white
1000 unites reflected off black
we consider the ratio not the total amount of light boucing off
What is lightness constancy?
We tend to perceive whites, grays, and blacks the same under varying illuminations
What is the principle that describes lightness constancy?
Ratio principle
What is the ratio principle?
Under even illumination, the ratio of reflected light guides perception of lightness
absolute amount of light doesn’t matter it is the relative amount of light reflected
What makes lightness constancy more complicated?
shadows
Which squares are reflecting the same amount of light (same lightness)?
A and B
A is darker so it reflects less light
B is in the shadow so it gets less light to reflect
What do the soft edges of shadows create?
Illumination edges
What do illumination edges do to our perception?
Our perception of colour depends on them
What is the difference between an illumination edge and reflectance edges? for lightness constancy
Illumination edges: same surface, different illumination
Reflectance edges different surface with same illumination
What guides the distinction between illumination edges and surface edges?
cues including penumbra of shadows and orientation of surfaces
What does the light that reflects off an object depend on for colour?
Reflectance - how much the object reflects each wavelength
Illumination - how much of each wavelength falls on object
What does incandescent light show on a wavelength chart?
higher wavelengths
What does sunlight show on a wavelength chart?
it is evenly spread out
What does led light show on wavelength chart?
mixed wavelength
What happens to the reflected light if you shine each type of light on a green shirt?
Incandescent –> higher wavelength
sunlight –> middle
LED –> lower wavelength
What is colour constancy?
We tend to perceive objects with the same reflectance as having the same colour under varying illuminations
What is an example of colour constancy?
can tell room is the same colour even if light is different
What is chromatic adaptation?
receptors for a colour fatigue/adapt after continuous stimulation
What happens if you put a red light on a green paper and on a person looking at it?
paper only looks slightly red (more yellow) because brain adapts to red to interpret the colour
What happens if you put a red light on a green paper and but not on a person looking at it?
paper looks more red
How does chromatic adaptation work in lush and arid environments?
Greeness decreases after being in a lush environment
Greenness increases after being in arid environment
Give example of how context influences colour constancy?
Grey spot in rubix cude looks blue in yellow light and yellow in bluelight
How does memory effect the colour we perceive? Explain the experiment?
Image of banana, orange, and 2 shapes were presented to people and they were asked to create a colour match to the grey
The grey for the orange was more orange
The grey for the banana was more yellow
It didn’t work for the shapes
How does context effect colour constancy? (general)
context is used to infer wavelengths of illumination
How does memory effect colour constancy?
Prior knowledge/expectations about colour may provide a top-down influence
What are the three factors that effect color constancy?
Chromatic adaptation
Context
Memory
In what areas of ventral pathway does colour get processed?
Retina (cone), LGN, blobs in V1, V2, V4 (specialized)
What type of cells for processing colour are in LGN?
Single-opponent receptive field cells
What are Single-opponent receptive field cells
Cells that have center surround sensitivity to colour. Center is excited by M cones (green) and inhibited by L cones (red) or for other colours too
What type of cells are found in V1 blobs?
Double-opponent receptive field cells
What are double-opponent receptive field cells?
One side is excited by M (green) and inhibited by L (red) and the other side is inhibited by M and excited by L
What does V4 do for colour processing?
higher/more subjective level of colour perception
What happens if you lesion V4?
Lesion can cause cerebral achromatopsia
What is achromatopsia?
cortical colorblindness due to brain damage
What are other cortical areas involved in colour perception? What did they do for the study?
Green area for colour specifically
Yellow area for shape, texture and colour
Ask people if colour is same or different to see which areas respond
What do researchers use to test colour perception abilities?
Ishihara plates
What are the 4 types of colour anomalies?
Monochromatism
Dichromatism
Anomalous trichromatism
Tetrachromatism
What is Monochromatism?
Only have rods, no cones (or one cone type)
True colour blindness (everything is grey)
Only know how much light not frequency info
What is Dichromatism?
Missing only one of three cone types
What are the three types of dichromatism?
Tritanopia
Deuteranopia
Protanopia
What is Tritanopia?
Missing S-cones
What is Deuteranopia?
Missing M-cones
What is Protanopia?
Missing L-cones
What is anomalous trichromatism?
Have all 3 cone types but one (or more) of the opsins is slightly different than normal (different frequencies)
What is tetrachromatism?
Have 4 cone types
What is considered congenital colourblindness?
monochromatism
What types of dichromatism are most common in males?
Protanopia
Deuteranopia
What colour blindness is protanopia and deuteranopia? what does it look like?
red-green colour blindness
stuff looks blue and yellow
What colour blindness is tritanopia? what does it look like?
green/blue colour blindness
looks pink (and a bit blue)
Why is dichromatism more common in males?
Sex-linked issues because genes related to cones are on the X chromosome
Males (XY) only have one chance to get a typical copy
Is anomalous trichromatism also more common in males?
Yes
What are the three types of anomalous trichromatism?
Protanomaly
Deuteranomaly
Tritanomaly
What is Protanomaly?
Malfunctioning L cones
What is Deuteranomaly?
Malfunctioning M cones
What is tritanomaly?
Malfunctioning S-cones
What does anomalous trichromatism look like?
Some colour diminishment
How do you get tetrachromatism?
Women who’s father has anomalous trichromatism due to an X-chromosome mutation may have both a typical copy of the gene from her mother and the anomalous copy
This results in 4 cone types
What are the 4 cone types in tetrachromatism?
3 regular and one mutated
L, M. S and L’
What are the spectral sensitivities of the 4 cone types?
L> L’ > M > S
How many colours can tetrachromats see?
Maybe 100 million
How many colours would a tetrachromat require inorder to match all stimuli in a colour matching experiment?
4
Do tetrahchromats perceive metamers in the same way as trichromats?
No, a metamer for a trichromat may look different for a tetrachromat
What would happen if there were two stimuli with the same S, M and L responses but the L’ cones have different responses?
A trichromat would perceive them as metamers (identical) but a tetrachromat would perceive them as different
What does colour in art teach us about colour on the page?
Colour on page is different from the colour we perceive
What is local colour?
colour of paint on page
Give examples of painters who mastered local colour
Monet in snow painting
Zurburan in orange painting (not typical orange colours)
Self portrait by Rembrant
What does the painting A Sunday afternoon on the island of la grande jatte show?
Playing with idea that colour on page has complicated relationship to how something is perceived
Uses colour for shadows
