6 Color perception Flashcards
Whats the function of color perception
- signaling function
- helps faciliatate perceptual organisation
- easier to detect objects
- color of familiar objects helps us recognize them
What did the prism do to white light
the white light split into different colors of the spectrum
the light was bent by the prism
red beams were bent only a little and blue beams were bent a lot
what did people thought the prism did
thought it added colors to the light
What do diff parts of the spectrum have?
diff properties that give rise to perception of diff colors
Which small part of the spectrum is visible to us?
400-700nm visible light
How is color of an opaque object determined?
determined by the wavelengths of light that are reflected from the objects
selective reflection
objects selectively reflect more light in a range of wavelengths
i.e. red or green
Chromatic vs achromatic colors
chromatic = when some wavelngths are reflected more than others (selective reflection), blue, red, green
achromatic = when the reflectance across the spectrum is equal, white grey black
What does a reflectance curve show?
percentage of light reflected at each wavelength in the visible spectrum
Why do tomatoes appear red and lettuce green?
tomatoes predominantly reflect long wavelengths and appear red
lettuce reflects medium wavelengths = appear green
What does it mean by selective and equal reflection of all wavelengths?
selective gives you chromatic colors
equal gives you achromatic colors
How is color determiend in transparent objects?
via selective transmission for liquids, glass etc
determined by the wavelengths that pass through the objects
only some wavelengths pass through the objects or substance
What happens when you mix paint or mix light?
subtractive color mixture for paint
* mixing paint causes fewer wavelengths to be reflected
additive color mixture for light
* mixing light causes more wavelengths to be reflected
What is substrative color mixture?
when mixing paint together, the only wavelengths reflected are those that are reflected by both paints in common
What is the color of mixture if the 2 paints have no common reflected wavelengths
result in little or no reflection across the spectrum
mixture will appear black
What is additive color mixing?
when colored lights are mixed together, it adds up the wavelengths of each light
mixing blue and yellow = green (paint)
mixing blue and yellow = white (light)
What are 3 dimensions of color perception?
hue - color type
saturation - intensity
lightness (value) - light to dark ratio of a color
What is munsell color space?
a coordinate system of the 3 dimensions
* hues arranged around the circumference
* saturation is the distance to the center
* value is the cylinders height
What is the trichromatic theory?
- the retina is sensitive to 3 principle colors: red, green, blue
- color vision is based on the combination of the 3 principle colors
What experimental evidence is there for the trichromatic theory?
Color matching experiment
* participants can adjust the proportions of 3 wavelengths to match any reference color
* 2 wavelengths allowed ppt to match some but not all colors
* at least 3 wavelengths is needed to match any other wavelength
What is a physiological evidence for trichromatic theory?
- found 3 different kinds of cone receptors
- short-wavelength cone (S): peak sensitivity: 419nm
- middle-wavelength cone (M): peak sensitivity: 531nm
- long-wavelength cone (L): 558nm
- each wavelength of light produces a unique pattern of activation across the 3 cone mechanisms
What is metamerism and metamers?
metamerism: the situation in which 2 physically different stimuli are perceptually identical
metamers: 2 identical fields
in the color matching experiment, 2 fields are physically diff (diff wavelength) but are perceptually identical
Why do metamers look alike?
because they both result in the same pattern of response in the 3 cone receptors
What is the mechanism behind metamerism?
- 530nm green light -> a large response in M and medium response in L
- 620nm red light -> a large response in the L receptor
- mixture leads to the same L, M and S cone response patterns as the 580nm light
these 2 wavelengths mixed tgt create the same pattern of activity in the cone receptors as the single 580nm light
What is the principal of univariance?
- the same photoreceptor can be excited to the same extent by different combinations of wavelength and intensity
- adjust intensity of 2 lights to cause single pigment to result in identical response (light appear same even if diff wavelength)
What are monochromats?
- people with only rods
- may not differentiate between 2 wavelengths in different intensities
- only see shades of grey
- suffer from principal of univariance
- only see in shades of lightness
- they can match any wavelength in the spectrum by adjusting the intensity of any other wavelength, since see them all as shades of grey
Why cant monochromats see chromatic colors?
they need more than one type of receptor
but they only got 5
What are dichromats?
- people with 2 types of cone pigment
- should have info about which wavelength is present
- can see chromatic colors, but cannot distinguish among all colors (may confuse)
How can dichromats be diagnosed?
by ishihara plates
Who are trichromats?
people with just 3 types of cone pigment
have normal color vision
Do colour perception depend solely on wavelength?
no
What are color aftereffects?
- stare at color here
- then look at a white screen
- will see other colors
Explain color after effects with the opponent process theory
If stare at green - message is green, longer you stare, the firing rate decreases, so when switched to white light (all wavelengths), now you get equal parts of red and green, but since green is firing slower, red signal temporarily overpowers it, then you see more red than what is there
Will notice if you move your eyes away from the dot or area, the illusion disappears because you were only tiring the cells in that particular part of the retina
What is the opponent process theory?
- color vision is based on the activity of 2 opponent process pairs
- red/green and blue/yellow opponent pair
- opposite colors cancel each other out to create white or grey when combined
- each color is made up of combinations of 4 primary colors: red, yellow, green and blue
What are physiological evidence for the opponent process theory?
explain how diff colors can be percieved in terms of input from cones
- opponent neurons in the LGN
- we can create opponent neurons using 3 cones input
opponent process theory vs trichromatic theory
- competeed with each other for a long time
- now thinks they just reflect diff stages of processing
- trichromatic theory focuses on retinal receptors
- opponent process theory focuses on opponent neurons in LGN (and other brain regions)
What is the color center in the brain
V4 - seems to be a critical region for color perception
What happens if V4 is damaged?
complete color blind (achromatopsia)
What did fMRI studies suggest?
suggest that color processing may not be limited to V4
adjacent areas may relate them
How does color perception depend more on wavelength? What are some examples?
- diff wavelengths may be perceived as the same color
- same wavelengths may be perceived as diff colors
examples
* color constancy
* simulataneous contrast effects
What is color constancy?
- reflected light depends on the light source and the reflectance properties of the surface
- but even with diff illumination, colors of objects are relatively constant
- critical for chromatic adaptation
What is chromatic adaptation?
ability to adjust to changes in illumination in order to preserve the appearance of object colors
e.g. adaptation to the red light selectively reduced the sensitivity of long-wavelength cones
stare at red, eye adapt to red, decreased redness of objects in environment
What is Uchikawa’s study on chromatic adaptation?
- baseline—paper and observer illuminated by white light = paper perceived as green
- observer not adapted—paper illuminated by red light, ob- server by white (the observer is not chromatically adapted) = percieved paper color as shifted towards red, color constancy does not occur in this condition
- observer adapted to red—both paper and observer illuminated by red light (the observer is chromatically adapted) = paper only shifted slightly to red
eye can adjust its sensitivity to different wavelengths to keep color perception approximately constant as illumination changes
perception of the object is shifted after adaptation, but not as much a when there was no adaptation
How is chromatic adaptation in the outside environment?
- diff dominant colors in diff seasons
- the eyes can adjust its sensitivity to diff wavelengths to keep color perception approximately constant in diff environment
- adaptation to green in lush environment would decrease the perception of green in that scene, same with yellow in arid enviornment
- tones down the dominant color in a scene
- comapre the lush and arid scene, colors are more similar after the chromatic adaptation
- adaptation also causes novel colors to stand out, so yellow becomes more obvious in the lush scene and green in the arid scene
What is the simultaneous contrast effect?
- a phenomenon that happens when 2 adjacent colors influence each other, changing our perception of these colors
- perception of an objects color is influenced by the colors surrounding it
- B looks lighter when its next to a darker patch
What is lightness constancy?
The fact that we see whites, grays, and blacks as staying about the same shade under different illuminations
color constancy for colors
Why is the dress viewed differently?
differences in how people interpreted the illumination was responsible for this effect
Illumination were rich in long wavelengths, (more yellow) -> the constancy mechanism will cause the visual system to decrease the effect of long wavelengths, so the blue will stay about the same (since blue objects usually reflect little long-wavelength light)
Under cooler light ->contains more short-wavelength blueish light -> discounting short wavelengths from the blue stripes would cause the blue stripes to be perceived as white and discounting short wavelengths from the black stripes would push perception of the black toward yellow
If the reason for “black becoming yellow” isn’t obvious, remember that a black object reflects a small amount of all wavelengths equally. Subtracting short wavelengths leaves the middle and long wavelengths associated with yellow light.
