Perception: exam 2
why do we say color is not a physical property but rather is a psychophysical property?
- color doesn't exist in the physical world -- we experience light at various wavelengths as color
3 steps to color perception:
1. Detection 2. Discrimination 3. Appearance
why does having 3 cone types solve the problem of univariance?
3 cone types are maximally sensetive to different wavelengths --- no stimulus will produce the exact same response on all 3 cones different wavelengths will create different patterns
range of wavelengths we can detect with our visual system
400-700 nm
S-cones are specialized for what size wavelength
420 nm ("blue")
rods are specialized for what size wavelength?
498 nm
M-cones are specialized for what size wavelength?
535 nm ("green")
L-cones are specialized for what size wavelength?
565 nm ("red")
cone-opponent cells
A cell type—found in the retina, lateral geniculate nucleus, and visual cortex—that, in effect, subtracts one type of cone input from another.
additive color mixture
A mixture of lights. If light A and light B are both reflected from a surface to the eye, in the perception of color the effects of those two lights add together. ex) red light +green light = yellow light
subtractive color mixture
A mixture of pigments. If pigments A and B mix, some of the light shining on the surface will be subtracted by A, and some by B. Only the remainder contributes to the perception of color. ex) smearing together red and green paint results in a brown color (not yellow)
in subtractive color mixing, the resulting color is due to what wavelengths (are/are not) absorbed
ARE NOT
HSB color space
Defined by hue, saturation, and brightness. Hue: The chromatic (color) aspect of light. Saturation: The chromatic strength of a hue. Brightness: The distance from black in color space.
RGB color space
Defined by the outputs of long, medium, and short wavelength lights in other words, defined by the outputs of red, green, and blue light sources (and their intensities)
metamers
Different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of physical differences. ex) a red and green light mixed together will give the same response as a medium wavelength light that looks yellow
the Red vs Green system uses what contrasts?
L-M or M-L excited by L in the center and inhibted by M in the surround of vice versa
two mathematical contrast equations the eyes use to see color
L-M or M-L) and ((L+M)- S) or (S-(L+M))
can our 3 cones help us distinguish certain wavelength mixtures from each other (or certain single frequencies)
NO--- even though trichromatic theory solves problem of univariance for single-frequency stimuli, can't distinguish certain wavelength mixtures from each other (or certain single frequencies)
the blue vs yellow system uses what contrasts?
Other cells excited by S-cone input in center, inhibited by (L + M)- s cone input in surround (and vice-versa)
kinds of color space
RGB color space HSB color space
photopic
Referring to light intensities that are bright enough to stimulate the cone receptors and bright enough to "saturate" the rod receptors (that is, drive them to their maximum responses). daylight
3 kinds of cones
S-cones (blue)-- short wavelengths M-cones (green)-- medium wavelengths L-cones (red)--- long wavelengths
principle of univariance
The fact that an infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make color discriminations based on wavelength. ex) rods
opponent color theory
The theory that perception of color is based on the output of three mechanisms, each of them resulting from an opponency between two colors: red-green, blue-yellow, and black-white.
trichomatic theory of color vision
The theory that the sensation of color results because three cones in the retina each of which responds primarily to a specific range of wavelengths any wavelength from about 420-660 nm will produce a unique set of three responses from each kind of cone also called Young-Helmholtz theory
color space
The three-dimensional space, established because color perception is based on the outputs of three cone types, that describes the set of all colors.
detection
Wavelengths of light must be detected in the first place
why can't we have reddish greens of greenish reds according to what we know about opponent colors and center-surround cells?
a cell will either favor L in the center and M in the surround (other the other way around) because of the center-surround set up, either L or M will be favored (both can't be stimulated at the same time)--- so can't have reddish greens or greenish reds
subtractive color mixing is also known as a form of ____ which uses ____
a form of filter; pigments
if R=G=B what is the result?
achromatic (black/grey/white)
if you want to remove the reddish tint, what color should you add and why?
add green because red and green are opposed
when lights are added together to contribute to color vision
additive color mixture
if you shine a blue light and a yellow light on a surface (that doesn't absorb any light) what is the resulting light color and why? what kind of color mixture is this?
additive color mixture--- blue+yellow= white the color will be white because: yellow contains a mix of long and medium wavelengths and blue contains shorter wavelengths. so blue + yellow give you a mixture long, medium, and short wavelengths which will appear white diff cones will be stimulated roughly equally
unique hues
any of four colors that can be described with only a single color term: red, yellow, green, blue
a unique green has the right balance of ___ and ___
blue and yellow
[L + M] - S and S - [L + M] compute ____ vs. _____
blue vs yellow
pair combinations for opponenet color theory
blue vs yellow red vs green
if you pass if yellow light through a blue filter that absorbs all but a middle range of wavelengths what is the result?
blueish/green
if white light passes through both a yellow filter and blue filter, what color is leftover and what is this called?
called subtractive mixing green leftover
why can we see more colors than are present in the color spectrum?
can have different combinations of additive and subtractive mixtures
under somewhat dim light, rods can't contribute to ____ but they can contribute to ____ or ____
can't contribute to color but can contribute to brightness or saturation
one type of photoreceptor ____ make color discriminations based on wavelength
cannot--- problem of univariance An infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor-- therefore you need several kinds of photoreceptors to discriminate between different wavelengths
what kind of cells are found in both the retina and the LGN
center-surround receptive fields
3d space that describes all sets of colors that is needed to explain color appearance
color space
according to Herring, a color could be a reddish ____ or bluish _____ but NOT reddish ____ or bluish ____
colors can be reddish yellow or blueish green but Not reddish green or bluish yellow
photoreceptors reposnible fro color vision?
cones
•how, exactly, does the brain combine the signals from the S, M, & L cones?
creates contrasts between outputs these are mathematical contrasts (not dark/light)
L-cones
detect long wavelengths (red)
step of color perception: discrimination
differences are taken between cone types, creating cone opponent mechanisms, which are important for wavelength mechanisms
discrimination
differentiating between different one wavelength (or mixtures of wavelengths) from another
explain how cones are not exclusively sensitive to certain parts of the spectrum?
even though L-cones are maximally sensitive to 565 nm, M-cones can detect this wavelength as well, so we see overlap of their spectral sensitivities
true or false: the brain uses contrasts of dark and light to combine signals from small, medium, and large cones
false!!-- contrasts are mathematical (not light dark)
true or false: a purple magenta is a result of a single wavelength on the spectrum of wavelengths humans can see
false-- results from a mixture of wavelengths
true or false: S-cones see blue, M-cones see green, and L-cones see red
false-- while it is tempting to call the different cones blue, green, and red, this isn't really correct for example, the L-cone's peak sensetivity is 565 nm which will show up as yellow, not red
true or false: different cones (LS, M, and L) are exclusively sensitive to different parts of the electromagnetic spectrum
false--- have overlap it's not this clear cut-- for example, 560 nm will look yellow even though we might expect it to look red
true or false: Mixing wavelengths changes the physical wavelength. so, if you mix 500nm and 600 nm lights, the result will be the average of the two (550 nm)
false--- mixing wavelengths doesn't change the physical wavelength you won't get the average or the sum, instead the physical stimulus contains both 500nm and 600 nm. color mixture is a mental event, not a change in physical light
true or false: the trichomatic theory of color vision solves the problem of univariance
false--- still have wavelengths that we cannot distinguish called metamers
step of color perception: appearance
further recombination of the signals creates color opponent processes that support the color opponent nature of color appearance
a unique green can be described as having no hint of ____ or ___ in it
has no hint of blue or yellow unique green is described only as being green
appearance
how we psychologically experience colors of light or objects and maintain that color stability overtime (even in changing light conditions)
what do we mean when we say we get "opposite" color results from adding vs. subtracting
if you pass white light through a "yellow" filter and and "blue filter, the remaining light that is not absorbed will be green but, if you do this additively and you can add a blue (short wave length) and yellow (medium/long waves) you will get white again (because you get a mix of s, m, and l)
how do hue cancellation experiments demonstrate the color opponent theory? start with a yellowish green and explain
if you start with a yellowish green you would add blue light to remove all of the yellowish traces (this suggested that blue and yellow were opposed)
How does subtractive color mixing work?
instead of adding, where all original light frequencies are retained, now color results from what's "left over" For example, if pigment A and B mix, some of the light shining on the surface will be subtracted (absorbed) by A and some by B So...only the remainder (i.e., which is not absorbed by either original pigment/filter) determines the perceived color This is why get "opposite" color results from adding vs. subtracting
brightness
intensity of light source
LGN
lateral geniculate nucleus. A structure in the thalamus, part of the midbrain, that receives input from the retinal ganglion cells and has input and output connections to the visual cortex.
M-cones
medium wavelengths, green
color mixture is a _______ event, not a change in physical light
mental
Different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of physical differences
metamers
if two sets of lights produce the same responses, they are ____ and must look identical. For example you can have a red and green light mixed together produce the same cone output as a medium wavelength that looks yellow. Both sets will look yellow even if they are physically different
metamers
mixtures of different wavelengths that look identical
metamers
a unique blue is a blue has no ____ or ____ tint
no red or green tint
§Idea that color perception based on the output of two major "opponent" mechanisms: qRed-green, blue-yellow
opponent color theory
if you only had one kind of photreceptor, light at 450nm and 625 nm each would result in the same response from a given photoreceptor. What does this situation demonstrate? why is this?
principle of univariance -- The fact that an infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make color discriminations based on wavelength. can't discriminate because both a shorter and longer wave length would produce exactly the same response
this helps explain why we don't see color in dimly lit scenes
principle of univariance--- all rods contain the same type of photo-pigment (rhodopsin) thus they all have the same sensitivity to wavelength
M-L is for ____ cones
red
a unique blue has the right balance or ___ and ____ wavelengths
red and green
a unique yellow has the right balance of ___ and ____
red and green
(L-M) and (M-L) compute ____ vs ____
red vs green
scoptic
refers to light sensetivities that are bright enough to simtulate the rod receptors but too dim to simtulate the cone receptors
at photopic conditions, the rods are _____ and so ____ contribute to color vision
saturated; don't
a cell that is excited by long wave lengths in its center and inhibited by medium wavelengths in its surround would be sentive to what contrasts?
sensitive to contrasts between long and medium wavelengths (l-M) or (M-L)
S-cones
shorter wavelengths also called blue cones
sensetivity of a cell to different wavelengths on an electromagnetic spectrum
spectral sensetivity
when only the remaining pigment contributes to color
subtractive color mixture
if you pass a white light through a yellow filter that absorbs shorter wavelengths what is the result?
take out blue, so have yellow leftover
saturation
the chromatic strength of a hue
hue
the dimension of color that is determined by the wavelength of light; what we know as the color names blue, green, and so forth the chromatic (color) aspect of light
spectral sensitivity
the sensitivity of visual receptors to different parts of the visible spectrum
why do we call unique hues pure colors?
they don't have red or green tint "added," or blue or yellow tint "added" there is no mixture of another color
step of color perception: detection
to detect color, light must be differently absorbed by 3 photopigments in the cones
any wavelength from about 420-660 nm will produce a unique set of three responses from each kind of cone which allows us to see color and solves the problem of univariance
trichomatic theory of color vision
significance of the finding that 3 mixing lights can match any reference light
trichromatic theory for color vision: 2 lights wasn't enough and 4 was more than was needed
true or false: there are hues that you can see that do not exist on the wavelength spectrum
true-- mixtures of different wavelengths allow us to see more colors ex) purples are a mixture of red and blue on a color circle
Herring noticed that all colors on the color circle could be represented by ___ pairs of ___ colors
two pairs of opposing colors
where does the light that we use to see come from? what kind of light is this?
typical light sources: the sun, lightbulbs, fire almost all light we use for color perception is reflected--- wavelengths that are not absorbed are reflected back
why don't rods show color?
we only have one variety of rods so all rods have the same senetivity to various wavelengths of light rods suffer from the problem of univariance
how can you explain how 3 cones work and can still not distinguish between all wavelengths leading to metamers?
when light hits the retina it is converted into 3 numbers by the 3 cones--- if numbers are sufficiently different from one another you will be able to discirminate -- however, if there is not a significant difference in numbers, those patches will be metamers- they will look idnetical even if wavelengths are physically different
a unique red has the right balance of ___ and ___
yellow and blue
if all 3 cones are stimulated equally what is the result?
you have s, m, and l wavelengths and so the color will look white
to have a mixture or red and green look yellow what is required?
you would need just the right amount of each color--- too much of one color might make it look a bit reddish or greenish
