S & P Ch 9

protanopia:

-1% males and .02% females -missing long wavelength pigment and sees more blue -wavelength at which they receive gray is called neutral point (492nm) -at wavelengths above the neutral point, they perceive yellow which become less intense at the long wavelength end

cone pigments

-1960s -found 3 different cone pigments: short-wavelength (S), middle (M), and long (L) -short max absorption at 419 nm, middle max absorption at 531 nm, and long max absorption at 558 nm

3 reasons the opponent-process theory wasn't accepted

-its main competition (trichromatic theory) was championed by Helmholtz, who had great prestige -Hering's phenomenological descriptions could not compete with Maxwell's quantitative color mixing data -there was no neural mechanism known that could respond oppositely at that time

actual light reflected by something depends on

-its reflectance curve -the illumination that reaches the sweater and is then reflected from it

some evidence for a single 'color center' in the cortex

-many neurons in the visual area called V4 respond to color -the phenomenon of cerebral achromatopsia like in Mr. I spares a person's ability to see but stops color seeing

selective transmission

-means that only some wavelengths pass through an object or substance -for things that are transparent like liquids or glass -so, cranberry juice selectively transmits long-wavelength light and appears red

how to determine the wavelengths that are actually reflected from the sweater?

-multiply the sweater's reflectance curve at each wavelength by the amount of illumination at each wavelength -ex. light reflected from sweater is slightly more long via incandescent light than from LED -with color constancy, this doesn't matter

anomalous trichromatism

-needs 3 wavelengths to match any wavelength as normal -but, mixes these wavelengths in different proportions -not as good at discriminating between wavelengths close together

Hurvich and Jameson's measurements for the opponent mechanisms

-noted that in cases of color deficiency, colors tend to drop out in pairs (blue and yellow or red and green lost) -complementary afterimages; red and green switch and yellow and blue switch -used hue cancellation to determine strength of the blue mechanism

principle of univariance

-once a photon of light is absorbed by a visual pigment, the identity of the light's wavelength is lost -this is why if the same amount of visual pigments are activated by different colored lights, they will appear the same -the receptor doesn't know the wavelength of the light, just the total amount it has absorbed -so, by adjusting the intensities of the two lights, we can cause the single pigment to results in identical responses

evidence for not a single color center in the cortex

-opponent neurons found in many areas including the primary visual receiving area (V1), the inferotemporal cortex (IT), and V4 -responding to color can occur in areas that respond to other visual qualities as well -when brain damage causes color blindness, it also causes other effects like prosopagnosia

Mr. I

-patient of Oliver Sacks in 1986; painter who became colorblind after a concussion in a car accident -sometimes difficult to distinguish one object from another

monochromats

-people with just one type of pigment that see in shades of gray -10/1 million people -no functioning cones -poor visual acuity and sensitive to light (rod system not designed to function in light and becomes overloaded)

unilateral dichromat

-person with trichromatic vision in one eye and dichromatic vision in the other -helps determine experience of a dichromate

reflectance curves

-plot the percentage of light reflected from an object at each wavelength in the visible spectrum -most objects don't reflect just a single wavelength of light -ex. tomatoes predominately reflect long wavelengths to our eyes, whereas lettuce mostly does medium

transmission curves

-plots of the percentage of light transmitted at each wavelength -% transmittance is on the vertical axis

opponent-process theory of color vision

-proposed by Ewald Hering (1834-1918) from University of Prague -based on phenomenological observations -that color vision is caused by opposing physiological responses generated by blue and yellow, red and green, and black and white

3 major forms of dichromatism

-protanopia: -deuteranopia: -tritanopia: (p and d most common; inherited through X chromosome, more common in males

2 developments in the 1950s that helped opponent-process

-psychophysical experiments by Hurvich and Jameson (1957) that provide quantitative measurements of strengths of each opponent mechanisms -physiological demonstrations of opponent neural responses in the retina and LGN

processing in these circuits occurs in two stages:

-receptors respond with different patterns to different wavelengths (trichromatic theory) -later, neurons integrate the inhibitory and excitatory signals from the receptors (opponent-process theory)

Hering's primary colors

-red, yellow, green, blue -they evolve into different colors as small amounts of the next primary color are aded to create the circle

table of wavelengths reflected and color perceived

-short: blue -medium: green -long and medium: yellow -long: red -long, medium, and short: white

functions of color perception

-signaling functions both natural and human-contrived: yellow banana is ripe and a red light means stop -role in perceptual organization: grouping objects together and segregating objects from their background -also helps us identify things we can see easily; people will recognize appropriately colored objects (yellow banana) faster and more accurately than inappropriately colored objects (purple banana) -helps us rapidly perceive natural scenes and the gist of scenes

2 types of opponent neurons in the cortex

-single-opponent neurons (resemble circle of one + in middle and - outside, vice versa); important for perceiving color within regions -double-opponent neurons (side by side with +/- and -/+); perceive boundaries between color; evidence for connection between color and form

trichromatic theory

-states that color vision depends on the activity of 3 different receptor mechanisms -Maxwell and Helmholtz provided evidence to support and it became the Young-Hemoltz theory

Thomas Young (1773-1829)

-suggested that the idea of link between each size of vibration and color won't work because a particular place on the retina can't be capable of the large range of vibrations required -proposed that the number is limited to three principle colors -began the birth of the trichromatic theory

artificial vs sunlight

-sunlight: contains about equal amounts of energy at all wavelengths (characteristic of white light) -artificial LED: more short wavelengths (slightly blue) -incandescent bulb: longer wavelengths (more yellow)

violet wavelengths

400-450 nm

blue wavelengths

450-490 nm

green wavelengths

500-575 nm

yellow wavelengths

575-590 nm

orange wavelengths

590-620 nm

red wavelengths

620-700 nm

if paper reflects long wavelengths of light, and absorbs short and medium wavelengths, then:

it appears red, since only the long wavelengths reach our eyes

value

light-to-dark dimension of color; value decreases as colors become darker

HSV color solid

named because its 3 dimensions are hue, saturation, and value

are wavelengths themselves colored?

no! it is how we receive them with receptors, etc that give color -our experience is created by the NS

what happens when both lights isomerize the same number of molecules, the result will be:

that the two spots of light appear identical

penumbra

the fuzzy border at the edge of a shadow

the colors of objects are determined by:

the wavelengths of light that are reflected from the objects into our eyes

if the blue and yellow paint did not share a wavelength in common, what would happen

there would be little or no reflection from mixing them, so it would appear black

trichromats

those with 3 visual cone pigments; provides additional ratios that allow additional discrimination of wavelength across the visual spectrum

Isaac Newton

(1642-1727) -studied light and color -made hole in a window shade, letting beam of light through -when he placed a prism in its path, the beam of white-appearing light was split into components of the visual spectrum -Newton thought that white light was made of many different colors -he placed a board in the path of different colored beams with holes so that only particular beams could pass through -each beam that passed through then went through a second prism

vision with one receptor type

-a person with just one pigment would perceive two lights (one 480nm and one 600nm) which a normal person sees as blue and orange -the single pigment absorbs 10% of 480nm light and 5% of 600nm light -since 480nm light will isomerize twice as many pigments as the 600nm light, it will cause a larger response in the receptor, resulting in perception of a brighter light

deuteranopia:

-affects about 1% males and .01% females -missing medium wavelength pigment -perceives blue at short, yellow at long, and neutral pont at about 498nm

color circle

-arranges perceptually similar colors next to each other around its perimeter -leaves out saturation and value (just hues) -Hering noted that the colors fit into 4 groups defined by amount of yellowness, blueness, greens, and redness

ratio principle

-as long as the ratio of reflectance of the object to the reflectance of surrounding objects remains the same, the perceived lightness will remain the same

hue cancellation

-because yellow is the opposite of blue and cancels it, they could determine amount of blueness in a 430nm light by determining how much yellow needs to be added to cancel all perception of blueness -once determined for 430nm, measurement repeated for 440nm and so on until no blueness-

achromatic color reflectance %

-black reflects 10% of light -gray reflects 10-70% of light -white reflects 80-95% of light

mixing blue and yellow paint

-blue paint absorbs long wavelength light and reflects some short and medium -yellow paint absorbs short and reflects some medium and long -when mixed, both paints still absorb the same wavelengths they absorbed when alone, so the only wavelengths reflected are those that are reflected by both paints in common -since medium are the only ones reflected by both paints in common, a mix of blue and yellow appears green

max responses documented by Hurvich and Jameson

-blue reaches max at 440nm and decreases until at 500nm it is 0 -yellow responds best to 500-700nm with max at 550nm -green responds from 490-580nm with max at 525nm *note: there is no color in the spectrum that simultaneously activates both blue and yellow or red and green

chromatic colors

-blue, green, and red -occur when some wavelengths are reflected more than others

color affected by surroundings

-color constancy works best when an object is surrounded by objects of many different colors -because the visual system uses the info provided by the ways objects are illuminated in the scene to estimate the characteristics of the illumination and make appropriate corrections

summary of connection between wavelength and color

-colors of light are associated with wavelengths in the visible spectrum -the colors of objects are associated with which wavelengths are reflected (for opaque) or transmitted (for transparent objects) -colors that occur when mixing are associated with which wavelengths are reflected into the eye (mixing paints causes less wavelengths to be reflected and mixing lights causes more wavelengths to be reflected)

what happens when you have two pigments?

-consider the ratios of responses of the two pigments to two wavelengths

Ishihara plates

-diagnose color deficiency

what is responsible for the three different absorption spectra of the pigments?

-differences in structure of the long opsin part of the pigments

using a color solid to determine results of mixing colors

-draw a line that connects two hues, like yellow and blue -any mix will fall along this line with the exact location depending on the amount of each light added to the mix -ex. if more yellow light, the yellow becomes less saturated; if equally mixed it is white

Newton's thought on how we perceive light

-each component of the spectrum stimulates the retina differently -thought the vibrations were sent down the nerves from the retina

how can both theories exist?

-each theory describes neural processes at different parts of the visual system -trichromatic theory describes processes in the retina and opponent-process describes processes in the LGN

color matching

-experimenter presents a reference color created by shining a single wavelength of light on a test field -the subject then matches the reference color by mixing different wavelengths of light in a comparison field until it matches *any reference color could be matched provided that the subjects were able to adjust the proportions of 3 wavelengths in the comparison field; 2 was only enough for some

when does color vision develop?

-first 3-4 mo of life -testing infants, you should use colors that are different but of the same brightness so only actual color differentiation can be measured -4 mo infants categorize colors the same way adults do

what are the 3 perceptual dimensions of color?

-hues -saturation -value *together contribute to why we can see so many colors

color solid

-illustration of the relationship between hue, saturation, and value -arranges colors within a 3D space -value goes up (lighter at top), saturation goes from in to out (more saturated at edges), and hue goes around the wheel

opponent neurons

-in retina and LGN -respond with excitation to light from one part of the spectrum and with an inhibitory response to light from another part of the spectrum -B+ and Y- / R+ and G- were found -later became S+ and ML- / L+ and M-

metamerism

-in which two physically different stimuli are perceptually identical -the two identical fields in color matching experiments are called metamers

mixing yellow and blue light

-the area where the lights are superimposed is perceived as white -because the two spots of light are projected onto a white surface, which reflects all wavelengths, all of the wavelengths that hit the surface are reflected to your eyes -all of the light that is reflected from the surface by each light when alone is also reflected when the lights are superimposed -the added-together light contains short, medium, and long wavelengths

chromatic adaptation

-the eye's sensitivity is affected by the color of the illumination of the overall scene -ex. adaptation to red light selectively reduces sensitivity of long wavelength cone pigment which decreases sensitivity to red light and causes to see reds and oranges as less saturated and bright -adaptation tones down the dominant colors in a scene and also causes novel colors to stand out

lightness constancy

-the fact that we see whites, grays, and blacks as staying about the same shade under different illuminations

the intensity of light reaching the eye from an object depends on 2 things:

-the illumination: total amount of light that is striking the object's surface -the object's reflectance: the proportion of light that the object reflects into our eyes *our perception of lightness is determined by the object's reflectance, not the intensity of illumination hitting the object

2 important things about the 2nd prism in Newton's experiment

-the prism didn't change the color appearance of any light that passed through it (so, unlike white light, the other colors weren't mixtures of colors) -the degree to which beams from each art of the spectrum were bent by the second prism was different (red only a little, yellow, violet the most bent); so each light must have different physical properties that give rise to our perception of different colors

what do reflectance curves look like for achromatic objects?

-they are flat, indicating equal reflectance across the spectrum -the different between gray, white, black is related to the overall amount of light reflected from an object -black paper reflects less than 10% of light that hits it and white reflects over 80%

why do metamers look alike?

-they both result in the same pattern of response in the 3 cone receptors -even though the lights in these two fields are physically different, the two lights result in identical patterns of physiological responses so they are identical as far is the brain is concerned

dichromats

-those that see with just two types of cone pigment; see chromatic colors but confuse some colors that people with 3 can discriminate between

the opposing theory on the circle of colors

-upper portion possesses redness, lower possesses greenness, left blueness, and right yellowness -there is no color that is yellowish and blueish or red and green -therefore, Hering proposed that our color experience is built from 4 primary chromatic colors that are arranged in two opponent pairs: yellow-blue and green-red

tritanopia:

-very rare, .002% male and .001% females -missing short wavelength -blue at short, red at long, and neutral at 570nm

color constancy

-we perceive the colors of objects as being relatively constant even under changing illumination

3 mechanisms for the opponent-process theory

-white + and black - responds positively to white and negatively to the absence of light -the red + green - mechanism responds accordingly -the yellow + blue - mechanism also -so, the more white, the less black and the more red, the less green

achromatic colors

-white, gray, and black -occur when light is reflected equally across the spectrum -reflecting all wavelengths of light produces white

how long did it take to have physiological evidence of the trichromatic theory?

100 years!

conservative estimate on how many colors we can tell the difference between

2 million

a person with only one visual pigment can match any wavelength in the spectrum by:

adjusting the intensity of any other wavelength

illumination edge

an edge where the lighting changes

reflectance edge

an edge where the reflectance of 2 surfaces changes; reflect different amounts of light

what is the most favored color?

blue

cerebral achromatopsia

color blindness

non spectral colors

colors that don't appear in the spectrum because they are mixtures of other colors, such as magenta (blue and red)

saturation

determined by the amount of white that has been added to a particular hue

desaturated hues

have a faded or washed-out appearance

Isaac Newton's 7 colors

red, orange, yellow, green, blue, indigo, violet -he wanted to harmonize with the 7 scale of music -violet tends to be excluded from spectral colors since we have a hard time distinguishing form blue

memory color

we judge things that we are more familiar with (stop sign, banana) as being more saturated in color than unfamiliar objects that reflect same wavelength

color vision may have evolved because:

we needed to perceive fruit from the background to eat

hue

what we would call the general color

additive color mixture

when mixing lights adds the wavelengths of each light in the mixture

subtractive color mixture

when mixing two paints subtracts all of the wavelengths except some that are associated with the resulting color

selective reflection

when some wavelengths are reflected more than others

when does a visual pigment isomerize?

when the molecule absorbs one photon of light