CHAPTER 9 | Perceiving *Colour*

transmission curves

plots of the percentage of light transmitted at each wavelength

chromatic colours

Colour with hue, such as blue, yellow, red, and green

spectral colours

Colours that appear on the visible spectrum

nonspectral colours

Colours that do not appear in the spectrum bc they are mixes of other colours - ie., the freaking magenta

Achromatic colours

Colours without hue. White, black and alll the grays between these two extremes are achromatic colours

dichromatism

people who are missing one of three cone pigments and hence experience some colours, but not as many as trichromats - needs only two wavelengths to match any other wavelength in the spectrum

selective transmission

When some wavelengths pass through visually transparent objects or substances and others do not. Selective transmission is associated with the perception of chromatic color. See also selective reflection.

memory colour

the idea that an object's characteristic colour influences our perception of that object's colour

value

the light to dark dimension of colour

Describe lightness constancy

the same way we perceive chromatic colours like red and green as constant even if illumination changes, we do the same for achromatic colours - Imagine a black Lab on the living room rug illuminated by a light - a small percentage of the light that hits the coat is reflected, and we see it as black - but when the lab goes outside into the sun, the perception of tightness remains the same - so the fact we see whites/grays/blacks/ as the same shade under dif illuminations = lightness constancy

How can we perceive millions of colours if we only have a spectrum of six or seven?

there are three perceptual dimensions of colour, which tog creates the large number of colours we can perceive colours like blue, green, and red = chromatic colours/hues - then there is saturation (amount of white) -value is the last one -> light to dark dimension

metamers

two lights that have different wavelength distributions but are perceptually identical

selective reflection

when an object reflects some wavelengths of the spectrum more than others

Describe the ratio principle?

when an object s illuminated evenly, then lightness is determined bye ratio of reflectance of the object to the reflectance of surrounding objects - as long as ratio remains the same, the perceived lightness will stay the same - ie blik square to surrounding white square is 9/90 under low illumination and 900.9000 in high - since ratio is same, perception of lightness is too

hering's primary colours

The colours red, yellow, green, and blue in the colour circle

additive colour mixture

The creation of colours that occurs when lights of different colours are superimposed

subtractive colour mixture

The creation of colours that occurs when paints of different colours are mixed together.

Define deuteranopia?

- 1% of males and 0.01% of females - results in the perception of colour - deuteranope is missing med wavelength pigment - perceives blue w/ short, sees yellow at long, and has neutral point at 498 nm

Describe protanopia?

- 1% of males, 002% of females - missing long wavelength pigment - so short wavelength = blue, and as wavelength is increased, it becomes less saturated until at 492nm, it is cray 0 the wavelength at which protanope perceives gray = neutral point - above this point, protanope perceives yellow, which is less intense at long wavelength end

What are the problems the visual system has to solve in lightness constancy?

- 1) illumination, the total l amount of light striking the surface - 2) the objects reflectance - when constancy occurs, our perception of lightness is determined not by intensity of illumination hitting an object, but the object's reflectance - so objects that look black reflect less than 10% of the leith - gray = 10 - 70% of light is reflectd - white = 80 - 95% of the light is reflected - so lightness is not just the amount of light reflected from object, ut to the percentage of light reflected from the object, which remains the same no matter the illumination

What is anomalous trichromatism?

- also needs three wars to match any wave - mixes these in dif proportions, so they are not as good at discriminating between wavelengths closer tog

What are the three major forms of dichromatism?

- protanopia -deuteranopia -tritanopia - common kind is the first two, and are inherited through the X chromosome since males only have one, a defect will cause colour deficiency, so girls are less likely to be colour deficient bc only one gene is required for normal colour vision - called sex linked bc women can carry gene w/o being colour deficient

give an example of lightness constancy?

- checkerboard illuminated by room light - white squares reflect 90%, black reflects 5% - if illumination is 100, white squares reflect 90 units and black reflects 9 - if we take it outside to bright sun, where illumination is 10 000 units, white squares reflect 9000 units of light and black reflect 900 - ut, even though black when outside reflects more light than white squares did when board was inside, the shares still look black - so, our perception is determined by reflectance , not amount of light reflected

Why is colour constancy an impressive achievement?

- colour constancy = when your friend wears a nice green shirt, and it looks blue both inside and outside - consider interaction between illumination (sun, or lightbulbs) and reflection properties of the object "blue shirt" 1. Illumination - wavelengths of sun and incandescent / LED bulbs - sunlight has approx equal amounts of energy at all wavelength which is a characteristic of white light - incandescents have more energy at long wavelengths, and LED have shorter waves - now what about the wavelengths of these illuminations and the green sweater? Reflectance curve as seater should reflect mostly short wavelength light - the actual light reflected from sweater depends on both reflectance curve and illumination that is reflected from it - we have to melt. sweater's reflectance curve at each wavelength by amount of illumination at each wave - shows that light reflected from shirt would have more long wavelength light when illuminated by incandescent then LED - fact we see the shirt as green even thought the wavelength of the light differ under different illuminations is colour constancy

What was Newton's idea about colour perception? What is our modern idea about colour being a construct of the ns?

- colours we see in response to dif wavelengths are not in the rays of lights, but that the rays cause a sensation of a colour - light rays are simply energy -> there is nothing blue about short wavelengths or red about long, we perceive colour bc of the way our ns responds to energy

What is infant colour vision?

- cones are poorly dev at birth, so newborns don't have great colour vision - colour vision develops early, and applicable colour vision is present in the first 3 -4 months of life CHALLENGES: Perception of light stem can vary on 1) its chromatic colour, and 2) its brightness - so you can present a colour to a colour deficient person and ask whether he can tell the difference between it and something else, and they will be able too, not bc of the colour, but bc one looks brighter - when stem w/ different wavelengths are use to test colour vision, their intensity should be adjusted so they have same brightness - so red patch needs t be lighter and allow patch darker

How do we perceive light in uneven illumination?

- illumination isn't usually even, bc of shadows cast by objects onto each other, or bc one part of an objet faces light and the other faces away - ie., shadow hitting a wall -> are the changes in wall appearance due to different wall properties, or the difference in the way it is illuminated - the problem for the perceptual system is that is has to take this uneven illumination into account - the perceptual system needs to distinguish between reflectance edges, and illumination edges - reflectance = where does the reflectance change - illumination edge = where does lighting change - the basic idea behind how our vis system figures out which edge is which is that our perceptual system uses number of sources of info to take illumination into account

Define titanopia?

- rare, affects 0.002% of males and 0.001% of females - a tritanope is missing short wavelength pigment - sees spectrum as blue at short waves, and red at long, w/ neutral point at 570 nm

What might be responsible for lightness constancy?

- ratio principle lightness perception in uneven illumination - shadows -orientation of surfaces

Describe Uchikawa's experiment on chromatic adaptation?

- isolated patches of coloured paper viewed in three conditions 1. baseline -paper and observer illuminated w/ white light 2. observer not adapted - paper illuminated by red light, observer by white (so observed isn't chromatically adapted) 3. observer adapted to red - both paper and observer illuminated by red Results: - baseline: green paper perceived as green - observer not adapted: paper is more red . No colour constancy bc the observer isn't adapted to red light illuminating paper - observer adapted to red: perception is shifted slightly to red, so it appears more hello - partial colour constancy -> perception of object shifted after adaptation, but not as much as hen there wasn'r adaption - eye can adjust its sensitivity to dif wavelengths to keep colour perception approx. constant

Describe memory and colour constancy?

- knowledge about usual colours of our enviro help our colour constancy - bc people know colours of familiar objects, they judge these objects as having more saturated colours than unfamiliar ones that reflect the same wavelengths - DEMO: - observers presented pics of fruits w/ characteristic colours (lemons, oranges, bananas) against grey background - observers viewed spot of light against background - when intensity and wavelength of the spot were adjusted so spot was physically the same as background, observers reported spot appeared the same gray as background - if the fruit had the same intensity and wavelength as background, fruits were reported to be slightly coloured - so banana that was same as background appeared yellowish - led Hansen to concl that observer's knowledge of fruit's characteristic colours changed the colours they were experiencing

What is colour? L

- not physical in the environment - really "no such thing" as colour (ah) - no unanimous account of where colour comes from/ is manifested in the brain

Describe the role of ns in creating colour experience?

- people like Mr I can see no colours (and neither can we at dusk) even though we are receiving the same stem as people w/ normal colour vision - many animals perceive either non colour or a greatly educed palette of colours, and other sense a wider range of colours than we do - the absorption spectra of a honeybee's visual pigments allows honey bee to see short wavelengths we cannot - there is no colour in wavelengths, so the bees ns will create the bee;s experience of colour - so we'll never know what they see - as always, perception is MORE THAN THE SUM OF ITS PARTS -> the perceptions are not in the molecular structures, but in the action of molecular structures on the ns

Describe the principle of colour constancy with eco valid examples (aka rooms and different seasons)

- room with yellow light - eye adapt to long wavelength rich lights, dec eye's sensitivity to long ones - dec sens causes long waves reflected from objects to have less effect than before adaptation, and compensate for greater amount of long length tungsten light in the room - yellow tungsten has small effect t - similar to enviro scenes, w/ dif dom colour sin seasons - same scene is lush in summer (lots of green), and arid (more yellow) in winter - Webster determined adaptation to green in lush scene would decrease perception of yellow in arid scene - so adaptation tones down dom colours, so if we compare perceived colours of lush and arid scenes, we see colours are more similar than before adaptation 0 adaptation causes novel colours to stand out, so yellow is more ova in lush and green in arid scene

Describe how the orientation of surfaces is important to telling the difference between an illumination and reflectance edge?

- the paper folded like a card, you first perceive an illumination edge - however, once you took away the orientation, you saw the shadowed white paper as being a grey papr - erroneous perception occurred bc viewing shaded corner through a small hole eliminated info about the conditions of illumination and the orientation of the corner - in order for lightness constancy to occur, it is important that the visual system has an adequate info about the conditions of illumination - without info, lightness constancy can break down and a shadow can be seen as darkly pigmented - sometimes we can be confused and fooled b the conditions of illumination, but most o the time we perceive lightness accurately (ie., St Mary statue)

How is colour a creation of the nervous system?

- the wavelengths of light we see ARE COLOURLESS - if we look at colour under dim illumination, we dark adapt and our vision is dominated by rods - so hues like blue/green/red are less distinct, and THEY DISAPPEAR INTO SHADES OF GREY - so our ns contructs colour from wavelengths through the action of the cones??!!!

How do we determine what dichromate see?

- unilateral dichromate -> trichrome in one eye, dichromate in the other 0 since both eyes are connected to the brain, they can lok at colour w/ dichromate eye and determine which colour it corresponds to in trichromatic eye

How do shadows give us info about whether something is a difference in reflectance or illumination?

- vis system needs to take the uneven illumination by shadows into account - needed to determine change in illumination is caused by shadow, not reflectance - vis system success bc although light is reduced by shadows, you don't see shadowed areas as gray or black - in case of wall, you assume that the shadowed and unshadowed areas are bricks w/ same lightness, but that less light falls on some areas bc of shadow - one thing the visual system may take into account is the shadows meaningful shape - another clue is provided by the nature of the shadow's contour - the ~penumbra~ - if you cover the penumbra, we see a change in the appearance of the shadowed area -> it seems like penumbra provides info to the visual system that the dark area next to the cup is a shadow, so the edge between shadow and paper is an illumination edge - however, masking off the penumbra eliminates ze blur, so the area covered by shadow is seen as a change in reflectance - so we need the penumbra for light constancy!

What is the effect of surroundings on colour?

- when surroundings are masked, most people perceive a green area to be more yellow under incandescents than daylight, so colour constancy breaks down when object is seen in isolation - number of investigators have shown that colour constancy works best when object is surrounded by many colours, usually happens in the enviro - the visual system sues info provided by the way objects in a scene are illuminated to estimate characteristics of illumination and make appropriate corrections

Describe Bernstein's study on infant colour vision?

-4 month old infants determined whether they perceived the same colour categories n spectrum as adults normal trichromatic vision see spectrum s sequence of colour categories, starting w/ blu, green, yelloww, orange, and red, w/ abrupt transitions between one colour and next - used habituation - 4 month olds habituated to 510 nm light (green), by presenting light a number of times and measuring how long the infant looked at it - dec in looking = habituation - after trial 15 of habituation, a 480 nm light presented - wavelength appears blue to adult, and is different than 510 nm lightt - infant's inc in looking time (dishabituation) indicates the perception of 480nm is a dif category than the 510 nm light - however, when procedure is repeated, by presenting 510 nm and then a 540 nm light, dishabituation doesn't occur, so 540nm is in thee same category for infants CONCL: 4 month olds categorise colours the same way adults do each wavelength was set in a way thaat looked equally bright to adults - not ideal procedure bc infants can perceive brightness differently than adults - later research has confirmed born stein's conch. that infants have colour vision - not possible to know whether infants are experiencing the same colour, although looking behaviour indicates they can tell difference between two wavelengths

opponent process theory of colour vision

A theory originally proposed by Hering, claiming that our perception of colour is determined by activity of two opponent mechanisms: a blue - yellow mechanism and a red-green mechanism - responses to the two colours in each mechanism oppose each other, one bing an excitatory and the other an inhibitory response this theory also incl a black and white mechanism, which is concerned w/ the perception of brightness

trichromatic theory of vision

A theory proposing that our perception of color is determined by the ratio of activity in three receptor mechanisms with different spectral sensitivities.

desaturated

Low saturation in chromatic colors as would occur when white is added to a color. For example, pink is not as saturated as red.

single opponent neurons

Neurons that increase firing to long wavelengths presented to the center of the receptive field and decrease firing to short wavelengths presented to the surround (or vice versa).

colour circle

Perceptually similar colours located next to each other and arranged in a circle

monochromatism

Rare form of color blindness in which the absence of cone receptors results in perception only of shades of lightness (white, gray, and black), with no chromatic color present

hues

The experience of a chromatic color such as red, green, yellow, or blue or combinations of these colors.

penumbra

The part of a shadow surrounding the darkest part - the fuzzy border at the edge of a shadow

reflectance

The percentage of light reflected from a surface.

lightness

The perception of shades ranging from white to grey to black.

reflectance edge

an edge between two areas where the reflectance of two surfaces changes

double opponent neurons

neurons that have receptive fields in which stimulation of one part of the receptive field causes an excitatory response to wavelengths in one area of the spectrum and an inhibitory response to wavelengths in another area of the spectrum, and stimulation of an adjacent part of the receptive field causes the opposite response Ane x if a double opponent responding is when response of one part of a receptive field is +M - L and the other adgacent part is +L - M

What is chromatic adaptation demo?

red square green square - shows the prolonged exposure to chromatic colour will mess wiht your colour perception adaptation to red light will reduce sensitivity of long wavelength cone pigmentt, decreasing sensitivity to red and causing you to see reds and oranges with adapted eye as less saturated than those viewed with right

illumination edge

The border between two areas created by different light intensities in the two areas.

colour blind

A condition in which a person perceives no chromatic colour can be caused by absent or malfunctioning cone receptors or by cortical damage

ishihara plates

A display of colored dots used to test for the presence of color deficiency. The dots are colored so that people with normal (trichromatic) color vision can perceive numbers in the plate, but people with color deficiency cannot perceive these numbers or perceive different numbers than someone with trichromatic vision.

lightness constancy

The constancy of our perception of an object's lightness under different intensities of illumination.

saturation

The relative amount of whiteness in a chromatic color. The less whiteness a color contains, the more saturated it is.

metamerism

The situation in which two physically different stimuli are perceptually identical. In vision, this refers to two lights with different wavelength distributions that are perceived as having the same color.

Describe the method of colour matching?

experimenter presents reference colour created by shining a single wavelength of light on a "test field" subj matches reference by mixing diff wavelength of light

What colours are in our visual spectrum

newton described the colours in his visual spectrum in terms of seven colours (roygbiv), bc he wanted to connect them to musical scales modern vision scientists are now excluding indigo from the list of spectral colours bc humans have hard time distinguishing it from blue and violet we also have non spectral colours (those that do not appear in the spectrum bc they are a mix of other colours) the number of colours we can differentiate is huge (thi of paint chips!) a conservative est. of how many colours we can discriminate is 2 mill

principle of univariance

once a photon of light is absorbed by a visual pigment molecule, the identity of the light's wavelength is lost This means the receptor doesn't know the wavelength of light that is absorbed, only the total amount of light it has observed

- What was interesting about the trichrome colour theory

*cool things* - it proposed physio mechanisms for colour vision based on psychophysical results before there where techniques avail to make the physic measurements - mechanism turned out to be generally correct!

Describe vision with one receptor type?

- - imagine a person w/ one pigment is perceiving 480nm light and one 600 nm light - which we see as blue/orange - absorption spectrum for single pigment indicates pigment absorbs 10% of 480nm light, and 5% of the 600nm one - when light is absorbed by retinal, the retinal changes shape via isomeriztion - the vis pigment molecule does this when the molecule absorbs one photon of light - isomerization activates the molecule and triggers process that activates visual receptor and leads to seeing the light - if the intensity of ea. light is adjusted so 1000 photons of light enters the one pigment, the 480nm light isomerizes twice as may visual pigment molecules as the 600nm light, and will cause a larger response = perception of a brighter light if we inc the intensity of the 600 nm light to 2000 photons, the light will also isomerise 100 visual pigment molecules - when the 100 480nm and 2000 600nm lights isomerise the same number of molecules, the result will be two spots of light that appear identical 0 the wavelengths of light being dif doesn't matter, bc once a photon of light is absorbed by pigment, the ID of the wavelength is lost - receptor doesn't know the wavelength of light, just the total amount it has absorbed, so if you adjust intensity of both lights, the single pig will result in identical responses, so they'll look the same colour - w/ one vis pigment, you can match any wavelength in the spectrum by adjusting intensity of other wavelength, and all of the waves are grey

opponent neurons

A neuron that has an excitatory response to wavelengths in one part of the spectrum and an inhibitory response to wavelengths in the other part of the spectrum.

What were the two developments that led to a greater acceptance of the opponent process theory?

- 1. Psychophys experiments by Hurvich and Jameson to provide quantitative measures of strengths of each opponent mechanisms 2. Physio demos of opponent rural responses in retina and LGN

What are the two stages of processing in the neural circuits for retinal colour coding?

- 1. receptors respond w/ dif patterns to diff wavelengths )trichrome theory) and then later, neurons integrate inhibitory and excitatory signals from receptors (opponent process) conclusion -> the signals for colour that are sent to the brain indicate the difference in responding of pairs of cones

Describe the method of hue cancellation?

- 430nm = blue mechanism has substantial strength, it looks violet - reasoned that bc yellow is opposite of blue and "cancels" it, they could determine amount of blueness in a 430nm light by determine how much yellow needs to be added to cancel the perception of blueness - once this has been found for 430nm light, the measurement is repeated for 440 nm and so on, until reaching the wavelength were there is no blueness - hue cancellation can be used to determine strength of yellow mech to determine how much blue needs to be added to cancel yellowness - for red and green, strength of red mech is determined by measuring how much green needs to be added to cancel perception of red, and the strength of the green by determining gif much red is added to cancel green

Describe the process of signals for colour indicating the difference in responding of pairs of cones?

- Neural level - +L - M neuron receives excitation from L cone and inhibition from M cone - it is responding to 500 nm and 600nm lights - 500nm light results in inhibitory signal of -80 and an excitatory signal of + 50, so response of the +l -M neuron = - 30 - 600 nm light results in inhibitory signal of -25 and an excitatory single of + 75, so response of +l _M neuron would be + 50 0 difference info = type of info sent by opponent neurons to brain

What are the different wavelengths of light?

- Newton thought prism separated dif coloured light particles while others thought prism separated light into dif coloured waves, and that last one is *correct* - 400 - 450 nm appear violet 450 - 490 nm appear blue 500 - 575 = green 575 - 590 = yellow 590 - 620 = orange 620 - 700 = red - so our colour vision depends on the wavelngths of light that enter our eyes

What is the trichromatic theory of colour vision?

- Newton's experiment - the he separated white light into its components to reveal the visible spectrum, he thought each of these components stem retina diff to let us perceive colour Newton's proposal that eye has "vibrations" that are sent to optic nerve and into the brain was on the right track in proposing that activity assoc. w/ dif lights gives rise to perceptions of dif colours Young - suggested newton's idea of a link between size of vibration and colour doesn't work, bc one place on retina cannot be capable of the large range of vibrations - he proposed that colour vision is based on three principal colours - that marks the birth of the trichrome theory, which says colour vision depends on activity of 3 dif receptor mechanisms - Young actually didn't test his ideas or publish research to support his theory, so Maxwell and Hermann Helmholtz provided the experimental evidence - different cones have different pigments they pick up - colour blindness caused by loss of cones

Describe the case of Mr I?

- Ollie Sacks was a neurologist who became famous and well known for intense case studies of unique cases of brain damage and psychiatric disorder - could draw important insights about the brain, mind, and perception - he made a relationship w/ a painter who became colour blind after suffering a concussion in a car accident - since the accident, he couldn't experience colours - his colour blindness = cerebral achromatopsia, caused by cortical injury after lifetime of experiencing colour, when colour blindness is usually caused at brith bc of genetic absence of one or more cone receptors - people with total colour blindness often say it is difficult to distinguish one object from another, like a brown dog against a light coloured road against irregular foliage - Mr I can now create striking b and w pics, he focused on contrasts in greys - his account shows the central placement of colour in our everyday lives

What is the evidence for the connection between colour and form?

- also, double opponent's side by side fields provide evidence that there is a connection between colour, and form - neurons can fire to bars even when intensity is adjusted so both lights look equally bright, so they fire when bar's form is only determined by differences in colour - this supports idea of bridge between colour and form processing - colour might help define edges and shapes of objects and areas!

What were the key findings of the colour matching evidence for the trichrome theory?

- any reference colour could be matched if the subj could adjust the proportions of 3 wavelengths in comparison field -> allows us to cover - 2 wavelengths allowed people to match some, but not all reference colours, and subj never needed 4 to match - since peofple / normal colour vis needed at least 3 wavelengths to match any other wavelength, conceal = colour vision depends on three receptor mechanisms w/ dif spectral sensitivities - light of particular wavelength stems each receptor mechanism to dif degrees, and pattern of activity in three mechanisms results in perception of a colour - ea wavelength is therefore ripped in the ns by its own pattern of activity in the three receptor mechanisms

Describe physio evidence of cone pigments?

- are there three mechanisms for colour perception, and if so, what are their physic properties? - researchers were able to determine there were 3 dif cone pigments -> short wavelength, w/ max absorption at 418nm, middle wavelength pigment, w/ max absorption at 531 nm, and long wavelength pigment, w/ max absorption at 558nm - differences in structure of long opsin part of pigments are responsible for these three different absorption spectra

Describe how Hering's opponent process theory was proposed?

- based on behavioural observations - opponent process theory was initially based on results of phenomenological observations - based on these, Hering proposed colour vision was actually caused by opposing physio responses gen by blue and yellow, red and green, and black and white - Lecture - visual system appears to be organized that colours are repped as opponents - as one colour is activated, the other colour is less activated

Describe the phenom evidence for opponent-process theory?

- based on people's experiences when looking at a colour circle (leaves out saturation and value) - all colours seem to fall into four groups defined by amount of yellowness, blueness, greenness, and redness in each group - Hering's description of the whole colour circle - he observed it can be divided into upper portion, where colour possess some red, lower where colours have some green, a left where colours have blue, and right where colours have yellow - while we can see yellowish reds, and yellowish greens (upper and lower right), and we can see bluish greens and bluish reds (lower and upper left) there is no colour anywhere in the circle that appears to be both yellowish and bluish, or any colour that appears to be a combination of red and green (and we can't imagine what that would look like) - so, proposal our colour experience is built from four primary chromatic colours arranged into opponent pairs -> yellow blue and red green, and also black and white

Communication and colour blindness (The Island of the colour blind)

- blue when sad - we understand things with colour metaphors, including emotions! - coordinating things like colour - safety and security is given from the colours - one of the most important comm is how much colour is used for scientific ideas - lines, dots, bars, are in DIFFERENT COLOURS, communicating data through colour is difficult to access as an achromat - colours are used to judge aesthetics, person w/ achromatopsia says there was beauty in the reef too!

Summarize the connection between wavelength and colour

- colours of light are assoc w/ wavelengths in the visible spectrum - colours of objects are assoc. with which wavelengths are reflected (for opaque objects) or transmitted (for transparent objects) - colours that occur when we mix colours are also assoc with which wavelengths are reflected into the eye - mixing paints causes fewer wavelengths to be reflected (Each paint subtracts wavelengths from the mix) mixing lights causes more wavelengths to be reflected (each light adds wavelengths to the mix)

How do we mix lights?

- if a light that appears blue is projected onto a white surface, and alight that appears yellow is projected on top of it, the area where the lights are superimposed is perceived as WHITE - *Kurtis voice* HHHUUghhhhH? -because the two lights are projected onto a white surface, which reflects all wavelengths, all of the wavelengths that hit the surface are reflected into observer's eyes - blue spot consists of a band of short wavelengths, so when it is alone, the short wavelength light is reflected into observers eyes - yellow spot consists of medium and long wavelengths, so when presented alone, these wavelengths are reflected into observers eyes - if the light that is reflected from the surface by each light when alone is also reflected when lights are superimposed - so, where the two spots are superimposed, the light from the blue spot and from yellow spot are both reflected, and the added light has short, med, and long wavelengths which results in the perception of light - additive colour mix

What is Zeki's evidence that there a single colour centre in cortex?

- if there is an are, that would make colour similar to faces, bodies, and please, which can claim the FFA, ESB, and PPA as specialised processing areas - idea of an area specialised for colour was put forth by Zeki based on: 1. finding that many neurons in a visual area called V4 responded to colour 2. phenomenon of cerebral chromatopsia, a condition caused by damage to brain which spares a person's ability to see colour - fact that the damage that typically results is near or identical to areas ID as colour areas supports idea of specialised module for colour perception

Colour switch demo?

- if we just used three cone pigments and processed waves of length based on mix and preferences, why would they switch? - the ring that opponent process is created is due to the consistency where red goes to green and blue to yellow - due to bleaching of pigments... I think

Importance of Hurvich and Jameson's experiments?

- important step to accepting opponent process theory - went beyond phenomenological observation by providing quantitative measurements of the strengths of opponent mechanisms

If there were two men who provided evidence for the trichrome theory... why is it the Young-Helmholtz and not the Maxwell-Helmholtz?

- it is now, however, called the young-Helmholtz theory bc Helmholtz' prestige in science comm and the popularity of his Handbook of PHysio, where he described idea 3 receptor mechanisms - However, maxwell's critical colour matching experiments is what allied us to have evidence for the trichrome theory

So, how do we see the colours of objects then?

- largely determined by the wavelengths of light that are reflected from objects into our eyes - chromatic colours (blue, green, red) happen when some wavelengths are reflected more than others - sheet of paper demo reflects long wavelengths and absorbs short and medium wavelengths, so only the long wavelengths reach our eyes - then, achromatic colours happen bc light is reflected equally across the spectrum - individual objects don't reflect a single wavelength of light -> ie., lettuce nad tomato reflectance curve. Both veggies reflect a range of wavel. but each selectively reflects more light in one part of the spectrum (tomatoes reflect long wavelengths of light into our eyes, whereas lettuce reflects medium wavelengths - pieces of paper that are achromatic have a flat reflectance curve = equal reflectance across the spectrum

Describe dichromatism?

- missing one of three cone pigments, so can see some colours - cannot distinguish as many colours as trichromats - need only 2 wavelengths to match any other wavelength in the spectrum 0 using the colour matching procedure can help determine the min number of wavelengths they need to match the other wavelengths - or, they use Ishihara plates - once the person is determined to be colour deficient, we still don't know what colours they see - cannot ask what they see, bc they might say red if we point to a strawberry bc they have learned that strawberries are red, but their perception of red won't be like ours

How do we see the colour of something transparent?

- only some wavelengths pass through the object - so cranberry juice selectively transmits the long wavelength light and appears red, whereas green jello transmits medium wavelength light and appears green - can plot transmission curves, they look similar to reflectance curves, but w/ percent transmission on vert axis

dichromats

A person who has a form of color deficiency. Dichromats can match any wavelength in the spectrum by mixing two other wavelengths.

Island of the Colourblind description?

- orange drink = gray, colour isn't something that would prevent him from drinking something off - he has congenital achromatopsia - red and green is common, but this man sees no colour - he is a monochromat, no cones! - cones are needed for fine detail, so he has poor visual acuity as well, comorbid w/ the vision - must rely on rods, which are designed for night vision, so he sees very well in the dark, but is blinded by bright light - they constantly blink/flutter their eyes -> usually to ensure that they aren't blinded by the bright light - he can only see levels of grey, and guesses at colours -> and he uses top down processing from his knowledge - things are interesting, but he can't see the colours - blue is the colour the sky "has" and means "sad," but he's an achromat and cannot see them - achromat is much more rare than the red/green, and thus is harder to understand - colour is independent from transparency - picks up language, the word for the colour of the shape is "yellow" - dressing is difficult!

Why the sky is coloured blue? L

- our visual system has calibrated to distinguish what is more and less important - in black and white, trying to pick berries is HARD - human visual system has developed/evolved ways of maximizing subjective experiences assoc. w/ highly valuable things, and emphasizing that over things that are less valuable - since our system has evolved to maximize the difference and made it easier for us, and find valuable items, system has calibrated itself such that all of the colours are related to one another - since we calibrated on earth w/ the trees and getting food, whatever happens in the sky, the sky is associated w/ the blue quality - therefore, those that live and eat different things should see things differently - if you look at primates, there is trichromatic vision like ours, whereas in bees, there isn't

What is additional evidence that is leading people to reject the idea of a colour centre?

- rejection of colour centre = favour for idea that colour processing is distributed across a number of different cortical areas that process info about colour and other info types Evidence - opponent herons have been found in many areas of cortex, incl. primary visual receiving area V1, the IT (assoc w/ form perception; and V4, which was originally proposed as the colour centre

How can we tell the difference between black, grey, and white?!

- relates to the overall amoutn of light being reflected from an object - black paper reflects less than 10% of the light that hits it, whereas the white paper reflects more than 80% of the light

What is the evidence for areas in cortex responding to colour and other visual qualities?

- showed pics of irregular objects resembling furry balls while subj brain activity was measured by firm subj had to press buttons o indicate whether two objects presented one after the other were the same or different in different trial blocks, subj were told to make their judgements based on same colour, shape, or texture - brain scans found areas that responded to shape, texture, and colour - there were also areas that responded just to colour, just to shape, or just to texture RESULT: there is an area that shows responding to colour can occur in areas that also respond to other visual qualities - effects of brain damage has shown when brain damage causes chromatopsia, it causes other effects as well, nil prosopagnosia.

What can plotting the strength of the mechanisms on the same graph (inverted) help us do?

- shows that blue opposes yellow, and green opposes red - can use to determine amount of each colour present at any wavelength in the spectrum - curves show that at 450nm, both blue and red mechanisms activated, so we see violet - at 600nm, both red and yellow are activated, so we get orange - no colour that simultaneously activates yellow and blue, or red and green

What are the functions of colour perception?

- signaling functions - natural and human made provides many colour signals that help us ID and classify things: we know a banana is BEST WHEN GREENER, and we know to stop when traffic lights are red - helps facilitate perceptual organization (groupign similar elements tog and segregating them from our backgrounds) - crucial to survival -> difficult time finding fruit if you're colour blind - link between good colour vision and the ability to detect coloured food led to proposal that monkey and human colour vision might have evolved so we could detect food - ie., Knut the science man says that picking berries is difficult, and he has to feel for the berries by the shape - also lets us recognize and ID things we can see easily (ie., when they changed the colour of foods, observers more rapidly and accurately ID the appropriate coloured objects) 0 knowing colours of familiar object might help us recognize them, and recognize natural scenes and perceive the gist of scenes LECTURE: why is the sky blue? - physics define it as due to electromag energy interacting with dif molecules in atmosphere, and wavelengths interact = blue - ehhh, gives us the electromag spectrum's behaviour in the atmosphere - but why is the perception blue??

Describe the two types of opponent neurons in the cortex?

- single opponent: +M - L neuron inc firing to med waves presented to centre of receptive field, dec firing to long wavelengths presented in surround most double opponent neurons have receptive fields wit side by side regions (like simple cortical cells!) - they respond best to med wavelength vertical bar presented to left of receptive field, and long wavelength presented to right - single opponent cells = important to perceive cool run regions - double opponent = perceiving boundaries between colours

Why does mixing blue and yellow = green?

- the blue blob absorbs long wavelength light and reflects short wavelength light, and a bit of medium wavelength light - yellow absorbs short wavelength light, and reflects medium and long wavelength light - 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 (whoa!?) - since medium wavelengths are reflected by both blue and yellow, it will create green - this is called subtractive mixing, bc blue and yellow subtract all of the wavelengths except those with green - both paints reflect some light in green part of spectrum - if our blue paint reflected only short wavelengths and our yellow only medium and long, mixing them would appear black bc they would have little ot no reflection across the spectrum

unilateral dichromat

A person who has dichromatic vision in one eye and trichromatic vision in the other eye. People with this condition (which is extremely rare) have been tested to determine what colors a dichromats perceive by asking them to compare the perceptions they experience with their dichromatic eye and their trichromatic eye.

How did Hering describe the whole colour circle?

- the trip around the circle can be described as a "journey" beginning with one of the primary colours (there is four, red, yellow, green, or blue) and evolves into dif colours as small amount of the next prim colour are added - so from red, you get more yellow, pass through orange and gold hellow, and reach yellow w/o a trace of redness. Then, as you add green we go from sulfur to canary to sap green, and then pure green. Then, by adding blue to green, e get into sea green, sea blue, and then reach a blue w/ any greenness, and then finally adding red to blue takes us back to beginning: blue violet, red violet, purple red, and true red - demos phenom descriptions, and sets the stage for idea that colour vis can be explained based on opposing colour pairs

What is the important things about the principles of neural circuitry for colour coding in the retina?

- their responses are determined by the wavelengths to which receptors respond best, and the arrangement of inhibitory and excitatory synapses - processing therefore has two stages

describe the physio evidence for opponent neurons?

- there was reports of "opponent neurons" in LGN of monkey - recorded (DeValois) from neurons that responded w/ excitatory response to light from one part of spectrum, and an inhibitory response to light from another - theree was a neuron Devalues called, for e.g., a +B-Y neuron bc it increased firing to wavelengths perceived as blue, and dec for wavelengths perceives as yellow - there can be +R-G near, inc firing for red and dec firing for wavelengths perceived as green - later +B-Y were called +S -ML bc +B = firing of short wave cone, and -Y = combined activity of long wavelength cones - +R -G are now called + l -M neurons

What were Hurvich and Jameson's Psychopaths Measures of opponent mechanisms?

- they noted that in cases of colour deficiency, colours tend to drop out IN PAIRS, so people lose the ability to perceive blue and yellow or to perceive red and green - phenomenon of complementary afterimages noted - red and green switch places, and blue and yellow switch places in the afterimage demo - these afterimages, since the colour on the opposite side of the colour circle is perceived = support for opponent process theory - they used method called hue cancellation to determine strength of the blue mechanism and provide quantitative measurements of the strengths of the mechanisms - after measuring how much yellow it took for the 430nm light to lose blue, they made similar measures at other wavelengths and found the blue curve, showing the strength of the blue mechanism reaches a max at 440nm and then dec, at 500nm strength = zero - 500nm light, which looks green, doesn't appear blue at all - did the same thing for yellow mechanism strength by increasing wavelength past 500n and adding blue until it eliminated all perception of yellowness - you get a yellow curve -> yellow mechanism responds to lights between 500-700nm, which max response is about 550nm - for red, shows considerable strength at long AND SHORT wavelengths -> which makes sense bc short wavelength light looks reddish blue - green mechanism cute indicates the mechanism responds to wavelengths between 490 nm and 580nm, w/ max response at 525nm

Describe the basic idea of the opponent process theory?

- three mechanisms proposed, which respond in opposite ways to diff intensities or wavelengths - White (+) beach(-) mechanisms responds positive to white and neg in absence - red Green mechanisms responds + to red and = to green - Yellow blue mech responds + to yellow and - to blue - the more white a light appears, the less black it appears, more red, less green, more yellow, less blue - LECTURE - challenge to imagine a reddish green vs a blueish yellow thing

Describe vision w/ two receptor types?

- two pigments - our first one that absorbs 480 and 600nm, and a second one that absorbs more 600nm light so the intensity that caused pig 1 to gen same response to the two wavelengths would cause pigment 2 to generate a larger response to the 600nm light - responses created by both tog could indicate a difference between the two wavelengths -consider ratios of responses of two pigments to the two wavelengths - 480nm light causes large pigment 1 response, smaller pig. 2 response, and 600nm light is vice versa - ratio of response remain the same for all light intensities -> always 10:2 (pig 1 - pig 2) for 480nm light and 5:10 for 600nm light - so, visual system uses ratio info to ID the wavelength of any light - constancy of ratio info occurs w/ three pigments, the basis of the trichrome theory that colour perception depends on pattern of activity in three receptor mechanisms

How can we explain the physio basis behind colour matching results that led to proposal of the trichrome theory?

- wavelength in one field is matched by adjusting the proportions of three different wavelengths in another fields - interesting, bc lights in the two fields are PHYSICALLY different, but they are perceptually identical = metamerism - the reason our meters look alike is bc they result in same pattern response in the cone receptors - when proportions of 620nm red light and 530nm green light are adjusted so the mix matches a 580nm light, it looks yellow, and the two mixed waves create the same pattern of activity in the receptors as a single 580nm light - green light causes a large response in M receptor, and 620 nm red causes large response in L, so together they result in large response in M and L and a smaller response in S receptor - pattern for yellow, so the same pattern as generated by 580nm light EVEN THOUGH THE LIGHTS ARE PHYS DIFFERENT, THEY RESULT IN IDENTICAL PATTERNS OF PHYSIO RESPONSES SO THEY ARE IDENTICAL AS FAR AS THE BRAIN IS CONCERNED, AND SO = PERCEIVED THE SAME

Describe the illusion w/ the A and B checkerboard where they are the same colour but don't look it?

- we don't have a single account, there is evidence to support trichromatic and opponent process - both are kinda needed to write for colour vision - three dif cone pigments is important, it constrains our vision - a is the same shade as B, even though it don't look it - colour is not a property of the universe, we have things like colour constancy where context messes us up!

Describe the physio evidence of cone responding and colour perception?

- we should be able to determine which colours will be perceived if we know response of each of the receptor mechanisms - we can sketch the relationship between the responses of the 3 receptors and our colour perception Blue is signalled by large response in S receptor, small response in M receptor, and an even smaller response in L receptor Yellow is signalled by a very small response in S receptor, and large response in M and L receptors White = equal activity in all receptors -if we think of wavelengths as causing certain patterns of receptor responding helps us predict which colours should result when we combine lights of dif colours - the patterns of receptor activity show that blue light causes high in S receptor, and yellow light causes high act in M and L, so combing both should stem all three equally = perception of white light

How can opponent responding be created by three receptor types?

- when trichromatic and opponents process theories were first proposed, they were seen as competitive - what was realised was that both theories were supported by physio evidence - each theory provides neural processes that take place in different parts of the visual system - trichromatic theory describes processes in the receptors ini retina, opponent process is more about the processing in the opponent neuron in the lGN - Process: -L-cone sends excitatory inputs to a bipolar cell -M-conesends inhibitory input to cell - creates +L - M (+r -G) cell that that responds w/ excitation to long wave that cause L cone to fire, and w/ inhibition to the the medium wavelengths that cause M cone to fire - +S -ML (or +B -Y) cell also receives inputs from cones - it receives excitatory input from the S cone and an inhibitory input from cell A, which sums inputs fromM and L cones - arrangement makes sense, bc we perceive yellow when both M and L receptors are stemmed - so cell A, which receives inputs from both receptors, causes "yellow" response

Why wasn't Hering's theory widey accepted?

-1. High, main competition was championed by HELMHOLTZ 2. Herings' phenomenological descriptions were not a great competitor w/ Maxwell''s quantitative colour mixing data 3. no neural mechanism known that could respond in opposite ways (at the time)

Describe Newton's famous colour experiment?

-made hole in window shade to let beam of light into the room - if prism 1 was in the bath, the beam of white appearing light was split into the components of the visual spectrum - many people thought this happened was bc the prisms added colour to light, but Newton thought white light was a mixture of diff coloured lights and the prism separated light into ind components - to support himself, newton placed a board in the bath of the dif beams - holes in the board only allowed certain beams to pass through - each beam that went through board then went into second prism - Two important things were noted about the light that passed through second prism 1. second prism didn't change colour appearance of light that passed through it - red beam still looked red. So unlike white light, ind. colours of spectrum aren't mixes of other colours 2. the degree to which beams from each spectrum were bent by second prism was different -> red beams were bent onl a litttle, yellow beams a lil more, and violet the most - CONCL: light in each part of spectrum has dif phys properties and these dif prop give rise to our perception of dif colours - humans perceive in a range of wavelengths, the different wavelengths we can percieve have dif colour phenomenologies

Describe monochromatism

-rare colour blindness hereditary - only occurs in 10 ppl out of 1 million -no functioning cones - see only in shades of lightness 0 actually colour blind, not just deficient - we can experience this by dark adapting our eyes - vision is controlled by rods, so world appears in shades of grey - match any wavelength in spectrum by adjusting intensity of any other wavelength - monochrome needs use one wavelength to match any colour in the spectrum - oor visual acuity - so sensitive to bright lights they have to wear sunglasses - rod system cannot function in bright light so it becomes overloaded

describe a colour solid?

3d space w/ systematically ranged colours HSV colour solid will be described (its the one w/ hue, sat and value) dif hues are arranged around circumference, w/ perceptually similar ones placed next to each other the order of use around cylinder matches order of colour in visible spectrum saturation = more sat colours on outer edge, and less saturated towards enter value = lighter colour at top, darker colours at bottom - its a coordinate system where our perception of any colour can be defined by hue, sat and value allows us to determine how different coloured lights will mix to produce new colours~! - you can see what would happen when you mix blue and yellow by drawing a line connecting yellow and blue hues - any mix of these will fall on the line, w/ location depending on amount of each light to the mix

monochromats

A person who is completely color-blind and therefore sees everything as black, white, or shades of gray. A monochromat can match any wavelength in the spectrum by adjusting the intensity of any other wavelength. Monochromats generally have only one type of functioning receptors, usually rods.

anomalous trichromatism

A person who needs to mix a minimum of three wavelengths to match any other wavelength in the spectrum but mixes these wavelengths in different proportions than a trichromat

trichromats

A person with normal color vision. Trichromats can match any wavelength in the spectrum by mixing three other wavelengths in various proportions.

ratio principle

A principle stating that two areas that reflect different amounts of light will have the same perceived lightness if the ratios of their intensities to the intensities of their surroundings are the same.

colour solid

A solid in which colours are arranged in an orderly way based on hue, saturation, and value

HSV colour solid

A solid in which colours are arranged in an orderly way based on their hue, saturation, and value

partial colour constancy

A type of colour constancy that occurs when changing an object's illumination causes a change in perception of the object's hue, but less change than would be expected based on the change in the wavelengths of light reaching the eye In complete colour constancy, changing an object's illumination causes no change in object's hue

complementary afterimages

An afterimage that is on the opposite side of the color circle from the inducing color

chromatic adaptation

Exposure to light in a specific part of the visible spectrum. This adaptation can cause a decrease in sensitivity to light from the area of the spectrum that was presented during adaptation.

hue cancellation

Procedure in which a subject is shown a monochromatic reference light and is asked to remove, or cancel one of the colors in the reference light by adding a second wavelength. This procedure was used by Hurvich and Jameson in their research on opponent-process theory.

cerebral achromatopsia

a loss of color vision caused by damage to the cortex

reflectance curves

a plot showing the percentage of light reflected from an object versus wavelength

colour matching

a procedure in which observers are asked to match the colour in one field by mixing two or more lights in another field

colour constancy

the effect in which the perception of an object's hue remains constant even when the wavelength distribution of the illumination is changed Partial colour constancy occurs when our perception of hue changes a little when the illumination changes, though not as much as we might expect from change in the wavelengths of light reaching the eye