The Visual System: Colour Perception

Front 1

What are the colours of the prism in order?

Back 1

Red, Orange, Yellow, Green, Blue, Indigo, violet
Richard of york gave battle in vain

Front 2

emission spectrum

Back 2

a plot of the energy content of light at each visible wavelength- sunlight has a continuous emission spectrum

Front 3

Complementary colours; why is sunlight white?

Back 3

any pairs of colours that produces white when mixed together. sunlight is white because it contains all the spectral colours mixed together

Front 4

White is a... what is another example?

Back 4

non spectral colour because no one specific wavelength produces it. purple is also non spectral and can only be made form red and blue

Front 5

What are the additive primary colours?

Back 5

red, green, blue- colour that cannot be generated through the mixture of other colours

Front 6

What are the types of light reflection by opaque objects?

Back 6

1) specular reflection: reflection at the surface, a simple mirror-like reflection that effects all wavelengths in an equal manner. Objects that are glossy display a lot of specular reflection
2) body reflection: light entering object is scattered and hen it emerges out it goes in all directions. this is what gives objects their perceived colours. whichever wavelength is primarily reflected out will be seen. If all lengths are reflected equally, the object will be white

Front 7

Lambertian Surface; reflectance spectrum; additive vs subtractive colour mixing

Back 7

perfectly matte surface; a plot of the amount of reflection as a function of wavelength- the spectra are all continuous and peak at different points; additive is mixing lights subtractive is mixing coloured things like paints

Front 8

The colour signal is dependent on what two things?

Back 8

the plot of the amount of energy contained in the light times the amount reflected as a function of wavelength. (the energy contained in the incident light and the amount as well as wavelength reflected by the object) blue paper will look different in different lighting conditions

Front 9

What are the subjective/subtractive primary colours? Talk about inks/paints

Back 9

cyan, magenta, yellow, the colours from which all others can be made. When you mix dyes inks paints together, they retain their absorption characteristics. So for example, yellow absorbs blue and reflects red and green and cyan absorbs red and reflects green and blue. mixing them will produce green (think about it). black= all of the three together equally

Front 10

Discuss coloured lights on coloured surfaces

Back 10

here we must look at additive and subtractive properties- wavelength of light and absorption of object
For example, a red light on a blue object will appear back because blue object absorbs red and green and reflects nothing. If we look more complicatedly at a secondary colours, a yellow light on a cyan object makes green because yellow= R+G and cyan absorbs red.

Front 11

What coloured object absorbs what? red, blue, yellow, cyan, magenta, white, black

Back 11

red= blue+green
blue= red+green
yellow (made of red and green) = blue)
cyan (made of green+blue)= red
Magenta (made of R+B)= green
white= reflects all
black= absorbs all

Front 12

HSV colour space, modification

Back 12

perceived similarity in colours can be represented by physical distance in a spatial map to produce a colour-space. Hue is represented in a circular manner. saturation radially and value vertically, making a cylinder!! The modification that needs to be made is that as value goes down, it encompasses a smaller range of hue and saturation therefore HSV is best represented by a cone

Front 13

Discuss the 3 dimensions of colour

Back 13

-hue (chromatic quality (colour))- a large number of spectra will represent the same hue
-saturation- (vividness of colour)- grey= a fully desaturated colour, flat profile. a highly saturated colour is one generated by light whose energy is contained within a narrow range of wavelengths
- brightness/ lightness (amount of energy) . 'value' is the perceived intensity. two spectra can produce same hue and saturation but have a different value or intensity

Front 14

Munsell system; CIE

Back 14

similar to HSV in that it is 3 dimensional , EXCEPT it uses colour chips laid out in a specific order, and certain hues like red display a broader saturation range while other like green have a reduced saturation range; International commission on illumination system gets around the problem of the first two models (representing 3d things on a flat surface) by representing colour space by a proportion instead of absolute level

Front 15

trichromatic theory

Back 15

the idea that colour vision is mediated by 3 separate light-detection systems in the eye (Thomas young)

Front 16

Monochromatic lights

Back 16

all of the energy is contained in a single wavelength (like RGB which are represented by 670, 560 and 440 respectively) and therefore has the physical characteristics to produce a highly saturated hue.

Front 17

what do people do in colour matching experiments? What are Metamers?

Back 17

subjects are told to to combine 3 primaries in any proportion in the 'matching field' in order to match the colour in the 'test field'; what we often end up with in these tests. any pair of lights or surfaces that are perceptually identical but are physically different in terms of their energy distributions across the wavelengths. they can be produced by any TRIO of primates

Front 18

Young- Helmholtz theory of trichromacy (different from just the Young Version)

Back 18

- the notion that colour sensations are generated by 3 different fibres in our eyes that wavelengths stimulate at different rates - not perfectly true obviously....

Front 19

colour afterimage, what did this propagate?

Back 19

- like staring at that US flag in the textbook- previously green areas appear red and yellow appear blue
- the trichromacy theory couldn't account for this- why does mixing yellow and blue light not produce a yellowish blue but rather a grey/ white?
- the opponent colour theory stated that yellow is like the fourth primary and that red and green, and yellow and blue, are not simultaneously processed by the visual system (they are in structured pairs)

Front 20

noctrnal bottleneck hypothesis

Back 20

the development of full colour vision amount mammals and notably primates came due to survival challenges for those species that made a transition to daylight life

Front 21

The Principal of Univariance

Back 21

essentially, each photon in a cone (S,L,M cones) produces exactly the same response regardless of wavelength and only the efficiency with which photons are captured is wavelength-dependent in the S,L, M cones. Neural output is proportionate to the amount of light absorbed in each cone. In the colour matching experiment, the metameric matches match occurs because both continuous (test) and discrete (match) spectra produce the same cone output signals! cones are colourblind!

Front 22

Protanopia ; Deuteranopia, Tritanopia

Back 22

dichromacy due to the presence of only S and M cones; only have S and L cones; only have L and M cones.
We call them colour-blind, but they are not. Loss of M or L cones results in loss or deficit in Red-green colour perception, whereas those with loss of S cant perceive yellow-blue differences

Front 23

Why trichromacy??

Back 23

why do humans have trichromacy (3 different types of cone photoreceptors)? Monochromacy is like total colour blindness, dichromacy is a limited version of colour vision and both of those would have huge problems with colour confusions. In monochromic, a colour must have the three primary colours in the exact right combination because of phodopsins finely tuned absorption spectrum . Any more than three would impact our spatial resolution

Front 24

Chromatic cancellation

Back 24

based on the opponent colour theory that certain colours cancel each other out (R+G and B+Y). For example, if you take orange which is made from yellow and red and keep adding green, the red component will eventually be lost and it will become yellow.

Front 25

neurobiological basis for the opponent theory? what are colour-opponent neurons?

Back 25

4 types of colour selective cells exist in the LGN:
- excitation by green but inhibition by red (G+/R-)
- excitation by red but inhibition by green (R+/G-)
- excitation by yellow but inhibition by blue (Y+/B-)
- excitation by blue but inhibition by hello (B+/Y-)
these neurons with the centre-surround profile but for colours. so for some, yellow will be in centre as excitatory with green around it others the opposite, etc. The extension of this is that the perception of red for example is largely the result of the R+G- neurons

Front 26

Adaptation of colour-opponent neurons

Back 26

after a long time of stimulation to a particular, they respond slower and so the opponent colour will briefly dominate. This causes the colour afterimage thing!

Front 27

Dual Process theory

Back 27

the idea that both the trichromatic and colour-opponent theories are both correct- the initial mechanisms for capturing spectral info occurs through light absorption in 3 cone types, however then the opponent theory takes over. this change happens in the retina by different inputs of cones upon ganglions.

Front 28

luminance ; what are the two channels in the retina?

Back 28

intensity of emitted/reflected source, encoded by ON/OFF and OFF/ON cells; chromatic channel - containing the colour-opponent neurons and achromatic/luminance channel-with the ON/OFF and OFF/ON cells (mostly driven by M and L cones)- these two channels are thus responsible for hue and brightness

Front 29

Which retinal ganglion cells are responsible for which colour-opponent and ON/OFF cells?.... so in the LGN....

Back 29

parasol ganglions encode luminance info- therefore ON/OFF and OFF/ON cells
Midget ganglion cells are for red-green processing, so R/G and G/R cells
Bistratified ganglion cells are responsible for blue-yellow colour processing, so B/Y and Y/B cells
SO magnocellular layer is primarily for luminance info from the achromatic channel and parvocellular layer is chromatic and for R/G processing. The Koniocellular layer takes inputs from bistratified cells and deals with Y/B colour

Front 30

magnocellular, parvocellular and koniocellular input to V1...

Back 30

mango (luminance info) goes to upper 4C, Parvocellular to lower 4C and Koniocellular to layer 3

Front 31

isoluminance, how to opponent neurons react?

Back 31

the luminance is constant across the stimulus and background but the colour varies (object and its surroundings reflect same amount of light but differ in colour). under these conditions, like isoluminace colour gratings, colour opponent neurons are silenced because the coloured light falling upon the inhibitory surround cancels the excitatory response from the centre How are we able to see them then?? double opponent neurons

Front 32

double opponent neurons (4)

Back 32

-neurons that are both spatially and chromatically opponent!!
-for example, one type has a receptive field centre excited by red but inhibited by green, and the surround is opposite.
-this is how we see isoluminant colours
- seen first only in V1 (high concentration in cytochrome oxidase blobs)

Front 33

CO strips in V2 (as opposed to the blobs in area V1)

Back 33

- there are thick and thin as well as pale
- V1 blobs project mostly to the thin and thick stripes, whereas inter blobs go to the pale stripes
- it is thought these stripes deal with higher level colour processing
- pale stripes have been implicated in form processing

Front 34

Difference threshold/ discrimination for hues? saturation? Brightness?

Back 34

less than 2nm change in wavelength generally, greater change is needed at high and low ends of spectrum; for saturation, different colours have different saturating powers. the threshold for saturation discrimination is at yellow light, meaning a greater amount of yellow light must be added to white to make it coloured than red and blue; For brightness, K value is about 2%

Front 35

colour contrast

Back 35

self explanatory, we rarely look at things isolated, everything is in contrast, however contrast is greatest when it is surrounded by its complimentary colour

Front 36

colour consistency (what is it, what is its constraints, why does it happen?), retinex theory

Back 36

coloured surfaces appear relatively unchanged despite changes in lighting conditions.
- doesn't happen in aperture conditions (isolated objects), only for scenes. happens because its all relative to context, or because of prior knowledge of typical object colours
retinex theory says that context alone doesn't account for the consistency but also that higher-levels play a role because we have existing info about the colour of things= Both retina + cortex!

Front 37

Bezold-Brucke Effect; abney effect

Back 37

increasing the intensity of coloured lights with wavelengths over 510nm causes the hue to shift more yellow whereas under 510 it shifts more blue; how hue change accompanies changes in saturation as well

Front 38

Hue and duration /size

Back 38

hue suffers with small objects and with durations of less than 200 ms

Front 39

McCollough Aftereffect (continent aftereffect)

Back 39

the aftereffect dependent on two different stimulus attributes like orientation and colour, LONG LASTING! much more so that the colour afterimage 4-5 days!

Front 40

colour vision is... (2)

Back 40

-the ability to discriminate
different lights only on the basis of their
spectral content
-is an entirely psychological property
of our visual experience and not an inherent
property of light itself or objects that reflect
light.

Front 41

colours with different wavelengths look identical if_____. colours with different wavelength distributions look different if_____.

Back 41

 Colours with different wavelength distributions look
identical if they produce the same ratio of absorptions in the L, M and S cone types
 Colours with different wavelength distributions look
different if they produce different absorptions and hence different patterns of activity in the three cone types

Front 42

purkinje shift

Back 42

 Going from photopic to scotopic conditions
makes green hues appear brighter than yellow
dimmer.