Theory L5 - Colour Constancy and the Brain

Question 1

What is light?

Answer 1

• SUN emits PHOTONs - which is both PARTICLE and WAVE - it is an electromagnetic wave particle.
• its colour is determined by the wave frequency.

Question 2

What colour is a high frequency wave?

Answer 2

blue / violet

Question 3

What colour is a lower frequency wave?

Answer 3

red / infrared / UV - can’t see. too short.

Question 4

What was Newton’s Prism experiment?

Answer 4

He used a prism to bend white light into different wave lengths - which made a rainbow.

White light contains all kinds of photons - blue, green, red.

Found that light will bend differently according to its wavelength.

Longer waves bend less = red. Super long waves = infrared (we can’t see it)

Question 5

What kind of waves bend less?

Answer 5

Longer waves.

Red.

Question 6

What leads us to have a yellow perceptual experience?

Answer 6

Receiving lots of red and green photons at once.

It mixes to create yellow.

Question 7

How do we see bananas’ yellow colour?

Answer 7

Banana absorbs its pigments (red, orange, green, blue, purple…), but NOT YELLOW!!!!

So the yellow photons bounce back, enters the eye and we see it.

WE SEE WHAT IT DOESN’T ABSORB.

it’s yellow because it absorbs everything except the yellow.

Question 8

Describe how the eye sees light?

Answer 8

Lighter enters the eye > interacts w/ photoreceptor cells > sends signals to ganglion cells > axons leave the eye

Question 9

What are the three layers of the retina?

Answer 9

• Ganglion cells
• Bipolar Cells
• Photoreceptors - rods & cones

Question 10

What is transduction?

Answer 10

This happens in the photoreceptor cells.
(Rods & Cones)

Refers to the change of physical energy of light into a biological signal

Question 11

What are cones?

Answer 11

• Type of photoreceptor cell
• 3 types of Cones
• Blue, Green & Red Cones - 440, 530 AND 560 cones, can be thought of as S, M, L.

they are MOST suitable for the specified wavelength, but can capture a range of other wavelengths. Lots of overlap between the cones.

Question 12

What wavelength is captured by all 3 cones?

Answer 12

450-500

The brain will use info from all 3 cones to register the colour. It uses the degree to which light is absorbed to register colour.

When the blue photons are absorbed more - blue cones more activated than others, then we see blue!!!

Question 13

What is the principle of univariance?

Answer 13

The cone receptors convey info about the quantities of light absorbed, not the actual wavelength of light.

The principle of univariance states that one and the same visual receptor cell can be excited by different combinations of wavelength and intensity, so that the brain cannot know the color of a certain point of the retinal image.

The principle of univariance is the fact that an infinite set of different wavelength–intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make accurate color discriminations by itself.

The brain looks at relative activation between the different cones to register colour.

Question 14

How does mixing paints work?

Answer 14

The appearance of objects is really the photos that are being reflected.

SO by mixing 2 diff colours that ABORB DIFFERENT LIGHT… only 1 type of light is reflected off.

so blue + yellow = green
bc blue light absorbs red, yellow absorbs blue, the result absorbs both, and thus reflects green.

Question 15

What colours are in printers?

Answer 15

Cyan - absorbs red - stimulates M and S cones
Magenta - absorbs green - stimulates Sand L cones
Yellow - absorbs blue. - stimulates M and L cones.

(stimulates the cones of what it reflects)

Question 16

What has Impressionism shown us?

Answer 16

• Renaissance - try to recreate the world as accurately as possible in paintings.
• Impressionists - tried to mix it up. Put colours close together in a blurred way to give a perceptual experience of an intermediate colour that wasn’t there.

red besides green = intermediate perceptual experience of a colour that wasn’t there.

made extremely colourful paintings!

• Monet’s painting of the cathedral - painted it the WAY it was on the retina, not the way it’s perceived

Question 17

What has Pointillism shown us?

Answer 17

If you put dots of colour really close together you get this recreation of a new perception.

You can’t see the different points from a particular distance

Question 18

What did Monet’s painting of the Rouen Cathedral show us?

Answer 18

• Different colours of the cathedral at Noon and Sunset
• At noon - heaps of diff photons - all present at equal probability
• Sunset - availability of blue photons is gone, but a LOT of red photons. Nothing to absorb all the red due to lack of blue

Impressionists found that the objects appeared a lot more red on the retina than they would at high noon - due to the availability of photons.

• we RESCALE the way we see things at night bc we don’t perceptually see the red.

Question 19

What is rescaling?

Answer 19

Due to long term associations, the brain rescales the perceptual experience to the way we learn it looks, rather than what is actually there.

We still perceive objects as having the same colour, irrespective of the lighting, where there could be a lack of the photons

For eg. At sunset, things are much more red, but we don’t perceive it.

Question 20

What are the perceptual qualities of colour?

Answer 20

• Intensity and wavelength = physicists

psychologists: (more interested in perceptual quality)
- Hue - the perceptual experience of the actual wavelength - colour

• Value – lightness/darkness - luminance, intensity, more of black = lower value
• Saturation – how much of the white and colour - low saturation when its a really light/white colour

Question 21

What is light scattering?

Answer 21

Light interactions with small particles in the atmosphere, causing it to scatter.

Question 22

Why is the sky blue?

Answer 22

Blue scatters the most - they hit particles and go different directions!
red and yellow photons go straight to the ground.

Question 23

Why are sunsets red?

Answer 23

Light has to travel a great distance from the horizon, which gives blue photons more opportunities to scatter.

= more red photons available.

Question 24

Why are clouds white?

Answer 24

Clouds are made of water.
When photos hit water, they all scatter - regardless of their wavelength.

When there is all sorts of photons = white.

Question 25

Why did we need different types of films?

Answer 25

Different types of films for cameras were needed because they did not rescale/recalibrate colours.

For outside - film less sensitive to blue photons were needed so that things didn’t seem so blue..(but blue @ noon)

Inside - neutral film could be used because there were more balanced colours inside.

Question 26

What is an example of perceptual rescaling in an experiment?

Answer 26

Rubrix Cube.

When observing the cube in context, the squares look colourful.

When individual squares are removed - they are different colours (e.g. grey).

The context of the cube provides info to the brain to perceptually rescale.

Question 27

What did Edwin Land’s Experiment show?

Answer 27

1. Showed paintings under white light, where a green and grey square was shown. Grey square activated all cones equally (S:100, M:100, L:100). Green preferentially stimulated M cones.
2. Changed the ambient lighting to red - where the green square now activated all cones equally (thus should appear grey), and the grey activated the L cone preferentially. BUT THERE WAS NO CHANGE IN PERCEPTION

Brain bases colours on context!!!! Dissociation between physical and perception!! PERCEPTUAL RESCALING

Question 28

what is Retinex theory?

Answer 28

• Retinal processing alone cannot explain colour perception
• Cortical processing must also play a role - taking in info about the context to rescale.

retina + cortex = Retinex.

Question 29

What have Semir Zeki’s Experiments shown us?

Answer 29

• Repeated Edwin land’s experiment in monkey why recording neurons in V4
• V4 neurons - colour specific firing/activation - high specificity.
• RED neurons that would fire when the stimulus was actual red, would ALSO FIRE when it was PERCEIVED AS RED under diff light!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

code for perceived colour!!!!! not specific wavelength

Question 30

What does Mary the colour scientist reveal?

Answer 30

• Can’t really measure what colour she is perceiving
• she will not be surprised by a blue banana
• she may not perceive the rose as red, according to what colour it is under.
• blue banana = might attribute blue with ‘yellow’ if she knows it’s meant to be yellow. and then perceive other things she knows to be ‘yellow’ as actually ‘blue’

allows for testing things that we can’t actually test.