Colour Perception

Question 1

What is colour good for?

Answer 1

• Scene segmentation: variations in colour often signal object boundaries
• Camouflage: animals use this fact to disguise themselves by colour markings
• Perceptual organisation: our visual system uses colour to group elements in a scene

Question 2

How can colour be useful in food identification?

Answer 2

• Ripe fruit
• Correct leaves
• Harmless or harmful berries
• Poisonous or venomous animals

Question 3

Give features of what colour is

Answer 3

• Visible light forms a narrow band of frequencies in the electromagnetic spectrum
• Within this band, different frequencies (or wavelengths) have different hues, ranging from red (long wavelength light) to violet (short wavelength light)
• Whole spectrum of visible colours covers just 400nm in terms of difference in wavelength – very sensitive in the band we can see
• Different objects absorb and reflect different wavelengths of light
• This gives them their ‘colour’
• The colour also depends on the light source

Question 4

What does the wavelength of the light reflected determine?

Answer 4

The hue which is seen

Question 5

Apart from wavelength what is perceived colour also determined by?

Answer 5

• Intensity of the reflected light (how bright it is)

- The saturation of the colour (how much white light is mixed in with the pure hue)

Question 6

What is the Trichromatic theory - Helmholtz?

Answer 6

• There are three receptor types and their combination of responses account for all colours
• Blue-sensitive cones maximally responsive to short wavelengths (S cones)
• Green sensitive cones maximally responsive to medium wavelengths (M cones)
• Red sensitive cones maximally responsive to long wavelengths (L cones)
• They’re no exactly red, green and blue so we now refer to them and S, M and L cones (the cones differentiate more between red and two shades of green)
• Short – around 419 nm
• Medium – around 521 nm
• Long – around 558 nm
• the colour you see is determined by the relative levels of activity in the three sorts of receptors

Question 7

What do animals with good nocturnal vision have?

Answer 7

Animals with good nocturnal vision have a reflective layer at the back of the retina – Tapetum lucidum

Question 8

What shape are rods and cones?

Answer 8

• Rods are long and thin

- Cones are fatter

Question 9

What do pigments in the cones determine?

Answer 9

Pigments in the cones determine which wavelengths they respond to best – when the pigment is broken down it changes permeability of cell and action potential is sent

Question 10

What is the cone distribution on the retina?

Answer 10

• There is a concentration of cones in an area called the fovea – a small pit in the (edge to edge: retina 32mm, fovea 1.5mm)
• Eye movements – image of object of interest falls on the fovea
• Fovea has the highest density of receptors – best acuity (1%-50%)
• No rods in the centre of the fovea

Question 11

Compare rods and cones

Answer 11

• Rods very sensitive, don’t need much intensity to be activated
• Cones need good lighting conditions to be activated
• Rods sensitive to intensity (black-white) only
• Cones responsive to different wavelengths – 3 types
• Rods found all over retina except centre of fovea
• Cones concentrated in fovea
• 20 times more rods than cones across whole retina

Question 12

What support is there for the trichromatic theory?

Answer 12

• Three primary colours combine to produce all possible colours
• Three forms of dichromatism (colour blindness)
• A mixture of green and red light produces same perception of yellow colour as monochromatic yellow light (metamerism) – we can’t tell the difference
• Afterimages: when you stare at a blue circle you see a yellow circle after: only red and green cones can respond to yellow. Blue cones/ channel fatigued/ adapted

Question 13

What is the opponent process theory?

Answer 13

• Output from different receptors can only signal i.e. red or green or blue or yellow
• 3 processes which are opponent in nature:
• Red-green
• Yellow-blue
• Black-white

Question 14

What is the output of the cones combined into?

Answer 14

• The output of the cones is combined into different channels:
• Achromatic system – all three channels, brightness
• Chromatic system: red/green channel, yellow/blue channel

Question 15

What is there at the level of the retina and what is there at the level of the lateral geniculate nucleus?

Answer 15

• Trichromacy at level of retina

* Opponency at level of lateral geniculate nucleus

Question 16

What support is there for the opponent process theory?

Answer 16

1. Non-existence of certain colours, e.g. bluish-yellow
2. Colour confusions in colour blindness (red and green)
3. Complementary afterimages
4. Colour context effects – perceive colours to be different dependent on their context
   - Relates to colour constancy – the brain isn’t just interested in the outputs of different cones, also interested in the nature of the object, uses information about the object in combination with the output of the cones to decide what colour it is
   - Two identical blues seem different in different backgrounds
5. Lightness Constancy
   - Two identical squares can appear grey and white
   - Top down processing due to top down processing of nature of light source

Question 17

Sum up trichromacy versus operant process?

Answer 17

• Both theories are correct
• Trichromacy at the level of the cones
• Opponent processes at the level of LGN and cortical cells

Question 18

Where does the LGN lie?

Answer 18

On the brainstem

Question 19

What are the different types of colour blindness?

Answer 19

• Anopias: insensitive to L, M or S wavelengths of light (missing a type of cone)
• Anomalies: Misalignment of L or M in trichromats (distribution or deficiency)

Question 20

What is dichromatism?

Answer 20

Missing cones

Question 21

What are the different types of anopias?

Answer 21

• Protanopia: L-cone pigment missing: 1.3% M, 0.02% F (need more ‘red’ in ‘red-green’ mixture to match ‘yellow’
• Deuteranopia: M-cone pigment missing: 1.2% M, 0.01% F (need more ‘green’ in ‘red-green’ mixture to match yellow)
• Tritanopia: S-cone pigment missing: 0.001% M, 0.003% F

Question 22

What is true colour blindness?

Answer 22

Achromatopsia

Question 23

How does colour blindness support both colour vision theories?

Answer 23

1. Whole fact of anopia points to 3 cone types

2. Opponent process theory supported by the fact that people who have trouble with red also have trouble with green etc.

Question 24

What are human tetrachromats?

Answer 24

• Some very rare humans (predominantly female) have 4 pigment cone types
• Can detect variations in hue that we normally can not