What is colour good for?
- 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
How can colour be useful in food identification?
- Ripe fruit
- Correct leaves
- Harmless or harmful berries
- Poisonous or venomous animals
Give features of what colour is
- 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
What does the wavelength of the light reflected determine?
The hue which is seen
Apart from wavelength what is perceived colour also determined by?
- 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)
What is the Trichromatic theory - Helmholtz?
- 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
What do animals with good nocturnal vision have?
Animals with good nocturnal vision have a reflective layer at the back of the retina – Tapetum lucidum
What shape are rods and cones?
- Rods are long and thin
- Cones are fatter
What do pigments in the cones determine?
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
What is the cone distribution on the retina?
- 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
Compare rods and cones
- 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
What support is there for the trichromatic theory?
- 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
What is the opponent process theory?
- 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
What is the output of the cones combined into?
- The output of the cones is combined into different channels:
- Achromatic system – all three channels, brightness
- Chromatic system: red/green channel, yellow/blue channel
What is there at the level of the retina and what is there at the level of the lateral geniculate nucleus?
- Trichromacy at level of retina
* Opponency at level of lateral geniculate nucleus
What support is there for the opponent process theory?
- Non-existence of certain colours, e.g. bluish-yellow
- Colour confusions in colour blindness (red and green)
- Complementary afterimages
- 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 - Lightness Constancy
- Two identical squares can appear grey and white
- Top down processing due to top down processing of nature of light source
Sum up trichromacy versus operant process?
- Both theories are correct
- Trichromacy at the level of the cones
- Opponent processes at the level of LGN and cortical cells
Where does the LGN lie?
On the brainstem
What are the different types of colour blindness?
- 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)
What is dichromatism?
Missing cones
What are the different types of anopias?
- 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
What is true colour blindness?
Achromatopsia
How does colour blindness support both colour vision theories?
- 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.
What are human tetrachromats?
- Some very rare humans (predominantly female) have 4 pigment cone types
- Can detect variations in hue that we normally can not
-
Introduction to sensation and perception26
-
The First Steps in Visual Perception25
-
Neural Processing and Perception15
-
Visual Cortex30
-
Form Perception28
-
Depth Perception26
-
Colour Perception24
-
Motion Perception21
-
Sound and the ear17
-
Touch and Pain32
-
Chemical senses and multisensory perception25
