Topic 5: Colour Flashcards
What reasons COULD cause people to see the dress differently?
- The image is over exposed
- They are looking at it on different screens and angles
- People see the same colour but give them different names
- Some people are colour blind
- Individual differences in colours processing
- Individual differences in experience
- Failure of colour constancy
Do different viewing conditions affect colour perception?
Yes, research has found change in image size and viewing angle affect it
What are all the types of photoreceptors?
Cones:
Blue (S-type)
Green (M-type)
Red (L-type, daylight vision only)
Rods:
One colour (night vision only)
What does each photoreceptor respond to?
Each photoreceptor type responds to a wide range of colours, but most strongly to one particular wavelength (colour)
What do you percieve with :
1. One cone type
2. Two cone types
3. Three cone types
- We can ‘see’ a wide range of colours, but cannot discriminate between them
- Can discriminate between those two colours, but not others. e.g.: blue and yellow cones mean you can discriminate between blue and yellow, but not green-yellow-orange-red
- Discriminate between all colours in the visible spectrum
What are types of colour blindness starting from most common to rare
- Mis-tuned cone type
- Dichromat - two cone types
- Tetrachromats - four cone types
- Monochromat - one cone type
- Rod monochromats - no cones at all
What is the most common type of colour blindness and why is it more common in men
Mis-tuned green cone. Because the red-green cone colour genes are carried on the X chromosome, red-green colour blindness is more common in men than women.
Common colour deficientcies
Protanopia (no red)
Protanomaly (miss-tuned red)
Deuteranopia (no green
Deuteranomaly (miss-tuned green)
Tritanopia (no blue)
What is Cerebral achromatopsia (Cortical colour blindness)
Stroke or similar injury that affects V8: patient can’t see colour at all
Photoreceptors, retina, LGN, V1, all intact
What are the colour pairs
- Red green
- Blue yellow
- Luminance
Colour opponency in retina and LGN
- Red-Green, Blue-Yellow appearance is misleading
- Red-Green is closer to what we would call reddish and greenish orange
- Yellow-blue is closer to lime-violet
The centre-surround properties of retinal ganglion cells provides the mechanism for colour opponency
These cells calculate the ratio of colours and provide edge detection mechanism.
Reminder about inhibition over time and space
Inhibition over time: Colour-after-effects
Inhibition over space: simultaneous contrast illusion
How does the water colour illusion work?
Colour filling in:
Colour centre-surround cells in the retina normally transmit only the colour edges. The cortex must then reconstruct the body colour of object
These images show only edges outlined in colour. They pass through the retina to the cortex, which automatically fills in the gaps with a sense of colour
What is colour constancy?
The colour of the light reflected from objects depends on their colour and the colour of the light that shines on them.
Normally, we don’t notice objects changing colour when the ambient light changes (from dawn to dusk!).
To do this, we must ‘discount’ or ‘ignore’ the light source.
We don’t really know for sure how this is done, but we do know where it’s done (i.e., a sub-section of area V4 also called V8).
Where does colour perception occur in the visual cortex?
Colour sensitive cells in V1 and V2 cluster into regions of cortex that are separate from the rest of the cells.
Colour signals are then passed to a sub-section of area V4 also called area V8.
V8 seems to be responsible for the conscious perception of colour.
