5. Colour vision Flashcards
What is the trichromatic theory of vision and who came up with it?
All colour sensations are produced by the activity of just three retinal photoreceptor types
- Thomas Young
Who demonstrated that clear white light was composed of seven visible colours?
Isaac Newton
Long wave sensitive (L) receptors respond to
Red colour
Medium wave sensitive (M) receptors respond to
Green colour
Short wave sensitive (S) receptors respond to
Blue colour
What % of cones in the retina are S cones?
~10%
The S-cone system is not involved in seeing much of…
Form and motion
Y chromosomes are from…
the father
X chromosomes are from…
the mother
How is colour blindness generally caused?
By missing or abnormal opsin genes.
- Almost always the L and M are the genes involved
____ genes are on the ___ chromosome, therefore, ____ are affected by colour blindness much more frequently
- Opsin
- X
- Men
Men have no ‘backup’ gene to rescue them if something goes wrong (Because Y-chromosome does not have an opsin gene but the X-chromosome does. Men have YX chromosomes, whereas, women have XX - 2 copies)
What do the opponent mechanisms in vision compute?
The difference (L-M) and sum (L+M) of Red and Green cones.
What is the ‘correlation’ between L and M cone responses?
0.99
What does the difference between M and L cones determine?
Whether the colour is Red or Green
What does the sum of M and L cones determine?
Whether the colour is Dark or Light
How is the opponent ‘blue/yellow’ axis calculated?
S-(L+M)
Which cones are affected in Protanopic (protoanomalous) colour blindness?
L cones (Red)
- results in damaged red/green system
- blue/yellow remains intact
Which cones are affected in Deuteranopic (deuteranomalous) colour blindness?
M cones (Green)
- results in damaged red/green system
- blue/yellow remains intact
Which cones are affected in Tritanopic (Tritanomalous) colour blindness?
S cones (Blue)
- results in damaged blue/yellow system
- red/green remains intact
Many animals only have two types of photo receptors:(___) cones
Allowing them to distinguish…
L & S cones
- Luminance channel (light/dark)
- One colour channel (blue/yellow)
the rarest (<1 in 1000) type of colour blindness
Most common type of colour blindness in humans is:
- ‘Anomalous trichromacy’ where…
Don’t confuse with Tritanopia which is the rarest (<1 in 1000)
Discrimination is poorer along red/green axis
The second most common type of colour blindness in humans is:
‘Dichromacy’
Where L or M cones are absent
- Protanopic (L - red)
or - Deuteranopic (M - green)
When did trichromacy evolve?
- Our primate ancestors were dichromats
L cone gene duplication occurred ~40 million years ago
What are ‘Ishihara plates’?
Test for colour blindness
- circles with smaller circles forming
- red/green patterns are masked by random luminance noise (so that luminance cannot be used as a cue for determining what number is drawn in one of the colours)
What is the Cambridge colour test?
It is an adaptation of the ‘Ishihara plates’ tests where a number is displayed.
In the Cambridge version, ‘C’ shaped figures are displayed on the plates, with the ‘C’s oriented in various directions (left, right, up or down) instead of numbers
What are the advantages of the Cambridge colour test?
- Can find ‘thresholds’ rapidly
- Can be adapted for use by animals
Which gene did John Dalton was found to lack following post mortem gene analysis of his eyeballs which were preserved?
Found to lack an M opsin gene
Primate colour vision:
- In primates of which world are some females trichromatic?
New world (South American)
- only some females have two types (alleles) of L cones (one able to be used as M cone)
In which world monkeys is trichromacy always present?
old
- all animals (X chromosomes have a gene duplication of an L-cone pigment to generate an M-cone pigment)
What happens when an opsin gene is inserted into the retina of a new world monkey?
The monkey becomes trichromatic following genetic engineering
- passes colour test following presentation only in parts of the retina that were altered
Why did trichromatic primate vision evolve?
Likely to support ‘frugivory’:
- The eating of fruits
Identifying edible foliage:
- lighter leafs are fresher
Retinal side closer to the nose is called:
Nasal retina
Retinal side closer to the outer side of the face is called:
Temporal retina
What is chromatic contrast?
Colour
What is achromatic contrast?
Luminance
Colour vision:
The Ventral Stream seems to be concerned with…
Object identity and ‘form’. (WHAT)
- The ventral stream has a strong representation of the fovea and a strong response to colour
Colour vision:
The Dorsal Stream seems to compute…
Action and location (WHERE)
- Dorsal stream regions respond WEAKLY to isoluminant colour
