Colour lights Flashcards
(22 cards)
Sensory signals in colour vision
Cones (S - blue, M - green, L - red). Colours should not be used to describe the physical properties of cones.
Where are opponent signals found?
Ganglion cells and LGN
What are the chromatically opponent pathways?
- S-opponent pathway (ancient): S-(L+M)
- L/M-opponent pathway (recent): L-M
Which cones are coded in which chromosomes?
L- and M-cones spectral sensitivities are coded on X chromosome.
S-cone is coded on chromosome 7.
What are the genetic causes of colour anomalies?
Unequal crossing over of genes produces missing or hybrid genes (during meiosis of the X chromasome).
Results of unequal crossing over of genes.
- Missing genes: a strand of DNA could be missing M- or L-cone gene.
- Hybrid genes: a strand of DNA could have hybrid L or M genes.
Deuteranope
Missing M-cone (X chromosome only has gene for M-cone).
Protanope
Missing L-cone (X chromosome only has gene for L-cone).
Anonymous trichromat
An individual with 3 different types of cones, but the genes for one of the cones is a hybrid genes (eg. hybrid L and normal M in X chromosome).
Dichromat
An individual with only 2 types of cones.
Anomalous trichromat colour matching
An anomalous trichromat looking at the colour mixture done by a normal trichromat would probably not see a match between the target and the mixture.
Anomalous trichromats can distinguish colours that normal trichromats can’t (and vice versa).
Boston et al. (2005) unique dimensions of colour vision for deuteranomalous indivuals
Simulated how different colour spectra would look on the opponent pathways of a normal trichromat vs. an anomalous trichromat (L’/(L’+L) - S/(L’+L)).
2 sets of spectra that would look similar to a normal trichromat look distinct to an anomalous trichromat.
Tetrachromat
An individual with 4 cone classes
Ingredients of tetrachromacy
- Spectral placement of hybrid placement nicely in the middle of M- and L- cone.
- Enough relative numbers of each cone type.
- Mosaic layout of each cone.
- Post-receptoral neural wiring that can make use of the extra cone type (eg. ganglion cells that can compare L’ and L’+L).
Genotype of a tetrachromat
Must be an XX individual.
One normal X chromasome (normal L and M).
One X chromasome with a hybrid gene (normal L and hybrid M; normal M and hybrid L).
Jordan et al. (2010) how did they try to find tetrachromats?
- Recruited the mothers of colourblind boys.
- Used a modified colour-matching task in which observers had to pick the odd-one-out.
Jordan et al. (2010) what was the task to find tetrachromats?
Pick the odd-one-out from mixtures of red and green colours vs. a pure yellow light.
Mixtures should be indistinguishable from yellow for a normal trichromat, but anomalous individuals with an extra cone-spectral sensitivity should be able to discriminate some of the colours from the target yellow.
The tetrachromat was able to make no errors and had the fastest response times (compared to controls).
John Sadowski (2013) aftereffects
A black and white image of a castle looks coloured after adapting to the complementary colours previously (happens at both cone-level and opponent pathways).
Does adaptation change colour matches? Imagine that a metameric match is made between the test light on the left and the mixture on the right and then you adapt to different lights (top vs. bottom).
The previous match will not look the same as before but it still matches.
This is because metamers match since cone signals match (so colour adaptation will be the same for both the test light and the mixture of 3 lights). The spectral distribution of the single-wavelength light is narrow, but the cone responses are a 3D vector.
What is the evidence for Hering’s (1878) opponent process theory?
Behavioural evidence for the special status of ‘unique hues’ (red, green, yellow, blue) is weak.
What is Hering’s (1878) opponent process theory?
An attempt to explain colour appearance.
- Every colour is described by the extent to which it appears red vs. green, yellow vs. blue, and black vs. white.
- These distinctions are encoded by innate and discrete neurobiological processes.
What is the link between Hering’s (1878) opponent process theory and physiological opponent channels?
Hering’s opponent colours do not align with the chromatic tuning of cells in the visual pathway.
