Lecture 1 and 2 - Colour vision Flashcards
What are the peak wavelength sensitivities of long, middle and short wavelength cones?
long - 560nm
middle - 530nm
short - 430nm
opsins are expressed in the…
outer cone segments and are bound to a chromophore
Spectral sensitivities of cones are determined by…
Opsins
Complete the sentence:
Small changes in the____________ __ _______ _______ that make up opsins can significantly shift the peak of the absorption spectrum
sequence of amino acids
Where are the genes that encode the L- and M- cones opsins found?
on the X chromosome at Xq28
Where are the genes that encode the S-cone opsins found?
on chromosome 7 (7q32)
The colour appearance of an object depends on:
1) The colour properties of the light source
2) The absorption/reflection properties of the object
3) The colour processing of the eye and brain
Rushton’s Principle of Univariance states that:
The effect of any photon of light absorbed by a photopigment is independent of its wavelength
(therefore lights of different spectral distributions will therefore appear identical if they produce the same absorptions in the three photopigments)
What are the three colour processing pathways
1) Luminance pathway where L and M cones are combined (L+M) (Black and white)
2) Red-Green pathway where L and M cones are subtracted (L-M)
3) Blue-Yellow pathway (S-(L+M))
The combination of cone signals (luminance, red-green and blue-yellow) takes place where
within the retina via different neural circuits leading to the 3 distinct pathways known as magno-, parvo-, konio-cellular pathways
Cone opponent pathways from retina to
cortex:
Magnocellular pathway
- Carries luminance/achromatic information
- Diffuse Bipolar Cells receive input from L + M cones
- Diffuse Bipolars then connect with Parasol Ganglion Cells, the axonal fibres of which transmit information to the lateral geniculate nucleus (LGN)
- and then input to layers 4Ca
Cone opponent pathways from retina to
cortex:
Parvocellular Pathway
- red-green chromatic information from the L and M cones in the fovea
- is transmitted via Midget Bipolar Cells ,
- The midget bipolars then connect with Midget Ganglion Cells the axons of which constitute the first section of the Parvocellular pathway
- input to layer 4Cβ
Cone opponent pathways from retina to
cortex:
Koniocellular Pathway
- S-cones has its own functional pathway whereby signals from the S-cones are fed via S-cone bipolar cells to bi-stratified ganglion cells
- combined inhibitory inputs from L+M are fed to ganglion cells via diffuse bipolars with horizontal cells providing necessary lateral interactions in this circuit.
- Bi-stratified ganglion cell signals are carried to the cytochrome oxidase blobs in layer 2/3 in V1
colour information is sent from V1 to
V2 which consists of 26 -34 parallel, pale, dark thick and dark thin cytochrome oxidase staining stripes perpendicular to V1.
Most of the neurons in the thin stripes are
selective for chromatic stimuli, double opponent and have connections with V4
Extra-striate area V4 forms the next important area in the colour pathway receiving the majority of inputs from V1, V2 (thin and inter-stripe regions) and sends projections to the…
infero-temporal cortex (IT)
what is the pre- frontal cortex important for?
- important in decision making and has strong
reciprocal connections with the anterior inferior temporal (IT) cortex, the final stage of the colour
pathway.
What is the Munsell system
- objective method of measuring and specifying colours
- contains over 1200 colour samples
- each Munsell colour is described by three variables: hue, value and chroma
How is colour represented in the Munsell colour system?
- Hue (dominant wavelength) is defined by different points around a circle.
- The value (lightness scale) is represented vertically.
-The chroma scale (which is equivalent to the amount of colour or saturation) is represented radially emanating out from the centre of the circle. - Minimally saturated (low chroma) colours are towards the centre, highly saturated colours (high chroma) radially further outwards
What is the CIE system of colour specification?
- international standards organisation for the
measurement of colours and lights. - Measurements in this system are based upon trichromatic colour matching characteristics of a standard ‘normal’ observer
- a test light of a certain colour is presented in one half of a bi-partite field and can be matched by a mixture of different amounts of three primary lights (red, green & blue) in the other half of the field.
- When a colour match is made the two halves of the field have different spectral radiation content but appear identical.
What are tristimulus values
- Three values that together are used to describe a colour and are the amounts of three reference colours that can be mixed to give the same visual sensation as the colour considered. Symbol: X, Y, Z. See also chromaticity coordinates.
How can we calculate chromaticity coordinates using X,Y and Z tristimulus values
x=X/X+Y+Z
y=Y/X+Y+Z
z=Z/X+Y+Z
X+Y+Z=1
what is normal colour vision known as
trichromatic colour vision
what is monochromacy
- when there is a single photoreceptor
- monochromats are totally colour blind
- they see changes in colour as changes in brightness
What is Dichromacy
- Two photoreceptors
- better colour vision that monochromats but still limited
- colour can be detected as long as the 2 cone types produce different responses to different wavelengths
what are colours which are physically different but appear the same called
metamers
What are the L, M and S cone opsin genes known as
L-cone opsin genes - OPN1LW
M-cone opsin genes - OPN1MW
S-cone opsin genes - OPN1SW
What is the percentage similarity between nucleotide sequences of L,M & S cones
L and M - 98% identical nucleotide sequence
they are only 40% similar to S-cone gene
How many exons do L and M cone opsin genes have and what is the most important one.
- 6 exons (functional regions of genes that code specific proteins)
- Exon 5 is most important as it encodes the amino acids that are responsible for the main spectral sensitivity differences between the L-and M-cone opsin
What is usually the first gene (upstream gene) in individuals with normal colour vision
The fist gene (most upstream) in the array always encodes an L pigment in individuals with normal colour vision
What controls whether the L or M genes gene is switched on
- A section of DNA on the X chromosome known as the locus control region (LCR).
- it is located upstream of the L and M cone opsin genes and dictates which of the two genes is expressed.
what is tetrachromacy
- referred to as super colour vision
- refers to the possibility that some females may have four cones
- This is due to the fact that OPN1LW and OPN1MW are on the X chromosome and females who are heterozygous (i.e have two different versions of the same gene)
could potentially have two different forms (the longer and shorter peak sensitivity variants) - 2 L cone variants, 1 M cone and 1 S cone
what are the prefixes given to defects characterized by the absence of L-, M- and S- cones
L-cones defects: protan-
M-cone defects: deutan-
S-cone defects: tritan-
what percentage of men do protan and deuatan defects commonly affect
8% to 10% of men in the USA
tritan defects are rare: 1 in 10,000 people
what is the most severe red-green colour deficiency
The dichromacies
what pigments have deuteranopes and protanopes lost
deuteranopes- lost their M pigment
protanopes- lost their L pigment
The milder forms of red-green colour deficiency are…
the anomalous trichromacies
what are the two kinds of anomalous trichromacies
protanomaly and deuteranomaly
Protanomalous trichromats are missing…
- the normal L cone pigment, just like the protanopes.
-However, they do have a form of trichromatic colour vision (as the name implies), but their trichromacy is not based upon normal L, M and S pigments. Instead they have an S cone pigment and 2 M (or M-like) pigments that have a small difference in their spectral peaks
Deuteranomalous trichromacy
- based upon 3 cone photopigments, an S cone plus 2 spectral subtypes of L cones .
- As a basis for 2 spectral types of L pigments, all those individuals with deuteranomaly have at least 2 different genes to encode the L opsins
what does the spectral proximity hypothesis propose?
- proposes that colour discrimination ability depends upon the size of the difference between the absorption spectra of the pigments
- the bigger the differences in the spectral peaks of these two photopigments, this would improve colour vision further
what are the four basic types of colour vision tests?
1) pseudoisochromatic plates - Ishihara and hardy rand rittler
2) hue discrimination tests - Farnsworth D15, Fransworth-munsell 100 hue test, city university test
3) Anomaloscopes - Nagel
4) lantern tests - Holmes-wright
