Lecture 1 and 2 - Colour vision

Question 1

What are the peak wavelength sensitivities of long, middle and short wavelength cones?

Answer 1

long - 560nm
middle - 530nm
short - 430nm

Question 2

opsins are expressed in the…

Answer 2

outer cone segments and are bound to a chromophore

Question 3

Spectral sensitivities of cones are determined by…

Answer 3

Opsins

Question 4

Complete the sentence:
Small changes in the____________ __ _______ _______ that make up opsins can significantly shift the peak of the absorption spectrum

Answer 4

sequence of amino acids

Question 5

Where are the genes that encode the L- and M- cones opsins found?

Answer 5

on the X chromosome at Xq28

Question 6

Where are the genes that encode the S-cone opsins found?

Answer 6

on chromosome 7 (7q32)

Question 7

The colour appearance of an object depends on:

Answer 7

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

Question 8

Rushton’s Principle of Univariance states that:

Answer 8

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)

Question 9

What are the three colour processing pathways

Answer 9

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))

Question 10

The combination of cone signals (luminance, red-green and blue-yellow) takes place where

Answer 10

within the retina via different neural circuits leading to the 3 distinct pathways known as magno-, parvo-, konio-cellular pathways

Question 11

Cone opponent pathways from retina to
cortex:

Magnocellular pathway

Answer 11

• 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

Question 12

Cone opponent pathways from retina to
cortex:

Parvocellular Pathway

Answer 12

• 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

Question 13

Cone opponent pathways from retina to
cortex:

Koniocellular Pathway

Answer 13

• 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

Question 14

colour information is sent from V1 to

Answer 14

V2 which consists of 26 -34 parallel, pale, dark thick and dark thin cytochrome oxidase staining stripes perpendicular to V1.

Question 15

Most of the neurons in the thin stripes are

Answer 15

selective for chromatic stimuli, double opponent and have connections with V4

Question 16

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…

Answer 16

infero-temporal cortex (IT)

Question 17

what is the pre- frontal cortex important for?

Answer 17

• important in decision making and has strong
  reciprocal connections with the anterior inferior temporal (IT) cortex, the final stage of the colour
  pathway.

Question 18

What is the Munsell system

Answer 18

• objective method of measuring and specifying colours
• contains over 1200 colour samples
• each Munsell colour is described by three variables: hue, value and chroma

Question 19

How is colour represented in the Munsell colour system?

Answer 19

• 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

Question 20

What is the CIE system of colour specification?

Answer 20

• 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.

Question 21

What are tristimulus values

Answer 21

• 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.

Question 22

How can we calculate chromaticity coordinates using X,Y and Z tristimulus values

Answer 22

x=X/X+Y+Z
y=Y/X+Y+Z
z=Z/X+Y+Z

X+Y+Z=1

Question 23

what is normal colour vision known as

Answer 23

trichromatic colour vision

Question 24

what is monochromacy

Answer 24

• when there is a single photoreceptor
• monochromats are totally colour blind
• they see changes in colour as changes in brightness

Question 25

What is Dichromacy

Answer 25

• 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

Question 26

what are colours which are physically different but appear the same called

Answer 26

metamers

Question 27

What are the L, M and S cone opsin genes known as

Answer 27

L-cone opsin genes - OPN1LW
M-cone opsin genes - OPN1MW
S-cone opsin genes - OPN1SW

Question 28

What is the percentage similarity between nucleotide sequences of L,M & S cones

Answer 28

L and M - 98% identical nucleotide sequence
they are only 40% similar to S-cone gene

Question 29

How many exons do L and M cone opsin genes have and what is the most important one.

Answer 29

• 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

Question 30

What is usually the first gene (upstream gene) in individuals with normal colour vision

Answer 30

The fist gene (most upstream) in the array always encodes an L pigment in individuals with normal colour vision

Question 31

What controls whether the L or M genes gene is switched on

Answer 31

• 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.

Question 32

what is tetrachromacy

Answer 32

• 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

Question 33

what are the prefixes given to defects characterized by the absence of L-, M- and S- cones

Answer 33

L-cones defects: protan-
M-cone defects: deutan-
S-cone defects: tritan-

Question 34

what percentage of men do protan and deuatan defects commonly affect

Answer 34

8% to 10% of men in the USA

tritan defects are rare: 1 in 10,000 people

Question 35

what is the most severe red-green colour deficiency

Answer 35

The dichromacies

Question 36

what pigments have deuteranopes and protanopes lost

Answer 36

deuteranopes- lost their M pigment
protanopes- lost their L pigment

Question 37

The milder forms of red-green colour deficiency are…

Answer 37

the anomalous trichromacies

Question 38

what are the two kinds of anomalous trichromacies

Answer 38

protanomaly and deuteranomaly

Question 39

Protanomalous trichromats are missing…

Answer 39

• 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

Question 40

Deuteranomalous trichromacy

Answer 40

• 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

Question 41

what does the spectral proximity hypothesis propose?

Answer 41

• 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

Question 42

what are the four basic types of colour vision tests?

Answer 42

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