5. Colour vision

Question 1

What is the trichromatic theory of vision and who came up with it?

Answer 1

All colour sensations are produced by the activity of just three retinal photoreceptor types

• Thomas Young

Question 2

Who demonstrated that clear white light was composed of seven visible colours?

Answer 2

Isaac Newton

Question 3

Long wave sensitive (L) receptors respond to

Answer 3

Red colour

Question 4

Medium wave sensitive (M) receptors respond to

Answer 4

Green colour

Question 5

Short wave sensitive (S) receptors respond to

Answer 5

Blue colour

Question 6

What % of cones in the retina are S cones?

Answer 6

~10%

Question 7

The S-cone system is not involved in seeing much of…

Answer 7

Form and motion

Question 8

Y chromosomes are from…

Answer 8

the father

Question 9

X chromosomes are from…

Answer 9

the mother

Question 10

How is colour blindness generally caused?

Answer 10

By missing or abnormal opsin genes.

• Almost always the L and M are the genes involved

Question 11

____ genes are on the ___ chromosome, therefore, ____ are affected by colour blindness much more frequently

Answer 11

• 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)

Question 12

What do the opponent mechanisms in vision compute?

Answer 12

The difference (L-M) and sum (L+M) of Red and Green cones.

Question 13

What is the ‘correlation’ between L and M cone responses?

Answer 13

0.99

Question 14

What does the difference between M and L cones determine?

Answer 14

Whether the colour is Red or Green

Question 15

What does the sum of M and L cones determine?

Answer 15

Whether the colour is Dark or Light

Question 16

How is the opponent ‘blue/yellow’ axis calculated?

Answer 16

S-(L+M)

Question 17

Which cones are affected in Protanopic (protoanomalous) colour blindness?

Answer 17

L cones (Red)

• results in damaged red/green system
• blue/yellow remains intact

Question 18

Which cones are affected in Deuteranopic (deuteranomalous) colour blindness?

Answer 18

M cones (Green)

• results in damaged red/green system
• blue/yellow remains intact

Question 19

Which cones are affected in Tritanopic (Tritanomalous) colour blindness?

Answer 19

S cones (Blue)

• results in damaged blue/yellow system
• red/green remains intact

Question 20

Many animals only have two types of photo receptors:(___) cones

Allowing them to distinguish…

Answer 20

L & S cones

• Luminance channel (light/dark)
• One colour channel (blue/yellow)

the rarest (<1 in 1000) type of colour blindness

Question 21

Most common type of colour blindness in humans is:

• ‘Anomalous trichromacy’ where…

Don’t confuse with Tritanopia which is the rarest (<1 in 1000)

Answer 21

Discrimination is poorer along red/green axis

Question 22

The second most common type of colour blindness in humans is:

‘Dichromacy’

Answer 22

Where L or M cones are absent

• Protanopic (L - red)
  or
• Deuteranopic (M - green)

Question 23

When did trichromacy evolve?

• Our primate ancestors were dichromats

Answer 23

L cone gene duplication occurred ~40 million years ago

Question 24

What are ‘Ishihara plates’?

Answer 24

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)

Question 25

What is the Cambridge colour test?

Answer 25

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

Question 26

What are the advantages of the Cambridge colour test?

Answer 26

• Can find ‘thresholds’ rapidly

- Can be adapted for use by animals

Question 27

Which gene did John Dalton was found to lack following post mortem gene analysis of his eyeballs which were preserved?

Answer 27

Found to lack an M opsin gene

Question 28

Primate colour vision:

• In primates of which world are some females trichromatic?

Answer 28

New world (South American)

• only some females have two types (alleles) of L cones (one able to be used as M cone)

Question 29

In which world monkeys is trichromacy always present?

Answer 29

old

• all animals (X chromosomes have a gene duplication of an L-cone pigment to generate an M-cone pigment)

Question 30

What happens when an opsin gene is inserted into the retina of a new world monkey?

Answer 30

The monkey becomes trichromatic following genetic engineering

• passes colour test following presentation only in parts of the retina that were altered

Question 31

Why did trichromatic primate vision evolve?

Answer 31

Likely to support ‘frugivory’:

• The eating of fruits

Identifying edible foliage:

• lighter leafs are fresher

Question 32

Retinal side closer to the nose is called:

Answer 32

Nasal retina

Question 33

Retinal side closer to the outer side of the face is called:

Answer 33

Temporal retina

Question 34

What is chromatic contrast?

Answer 34

Colour

Question 35

What is achromatic contrast?

Answer 35

Luminance

Question 36

Colour vision:

The Ventral Stream seems to be concerned with…

Answer 36

Object identity and ‘form’. (WHAT)

• The ventral stream has a strong representation of the fovea and a strong response to colour

Question 37

Colour vision:

The Dorsal Stream seems to compute…

Answer 37

Action and location (WHERE)

• Dorsal stream regions respond WEAKLY to isoluminant colour