VF - Colour Vision 1 - Week 8

Question 1

What is the true organ of vision?

Answer 1

The brain, because the eye only detects the light, but the brain is what processes and sees.

Question 2

Explain Newton’s experimentum crucis, the setup he used, and the point he made.

Answer 2

He split white light into a rainbow with a prism, then isolated a single wavelength of light, ie red.
Then passed this light through a prism, to show the prism didnt inherently change the wavelength of light.

Question 3

Define the following:
Sunlight
Skylight
Daylight

Answer 3

Sunlight - light directly from the sun
Skylight - light indirectly coming from the sky
Daylight - sunlight + skylight

Question 4

Given the changes in illumination, why doesnt the colour appearance of objects change?

Answer 4

This is due to colour constancy.

Question 5

Define metamers.

Answer 5

Two very different spectrum of light that appear to be the same.

Question 6

Define subtractive colour mixing with light spectrums.

Answer 6

When two light spectrums overlap, in subtractive mixing, keep the overlap, but remove the non-overlapping regions.

Question 7

Define additive colour mixing with light spectrums.

Answer 7

When two light spectra overlap, simple add the intensities together.

Question 8

Explain the young-helmholtz trichromatic theory.

Answer 8

They theorised that it is impossible for each retinal cell to contain an infinite number of particles, one for each every possible wavelength of light.
Instead, it would be limited to the principle colours red, yellow, and blue.

Question 9

Define the opsin and chromophore.

Answer 9

Opsin - protein in which the chromophore sits in. 7 transmembrane a-helices.
Chromophore - 11-cis-retinal within the opsin.

Question 10

Are differences in photopigments due to differences in chromophores, opsins, or both?

Answer 10

Opsins only (amino acid sequence differs).

Question 11

Do photopigment absorption curves match photoreceptor responses in living, intact retinas?

Answer 11

No.

Question 12

Define photopigment photosensitivity.

Answer 12

Probability that a photopigment will photoisomerise depending on the wavelength of light.

Question 13

Describe a colour matching experiment.

Answer 13

A test wavelength is matched by a mixture of three primary lights

Question 14

Who performs colour mmatching, and what is the purpose of colour matching?

Answer 14

Normal trichromats and colourblind dichromats.

Difference between the two populations allows measurement of L and M cones uncontaminated by other cone responses.

Question 15

Summarise the principle of univariance.

Answer 15

Photoreceptors probvide change of its dark current to the visual system but photoisomerisation depends on two dimensions:

• Intensity of light
• Wavelength of light

A single cone cannot distinguish changes in wavelength from changes in intensity.

Some changes in light produce no change in the cone response.

What cones can’t do individually, they can do as a
team.

Question 16

What is the purpose of prereceptoral filtering?

Answer 16

Reduces chromatic aberration at the fovea, and shields photoreceptors from UV light.

Question 17

What wavelengths of light does the cornea filter out?

Answer 17

<300nm

Question 18

What wavelengths of light does the lens filter out?

Answer 18

<430nm

Question 19

What wavelengths of light are filtered out by macular pigments?

Answer 19

430-500nm

Question 20

Human night vision (scotopic vision) depends on a single class of photoreceptor (rods) and can be used to illustrate the principle of univariance. Rod spectral sensitivity spans the range of approximately 400-650 nm. Which of the following statements is most accurate with respect to matching experiments carried out under scotopic conditions:
a) A normal observer can adjust the intensity of a 450 nm light to match
exactly a light of 548 nm.
b) A normal observer can adjust the intensity of any light between 420 nm
and 610 nm to match exactly a light of 548 nm.
c) A normal observer can adjust the intensity of any mixture of two lights
between 420 nm and 610 nm to match exactly a light of 548 nm.
d) All of the above are all correct.

Answer 20

D.