Colour Vision

Question 1

What is wavelength?

Answer 1

Wavelength is the distance between the crests in nm

Question 2

What is intensity?

Answer 2

The wave’s height
Approximates to brightness

Question 3

What’s the difference between short, medium and long wavelengths?

Answer 3

Short looks purpleish/blueish
Medium looks greenish
Long looks reddish

Question 4

Describe the reflection of light:

Answer 4

Objects either reflect or absorb light
They appear to be the colour of the light they reflect
They look black if they absorb lots of light at all wavelengths and reflect little
Appear white if they absorb little light and reflect lots of all wavelengths
Yellow if they absorb all else, and reflect only yellow

Question 5

What are additive colour mixtures?

Answer 5

Adding various pure wavelengths of light together forms new colours.
Short (blue) + long (red) –> purple
Short (blue) + medium (green) –> turquoise
Medium (green) + long (red) –> yellow
Short (blue) + medium (green) + long (red) –> WHITE

Question 6

What are subtractive colour mixtures?

Answer 6

Example of subtractive colour mixtures is mixing paint - as paint reflects more than just a single wavelength of light –> therefore the more colours you add, the more light is absorbed.

Question 7

Compare the photoreceptors - rods and cones:

Answer 7

Rods
- only one type
- none in central fovea
- high sensitivity
- night vision
- scotopic (rods are sensitive enough to engage in night vision, cones are not due to the principle of univariance)
Cones
- several types each tuned to various wavelengths
- mostly in fovea
- lower sensitivity
- day vision
- photopic

Question 8

Describe the principle of univariance:

Answer 8

At low luminance, cones are not sensitive enough for night vision and rods must do all the work.
This is because rods have a broad wavelength tuning

Question 9

What happens in photopic colour vision

Answer 9

The rods are all bleached out (over stimulated)

Colour vision in daylight is good, humans can reliably discriminate at least 200 wavelengths.

Cones have a two-receptor system (compared to univariance) which allows them to better detect colour, as well as small wavelength tuning curve peaks.

Question 10

Describe the two receptor system

Answer 10

Colour is represented by the relative activation of the 2 cone channels (M and L)
As wavelength changes, pattern of relative activation changes:
- short wavelengths: more activity in M than L cones
- medium wavelengths: equal
- long wavelengths: more activity in L than M cones
i.e. relative activation remains the same

This allows us to discriminate a wavelength from an intensity change, compared to the univariance of cones where various intensities of different wavelengths can appear the same.

Question 11

What is a metamer

Answer 11

psychophysical colour match between two patches of light that have different sets of wavelengths.

Question 12

What is colour deficiency and what are the different types?

Answer 12

Anomalous trichomats: have 3 types of cones, but responses are atypical due to a deficiency in one cone type –> encompasses protanomaly (long), deuteranomaly (medium), tritanomaly (short)

Dichromats: lacking one cone type entirely (protanopia = long) etc

Monochromats: truly colour blind, only have 1 type of cone and lack colour vision. Rod monochromats have no cones at all.

Cerebral achromatopsia: damage to V4

Question 13

Describe colour opponency

Answer 13

3 axis
- red-green
- blue-yellow
- black-white

Question 14

What is the physiology of opponent processing

Answer 14

• cells have been found in LGN and V1 which show antagonistic cone inputs
• some show a centre surround arrangement (double opponent cells)
• R/G cells are parvocellular
• BY are koniocellular
• BW are magnocellular