Chapter 5 - Trichromatic Colour Representation

Question 1

How are humans unique in colour perception?

Answer 1

Humans have three types of cones where most other mammals have 2 cones (these are dichromats)`

Question 2

How many cones must be stimulated in order for a percept to occur?

Answer 2

Two

Question 3

In what way can we think about a cone’s peak sensitivity?

Answer 3

We can think about a cone’s peak sensitivity as the probability it will absorb a photon of a particular wavelength

Question 4

What thing does a cone only respond to?

Answer 4

The energy it absorbs
- all wavelengths of light can produce identical responses from a cone if the energy by the cone is the same for these wavelengths (no single cone can differentiate between a change in wavelength and a change in intensity – luminance)

Question 5

What would happen if a retina was covered with only a single cone type?

Answer 5

the person would be truly colour blind and cone responses would only indicate the presence of light, therefore no colour information would be extracted

Question 6

What does the relative sensitivity of a cone describe?

Answer 6

The likelihood it will absorb a photon

Question 7

What process occurs in order to allow for spectral discrimination?

Answer 7

A comparison process based on the different responses from both cone types

Question 8

What is opponent colour representation?

Answer 8

A comparitive process must be taking place in order for a cone to correctly signal for colour
- some combinations do not exist, such as reddish green or bluish yellow)

Question 9

What did Herring and Mach propose?

Answer 9

That colour perception stems from the output of three mechanisms (not cones), each of them resulting from an opponency between

• red/green
• blue/yellow
• black/white

Question 10

What are unique colours?

Answer 10

Pure colours

Question 11

What wavelength describes pure blue?

Answer 11

477nm

Question 12

What wavelength describes pure green?

Answer 12

510nm

Question 13

What wavelength describes pure yellow?

Answer 13

580nm

Question 14

What wavelength describes pure red?

Answer 14

It does not exist as a point on the spectrum

Question 15

What do the opponent colours depend on?

Answer 15

The connections of cones and the ganglion and bipolar cells (both ON and OFF)

Question 16

What are the preferred wavelengths of the S, M and L cones respectively?

Answer 16

S = short (blue)
M = medium (green)
L = long (red)

Question 17

Describe the receptive fields of the LM pathway

Answer 17

colour opponent ON and OFF regions

Question 18

Describe the receptive fields of the S-ML pathway

Answer 18

• spatially uniform

- excited by blue, inhibited by yellow (and vice versa)

Question 19

Describe single-opponent cells?

Answer 19

Cone-opponent but not spatial-opponent

Question 20

Where are the colour opponent receptive fields concentrated?

Answer 20

In the parvocellular and koniocellular layers of the LGN

Question 21

Where are L-M (red-green) opponent neurons mostly found?

Answer 21

In the parvocellular layers (3-6)

Question 22

Where are S-LM (blue-yellow) opponent neurons mostly found?

Answer 22

in the koniocellular layers

Question 23

What do the magnocellular layers contain?

Answer 23

Achromatic receptive fields (luminance based)

Question 24

What does the koniocellular layer project to?

Answer 24

Layer 3 of the striate cortex directly

Question 25

What does the magnocellular layer project to?

Answer 25

Layer 4Calpha of the striate cortex

- sends axons to layer 4B

Question 26

Where are double opponent cells found?

Answer 26

At the striate cortex

Question 27

What will double opponent cells do?

Answer 27

Decrease their firing rate if their preferred colour in the centre (L+) is presented in the surround
- ideally suited to detect colour differences in environment (even when a surface has the same illumination-contrast/intensity)

Question 28

Describe simple double-opponent cells?

Answer 28

• orientation selectivity based on spectral contrast (difference in colour) and not contrast energy (luminance)
• make up roughly 10% of cells in the cortex

Question 29

Describe a complex cell that responds to spectral not energy contrast?

Answer 29

• orientation and direction selective

- responds strongly to red/green contrast

Question 30

Describe how spatial energy and spectral contrast work together to create shapes in colour?

Answer 30

• energy contrast offers higher spatial resolution

- spectral contrast offers the colour of the object

Question 31

What is the last area sensitive to colour?

Answer 31

V4 (dorsal and ventral have similar pathways at this point)