Visual Perception - colour perception

Question 1

Learning Objectives

Answer 1

Describe how we see colour.

Evaluate colour opponency and its uses.

Explain chromatic processing at each stage in the visual pathway.

Evaluate some defects of colour vision.

Question 2

How does colour vision help us?

Answer 2

Discrimination and Detection.

Important in many key tasks:
When choosing what to eat.
Scene segmentation.
Visual memory.
Mating rituals.
Camouflage.

Question 3

What is hue?

Answer 3

Hue (H): the quality that distinguishes red from blue, i.e., the hues of the rainbow.

Question 4

What is brightness?

Answer 4

Brightness (V): the perceived intensity of light (sometimes lightness).

Question 5

What is saturation?

Answer 5

Saturation (S): characterizes a colour as pale or vibrant.

Question 6

Why do objects appear coloured?

Answer 6

because they reflect different wavelengths of light from different parts of the visible spectrum.

Question 7

What is colour is a property of?

Answer 7

Colour is therefore a property of our neural apparatus. For an object to appear coloured, we need to have the correct photoreceptors and neurons.

Question 8

What does colour perception arise from?

Answer 8

Colour perception arises from the ability of certain light rays to evoke a particular pattern of neural responses in the visual system.

Question 9

What is a Metamer?

Answer 9

A metamer is a sensory stimulus that is perceptually identical to another stimulus, but physically different.

Question 10

What is an example of a metamer?

Answer 10

For example, Newton demonstrated a light that appeared orange was indistinguishable from a light produced by combining a red light and yellow light – they are colour metamers.

This suggests the visual system is producing identical neural responses to physically different stimuli.

Question 11

Are Metamers unique to the visual system?

Answer 11

No, This is not unique to the visual system!

The way we encode mint is the same way we encode cold (temperature) – they are flavour metamers.

Question 12

Why can/cant we tell visual stimuli apart?

Answer 12

if you can discriminate between two lights (they appear different), then the neural representation of these stimuli must differ.

If you cannot tell visual stimuli apart, then the physical property that makes them different is not being encoded by the visual system!

Question 13

So, what is the connection between the physical stimulus and our perception of colour?

Answer 13

Since the photoreceptors are the first stage in the processing of visual information, it is likely that the answer lies here.

Furthermore, we already know rods are colourblind, so we need to look at cone cells and the properties of their photopigments.

Question 14

What are the three cone types?

Answer 14

We have 3 cone types.

S cones (short λ, blue); peak absorption at 420nm.

M cones (medium λ, green); peak absorption at 530nm.

L cones (long λ, red); peak absorption at 565nm.

Question 15

What is the principle of univariance? (condense this cba now xxx)

Answer 15

Consider a hypothetical photoreceptor with a single photopigment.

The graph shows the proportion of light absorbed by the photoreceptor (expressed as a percentage of the peak absorption), as a function of the wavelength (λ) of the incident light.

At λA, it absorbs about 25% of the incident light

At λB, it absorbs about 50% of the incident light

If the intensity of λA is the same as λB, then there will be a different response from the cell to the different light

But if the intensity of λA is about 2x the intensity of λB, then the response from the cell will be the same to both.
Therefore, any single photopigment is colour-blind, since an appropriate combination of λ and intensity can result in an identical neural response - this is the principle of univariance

Question 16

How do we differentiate between wavelengths and intensities?

Answer 16

We need a comparison of signals from two or more cone classes, each with a unique spectral sensitivity.

wavelength discrimination improves with the number of cone classes.

Question 17

How many pigments do different species have?

Answer 17

Some non-primate mammals that rely heavily on sound and smell only have 2 pigments – they are dichromats.

Some birds that rely heavily on vision can have up to 5 pigments – they are pentachromats.

Question 18

What is it called when mammals have two pigments?

Answer 18

Dichromats

Question 19

What is it called when birds have five pigments?

Answer 19

Pentachromats

Question 20

What is it called when humans have 3 pigments?

Answer 20

trichromats

Question 21

How do we balance neural activity?

Answer 21

The balance of neural activity – how much each cone class is activated- from each of these receptors is sufficient to represent the vast array of natural colours we encounter.

Question 22

What interesting points can be seen from the cone mosaic (a recreation of the layout of cone cells on the retina, colour coded for pigment)

Answer 22

There are far fewer S cones (bleu) than M or L

There are no S cones in the fovea

They are randomly distributed, but clumping is common.

The layout and relative proportions of cones is largely individual, e.g., some will have roughly equal amounts of L and M comes, while others will have a L:M ratio of 4:1.

Question 23

How are Topographical patterns of cone cells created?

Answer 23

These images were created using a super high-performance optical system called a TSLO, and a retinal adaptation paradigm using high-intensity-coloured lights.

Question 24

What is opponent coding theory?

Answer 24

Colours are grouped into opposing pairs (blue and yellow, red and green).

Question 25

Why is it hard for us to imagine a reddish green?

Answer 25

This is evident from the colour afterimages:

adapting to one colour produces its opponent in the afterimage.

This is purely a result of our physiology;
There is no link between opposing
colours in the physical spectrum!

Question 26

What RFs do parvocellular RGCs have?

Answer 26

chromatically opponent RFs (centre-surround antagonism).

Question 27

What is the physiology of oppency?

Answer 27

The centre may be excited by red light, while the inhibitory surround is excited by green light (top left).

There are both ON and OFF versions of this
This arrangement also exists for blue/yellow

There are also cells that respond to red light switched on anywhere in the RF, or green light switched off anywhere in the RF (bottom left).

Question 28

Where do the LGN layers 1 and 2 get their input from?

Answer 28

M RGCs: input for achromatic luminance channel

Question 29

Where do LGN layers 3-6 get their input from?

Answer 29

P RGCs: input for the two chromatic channels, called cardinals.

Question 30

What do cells prefer at the LGN?

Answer 30

At the LGN, nearly all cells prefer stimuli that are modulated along the cardinal directions of colour space, i.e., red-green (0-180 hue angle) or blue-yellow (90-270).

Question 31

What do cortical cells show a preference for?

Answer 31

cortical cells show a preference for a wide range of hues, not just the cardinals

Tuning width remains fairly consistent across cortical areas (V1, V2, V3).

Question 32

What is a double opponent RF?

Answer 32

Some cortical cells have double opponent RFs. The centre is excited by red and inhibited by green, while the surround is excited by green and inhibited by red

Question 33

What is colour constancy?

Answer 33

is the ability to assign a fixed colour to an object even though the actual spectral information entering the eye changes in different illumination conditions

Question 34

What is a percieved change in colour called?

Answer 34

chromatic induction

Question 35

What is colour vision deficiency?

Answer 35

can be congenital or acquired.

Question 36

What is aquired CVD typically due to?

Answer 36

CVD (cerebral achromatopsia) is typically due to damage to V4

Question 37

What is Congenital CVD due to?

Answer 37

X-linked recessive gene:

XY chromosomes: 8% chance of colour blindness.
XX chromosomes: 0.5% chance of colour blindness.

Question 38

What cones does congenital CVD effect?

Answer 38

People affected have normal cone numbers, but fewer photopigments available.

Congenital CVD usually affects M or L cones, rather than S cones.

If M or L cones are missing, then green and red will be confused.

If S cones are missing, blue becomes hard to distinguish.

Question 39

What is a dichromat?

Answer 39

Lacking a pigment makes you a dichromat (rather than a trichromat).

Question 40

What is an anomalous trichromat?

Answer 40

It is more common to be an anomalous trichromat (so all three are present, but one does not work optimally).

Question 41

What are Ishihara colour plates?

Answer 41

Each dot in the images is different only in hue – not luminance.

The combination of numbers that are visible to a patient is indicative of the type of deficiency.

Question 41

Colour perception summary=

Answer 41

Colour perception aids visual judgements.

Colour is a purely psychological property that comes from the interaction of wavelength and our visual machinery.

Cells in the LGN show chromatic opponency and are tuned along cardinal hue axes.

Cortical cells are tuned to a wide variety of hues and can show double opponent responses.

An individual may be missing a photopigment (or one may be atypical), resulting in colour vision deficiency.