7. Colour Vision

What other animals have colour vision like humans do?

- Catarrhine & Platyrrhine monkeys (trichromatic).
- Tropical fish (tetrachromacy)
- Pigeons (Pentachromats)

What is scene segmentation?

Variations in colour often signal object boundaries

What is camouflage?

Animals use this fact to disguise themselves by colour markings

What is perceptual organisation?

Our visual system uses colour to group elements in a scene

What is colour?

Visible light forms a band of frequencies in the electromagnetic spectrum. Different frequencies have different hues, ranging from red (long wavelength light) to violet (short wavelength light)

How much does visible light cover within the whole electromagnetic spectrum?

400 nm (1mm= 1,000,000nm)

What is percieved colour determined by?

- intensity of reflected light
- saturation of colour (how much white light is mixed in with the pure hue)

What is a hue?

A colour/ shade

What are three properties of light?

Wavelength, intensity, spectral purity

What are three psychological attributes to light/ colour?

hue, brightness, saturation

What is Trichromatic theory?

(Young-Helmholtz)
We have three receptor types and their combined responses account for all colours. Different receptors respond best to different wavelengths of light.

What are S-cones?

Blue-sensitive cones maximally responsive to short wavelengths

What are M-cones?

Green-sensitive cones maximally responsive to medium wavelengths

What are L-cones?

Red-sensitive cones maximally responsive to long wavelengths

Why do cats & dogs have better night vision?

They have a reflective layer where there cones are located

What is the explanatory gap?

The difficulty in explaining how physical properties are subjectively experienced e.g. is your red the same as my red?

Where are cones concentrated?

In the fovea (1.5mm) - a small pit in the retina

Where does an object of interest fall in the eye?

In the fovea

How many rods are in the centre of the fovea?

Zero

Which area has the highest density of receptors?

The fovea, best acuity (1-50%)

Rods

Very sensitive so don't need much intensity to be activated. They are only sensitive to black/white.

Where are rods concentrated?

All over the retina except the centre of the fovea

How many more rods than cones are there in across the whole retina?

20 times more rods than cones

What do the three receptors respond best to?

They respond best to long wavelength (yellow/red), medium (green) or short wavelength light (Blue). The colour we see is determined by the relative levels of activity in the three sorts of receptors

Support for Trichromatic theory

- Three primary colours combine to produce all possible colours.
- There three forms of dichromatism (colour blindness)

What did Hering discover (opponent process theory?

When people are presented with colour samples and asked to pick out those that are pure, people normally pick red, green, blue which is predicted by trichromatic theory. But, people also picked yellow. Cones and fatigue were not understood so it was unclear how trichromacy could explain afterimages

What are after images?

false images that stay visible even after the original light stimulus is gone

Who proposed the Opponent Process Theory?

(Hering (1920), Hurvich-Jameson)

What are the three receptor types of the Opponent Process Theory?

1. Red-Green
2. Yellow-Blue
3. Black-White

These pairs produce a combination of colours

e.g. Red-Green receptor will signal either Red or Green but not both

Support for Opponent Process Theory

- Non-existence of certain colours e.g. bluish-yellow
- Colour confusions in colour blindness (red or green)
- Complementary afterimages (e.g. staring at a blue circle and seeing a yellow afterimage)
4. Colour context effects

Who proposed colour blindness?

John Dalton (1974) who suffered for a form of it himself.

What is Anopias?

Insensitive to L, M or S wavelengths of light (missing a type of cone)

What is Anomalies?

Misalignment of L or M trichromats (distribution or deficiency)

What is Dichromatism?

Missing one of three cones

What is Protanopia?

L-cone pigment is missing (1.3% males, 0.02% females)

What is Deuteranopia?

M-cone pigment is missing (1.2% males, 0.01% females)

What is Tritanopia?

S-cone pigment is missing (0.001% males, 0.003% females)

What is Protanomaly?

L-cone pigment deficiency (1.3% males, 0.02% females) e.g. need more red in 'red-green' mixture to match yellow

What is Deuteranomaly?

M-cone pigment deficiency? (5% males, 0.35% females) e.g. need more green in 'red-green' mixture to match yellow

What are human tetrachromats

Very rare humans (mostly females) who have four pigment cone types. They can detect variations in hue that we normally cannot

Which animal has 16 receptors and trinocular vision?

The mantis shrimp