Colour vision-Snowden

How do we perceive vision?

Photons bounce off of objects into the eye via the pupil, what we capture is turned into vision and we perceive what the photons represent

What cells put together the information from many photoreceptors?

Retinal ganglion cells

What does the centre surround receptive field do?

Gets rid of most of the info from photos so we don’t overwhelm the system, keeping differences in illumination

After the centre surround receptive fields sort the info…

Retinal ganglion cells send the information back to LGN

The magnocellular levels of LGN..

Process fast signals, wider information in less detail

The parvocellular cells in LGN process…

Colour vision, slower processing, fine detials

How many visual areas do we have

35

Which area is thought to be associated with colour vision, and which with colour?

• V8

• MT

Light comes in … intensity is determined by …

• Photons

• Photons arriving at one time

Light is a part of the

electromagnetic system

Why don’t we see infrared like snakes?

Bc we are warm blooded so our blood vessels in our eyes would dominate our vision

Why do we perceive a ‘rainbow’ when white light is passed through a prism?

The wavelengths that make up the white light get split, so we see the individual wavelengths

What nm is the short wavelength?

400nm

What is the visible light range in nm?

400-750nm

Why do objects of certain colours appear the way they do?

They absorb other wavelengths and reflect the one they appear as

What is an example of how colour vision is a product of evolution?

Bowmaker 1994: The deeper in a lake, the shorter the wavelength their cones peaked at

Ender 1993

Human peak sensitivity is around 550nm (green), related to the amount of light present in the forest during primate vision evolution

What is an example of colour subtracting?

Ink in newspapers- absorbs red wavelength and repells the others

Colour mixing experiments

• Any single wavelengths can be perfectly matched with a mix of 3

Trichromacy theory of vision

The trichromacy theory of vision states that color vision is based on three colors: but it can be any 3!

Linked to the 3 different cone types

4 basic photoreceptors

• Rods

• Short cones

• Medium cones

• Long cones

When observing the photoreceptors you will always see?

One of them peaking

Proportion of photoreceptors

• Mostly rods

• About even split of red/green cones

• Blue- rarest

The principle of univariance

The strength of a response is governed by intensity and wavelength but represented by one number

How does having more than one type of receptor help this problem?

Any light will now stimulate the different receptors in different ratios, taking output from one set of cones will help you determine colour.

Ratio of firing changes quicker…

…In the middle

Dichromat

Having a 2 cone colour system

Acuity

The level of detail that can be perceived

Why is acuity reduced when more cones are introduced?

Because acuity is calculated from taking info from neighbouring cones of the same type

Why don’t we need many blue cones?

Chromatic aberration: Blue light is always out of the eyes focus, therefore you don’t need as many cones as there’s less fine detail to be encoded

Explain the primordial colour system

Comparing blue to yellow

Very old system

Chromatic with little spatial resolution, made foraging hard

Explain the second subsystem

The l cones split into L and M

Only in old-world primates

Why is the second subsystem helping for foraging?

Mollon 1989- could not distinguish a berry on a bush

How else is the second subsystem helpful?

Reproduction (Domb and Pagel, 2001) animals signal by colours in their faces or genitals when they are sexually ready

Sumner and Mollon 2000

Species of monkey who ate yellow fruit

Found: the spacing of cone sensitivity curves was optimal for discrimination of fruit signal from leaf signal

Lovell et al 2005

The red/green system has colour constancy

Monochromat

Only 1 type of cone

Protanopes

No long cone (red/green colour blind)

1.0%-0.02% Females

Deuteranopes

No middle cone

1.2% Males, 0.1% Females

Tritanopes

Lack of short wavelength cone

Very rare

Protanomaly

Abnormal long cone

1.0% Males, 0.02% females

Deuteranomaly

Abnormal medium cone

1.9% Males, 0.04%

Rod monochromacy

Total lack of cones

Birch 1993

Cone anomalies and deficits are genetically inherited and sex-linked

How can blue cone deficits be acquired?

Diabetes and drug taking

Livingstone and Hubel, 1988

Through V1 there are patches of cells where that are all responsive to the wavelength of a stimulus

From these cells, colour is converted onto the pre-striate cortex

Which area is suggested to be important in achieving normal colour perception?

Shipp and Zeki 1985- Area V4

What is opponent coding?

Encoding colour activation by comparing activities of cones types

Colour aftereffect?

Adapting a patch of the retina to a given colour reduces its sensitivity to that colour due to fatigue, therefore the opposite channel fires more

de Monasterio, 1975

Cells in parvocellular layer of LGN show red-green colour opponency

The blue-yellow signals get processed where in LGN

The konio-cellular pathway

Explain this diagram

• We start with trichromacy with long, medium and short cones

• In LGN these cones have colour opponency because L and M are opponents

• We then get colour opponency in the visual cortex

• In the higher cortex we categorise into colours

What is cerebral achromatopsia?

Not seeing any colour

Cowey and Haywood 1995

• Investigated cerebral achromatopsia

• Proved that they were truly experiencing a lack of colour

• From lesioning monkey revealed it is not a result of V4 damage

Hadijikhani 1998

Monitored brain looking at black/white vs red/green grating while controlling for luminance, found:

• Activiry in V8, V1, V2, V3 BUT NOT V4

Colour constancy

Using the colour of illuminating light to interpret the colour of an object

How does the visual system discount illumination to get the true colour of an object

Looking at lots of objects in a scene and seeing what wavelengths are being mostly reflected

Land 1983: Mondrian pattern

Lit the pattern with different wavelengths

Found: If a single patch was presented it appeared to change colour with illumination, when included in patterns this was no observed

Illumination and the cortex

• Cells in V1 respond differently as illumination changes

• Cells in V4 maintain responses despite illumination changes