Seeing - Colour Vision (L2)

What did Isaac Newton (1642-1727) do in relation to colour vision? (Wade, 2024)

Bough a prism from the 1665 Stourbridge fair and during the plague asked if the prism created colour or if some types of light are fundamental.

Found that white light is a mixture of wavelengths and refraction depends on those.

Object colour depends on lighting and you can mix primaries to produce other colours.

What did Thomas Young (1773-1829) do? (Wade, 2024)

In an appendix to his Göttingen dissertation there are four pages added proposing a universal phonetic alphabet.

Thomas Young was the first to show that some of the hieroglyphs on the Rosetta Stone wrote the sounds of a royal name, that of Ptolemy.

Why is it difficult to measure colour vision? (Wade, 2024)

Despite the fact that we obtain seven primitive distinctions of colours, different proportions in which they can be combined to create a variety of colours.

Discussed by Young (1802a)

What is the trichromatic theory of vision? (Wade, 2024)

All colour sensations are produced by the activity of just three retinal photoreceptor types.

What did Roorda and William (1999) do in relation to colour and cones? (Wade, 2024)

Photographed the individual photoreceptors in the retina.

If a subject adapts to a long-wavelength red light, the red cones become most bleached and when we photograph the retina after this in white light the red cones don't respond as well as they did before the adaptation and so look darker.

This way we can map out the individual red, green and blue cones in the living retina.

What are metamers? (Wade, 2024)

Lights that look the same even though they have different spectra.

Why do lights with different spectra look the same? (Wade, 2024)

They drive photoreceptors the same way.

How can different people maintain the same experience of colour even when they have vastly different L-M cone ratios? (Wade, 2024)

The light spectras would likely be metamers and so whilst they are different, they push enough cones in the correct direction so that perception is similar.

Where are the L and M cone (opsin) genes typically found? (Wade, 2024)

On the X chromosome.

What are the consequences of having only a single copy of the X-chromosome and therefore only one copy of the L and M-cone genes? (Wade, 2024)

You are more likely to be colour blind, this is why it is more common in men than in women as they don't have a backup chromosome.

How does colour blindness have genetic causes? (Wade, 2024)

It is generally caused by missing or abnormal opsin genes.

This is typical to L and M opsins.

What is colour blindness caused by at its most basic level? (Wade, 2024)

The loss of one cone type knocking out a dimension of colour.

e.g. If all M become L, then the L-M dimension disappears, knocking out red/green.

What is opponent processing theory? (Wade, 2024)

Absorption spectra are not evenly space and L and M cones are especially close, so they convey almost the same information.

It is inefficient to send L, M, and S signals straight to the cortex.

Instead, the retina computes three combinations of those signal:
- Black/White
- Red/Green
- Yellow/Blue
Losing either of the L or M opsins damages the opponent red/green system (leaving blue/yellow).

Losing the S cone opsin leaves only the red/green system.

Can animals be colour blind? (Wade, 2024)

Kind of, many animals only have two photoreceptors: L and S. Like human 'dichromats', these animals have one 'color' channel and one 'luminance' channel.

They can distinguish light/dark and blue/yellow but not red/green.

How is colour blindness presented in humans? (Wade, 2024)

Is not usually the absence of color vision. Instead, people lose a dimension of color vision.

What is anomalous trichromacy? (Wade, 2024)

Where discrimination is poorer along the red/green axis then 'dichromacy' where L or M is absent.

What is tritanopia? (Wade, 2024)

The rarest type of human colour blindness (<1 in 1000) where there are no S cones.

How many alleles are required for colour blindness? (Wade, 2024)

Two

How can you test colour blindness? (Wade, 2024)

Ishihara plates

What are ishihara plates? (Wade, 2024)

The desired pattern being tested is masked by random luminance noise so that small luminance cues cannot be used (lighting/printing).

What did Robert Boyle describe in relation to colour vision? (Wade, 2024)

A gentlewoman who had trouble locating violets in the grass.

Why is colour vision so important? (Wade, 2024)

It allows us to identify objects in dappled lighting because shadows do not alter colour like they do luminance.

When did trichromacy evolve? (Wade, 2024)

Our primate ancestors were dichromats, the L cone gene duplication occurred about forty million years ago.

At some point a monkey was born with three different photoreceptors.

It is possible that this animal immediately acquired the ability to differentiate between red and green.

Why would colour vision have been especially useful for primate ancestors? (Wade, 2024)

A luminance system cannot detect fruit amongst leaves, so the development of a red/green system allows for easier identification of food in these settings.

In relation to colour vision, what is the role of the ventral stream? (Wade, 2024)

Object identity and 'form'.

It has a strong representation of the fovea and a strong response to color.

In relation to colour vision, what is the role of the dorsal stream? (Wade, 2024)

Motion, action, and location.

Some regions here (like hMT) respond very weakly to pure color.