Lecture 2 Notes: Colour Vision

Colour Vision

Introduction

  • Lecture by Alex Wade from the Department of Psychology, University of York.
  • Email: alex.wade@york.ac.uk

Topics Covered

  • Brief history of color vision.
  • Photoreceptors and the retina.
  • Color blindness.
  • Color processing in the cortex.
  • No extra examinable reading; resources available on VLE (PDF, PPT, and papers).

Early Color Science

  • What is light?
  • What is color?
  • How do objects have color?
  • What are the ‘primary’ colors and why?
  • What do eyes do?

History of Color Vision

  • Early research had a fundamental error by not considering both physics and biology.
  • Color vision depends on physics (light properties) and biology (eye and brain processing).
  • Understanding both is crucial to explaining human color vision.

Isaac Newton (1642-1727)

  • Newton's question: Does the prism create 'colour'? Or are some types of light 'fundamental'?
  • White light is a mixture of “wavelengths”.
  • Refraction (bending) depends on wavelength.
  • Object color depends on the lighting.
  • You can mix ‘primaries’ to produce any other color.
  • Newton described the physics of color.

Newton's Experiment

  • White light is passed through a prism.
  • A critical experiment involves using a screen to let only one color pass through a second prism.

Outstanding Questions

  • Why are only three colors required to match any other color?
  • Which were the ‘special’ spectral colors?
  • How to decide what are the special colours?
  • How did color get into your head?

Thomas Young (1773 – 1829)

  • Polyglot: Knew Greek, Latin, French, Italian, Hebrew, Chaldean, Syriac, Samaritan, Arabic, Persian, Turkish, and Amharic.
  • Proposed a universal phonetic alphabet.
  • Deciphered the Rosetta Stone: Showed that some hieroglyphs wrote the sounds of a royal name (Ptolemy).

Young's Theory

  • It is almost impossible for each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation.
  • It becomes necessary to suppose the number limited, for instance, to the three principal colours, red, yellow, and blue.
  • Each of the particles is capable of being put in motion less or more forcibly, by undulations differing less or more from a perfect unison; for instance, the undulations of green light will affect equally the particles in unison with yellow and blue and produce the sensation of green.
  • From three simple sensations, with their combinations, we obtain seven primitive distinctions of colours; but the different proportions in which they may be combined, afford a variety of tints beyond all calculation.

Trichromatic Theory of Vision

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

Cones in the Retina

  • Adaptive optics reveal different proportions of red and green cones in the foveas of different people.
  • Individuals can have ‘normal’ colour vision despite these differences.
  • See Roorda & Williams (1999) Nature 397 520-522.

L, M, and S Cones

  • Three types of cones: L (long wavelength, red), M (medium wavelength, green), and S (short wavelength, blue).
  • Each cone type has a different relative proportion of light absorbed across the spectrum.

Metamers

  • Lights that look the same even though their spectra are different.

  • They look the same because they drive the photoreceptors in the same way.

  • Humans can match any color by mixing three ‘primaries’ because you have three classes of cones.

  • All colors are represented by the amplitudes of responses in these three cone classes.

  • There are infinitely many spectra that will give rise to the same colors. These are called ‘metamers’.

Genetics

  • Where do the three classes come from?
  • And what happens when one goes missing?

Brief Intro to Genetics:

  • Humans have 23 chromosome pairs (think “23 and me!”). In almost all cells they come in pairs (‘diploid’).
  • Germ-line cells (sperm, eggs) are ‘haploid’: one of each type.
  • The sex chromoxomses (‘X’ and ‘Y’) are special.

Opsin Genes

S-Cones

  • S cones: a “fossil” of the original color vision system
  • Not many of them (only about 10% of the cones)
  • Don’t see much form or motion through S-cones
  • We shall speak of them no more….

Sex Chromosomes + Opsin Genes:

Consequences

  • 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?

Mutation

Color Blindness

  • (Generally) caused by missing or abnormal opsin genes
  • Almost always, the genes involved are the L and M opsins
  • Because these are on the X chromosome, men are affected by color blindness far more than women
  • Men have no ‘backup’ gene to rescue them if something goes wrong.

Effect of Color Blindness

  • Usually caused by loss of one cone type
  • Knocks out a ‘dimension’ of colour vision
  • What is a dimension of colour vision?

Opponent Processing

  • Absorption spectra not evenly spaced
  • L and M cones are especially close….
  • …so they convey almost the same information

Opponent Channels

  • Inefficient to send L, M and S signals straight to cortex
  • Instead, the retina computes three combinations of those signals:
    • L+M, L-M and S-(L+M)
    • Those are ‘black/white’, ‘red/green’ and ‘yellow/blue’

Opponent Channels diagram

Red or Green?

Colour Blindness

  • If all your ’M’ cones become ‘L’…
  • …the L-M dimension disappears
  • You can still discriminate dark/light and yellow/blue…
  • …but not red/green

Losing A Dimension

Faulty opsin genes cause color vision deficits

  • 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.

Colour Blindness In Animals

  • 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.

Color Blindness in Humans

  • Is not usually the absence of color vision
  • Instead, people lose a dimension of color vision.
  • Most common is ‘anomalous trichromacy’ where discrimination is poorer along the red/green axis
  • ..then ‘dichromacy’ where L or M is absent
  • Rarest type (<1 in 1000) is tritanopia where individuals lack S cones

Genetics of Colour Blindness

More About Color Blind Genetics

Tests For Color Blindness

  • ‘Ishihara plates’ – Note, the red/green ‘pattern’ is masked by random luminance noise so that small luminance cues due to printing/lighting errors cannot be used.

Why Color Vision?

  • '…that when she had a mind to gather Violets, tho' she kneeled in that Place where they grew, she was not able to distinguish them by the Colour from the neighbouring Grass, but only by the shape, or by feeling them.'
  • '…the fruit on trees when red, I cannot distinguish from the leaves, unless when I am near it, and then from the difference of shape rather than colour'
  • Mollon,, “Tho' she kneel’d in that place where they grew..”, J Exp Biol 1989

What Is Color Vision Good For?

Simulated Fruit

  • Normal vision vs. vision with different types of color blindness (protanope, deuteranope, tritanope).

When Did Trichromacy Evolve?

  • Our primate ancestors were dichromats
  • L cone gene duplication occurred about 40 million years ago.

Trichromacy Evolution

  • At some point…
  • An ‘old world’ monkey was born with three different photoreceptors. It is possible that this animal immediately acquired the ability to differentiate between red and green.

Yes But Why?

Luminance System

  • Luminance system cannot detect fruit amongst leaves

Development of red/green system

  • Development of red/green system for this task.

Primate Color Vision

  • Trichromatic primate color vision has evolved to support ‘frugivory’: the eating of fruit. In this sense, primates are useful to trees just as honeybees are to flowers.
  • Petroc Sumner, “Colour Vision: Why Are We Primates Unique?”, Eye News 2002, ( Perspectives in Basic Science)
  • Sumner and Mollon “CATARRHINE PHOTOPIGMENTS ARE OPTIMIZED FOR DETECTING TARGETS AGAINST A FOLIAGE BACKGROUND”, J Exp Biol 2000

Into The Brain…

  • Visual pathway from the eyes to the brain, including the optic chiasm, lateral geniculate nucleus, superior colliculus, pulvinar nucleus, optic radiation, and primary visual cortex.

In The Brain…

  • Brain areas involved in visual processing: V1, V3a, hV4, LOC, hMT+.

In The Brain

  • Many areas respond to both color and achromatic contrast.
  • The ‘ventral stream’ seems to be concerned with object identity and ‘form’. It has a strong representation of the fovea and a strong response to color
  • The ‘dorsal stream’ seems to be involved in motion, action and location. Some regions here (like hMT) respond very weakly to pure color

Summary

  • Color comes from both physics and biology
  • Three cone type are used to represent all colours
  • Colour signals are transmitted as sums and differences of cone activations
  • Color blindness is X-linked and causes red/green confusion
  • Trichromatic (3 cone) vision helps primates find fruit
  • Some cortical neurons (and areas) have strong biases for colour or luminance stimuli