L6: Colour Vision

What is light made up of

Photons (balls of light), radioelectic waves

What do retinal ganglion cells signal

Only changes in the world

What is roughly our visible spectrum of light

400-800 nm

How do wavelengths vary

• Wavelength varies smoothly and evenly

  • BUT our perception of colour does not (i.e. if you look at above spectrum of visible light, appears much bigger red area than yellow)

    • Not a product of light, but of what is inside of our eyes

How many colours do we need to make any other colour

Three

Trichromacy theory

• Three receptor types

  • We can make one colour by mixing three colours

How many types of photoreceptors do we have to help us see the colour spectrum and what wavelength are they

4

• Blue cones (short wavelength)

• Rods

• Green cones (medium wavelength)

• Red cones (long wavelength)

How does any cell (photoreceptor) signal what is going on

Through its rate of fire

• Can only either fire an action potential or not

• Response is unidimensional

Principle of Univariate

When a single type of cell is firing, you cannot tell if it’s from change in wavelength OR change in intensity

• Cell response is unidimensional (can only go up or down) BUT the world is multidimensional (i.e. light can change in wavelength and intensity)

• So if we only have one type of photoreceptor, we must suffer the principle of univariate

  • This is what happens as night when we only have rods

HOW can we get over the Principle of Univariance?

By having more than one type of receptor

Two Receptor System

• Each stimulus activates both receptors but in different ratios

• This ratio does not change with changes in intensity

• So colour can be calculated by taking the ratio of activity in the two channels

  • By looking at the ratio of activity between the two receptors can tell colour (wavelength)

• By looking at total amount of activity (magnitude), can tell the intensity 

• (NOTE) This will produce greatest colour-change sensitivity when this ratio is changing the fastest

• Colour perception depends on the activity of two separate receptor systems

• From experiments (Graph one with two triangles, they represent different receptors), find that we peak in being able to tell colour change when one is going up and the other is going down (right in the middle), this is because there’s rapid ratio change occurring

  • Experiments measure delta lambda (how different does a colour have to be for an individual to repliable notice it)

What are the two theorised colour vision system

1. Primordial

   • Most mammals are dichromatic, meaning they have a few S cones and lots of M cones (a blue/yellow system)

   • They have two photoreceptor cones

   • This system is phylogenetically old (500 million years)

   • Purely chromatic channel with little spatial resolution

2. Second subsystem

   • About 10 million years ago the L (long wavelength) cones split into two

     • Red/green system

   • Only old world primates (though nwms [new world monkeys] also have a similar system that evolved independently)

   • Co-evolved with certain fruits so that we could forage them

What does having three types of cones mean for the colour perception system

Three types of cone means two peaks in the spectrum we can see really well

Diagram of colour blindness

What are the three types of ‘colour blind’ people

• Protanopes

  • Mostly males as gene that encodes this is on the X chromosome (males only have one so if faulty screwed, females have two so if only one is faulty you won’t be colourblind)

• Deutranopes

• Tritanopes

Anomalous trichromacies

Have three photoreceptors but ones a bit faulty, not as sensitive as it should be

What animal is believed to have the best colour vision

Mantis shrimp

What are the two subsystems of colour vision (Opponent Processes)

• Comparison of short with long/medium (blue/yellow)

• Comparison of medium with long (red/green)

Cerebral Achromatopsia (Boyle, 1688)

People who don’t see colour

• Believe to be due to problems in V8

Colour constancy

Our ability to work out ‘colours’ despite large changes in the wavelength of the illuminant (lighting)