How is it that we are able to see in colour?
The process starts with light hitting the photoreceptors in the retina. As you might remember from my last post, there are two different types of photoreceptors: rods and cones. Rods are used when are eyes are adapted to the dark, while cones are used in daylight, and are involved in distinguishing different wavelengths of light which represent colour.
There are three different types of cone cell, each most sensitive to a different wavelength of light. These sensitivities can be plotted to give ‘spectral sensitivity curves’ like the ones shown below.
As you can see, there is a cone which responds mostly to short wavelengths of light, which are blue. The medium wavelength cone responds more to green, while the long wavelength cone responds mostly to red light. The black curve shows the spectral sensitivity of rod photoreceptors.
Have you ever wondered why the two most common colours to be affected by colour blindness are red and green or why this condition is more common in men than women?
Basically, the gene which determines the sensitivities of the cones is on the X chromosome. As you can see from the spectral sensitivity curves above, the red and green cones are quite close to each other on the spectrum, and are also thought to originate from a single ancestral pigment gene. They are about 96% alike, and the combination of these factors means that alterations are likely to occur. As females have 2 X chromosomes, then it is unlikely that both with be altered, whereas males only have one. This means men have a higher chance of being colour blind.
The signals from the photoreceptors are processed further in the retinal ganglion cells.The majority of these neurons are colour-opponent cells: a response to one wavelength in the centre of its receptive field can be cancelled by showing another wavelength in the receptive field surround. Two types of opponency are found: red versus green and blue versus yellow. For example, a cell with a red ON centre and a green OFF surround will fire if a red light in shone on the centre and the response to red is only cancelled by green light in the surround.
This diagram shows how signals from the cones are processed in the colour-opponent ganglion cells:
There is also an area in the visual cortex in the brain that is specialised for processing colour (the red area in the diagram below). It is known as V4 as has been shown to be active when people are processing coloured stimuli.
Hopefully you now know are bit more about how we are able to see in colour – if you have any suggestions for other topics you’d like me to write about then please let me know in the comment box below 🙂