Color Vision

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.

Colour Blindness:

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 🙂

The visual system

How is it that we can see the world around us? It’s quite a bit more complicated than most people think – not simply light hitting the retina and an inverted picture being turned the right way round. Here’s an overview of how we are able to see.

First of all, light enters the eye and hits the retina at the back of our eye.

The retina is made up of several layers of different cells which detect and then start to process the visual input. The cells which respond to light and colour are called photoreceptors. There are two different types of photoreceptors: rods for detecting light and dark, and cones, which detect colour. Cones are concentrated in the fovea, which is the area of the eye used for fixating on stimuli.

The diagram above shows the organisation of cells in the retina. The ganglion cells take the output of the retina to the brain via the optic nerve. The optic disk (see diagram of the eye) is where these projections leave the eye – another name for this area is the blindspot as there are no photoreceptors in this area.

The information from the retina is projected to the Lateral Geniculate Nucleus (see below). From there, it travels to the striate cortex in the occipital lobe – this area is also known as the primary visual cortex or V1.

From the primary visual cortex, the visual information travels through other areas in the occipital lobe, where aspects such as colour and motion processing occur.

After processing in the visual cortex, the information is projected to other areas of the brain. There are then two ‘streams’ in the brain which are specialised for different aspects of vision:

1. the dorsal stream projects towards the parietal lobe and is important in identifying object location.

2. the ventral stream projects towards the temporal lobe and is important in identifying and recognising objects.

These were identified by Goodale and Milner in 1992.

I hope you liked this post on the visual system, check back soon for my next posts about visual disorders and colour vision 🙂