MadSci Network: Anatomy Query:

Our peripheral vision works essentially the same as our central vision. The major difference is the quality of what we see or the 'acuity'. In our retina there are two classes of light receptors - the cells that detect and transmit the light that we see. Both cell types can be found in the centre of our retina (the fovea) and the periphery of the retina but there are differences in the distribution of these. Clustered around the fovea are 'cones' - these are the cell types that pick up different wavelengths of the light spectrum. Thus they are often referred to as 'colour-receptors'. They are very densely packed here, thus able to detect very fine details. They are much less densely packed in the periphery. The other cell type are known as 'rods'. On a normal sunny day these don't do much work, they are used for detecting very dim light and are the ones that kick in when night falls. One fun way of using your 'rods' is when you are looking up at stars in the sky. You might have noticed that you can see very faint stars much better if you don't look directly at them.... in this case you are letting the dim starlight fall on the periphery of your retina rather than the fovea. This is where the rods are more densely packed. Thus the 'rods' are being stimulated instead of the cones (which aren't working too well as its nighttime) - so you see the star! Signals from your entire retina are passed onto the rest of the pathways in your visual system, but as the most important part of what you see is what you are focussing on, then the majority of the processing is taken up by what falls onto your fovea and thus the cones. So larger parts of your brain are taken up by processing whats in the centre of your vision rather than the periphery. It might not seem like this, but try focussing on a point in front of you - ie the computer screen.... Focus on the word 'screen' you have just read - you won't actually be able to discriminate what the words say even 3 or 4 lines above or below that (i.e. your periphery). So this gives you an idea of how important the brain thinks our peripheral vision is - but theres no need to worry - thats why we can move our eyes and our heads!

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