Eyes in the back of your head

Have a look at the photograph on the right. I took it during a recent walking holiday in the Lake District and my purpose in including it here is to demonstrate the vast amount of information the image contains - the clouds and weather conditions, the colour and texture of the rocks, the shading on the hillsides, the sense of perspective. When stored on a computer, this photograph occupies more than 5mb of disk space!

Now imagine how much computer disk space would be needed if, instead of taking a single photograph, I had taken photographs every step of the way. The amount of information gathered would be vast, yet this is precisely what your eyes are doing every day for the whole of the time you are awake. How then does your brain manage to process this huge quantity of information?

The reason is that the camera and humans behave very differently. When a camera takes a photograph it records as much information as it can, which is why the quality of the lens and the number of megapixels are important factors to consider when buying a camera. When we look at the same scene we simply don't have the time or the capacity to take it all in, so instead our brain uses the information from the eyes as clues, compares that information to its stored memories of similar visual information and then jumps to a conclusion as to what it is we are looking at.

For example, look again at the photograph and answer the question; was it raining?

Clearly the answer is "no". But how do you know that as, even if it were raining, it is unlikely that the camera would have managed to record the individual raindrops falling from the sky? The reason you know it is not raining is because your past experience tells you that the clouds do not look like rain clouds, that the water in the small pool would be disturbed if it were raining and that the rocks would be darker in colour if they were wet.

The reason you know it is not raining therefore, is not because the photograph proves it is not raining, but because the photograph differs from your knowledge of what it would look like if it was raining and because it is consistent with your knowledge of what it should look like in dry, slightly cloudy conditions.

In other words, your memories and past experiences are extremely important in determining your perception of what it is you think you see. This is why optical illusions can fool us and why the Police treat witness statements with great caution unless they are verified by several people.

Seeing therefore has at least as much to do with your brain as it does your eyes - which is why you can "see" vivid images in your dreams even though your eyes are closed and why you can bring to mind the face of a friend, just by thinking about them.

Recently scientists have begun to realise that the ways in which the brain processes visual information is even more complicated than first thought. At one time it was assumed that all information from the eyes was passed to the visual cortex, which is located to the rear of the cerebral cortex, in the part of the brain that deals with our conscious thoughts. However, it is now known that information from the eyes also passes subconsciously from the eyes into other parts of the brain.

The people credited with first recognising this phenomenon known as "blindsight" were the neuroscientists Mel Goodale and David Milner. Their discovery resulted from a patient known as D.F. who had damage to a particular area of the brain known as the ventro-lateral occipital region - a section of brain tissue within the area of the brain responsible for vision.

At one point during the neurological testing, David Milner held up a pencil and asked D.F. if she could identify it. Obviously she couldn't as the damage to the brain had effectively rendered her blind. Suddenly, and much to Milner's and Goodale's surprise, she reached out and grabbed the pencil in order to inspect it more closely. But how was this possible? How was she able to accurately pinpoint exactly where the pencil was located in space and to fluidly grasp and manipulate the pencil, despite having no conscious ability to see it?

Goodale then performed an experiment with D.F. to confirm his suspicion. He placed two 3-D rectangles of different sizes next to each other and asked D.F. to tell him which one was the larger of the two. Of course, she couldn't do this. He then asked her to pick each one up and measured the distance between her fingers as she grasped for each. Her fingers accurately estimated the sizes of each object as she reached for them, opening just enough to grasp each one. This is a task we unconsciously perform every time we reach out to pick up any object. Our fingers automatically open just wide enough to grab the object.

In D.F.'s case, it was as if one part of her brain knew which was the bigger rectangle, even though she couldn't consciously state that knowledge. Goodale and Milner concluded therefore that the brain contained of two independent visual processing areas; one for conscious perception and one for subconscious action.

Since their findings were published many other similar cases have come to light; such as the person who was blind but could accurately identify the emotions on the faces of people, or the recent case of a man in Norway who, blinded by a stroke, can still navigate his way around obstacles placed in his path.

The interesting point regarding this second aspect of seeing is that the subconscious information is processed by the brain many times quicker than the conscious information. Most people will have experienced this for themselves when startled by something such as a falling object or an imminent threat. Examples might be when you instinctively raised an arm to protect yourself before you consciously realised that something was flying towards your face, or when you reached out your had to catch a falling object without even thinking about it.

This fast-path into the brain is therefore very important for human survival as the conscious processing of information is far too slow where emergencies are concerned. Moreover, because this fast-path routes information directly into the lower limbic area, the part of the brain that deals with emotions, it is also worth noting that when you meet someone for the first time, you will have formed an initial impression of them based on your instincts well before your conscious brain has even seen them.

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Published: April 2010

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