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A PHYSICIST WRITES . . .
(July 2003)
Back in April I started to describe to you how we actually ‘see’ what we are looking at. I got as far as the image of the scene in front of you that is projected upside-down (as in a camera) onto the back surface of each eye — the retina. You may find the rest of the story hard to believe.
A hundred million cells in each retina react to the light by sending electrical signals into several parts of the brain. Each part has a different basic job to do: sensing movement, recognizing all the colours in the image, detecting lines at different angles, locating the boundaries between light and dark areas, and so on. Let’s call this the first stage of visualizing.
In the second stage all these details are brought together so that the shapes in your view ahead can be identified. This involves matching them against your vast memory of shapes and objects. At the same time the information from the two eyes is combined, enabling the brain to judge the solidness of things and how far away they are.
So far, amazingly, this continuous rapid brain-work has been entirely subconscious. All you are aware of is the final stage of visualizing (the end of the reassembly-line, if you like) in which you perceive your surroundings, in full 3D as if seeing them through an open window. But this is a complete illusion, created entirely by messages between brain cells. There are no eyes and no pictures inside the brain!
How was all this discovered? Valuable clues came from simple tests on brain-damaged people. The Reith Lectures on Radio 4 last April were given by a neuro-scientist. He reported on a patient who had unfortunately suffered an injury to the part of the brain where the final stage of visualizing takes place.
As a result this man was quite unable to see. But when asked to point to something, he could do it — without knowing how! His brain subconsciously identified what his eyes were looking at, in the way I described above, but there the assembly-line stopped. This phenomenon is called blind-sight.
The lecturer then caught my attention even more, by pointing out that if you are driving while (for example) talking intensely with your passenger, you stop being conscious of your actions, the roads, the traffic and so on, unless a hazard appears or something else unusual happens. You are (he said) driving using blind-sight, in other words relying on the subconscious brain just as his patient was. I was not sure whether to feel alarmed or reassured at this news.
He went on to say that you cannot imagine the opposite scenario: paying conscious attention to driving while subconsciously talking to your passenger. Evidently this scientist is not an advanced driver, because it seems to me that is exactly what you are trying to do when you volunteer to give a commentary as you drive (not that I claim to be at all expert at it).
Sometimes when I am rummaging in my toolbox for a particular screwdriver, I sense it is there before I am able to locate it — is this another example of blindsight in action, or is it just a consequence of possessing too many tools?
Peter Soul
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