Memories are made of a million million connections: A prize-winning scientific book reveals how the brain is being constantly rewired, reports Steve Connor

AS STEVEN ROSE, author of The Making of Memory (Bantam Press), this year's winner of the Science Book Prize, says: 'No memory within a computer could survive such a complete turnover of all the machine's constituent parts. Somehow, just as the shapes of our bodies persist despite the ceaseless ebb and flow of their molecular components, so do our memories, embedded in the structure and processes of the brain.'

Professor Rose, a neurobiologist at the Open University, has spent most of his professional life trying to understand how the brain remembers. He works on the brains of new-born chicks, who at a few days old can be trained simple memory tasks, such as learning to avoid coloured beads from the memory of having once pecked at one soaked in a bitter substance.

Sophisticated instruments that can detect minute amounts of substances in the chicks' brains help him to compare a chick with a certain memory of an event with one that has no such memory. The results are fascinating. It appears from his work that memories do not stay put in one place after they are formed, but divide into multiple copies of themselves and wander off around the brain, perhaps with different parts of the experience stored in different places, in much the same way that a secretary might photocopy an important document and send copies to relevant departments.

Scientists now believe in a 'connectionist' view of the brain, where the millions upon millions of nerve cells act as a vast network or grid. Professor Rose and his colleagues suggest that sensations, from a bitter- tasting substance to the death of a loved one, stimulate electrical activity in nerve cells that trigger new connections to sprout between the cells. This constant rewiring of the brain, and the need to keep it primed for electrical activity, could account for why it is such an energy-hungry organ. Could the new connections also be the physical manifestation of memory?

The truth, no doubt, will prove more complex that this. The power of the human brain remains one of the greatest achievements of natural selection. Each of us can, for instance, recognise up to 10,000 human faces - one Roman general, Publius Scipio, was said to be able to put names to his entire army of 35,000 men.

But most of us are not Scipio-like. Somehow the brain carefully filters out the important things in life for long-term storage. The trivia gets weeded out, or so we hope. (How many times has an idiotic tune haunted you for days on end?)

For all its importance, the brain remains an enigma. We know it consists of about ten thousand million nerve cells and there are ten million million connections and pathways between these cells. We also know a little about the enormous amount of energy consumed by the brain. Despite weighing only a 50th of the body's weight, the brain uses about a quarter of the energy we need in a typical day. An ounce of brain burns more calories than an ounce of muscle uses during exercise. Being conscious is harder work than running to work.

This is all very well, but what does it tell us of memory? The answer is, nothing very much. The molecules of the brain, like those in the rest of our body, are changed completely many times over during our lifetime. The connections between nerve cells are broken and replaced perhaps thousands or millions of times. And yet our memories remain, we hope, intact.

To complicate matters further, Professor Rose also suspects that the way we remember changes in the transition from child to adult. Childrens' minds, he says, are more open. They see and hear 'eidetically', meaning they remember in a pictorial form. This is lost in older life. A 30-year-old man has memories of being a 10-year- old child that are quite different from the way at 50-year-old man remembers being 30 - even though the time difference is the same.

We thus return to the brain as a filter of perceptions. Adults use their brains as much to get rid of experiences as to remember them. Whether we will ever really understand how it does this will remain conjecture for some time to come.

Professor Rose believes we should eventually learn the secrets of our own minds, but only by first looking at simpler forms of life. 'In researching my chicks' memories, I can begin to make sense of my own,' he writes.

'The Making of Memory', Steven Rose, Bantam Press, pounds 16.99.