Brains often unwitting accomplices

To assist the reader's digestion of the facts (regrettably poorly outlined by the authors in parts), each chapter is laced with short (and not so short) anecdotes. One of the more interesting stories revolves around the hippocampus, and its connection to short-term memory.

A few years ago neuroscientists at University College, London, compared the brains of 50 taxi drivers to 50 people with other jobs, in order to determine if the intensive, roughly two-year study taxi drivers must undertake to qualify as a London cabbie had any effect on the size of the brain.

The study, published in the Proceedings of the National Academy of Sciences (2000;97(8):4398-4403), revealed that only one part of the brain was different between the two groups: the hippocampus. The posterior hippocampus of cabbies was on average 7 per cent larger than in the other group, and the anterior hippocampus was 15 per cent smaller. The longer a driver operated a taxi, the larger the disproportion.

This raises the question: does acquiring and utilising the detailed knowledge of London's urban landscape make the hippocampus grow? The differences in hippocampus between the two groups were attributed to active neurons, which secrete growth factors known as neurotrophins. Neurotrophin secretion is a key event in early development.

Another question raised was: what changes in the brain when we learn something? The authors advise that storage of new information is manifested by changes in the strength of connections between neurons, and by changes in which connections are made, rather than in changes of the size of a brain's structure per se.

Given that that hippocampus is widely known to be involved in spatial navigation in humans, researchers observed the hippocampus of other animals for comparison. When studying rats running around a maze, it was observed that neurons of the hippocampus fire only when the rat is in a particular location. Because the rat hippocampus contains millions of neurons, each place in the maze is then associated with hundreds or thousands of neurons that fire when the rat is there, but not before or after. Taken in aggregate, the neurons (both firing and not firing) sketch a landscape composed of place cells, laying out map-like where the rat is.

Similar results were obtained when researchers observed humans playing video games that were set in an urban environment. Again, each place in that virtual world was uniquely recognised, further reinforcing the earlier findings from the taxi study on the workings of the hippocampus.