Miércoles 22 de Febrero de 2006, Ip nº 141

Wisdom or senility
Understanding how the brain ages could help to slow deterioration

AT WHAT point does an ageing mind become a liability and not an asset? The answer depends on what that mind is asked to do. If the task requires a wealth of knowledge and experience, then the elders have it. If the job needs sharp and fast thinking, youth triumphs.

The inherent differences between people mean that a quick-witted 60-year-old can outperform a slow-minded 25-year-old in tasks at which youth should prevail. But, in general, from the age of 20, you can see a decline in a person's raw mental agility—recalling a list of objects, grouping objects into classes and replacing words with numbers (or vice versa). The speed of reaction slows, too.

However, in the real world, the acquisition of knowledge will obscure the consequences of age-related decline. Performance in many jobs depends on how much you know and how well you know it, both of which increase well into your 60s. Knowledge tends to decline sharply after 65, but that may be the consequence of retirement rather than its cause.

As the brain grows older, it undergoes complex and poorly understood changes—from the biochemical, to the molecular, structural and functional—that lead it to shrink. The brains of those aged 40 and older decrease in volume and weight by 5% every ten years. This shrinkage, though universal, affects people's mental performance in very different ways. Similarly, other changes in the brain—such as the gradual decrease in the supply of oxygen and sugar—leave some peoples' memories unaffected, while others are searching for their glasses.

Much wisdom about the ageing brain has recently been overturned. It was thought, for example, that the loss of neurons led to forgetfulness, culminating in Alzheimer's disease. Researchers now blame the loss of synapses—connections between neurons. It had been thought that new brain cells grew only in young children, and that thereafter every bump on the head or glass of wine led to a loss of intelligence. Recent work has shown that, given intellectual stimulation, new neurons will grow in adults' brains.

No single mechanism adequately explains why brains age. One theory blames oxidative stress, caused by the onslaught of nasty molecules known as free radicals. Another identifies the influx of calcium into the brain cells. Brain ageing could be caused by changes in the way genes are expressed; or dysfunction among the neuron's mitochondria, which provide cells with energy.

Worryingly for some, evidence is emerging that high stress levels contribute to intellectual clumsiness. Chemical transmitters known as glucocorticoids increase blood pressure and damage the hippocampus, the area vital to learning and memory. Recently, young people with high blood pressure have been shown to be just as susceptible to mental decline as elderly people are. Researchers think that raised stress levels help people cope with the emergency but, over days and weeks, prevent them from learning new information and remembering it. As Jonathan Seckl, of the University of Edinburgh, in Scotland, puts it, “When you are in a fire, you concentrate on where the exit is, not...what the woman next to you is wearing.”

What can be done to promote healthy ageing? You can lower your blood pressure, perhaps through physical exercise, and mental workouts, such as crosswords and sudoku—older people with a history of complex mental activity are less likely to suffer mental decline. But the best protection is to have been born with a big brain.

  16/02/2006. The Economist.