In DBS, an electrode is implanted into a particular area of the brain, which, when stimulated, significantly alleviates movement disorders such as the tremors of Parkinson's disease. No one knows how it treats depression.
One theory is that the stimulation leads to alterations in neural pathways and neurotransmission that mimic the work of antidepressants.
Barry Beyerstein, professor of psychology at Simon Fraser University in British Columbia, has collected evidence on the use of DBS. The whole brain has now been probed by electrodes and the fact that no large "silent" dormant area has been found is, he argues, the final proof that there are no large untapped reservoirs.
This flies in the face of what many self-help authors and motivational speakers tell us: that we routinely use only 10 per cent of our brain. This myth, trotted out as a self-evident truism, is vital to the self-helplife-coach industry, which needs a neuroscientific scapegoat to explain why people are not as productive and happy as the positive thinkers maintain they ought to be.
Professor Beyerstein has made it his personal mission to dispel the 10 per cent myth. His evidence suggests that decades of work using electrodes to stimulate deep brain structures has failed to uncover any "freeloading" nine-tenths. The 10 per cent belief betrays a basic misunderstanding of brain function, he says.
He argues that, because the brain has been shaped by natural selection and is one of the most metabolically expensive organs to grow and run, it makes no evolutionary sense for our bodies to build massively under-utilised tissue and so squander precious metabolic resources. At 2 per cent of total body weight, the brain accounts for 20 per cent of our resting oxygen consumption. As Professor Beyerstein puts it: would you pay huge bills to heat all 10 rooms of your home if you never strayed beyond the kitchen?
But why is he so exercised by the 10 per cent myth? After all, surely it's relatively harmless and perhaps even inspirational and motivating: it might even get us using our brains more effectively. He argues that it's a dangerous idea because it's used by those trying to sell some scientifically dubious product, seminar, book or idea.
But the 10 per cent myth seems to be maintaining its attractiveness and is proving difficult to extinguish. Television science documentaries could be partly to blame. Many tell the stories of patients who remain remarkably high functioning, even though a progressive brain disease has wiped out vast portions of their cortex. But this is actually a testament to the remarkable ability of the brain to reorganise and survive. New interconnections develop between neurons, allowing some adaptation to loss of tissue, a response that in itself must mean there is no bank of unused cells waiting to be drawn on during times of brain overdraft.
All the more reason to be careful and look after what brain you have, by not making any unnecessary withdrawals.
Dr Raj Persaud is Gresham professor for public understanding of psychiatry, and director of the Centre for Public Engagement, King's College London.
His latest book is The Motivated Mind (Bantam Press). Email: email@example.com