IN 1953 a physician named Louis Sokoloff laid a 20-year-old college student onto a gurney, attached electrodes to his scalp and inserted a syringe into his jugular vein.

For 60 minutes the volunteer lay there and solved arithmetic problems. All the while, Sokoloff monitored his brainwaves and checked the levels of oxygen and carbon dioxide in his blood.

Sokoloff, a researcher at the University of Pennsylvania in Philadelphia, was trying to find out how much energy the brain consumes during vigorous thought. He expected his volunteer’s brain to guzzle more oxygen as it crunched the problems, but what he saw surprised him: his subject’s brain consumed no more oxygen while doing arithmetic than it did while he was resting with his eyes closed.

People have long envisaged the brain as being like a computer on standby, lying dormant until called upon to do a task, such as solving a Sudoku, reading a newspaper, or looking for a face in a crowd. Sokoloff’s experiment provided the first glimpse of a different truth: that the brain enjoys a rich private life. This amazing organ, which accounts for only 2 per cent of our body mass but devours 20 per cent of the calories we eat, fritters away much of that energy doing, as far as we can tell, absolutely nothing.

“There is a huge amount of activity in the [resting] brain that has been largely unaccounted for,” says Marcus Raichle, a neuroscientist at Washington University in St Louis. “The brain is a very expensive organ, but nobody had asked deeply what this cost is all about.”

Raichle and a handful of others …