The team hope their work could aid diagnosis of attention disorders

Now scientists believe they have shown the brain remains in listening mode even when the only sound is silence.

French researchers showed parts of the brain related to hearing were activated in subjects listening out for a sound.

They hope the study, in the Journal of Neuroscience, may aid work on attention deficit disorders which are linked to problems in the same part of the brain.

They also hope it could improve rehabilitation programmes and treatments for such disorders.

'First study'

Researchers Pierre Fonlupt and Julien Voisin at Inserm, France's national research institute, watched the brains of 11 subjects through a magnetic resonance scan while they were at rest, listening to silence and listening to an actual sound.

In the listening-to-silence trial, subjects were told they would eventually hear a sound in the right or left ear indicated by a glowing arrow.

Their work suggested that while the subjects were listening to silence, the part of the brain related to sound - the auditory cortex - was activated on the opposite side to where they were expecting the sound.

They also found the auditory cortex was activated when the subjects were listening to actual sounds.

Two other areas in the front of the brain, also linked to hearing, were also activated regardless of which side the sound was due to be heard, the study found.

What I found most remarkable is that the areas of the brain that are shown to be affected in the research are exactly the areas affected in ADHD

Dr Katya Rubia

Mr Fonlupt said: "This is the first study in humans to show that the auditory cortex is activated when a subjects is attending to and listening to silence, when expecting an upcoming sound."

The results seem to parallel earlier studies showing parts of the brain linked to seeing were activated when subjects were expecting to see a stimulus.

"These findings may spark further research aimed at uncovering more selective pre-activations of auditory and other sensory areas," said Mr Fonlupt.

Robert Zatorre, of the Montreal Neurological Institute of Canada, said: "This finding tells us something about the mechanisms in the brain that are important for attention.

Noisy

"When you expect an event to occur, your brain is highly sensitised to that possibility."

Dr Katya Rubia of the Institute of Psychiatry said the work was very interesting and may aid the clinical understanding of attention deficit hyperactivity disorder in which she specialises.

She told the BBC News website: "What I found most remarkable is that the areas of the brain that are shown to be affected in the research are exactly the areas affected in ADHD."

She suggested similar listening tests as the ones undertaken by the subjects may help to train the brain to focus.

Sufferers of the disorder find it difficult to distinguish the relative importance of sounds around them and often describe their world to be too noisy to concentrate.