Washington, Feb 15 (IANS) The phrase 'perk up your ears' made more sense last year after scientists discovered how the quietest sounds are amplified in the inner ear before transmission to the brain.

When a sound is barely audible, extremely sensitive inner-ear 'hair cells' - neurons equipped with tiny, sensory hairs on their surface - pump up the sound by their very motion and mechanically amplify it.

Richard Rabbitt of the University of Utah reported last spring on the magnification powers of the hair cells.

Now, Rabbitt and Marine Biological Lab (MBL) senior scientist Stephen Highstein have evidence that hair cells perform similarly in another context -- in the vestibular system, which sends information about balance and spatial orientation to the brain.

'The bottom line is we have 'accelerometers' in the head that report on the direction of gravity and the motion of the head to the brain,' says Highstein.

'What we found is they respond with a greater magnitude than expected for very small motions of the head. This brought to mind a similar amplification of very small signals by the human inner-ear cochlea.'

'And, in fact, the vestibular system and the cochlea have a sensory element in common: the hair cells,' Highstein adds.

Rabbitt and Highstein found that, in both the auditory and the vestibular systems, the hair cell response exhibits 'compressional nonlinearity': The lower the strength of the stimulus, the more the hair cells 'tune themselves up to amplify the stimulus', Highstein says.

Toadfish was used for this study. 'What's interesting is the bony fishes evolved some three to four million years ago; subsequently this feature of its hair cells was apparently co-opted by the mammalian cochlea,' says an MBL release.

Evolution conserved this feature, and the mammals later used it to improve hearing sensitivity, Highstein says.