The cause of tinnitus - the distracting whoosing and grinding sounds experienced by up to one in four people - has been confirmed by scientists in the US and China.
Tinnitus has plagued some notable figures in history including, famously, Beethoven, who eventually became completely deaf. Sufferers describe the symptom of experiencing sounds, heard only by themselves, which range from whistles and ringing noises, to repetitive, rhythmic mechanical percepts. The sounds are intrusive and disturbing, and can be sufficiently distracting to drive some people to suicide.
What unites the majority of cases is a history of exposure to loud sounds and previous hearing damage. This led initially to speculation that the phantom sounds experienced by tinnitus sufferers were originating in the organ of hearing - the cochlea - in the inner ear. "But that is hard to reconcile," explains University of Buffalo scientist Richard Salvi, "with the fact that cutting the nerve between the ear and the brain doesn't make the tinnitus go away."
Instead, hearing specialists suspect that tinnitus arises from within the brain's hearing system itself and is actually the brain adapting to damage to the cochlea, in the same way that amputees experience phantom pain following the loss of a limb.
To probe how this might be happening, Salvi and his colleagues triggered a state of tinnitus in mice by giving the animals high doses of aspirin. "We trained the animals to press one lever when they heard silence, and a different lever when they experienced [tinnitus] sounds."
Tinnitus-afflicted animals were then examined using brain recording techniques to eavesdrop on the electrical activity in several areas of their nervous systems. These auditory centres showed higher levels of activity compared to normal, corresponding, the team think, to an amplification process.
By attempting to tune in to weak signals coming from the damaged inner ear, the brain increases the neurological noise intrinsically present in the system, resulting in the perception of sounds, which are heard as tinnitus. "In the mice, the hearing centres in the brain increase their activity. It's a bit like turning up the radio to hear a quiet station: the hiss also goes up," explains Salvi.
Interestingly, the new study, published in the journal eLife, also revealed changes in activity in other parts of the nervous system, including the brain's emotional centres, which could help to explain the distressing impact of tinnitus on the moods of sufferers...