What causes tinnitus?
As many as one person in every four is affected by tinnitus. This is a phantom ringing or mechanical sound experience which occurs in the absence of any real sound input, and often in people with prior damage to their hearing. So, why does it happen? Richard Salvi explained his findings to Chris Smith...
Richard - Well, because tinnitus is often perceived as coming from one of your damaged ears, the original theories were tinnitus originates from abnormal activity in your inner ear that's been damaged. However, studies have been done where the auditory nerve that connects the ear to the brain - when that's severed because of a surgical intervention, the tinnitus persists. And this lead people to believe that tinnitus might actually be generated somewhere in the central nervous system, maybe in the auditory pathways.
Chris - And how have you sought to find out which of those two it is with these experiments?
Richard - In these animal experiments, we have a way of inducing tinnitus with a drug that's commonly used by most people in the world - Aspirin. If you take a really, really high dose of Aspirin, it will induce this phantom sound of tinnitus. So, we induced that in animals. We have behavioural techniques where we can interrogate the animal. We can ask the animal, "Are you hearing silence? Are you hearing a sound?"
Chris - And what is that test? How do you do that?
Richard - We train the animal initially to press the right bar under two sound conditions - when they hear quiet or when they hear an amplitude modulated noise, a noise that would sound like "thhssssk, thhssssk, thhssssk, thhssssk, thhssssk, thhssssk, thhssssk, thhssssk!!", or press the left bar if they hear just a steady sound. "thhhhhhhhhhhhssssk!" After the animals learn that task, then we induce tinnitus by giving them a high dose of Aspirin. And on the quiet trials, if they have tinnitus, they no longer hear quiet and they shift their behaviour from the right bar, to the left bar which is associated with the steady sound. So, they're tricked into believing quiet is steady sound.
Chris - And how does that help us to understand physically and physiologically what tinnitus is?
Richard - Once we know that the animals are perceiving this phantom sound, then we can do electrophysiological studies. We can put electrodes into the different parts of the auditory brain or other non-auditory pathways and see what's happening to the neural activity, how it's changed. What we found is parts of the central auditory pathway became very hyperactive when we played a sound to them, whereas, their inner ears became less active. So, it looked like the brain was compensating for the peripheral hearing loss.
Chris - It's rather like if I've got my radio tuned in and there's a little bit of static or interference in the background, and the conversation is rather quiet, I turn up the radio to hear better. But there's still an inherent hiss there which will become louder, and that's what's happening in these animals.
Richard - That's exactly the analogy that I use to explain this. By turning up the volume control, you hear all the static background in your radio station.
Chris - Does this give us any insights then into how we can manage tinnitus better? Now you understand a bit more and you have some physiological proof as to what appears to be going on in the auditory system. Can we shut this off?
Richard - Well, there's ways of mitigating it. One of the most effective ways is if somebody's completely deaf and has tinnitus, you can put in a prosthetic device called a cochlear implant which electrically stimulates the stump of the auditory nerve. And when you turn the cochlear implant on and put information back into the central auditory nervous system, in about 90% of the people, the tinnitus disappears.
Chris - What about people who are the reverse of deaf? Because there are people who hear sounds and they experience them, perceive them, as distractingly loud.
Richard - You're describing a phenomenon we refer to as hyperacusis. So, if you turn up the central gain mechanism, a really weak signal now that comes in from the periphery and reaches the central nervous system, if you've got your volume control turned up too high, what's gonna happen - sounds will be, sound super loud to you. So, we think this central gain hypothesis is a way of explaining both tinnitus and hyperacusis.
Chris - And if one explores the auditory system when these processes are happening, is it just a discrete zone that's affected or do other brain regions affect the process too? So, for instance, is someone sensitive to sound when they're feeling emotionally sensitive, for example?
Richard - Yes. In our study, we found not only the auditory parts of the brain became hyperactive, but an area called the amygdala that assigns emotional significance to sound. A second area that became activated was the reticular formation. This is an arousal area that gets you excited. And a third area was the hippocampus. This is an area that's involved in spatial navigation. We think all of this network might play an important role in tinnitus and hyperacusis.