2017 ATRR: The neuroscience of tinnitus
Paul Chinnock reviews the current state of research
Neuroscience is the scientific study of the nervous system. It has rapidly expanded in recent years as new techniques have been developed. Probably it is research using various imaging technologies (‘scans’, including MRI and PET) that have received most publicity, but neuroscientists also use a wide range of other procedures including some from the worlds of molecular biology and genetics.
Can neuroscience help discover what exactly is going on in the nervous system, particularly in the brain, when someone is experiencing tinnitus? This topic was discussed in an article (by Dr William Sedley) in the BTA Annual Research Review 2017 and I have summarised the article here – the article can be downloaded in full at the bottom of the page.
Some of these studies were conducted in animals, where various techniques (noise or toxins) were used to damage hearing and to attempt to cause tinnitus. Researchers have to make judgements as to whether the animals are actually experiencing the condition. Although the results from such research have so far been inconclusive, and sometimes contradictory, it is thought that they may be relevant to tinnitus in humans and could therefore help investigate what causes the condition and to evaluate potential treatments. For example, one study found that long or intense periods of noise are more likely to give mice hyperacusis, whereas shorter or milder periods seem to lead to tinnitus. In another study, rhythmic electrical stimulation of the brain led to the disappearance of ‘behavioural evidence’ of tinnitus in rats.
The electrical currents caused by brain activity can be detected by EEG (electroencephalography) devices and it had been hoped that EEG measurements would turn out to show different patterns in tinnitus patients than in people without the condition. Disappointingly, however, the most recent research has not shown up any such differences. Another disappointment has been the failure to find any consistent differences between people with or without tinnitus in terms of the physical structure of the brain; where any such differences have been found, they seem to disappear when factors such as ageing and hearing loss are accounted for.
One study, however, found that a particular type of ‘brain wave’, detectable by EEG following unexpected events, occurs at a reduced level in tinnitus patients. In other research, tinnitus patients who had mild hearing loss were compared with others whose loss was more severe. EEG results showed increased levels of electrical activity in the auditory areas of the brain in the mild hearing loss patients, while for the others an increased level was identified elsewhere, in a part of the brain dealing with memory. It is looking likely that it is not only the auditory areas of the brain that are involved in tinnitus; activity in other regions also plays a part.
While some people with tinnitus do have hearing loss, others do not. It has been suggested that tinnitus patients who have normal results in standard hearing tests actually have a ‘hidden’ hearing loss, detectable when sound stimulation fails to produce the expected level of electrical activity in the auditory nerve. Some recent studies appear to back up this theory but others do not. However, research in this area is yielding some interesting findings, which could ultimately lead to a better understanding of the relationship between hearing loss and tinnitus.
So far neuroscience has only given us small clues to help understand what is going on in tinnitus. William Sedley describes this research as looking for a needle in a haystack. However, in time, it could produce useful results.