Tinnitus: What brain changes underlie the ringing in your ears?
You are enjoying a relaxing moment lying down in the grass when you first hear it. A buzzing noise, likely coming from an insect drawn to the warmth of your body. You snap out of your serenity and start swatting in all directions. That is so annoying, you think to yourself. Eventually, you catch the fly, the buzzing stops, and you resume feeling the warm sun on your skin. Unfortunately, for the one in ten individuals that suffer from tinnitus, the buzzing sound never stops.
Tinnitus is the perception of a sound in the absence of an external sound source – a phantom noise. Many individuals with tinnitus hear a buzzing, ringing, hissing, or humming noise constantly, even when in a silent room. As you can imagine, hearing this phantom noise can significantly interfere with an individual’s daily life. In extreme cases, it can cause difficulty in simple tasks such as hearing, concentration, and even thinking! When experienced long-term, it can even lead to insomnia, depression, and anxiety.
Some common correlates of tinnitus include age-related hearing loss, certain prescription drugs, and exposure to loud noise. It can also get worse with other conditions like ear infections, stress, and high blood pressure. Interestingly, it is not the ear that is responsible for tinnitus; the problem lies in the brain. However, the brain changes underlying tinnitus remain unknown, which prevents us from finding a cure.
Previous human and animal research on tinnitus has found changes in brain activity in response to sound. When a sound is heard, the area of the brain that processes this information is called the auditory cortex. Some studies have found patients with tinnitus to have increased brain activity in the auditory cortex in response to sound – a phenomenon also termed as increased sound-evoked activity. However, scientists have not yet found a causal relationship between increased sound-evoked activity in the auditory cortex and tinnitus. Does tinnitus cause increased sound-evoked activity, does increased sound-evoked activity cause tinnitus, or do the two simply co-exist? In a recently published article, Dr. Sarah Hayes, AuD, PhD, at Western University carried out a variety of experiments to answer these questions.
To reliably measure and manipulate brain activity, Dr. Hayes used a rodent model for these experiments. You may wonder, how do we know if rats have tinnitus? In fact, they can tell us! Rats are trained in specialized chambers to indicate if they can hear sound (by moving to the left side of the chamber for a treat) or not (by moving to the right side of the chamber for a treat). Thus, if they tell us that they hear sound without us playing any noise, we can infer that they have tinnitus.
To explore the relationship between increased sound-evoked activity in the auditory cortex and tinnitus, this research group performed two experiments. First, they looked at animal models of tinnitus to see if they exhibit increased sound-evoked activity. Second, they artificially increased sound-evoked activity in the auditory cortex to see if it caused tinnitus.
In the first experiment, to see if tinnitus caused increased sound-evoked activity in the auditory cortex, the researchers first needed to induce animal models of tinnitus. If you recall, exposure to loud noise and certain drugs can cause tinnitus. For this reason, rats were either noise exposed or given a drug called salicylate. In the specialized chambers, both rats that were noise exposed and given salicylate exhibited tinnitus.
To then see if these rats exhibiting tinnitus also experienced increased sound-evoked activity, the brain activity in the auditory cortex was recorded while sound was being played. From this, the researchers interestingly found that only the animals that were given salicylate showed increased sound-evoked activity, not the rats that were exposed to loud noise, although both models exhibited tinnitus. This tells us that increased sound-evoked activity in the auditory cortex is not necessary for the development of tinnitus, and instead, may be a result of only certain causes of tinnitus such as exposure to drugs.
In the second experiment, to see if increased sound-evoked activity in the auditory cortex causes tinnitus, the researchers used a drug called gabazine. This drug has an overall effect of increasing brain activity in the auditory cortex. After drug administration, the rats were then put in specialized chambers and interestingly, they did not show evidence of tinnitus. This finding shows that increased sound-evoked activity in the auditory cortex does not directly cause tinnitus, it can exist on its own.
Overall, Dr. Hayes and colleagues found that the relationship between increased sound-evoked activity in the auditory cortex and tinnitus is quite complex. While the two can co-exist, one can also exist without the other (see Table). When researchers in the future go to explore further treatment options, these findings will be important for them to consider.
Summary of research findings