Some new findings are suggesting that tinnitus and depression are linked in the brain’s limbic system.

The thinking used to be that depression was a natural reaction to the sudden onslaught of continual noise in the head and ears that robs people of sleep, concentration and the enjoyment of life. But these new findings seem to show that the link between tinnitus and depression go much deeper.

Researchers from the National Institute on Deafness and Other Communication Disorders (NIDCD) have theorized that the human limbic system can make some people more vulnerable to tinnitus than others and perhaps cause depression.

The researchers’ theory is that the limbic system—a linked network of brain structures involved in emotion, behavior, and long-term memory—acts as a gatekeeper to keep the tinnitus signal from reaching the auditory cortex, the part of the brain that mediates our conscious perception of sounds. In people with tinnitus, they suggest, the gate has “broken.”

In previous studies, Josef Rauschecker, PhD, DSc, and his colleagues in the Department of Neuroscience, the Division of Audiology, and the Department of Otolaryngology at Georgetown University had noted a significant loss of volume in the medial prefrontal cortex (mPFC), an area located in the frontal lobe of the brain, of people with tinnitus. The finding was intriguing because although this isn’t an area of the brain that processes sound, it is an area that has the potential to modulate sensory information—such as sound—because of its intimate connections with key structures in the limbic system.

The fact that tissue loss in this particular area was a prominent feature was puzzling, but changes had been noted before in other structures associated with the limbic system in people with tinnitus. Previous researchers had tended to interpret the involvement of the limbic system as a reflection of the emotional reaction to the sound. The depression that often accompanies tinnitus (and that has deep roots in the limbic system) was seen as a normal reaction to an upsetting condition that wouldn’t go away.

However, Rauschecker’s team wondered if the damage they saw in the mPFC wasn’t a consequence of tinnitus, but instead a part of its cause.