Has the origin of tinnitus been found?

To hear a unified sound, our brain synchronizes using gamma waves. This discovery made by neurolinguists from the University of Zurich (UZH) leads to new approaches in the treatment of tinnitus. The results of their work have been published in the PNAS.

Each hemisphere has a role

How come we don’t hear everything twice when our ears are placed on opposite sides of our head and most sounds don’t reach both of our ears at exactly the same time? “While this helps us determine which direction sounds are coming from, it also means our brains have to combine information from both ears. Otherwise we would hear an echo”, says Basil Preisig, researcher at the Department of Psychology at the University of Zurich. In addition, the sound that enters through the right ear first reaches the left hemisphere of the brain and that which arrives through the left ear first affects the right hemisphere. When processing speech, each hemisphere has its role: the left is responsible for distinguishing between phonemes and syllables, while the right side recognizes the prosody and rhythm of speech.

Although each hemisphere receives information at a different time and processes different speech characteristics, the brain manages to integrate what it hears into a unified speech sound. This mechanism was previously unknown. Previous studies have indicated that measurable oscillations caused by the brain, gamma waves, are involved in this process. In this new study, the researchers demonstrated that the process of integrating what we hear is directly related to gamma wave synchronization.

Gamma waves organize synchronization

For this research, the scientists asked 28 volunteers to repeatedly solve listening tests: an ambiguous syllable (a speech sound between ga and da) was played in their right ear while a click containing a fragment da or ga syllables was played unnoticed in the left ear. Depending on what was played in their left ear, participants heard ga or da and had to report the sound they heard. During the test, the researchers tracked brain activity in both hemispheres using functional magnetic resonance imaging (fMRI).

During the experiments, the researchers disrupted the natural activity pattern of gamma waves by stimulating both hemispheres of the brain with electrodes attached to the head. This manipulation affected the participants’ ability to correctly identify the syllable heard. fMRI analysis showed that there are also changes in the activity of neural connections between the right and left cerebral hemispheres with a change in the strength of the connection according to the influence of the rhythm of gamma waves by electrical stimulation in both cerebral hemispheres synchronously or asynchronously.

A way to treat hearing problems

The synchronization of gamma waves appears to serve to balance the different inputs from the two hemispheres of the brain, providing a unified auditory impression. “Our results suggest that gamma wave-mediated synchronization between different brain areas is a fundamental mechanism for neuronal integration.”, noted Basil Preisig. “Furthermore, this research shows for the first time, using human hearing as an example, that the connection between the two hemispheres of the brain can be successfully modulated by electrostimulation.”, adds Alexis Hervais-Adelman, head of neurolinguistics at UZH.

These results represent a promising avenue for treating hearing disorders, such as tinnitus. “Previous studies show that disturbances in the connection between the two hemispheres of the brain are associated with auditory phantom perceptions such as tinnitus and verbal auditory hallucinations.concludes Basil Preisig. Thus, electrical brain stimulation may present a promising avenue for the development of therapeutic interventions..”