How does our brain form an overall impression from various pieces of information? In the case of hearing, researchers have now been able to show that the slightly different hearing impressions of the two ears are synchronized with the help of gamma waves. In this way we can perceive what has been said as uniform speech sounds. This proof was achieved by specifically influencing the hearing system using electrical stimulation. The results are therefore also of importance for the development of therapies against the perception of phantom noises in tinnitus, say the scientists.
Our ears are on opposite sides of the head and most of the sounds reach the auricles with a slight delay. They give our brain a slightly different input and there is an important reason for this: “This helps us to determine from which direction noises come. However, this also means that our brain has to unite the information from both ears. Otherwise we would hear an echo, ”says Basil Preisig of the University of Zurich. As he explains, there is also a need for further synchronization, because the information from the right ear first reaches the left hemisphere and that from the left ear first reaches the right. Studies have already shown that the two different hemispheres of the brain take on different tasks in speech processing: the left side is responsible for analyzing the syllables, while the right side records the speech melody.
On the trail of the integration process
In order to create the impression of a uniform speech sound, the brain must therefore combine the information that has been shifted over time and the analysis results from the two hemispheres of the brain. Earlier studies have already provided indications that special vibration patterns of nerve activity in the brain play a role: the so-called gamma waves. These are signals in the frequency range above 30 Hertz, which were determined in connection with certain brain activities. For the first time, Preisig and his colleagues wanted to directly demonstrate the extent to which there is a connection between the integration of what is heard and the synchronization through gamma waves.
To do this, they carried out experiments with 28 volunteers who were asked to repeatedly solve a listening task. You heard an ambiguous syllable in your right ear – a speech sound between “ga” and “da”. At the same time, there was a subtle click in the left ear that contained either a fragment of the syllable da or ga. For each repetition, the subjects had to state what they had heard. It was shown that, depending on which click sounded, the test subjects either perceived one or the other syllable. During this process, the scientists also recorded the activity in the two hemispheres using functional magnetic resonance imaging (fMRI). The decisive aspect of the experiments was then electrical stimulation, which was conveyed to the test subjects by means of electrodes on the head: Through these, the scientists specifically disrupted the natural activity pattern of the gamma waves that arose in the two halves of the brain during the tasks.
Detection procedure with application potential
The manipulation, they report, affected the subjects’ ability to correctly identify the syllable they heard. In this context, the fMRI analysis made it clear that this led to changes in the activity of the nerve connections between the two cerebral hemispheres: Depending on whether the researchers created the rhythm of the gamma waves using electrical stimulation in the two hemispheres to be synchronous or asynchronous, changed the strength of the connection. Corresponding disorders were then associated with a deterioration in the ability to integrate, the tests showed.
The researchers now see these results as evidence that the various inputs of the two hemispheres of the brain are compared with each other through the synchronization of the gamma waves, which leads to a clear acoustic impression. “Our results thus also support the explanation that the synchronization between different brain areas mediated by gamma waves is a fundamental mechanism for neuronal integration,” says Preisig.
His colleague Alexis Hervais-Adelman from the Institute for Psychology at the University of Zurich adds: “In addition, using the example of human hearing, the results show that the connection between the two halves of the brain can be successfully modulated by electrical stimulation”. As he explains, this possibility could have important clinical significance: “Earlier studies show that disorders of the connection between the two hemispheres of the brain are associated with auditory phantom perceptions such as tinnitus and hearing voices,” said Preisig. “This means that electrical brain stimulation could represent a promising way of developing therapeutic interventions,” the scientist hopes.
Source: University of Zurich, specialist article: PNAS, doi: 10.1073 / pnas.2015488118