A team of researchers led by psychologist Christoph Herrmann wants to improve speech comprehension with a special form of electrical brain stimulation. In a joint project funded by the Federal Ministry of Education and Research, the Oldenburg researchers and their partners want to develop a new hearing aid.
"Peter gives four white cups". The test subjects hear simple sentences like this. But their task is not easy. Anyone sitting on Prof Dr Christoph Herrmann's chair in the laboratory - electrodes on their head, headphones on their ears - has to understand the words despite the noise. Because that's what it's all about: the psychologist and his colleagues want to develop a device that will help them to make speech more comprehensible for people with hearing impairments.
Making out the sentences of a conversation partner from the babble of voices in a restaurant, for example, is a challenge, especially for older people and those who wear hearing aids. Scientists refer to this as the cocktail party effect. The problem is that even modern hearing aids cannot compensate for this, as the devices do not receive any information about which conversation partner the wearer is trying to follow.
Sharpening perception in a targeted manner
"In these situations, transcranial electrical brain stimulation could provide a remedy," says Herrmann. The idea behind this method is to use an electric current to sharpen the perception of a specific sound source in the brain from the outside. Other researchers have already identified the method as promising using mathematical models and in experiments with rats. In a recently published study, Herrmann, together with his colleague Dr Anna Wilsch and other colleagues from Lübeck and Salzburg, showed that this method also works in principle in humans.
In order to improve speech comprehension with transcranial electrical brain stimulation, a computer programme developed by the Oldenburg researchers first evaluates a sound signal reaching the ears, for example a spoken sentence, and calculates the so-called envelope curve. This is the rough structure of the sound. This signal is transmitted as a weak alternating electric current via two or more electrodes attached to the scalp through the temporal lobe - the region in which the brain processes auditory information. "In this way, we synchronise the electrical brain activity, which can be measured during hearing, with the external current source and thus amplify perception," explains Herrmann.
Improving speech intelligibility
The strength of the current that the test subjects receive during the experiment is just high enough for them not to feel anything. In the current study, a total of 19 young, healthy test subjects received such stimulation while listening to sentences consisting of five words - overlaid with noise of varying intensity. The scientists also carried out control measurements in which either no current at all or only a slight direct current flowed through the electrodes. Neither the test subjects nor the scientists knew which experimental situation was being played out.
One result: compared to the control measurements, the test subjects actually understood the sentences significantly better when they received transcranial brain stimulation, despite the noise. However, for optimal stimulation, the researchers need to apply the electrical signal with a time delay to the sound signal. "It takes a while before a signal from the ear reaches the auditory centre of the brain, the auditory cortex. But the electrical stimulation only takes effect once the signal has reached the cortex," explains Herrmann. At the beginning of the experiments, the scientists did not know the duration of this delay. Through various test runs, they discovered that this time delay is in the range of tenths of a second and varies from person to person.
In the meantime, the scientists have also carried out tests on people with hearing loss. "The gain in speech intelligibility was even greater in these people than in healthy people," says Herrmann. Ultimately, the aim is not to make the brain of a healthy person more efficient. "We want to help those affected." The psychologist has applied for a patent for the principle of brain stimulation.
Linking EEG, brain stimulation and hearing aid
In order to actually develop a device, Herrmann is also cooperating in the joint project "Mobile EEG-based brain stimulation to improve hearing" with a manufacturer of brain stimulators, a hearing aid manufacturer, the Oldenburg Hearing Centre, the Fraunhofer Institute for Digital Media Technology and the University of Siegen. The Federal Ministry of Education and Research (BMBF) is funding the project with a good two million euros over three years.
Before the technology is suitable for everyday use and compact enough, Herrmann and his partners still have a few hurdles to overcome. For example, it is not clear how long the effect of the brain stimulation will last. The Oldenburg scientists are also working on more realistic dialogue situations. At the end of the project, the researchers also want to measure brain activity using electroencephalography (EEG) during the speech comprehension tests. "In the EEG, we can see which of several speakers someone wants to concentrate on," says Herrmann.
The researchers want to pass this information on to existing hearing aids. This is because they can already calculate the direction from which the sound is coming from the differences in level and duration. "If the hearing aid also knows that its wearer wants to listen to a speaker on the left, even though they are looking straight ahead, then it could use this information accordingly." Ultimately, the researchers want to show in the project that it is feasible to couple EEG, brain stimulation and hearing aid with each other so that hearing impaired people can actually understand speech better in complex situations. The end result will not be a finished, wearable product. "But our goal is to at least develop a prototype," says Herrmann.