Electrical currents aid in speech comprehension
Scientists from Maastricht University (UM) and the University Medical Centre Groningen (UMCG) have managed to influence speech comprehension by sending electrical currents through the skull to stimulate the brain area involved in hearing. Previously, UM researcher Lars Riecke demonstrated that applying an electrical current to the brain can help experimental subjects hear beeps. This latest study shows that electrical currents can actually influence the processing of human speech in the brain. The results were recently published in the journal Current Biology.
Some people can hear everything that is said to them, but not understand it. It is not unusual to have difficulty following conversations in a large group or a noisy setting. But people who wear hearing aids in particular know how annoying it is to have to talk to others with a lot of background noise around. Hearing aids amplify not only the target noise, but all other sounds as well. One idea is to piggyback on the principle of a cochlear implant, which can be fitted surgically in people with severe hearing impairments. A cochlear implant converts actual sounds into electrical currents, which are then sent to the auditory nerve. “It occurred to me years ago, why would you not also be able to do that at brain level?” asks Riecke. “Simply deliver sounds to the brain area involved in hearing in the form of an electrical current applied directly to the skull. If that works, patients may be able to avoid having an operation.”
The scientists in Maastricht and Groningen used a technique called transcranial Current Stimulation (tCS). Electrodes are placed on the head above the brain area involved in a certain cognitive function. Next, a weak electrical current of approximately one milliampere – a very safe level, barely enough for the subject to feel – is passed through the electrodes. Several years ago, Riecke demonstrated that sending an electrical current with a particular waveform through the skull influences the subject’s ability to hear sounds with that same rhythm. Now, it appears that applying an electrical current with the more complex waveform of a human speech signal influences the processing and comprehension of speech in the brain. This finding offers interesting prospects for the improvement of hearing aids.
The researchers’ aim in sending an electrical speech signal through the skull was to better attune the brain activity (brainwaves) to the slow sound waves of acoustic speech. To show that this aids in speech comprehension, the scientists developed two experiments. The first is known as the cocktail party scenario. Experimental subjects heard a male and a female voice both talking at once. They were instructed to listen only to one of them – the man, for example – and then to repeat what the man had said. In the second experiment the man and the woman could be heard speaking separately. In both scenarios, the subjects’ comprehension of what was being said improved demonstrably if the electrical current was applied at a specific moment. “It’s similar to the scaffolding effect of lip-reading: if the electrical and the acoustic speech signal are sent at the same time, the speech is easier to understand”, Riecke explains. The next step is to perform this type of research in patients who have trouble with speech comprehension.