In the complex search for the cause of age-related hearing loss, researchers at the University of Oldenburg have discovered a promising approach. They have deciphered the code with which certain sounds are transmitted from the ear to the brain. They report this in the scientific journal eNeuro. With this knowledge and further experiments, they want to find out in future whether and how this code changes with age.
Neuroscientists Dr Amarins Heeringa and Prof Dr Christine Köppl have investigated what information the auditory nerve of Mongolian gerbils transmits to the brain as electrical stimuli when different sounds are played to the animals. Because a major challenge in hearing is to distinguish the desired information from other background noises that constantly surround humans and animals, they not only played various monosyllabic speech samples to the animals, but also simultaneously played background noises that resembled a babble of voices.
This soundscape of speech samples and background noise reaches the middle ear as a sound wave that causes the eardrum to vibrate. The vibrations are transmitted via the auditory ossicles to the cochlea and finally to the sensory cells, which convert the vibration into electrical impulses, which in turn are transmitted by the auditory nerve. The ear thus translates sounds into a code of electrical impulses, which is decoded in the brain and ultimately becomes the conscious perception of what is heard.
Finding the exact information in this complex code, which transmits the entire soundscape, with which the different vowel sounds are transported, presented the experts with a particular challenge. As they discovered, the different sounds do not cause different numbers of nerve impulses, but rather different lengths of pauses between these impulses.
The neuroscientists depicted these different time patterns graphically and recognised differences depending on whether a sound was played with an "a", an "e" or an "i". However, the patterns for the sounds "e" and "i", which also sound similar and are therefore easily confused, were similar. From this, the researchers conclude that the "interrogation" of these similar-sounding sounds is already caused in the ear and not only during decoding in the brain.
A partner study by Carolin Jüchter, Dr Rainer Beutelmann and Prof Dr Georg Martin Klump, also from the University of Oldenburg, which was published in the journal "Hearing Research", had previously shown that Mongolian gerbils can actually distinguish between the different vowels in human speech. The team taught the animals to leave their waiting position on a platform when, in an environment with background noise, they realised that a repeatedly played speech sample suddenly changed. In a comparative test, human test subjects also had to indicate such sound changes. Both humans and animals found it easier to distinguish between vowels than between consonants. And it also turned out that the animals actually had problems distinguishing the vowels in practice, which had similar timing patterns in Heeringa's experiments. Despite minor differences in humans and animals, the researchers conclude that the Mongolian gerbil is suitable as a model for studying human hearing.
The two first authors of the studies, Amarins Heeringa and Carolin Jüchter, work in the "Cochlear and Brainstem Physiology" and "Zoophysiology and Behaviour" departments, which collaborate in the Hearing4all cluster of excellence, among others. Their joint research interest is particularly focussed on the question of what exactly changes in the ear with age-related hearing loss. In order to find answers to this question, they also want to carry out joint studies on older Mongolian gerbils.
