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A team of neuroscientists of the University of Oldenburg has discovered how similar sounds are converted into neural patterns. The study is based on investigatons of electrical stimuli in the auditory nerve of Mongolian gerbils. The results may aid future studies on the causes of age-related hearing loss.
In the search for the causes of age-related hearing loss, researchers at the University of Oldenburg have followed a new, promising method. They have deciphered the code that is used to transmit speech sounds from the ear to the brain, as they report in the scientific journal eNeuro. Based on these data and further experiments, they aim to find out whether and how this code changes with age.
Neuroscientists Dr Amarins Heeringa and Prof. Dr Christine Köppl have investigated the electrical stimuli that the auditory nerve of Mongolian gerbils transmits to the brain when they hear different sounds. A key challenge in hearing is to separate important information from background noise that constantly surrounds people as well as animals. To understand how vowels are encoded by auditory nerve fibres the Oldenburg scientists presented the animals with various monosyllabic speech samples, embedded in background noises that resembled a babble of voices.
Pauses between spikes provide information on vowels
Put simply, the ear of vertebrates converts acoustic signals into electrical signals. This happens in the sensory cells in the inner part of the ear, the cochlea. The auditory nerve receives these signals from the sensory cells of the cochlea and transmits them as a series of electrical impulses, so-called spikes, to the brain. These are further processed in the brain and ultimately lead to a conscious perception of what has been heard.
The experts set out to detect the patterns representing different vowel sounds in this complex signal. They found that the pauses between the spikes rather than the number of nerve impulses itself provide the information on how to discriminate different vowels. The neuroscientists charted these different time patterns, known as spike timing patterns, in graphs, and were able to identify differences depending on whether a sound containing an "a", an "e" or an "i" was played. The patterns for the sounds "e" and "i", which were easily confused in German by gerbils and humans, resembled each other. The researchers conclude that the "mishearing" of these similar-sounding vowels originates already in the ear rather than later during further processing in the brain.
A complementary study published in the scientific journal Hearing Research by Carolin Jüchter, Dr. Rainer Beutelmann and Prof. Dr. Georg Martin Klump, also from the University of Oldenburg, had shown that Mongolian gerbils can distinguish between the different vowels in human speech. The team taught the animals to leave a waiting position on a platform when they perceived a sudden change in a speech sample that was played over and over again in an environment with background noise.
Mongolian gerbil suitable model for research on human hearing
In a similar experiment, human listeners were asked to identify the same sound changes. It turned out that both humans and animals found it easier to distinguish between vowels than between consonants. Interestingly, in these experiments the gerbils had problems distinguishing between the same vowels as in Heeringa's experiments. Despite minor differences between humans and animals, the researchers conclude that the Mongolian gerbil is a suitable model for research on human hearing.
The two lead authors of the studies, Amarins Heeringa and Carolin Jüchter, work in the research groups "Cochlea and Brainstem Physiology" and "Zoophysiology and Behaviour", which collaborate within the Cluster of Excellence Hearing4all. They are particularly interested in understanding the physiological changes that occur in the cochlea during age-related hearing loss. Their next step will be to study older Mongolian gerbils.
Amarins Nieske Heeringa (amarins.nieske.heeringa@uol.de)
Carolin Jüchter (carolin.juechter1@uol.de)
Department of Neuroscience
University of Oldenburg,
https://doi.org/10.1523/ENEURO.0474-21.2021
https://doi.org/10.1016/j.heares.2022.108472
https://uol.de/en/neurosciences
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