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The brain can learn to hear again

The world of sounds should not be closed to deaf people. So-called cochlear implants compensate for the lack of sensory cells in the inner ear and can restore hearing to many of those afflicted with deafness. Yet the procedure still has its failings. Prof. Robert Illing from the neurobiologial research laboratory at the Department of Otorhinolaryngology at the University of Freiburg explores the plastic properties of the nervous system that facilitate the application of these implants at the same time as having limitations. His findings help to boost the use of the brain’s ability to learn and to optimise successful therapy.

Most deaf people lack hair cells within the cochlea, the sensory apparatus of the inner ear. This means that the sound that reaches the spiral channel of the organ is not detected and translated into an electrical signal. This deficit can be congenital or it can appear, for example, as a result of an infection. The result is always the same: The brain remains isolated from the world of sound. It “hears” nothing.
Cochlear implants, which are being successfully used by the Freiburg Department of Otorhinolaryngology, can compensate for this defect. The implants receive sound waves via a microphone on the outer ear, which are forwarded to a cluster of 24 delicate electrodes inside the cochlea. The electrodes stimulate the neurons of the acoustic nerve and – depending on the received pitch – the current reaches other populations just as it would in an intact ear. In this manner, a cochlear implant represents a bridge between the outside world and the brain and provides a crude image of the audible environment with differences in pitch and volume.

“The therapy can be extremely successful, especially for young patients with relatively plastic brains,” says Prof. Robert Illing of the neurobiologial research laboratory at the Department of Otorhinolaryngology. “In an ideal scenario, a congenitally deaf child can develop normal speech”. However, the enjoyment of music will still be limited and deaf adults are even more restricted. Older patients complain that other peoples’ voices sound distorted. In many cases, doctors are satisfied when patients are able to react to alarm signals such as oncoming cars or horns.
Via the red synapses (arrow), sensory signals are transferred from the inner ear to the green synapses (arrow head) in the brainstem of a rat. They change their structure and function as a result of deafening and stimulation through a cochlear implant. Sc
Via the red synapses (arrow), sensory signals are transferred from the inner ear to the green synapses (arrow head) in the brainstem of a rat. They change their structure and function as a result of deafening and stimulation through a cochlear implant. Scale: 50 micrometres. (Figure: Work group Prof. Robert Illing)
The reason for these limitations is that the brains of deaf people are not used to the multitude of stimulations from the audible environment. If the time that elapses between the onset of deafness and the implantation is too long, the nervous system restructures itself and the circuits used for hearing take on new functions. They “forget” how to hear. Children who are born deaf never develop these circuits at all. Thus, cochlear implants are intended to stimulate the neurons of the acoustic nerve in such a way that synapses become reorganised and connect all over again. Scientists aim to force the brain to use its entire plastic capacity and to get used to the electrodes in the inner ear. However, in order to achieve this, scientists need to know which pattern of stimulation best mobilises the plastic processes of the synapses.

Reorganisation processes of the synapses

“The audible world consists of complicated patterns of different pitches, volumes and their temporal course,” says Illing. “The electrodes within the cochlea must translate this into a language that the brain can understand”. In order to understand what kind of input makes the neurons within the acoustic nerve especially “adaptive”, Illing and his group are examining the brains of deaf rats after receiving a cochlear implant. The scientists vary the intensity, the frequency and the temporal course of the stimuli that are transferred from the electrodes to the neurons within the rat’s brain. The aim is to encourage the brain to undergo intense reorganisation.
As a result of a change in the sensory stimulation, molecules (arrow heads) have appeared at the axon terminal in the auditory pathway of a rat and they play an important part in the experience-dependent reorganisation of synapses. v: synaptic vesicle, m:
As a result of a change in the sensory stimulation, molecules (arrow heads) have appeared at the axon ending in the auditory pathway of a rat and they play an important part in the experience-dependent reorganisation of synapses. v: synaptic vesicle, m: mitochondrium. Scale: 50 nanometres. (Figure: Work group Prof. Robert Illing)
Illing and his colleagues have realised that connections within the acoustic nerve change or even arise anew on the basis of biochemical processes that begin after stimulation at the synapses. Initially, genes that encode something like the first concept for reorganisation are switched on. These genes encode regulatory proteins that in turn orchestrate the production of additional regulators as well as structural proteins. Finally, the resulting products change the structure of a synapse as well as its electrophysiological behaviour. Illing and his colleagues are able to visualise these newly developed proteins with the aid of current microscopic methods and can thus assess the extent of a construction site.

“Our experiments have shown that the plasticity within the acoustic nerve particularly depends on the temporal structure of the stimulation,” says Illing. “It depends on the sequence of sounds and not on their volume”. Thus, cochlear implants are particularly aimed at improving the temporal coding of sound information before it is transmitted to the neurons in the acoustic nerve. If this is achieved then deaf people can look forward to better hearing in future.

mn – 3rd June 2008
© BIOPRO Baden-Württemberg GmbH
Further information:
Prof. Dr. R.-B. Illing
Neurobiological Research Laboratory
Department of Otorhinolaryngology at the University of Freiburg
Killianstr. 5
D - 79106 Freiburg, Germany
Tel.: +49 (0)761 270 4273
Fax.: +49 (0)761 270 4075
E-mail: robert.illing@uniklinik-freiburg.de
Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/the-brain-can-learn-to-hear-again