Basic Science Advances Promise Cochlear Implant Gains
by James Cavuoto, editor
February 2021 issue
The cochlear implant industry has experienced significant growth in the last 20 years, as industry leaders Cochlear Ltd., Advanced Bionics, and Medel have offered a steady stream of enhancements to their devices and sound processing algorithms. Still, the field has been limited somewhat by the number of addressable auditory channels—increasing the number of electrodes doesn’t necessarily deliver a more robust frequency spectrum.
New research from Sweden promises to offer CI vendors more opportunity to deliver broad-spectrum sound to their users. A team at Uppsala University created the first 3D map of the auditory nerve showing where the various sound frequencies are captured. Using a synchrotron X-ray imaging process, they were able to map the neural connections in the cochlea and find out exactly how the frequencies of incoming sound are distributed. The study was published in Scientific Reports.
“This can make treatment with cochlear implants,” said Helge Rask-Andersen, professor of experimental otology at Uppsala. Sound waves have differing frequencies—that is, the number of vibrations they make every second varies according to whether it is a high-pitched sound, which causes more vibrations per second, or a low-pitched one, which results in fewer. The human ear can perceive frequencies of between 20 and 20,000 Hz.
When the sound waves are captured by the cochlea of the inner ear, fibrous connective tissue and sensory cells separate the various frequencies. High-frequency sounds reach the sound-sensitive hair cells in the lower part of the cochlea, while low-frequency sounds are absorbed in the corresponding way in the upper parts of the cochlea.
The researchers have now studied the details of this process, almost down to the cellular level. Since the synchrotron X-ray radiation is too strong to be used on living human beings, donated ears from cadavers were investigated instead. This research made it possible to work out the locations of the various frequencies in the cochlear nerve, and enabled the creation of a three-dimensional tonotopic frequency map.
“This kind of map is comparable to a piano, with the keys being analogous to all the similarly coded frequencies. Unlike the piano, which has 88 keys, we have about 3,400 internal auditory hair cells, all of which encode distinct frequencies. The hair cells are attached to a 34-mm-long basilar membrane, and are also tuned by 12,000 outer hair cells so that we can hear every volume level. This information is mediated to the brain via 30,000 precisely tuned fibers in our hearing nerve,” said Rask-Andersen.
Human ear canals and nerves are not entirely uniform in appearance. The researchers therefore think the new knowledge may prove immensely important for people who, owing to grave hearing impairments, have CIs inserted. Showing exactly what the patient’s cochlea looks like enables the technology to be individualized better and each area stimulated with the right frequency.