By Eva Borras
Hearing and communicating disorders are more common than we might think. According to the World Health Organisation, disabling hearing loss affects over 466 million people – 5% of the world’s population – and this number is expected to grow to 900 million by the year 2050 (Deafness and hearing loss, 2020).
The cochlea is the spiral shaped bone in the ear that contains the receptor organ for hearing. On a fully functional cochlea, there are 15,000 sensory hair cells. Sound waves through air reach the tympanic membrane and cause vibrations in the middle of the ear, forming a wave of displacement that is detected by the hair cells in the membrane (Wilson, 2008). The brain modulates the cells in order to listen to noises in our surroundings, ‘focus’ in noisy environments, and balance left and right hearing. However, if hair cells in the cochlea are severely damaged or destroyed, there are not enough electrical signals created for the brain to interpret. When this occurs, there is hearing loss, and the use of a hearing aid that amplifies the sound is of little or no benefit at all. To treat this type of hearing loss, cochlear implants were developed.
Cochlear implants are electronic devices that have been used to treat profound bilateral hearing loss, having almost half a million users in 2017 (What is the CI, 2020). Initially, cochlear implants were designed to be single-channelled (channel meaning a path by which electricity is conducted in the wire or electrode), with 1 channel in 1 wire (Mudry and Mills, 2013). During the mid-1980s, the design was improved to consist of multiple channels of processing and stimulation in the cochlea, based on pacemaker technology (Wilson, 2008).This way, cochlear implants provide a clearer representation of surrounding sound to those with a damaged cochlea.
A cochlear implant consists of four main parts. These are: the external microphone, the sound processor worn outside the ear, the transmitter and receiver pair, and finally an electrode array, composed of 12-24 electrodes.
Similar to the hair cells in a functioning ear, the high frequencies of sound (10kHz) are generated by the electrodes at the beginning of the electrode array (near the base of the cochlea), while the electrodes at the end of the array (near the apex) generate low frequency sounds (100Hz) (Wilson, 2008).
Once the sound from the environment has been picked up by the microphone, it is passed to the audio processor, arranges the sound captured in the form of voltage output from the microphone (Clark, 2006). Then the transmitter converts acoustic sounds into electrical impulses and sends it to the internal receiver through the skin to the inner ear. From here, it is passed to the electrode array which produces currents that are passed along the cochlear or auditory nerve, which in most cases is unharmed, providing the brain with the auditory information that allows the user to ‘hear’.
When it comes to restoration of function, the most effective neural prosthesis developed to this day is the cochlear implant. However, there is still much room for improvement.
One of the reasons present-day implants do not have very high channel independence is because of the reduced number of functional channels. There are overlapping excitation fields created from different electrodes (“cross-talk”), which does not allow for there to be more than 4 to 8 functional channels, even if the number of stimulating electrodes is increased. In the future this issue could be solved by either placing electrodes in a closer position to the targeted neurons or by designing a drug treatment to encourage growth of neural tissue toward electrodes, for example (Wilson, 2008).
Due to channel cross-talk present with the current electrical implants, there is a limit to the quality of the sound. This is because each electrode contact in the implant creates an electrical current stimulation that spreads over a wide area, thus causing poor speech recognition when there is background noise. As a result, the individual detects sounds in less detail.
With the aim to provide a more realistic sense of hearing, a study by Keppeler et al shows significant progress with the use of optogenetic stimulation instead of electrical. A 10-LED-chip multichannel optical cochlear implant was designed to stimulate the cochlea by making use of the fact that cells responsible to different frequencies are found in different places in the auditory system. Once it was tested over weeks in vitro to be stable, silicone-moulded optical implants were inserted into transgenic rats via a cochleostomy. The rats expressed the sensory photoreceptor channelrhodopsin (able to control phototaxis). Optogenetic stimulation by behavioural and electrophysiological experiments was carried out and recordings from auditory midbrain showed increase in spectral selectivity. The results show that when the device was turned on in a deaf animal, the mean response rate to stimulus was 92% compared to 50% when it was off, significantly better. Additionally, when the spread of excitation with LED-based cochlea implants (1.9/2.4/3.4/4.1 octaves for a d′ of 1.5/2/2.5/3) was measured to be larger than mono- and bipolar electrical implants (1.6/3.6/6.5/6.9 and 2.2/4.2/6.2/6.9 octaves, respectively), this demonstrated that optogenetics increased spectral quality.
Despite the obvious signs of this device’s success shown in this study, it was reported that issues in the design still had to be addressed before a clinical trial could be carried out, such as the large LED size and the wide range of light emitted. (Keppeler et al., 2020).
In conclusion, it seems optical stimulation has the potential to provide a pivotal advance to resolve the number one challenge for cochlear implant recipients – understanding conversations in a noisy environment (Gil Gonzales, 2019).
Clark, G., 2006. The multiple-channel cochlear implant: the interface between sound and the central nervous system for hearing, speech, and language in deaf people—a personal perspective. Philosophical Transactions of the Royal Society B: Biological Sciences, 361(1469), pp.791-810.
Eurociu.eu. 2020. What Is The CI. [online] Available at: <http://eurociu.eu/what-is-the-ci-> [Accessed 8 September 2020].
Gil Gonzales, A., 2019. The Future Of Cochlear Implants. [online] Medium. Available at: <https://medium.com/swlh/https-medium-com-a-gg-the-future-of-cochlear-implants-5d9febbfff82> [Accessed 8 September 2020].
Keppeler, D., Schwaerzle, M., Harczos, T., Jablonski, L., Dieter, A., Wolf, B., Ayub, S., Vogl, C., Wrobel, C., Hoch, G., Abdellatif, K., Jeschke, M., Rankovic, V., Paul, O., Ruther, P. and Moser, T., 2020. Multichannel optogenetic stimulation of the auditory pathway using microfabricated LED cochlear implants in rodents. Science Translational Medicine, 12(553), p.eabb8086.
Mudry, A. and Mills, M., 2013. The Early History of the Cochlear Implant. JAMA Otolaryngology–Head & Neck Surgery, 139(5), p.446.
Who.int. 2020. Deafness And Hearing Loss. [online] Available at: <https://www.who.int/news-room/fact-sheets/detail/deafness-and-hearing-loss#:~:text=Over%205%25%20of%20the%20world’s,will%20have%20disabling%20hearing%20loss.> [Accessed 8 September 2020].
Wilson, B., 2008. Cochlear implants: Current designs and future possibilities. The Journal of Rehabilitation Research and Development, 45(5), pp.695-730.