How to make a fake auditory signal?

Question

My question is about making fake auditory signals. The ear collects sounds from the environment, which are transformed into a neural signal by the hair cells in the inner ear. This signal is sent through the auditory nerve to the to brain, where it is decoded into a hearing sensation.

Now can we make fake signals and feed them into the auditory nerve to bring it to the brain? E.g., instead of the ear collecting sound, could we use an electrical instrument for collecting sound and convert this signal into a neural signal in the auditory nerve?

Answer 1

Devices that bypass the hair cells in the inner ear and directly stimulate the auditory nerve are called cochlear implants. Cochlear implants are used to treat deafness caused by the loss of hair cells in the cochlea. The hair cells are the sensory cells that convert sound vibrations into electric neural signals (Purves et al., 2001). With state-of-the-art devices, the better performing implant wearers are able to talk over the phone(!) An exclamation mark is in place, because it means they can understand the spoken word without the need for lip reading or sign language. Hence, cochlear implants are the answer to your question, but they definitely are not designed to generate fake signals. Instead, they are used as an effective treatment for deafness and are capable of transmitting meaningful speech information.

Cochlear prosthetics consist of an array of typically 20-24 electrodes that are inserted in the scala tympani of the cochlea. The inner ear is tonotopically organized, which means that high frequencies are coded in the base of the cochlea, while low frequencies are encoded in the tip. Hence, each electrode stimulates a separate frequency band (Fig. 1). By using a microphone and sending the acoustic signal through a filter bank, a number of auditory frequency bands can be obtained equal to the number of electrodes in the implant. After this speech processing step, the acoustic frequency bands are converted into trains of biphasic pulses. These pulse trains are sent to the electrodes in the cochlea, which then activate the auditory nerve directly, effectively replacing the degenerated hair cells in the inner ear (Stronks, 2010).

^{Fig. 1. Cochlear implant. Source: Mayo Clinic.}

<sub>References
- Purves et al., Neuroscience (2001) 2^nd ed.
- Stronks (2010). PhD thesis, Utrecht University</sub>

Answer 2

As @AliceD mentioned, cochlear implant is one of the earliest achievements of neural engineering. However, there are orders of magnitude more inner hair cells (IHC) and even more auditory nerve fibers (AN) in human cochlear than the current cochlear implants offer electrodes. If you are interested in a more detailed model of IHC to AN signal transmission, there are tons of research, for example:

• Meddis, R. (1986). Simulation of mechanical to neural transduction in the auditory receptor. The Journal of the Acoustical Society of America, 79(3):702-711.
• Walsh, E. J. and McGee, J. (1987). Postnatal development of auditory nerve and cochlear nucleus neuronal responses in kittens. Hearing Research, 28(1):97-116.
• Sumner, C. J., Lopez Poveda, E. A., O'Mard, L. P., and Meddis, R. (2002). A revised model of the inner-hair cell and auditory-nerve complex. The Journal of the Acoustical Society of America, 111(5):2178-2188.
• Heinz, M. G., Colburn, H. S., and Carney, L. H. (2001). Evaluating auditory performance limits: I. One-Parameter discrimination using a computational model for the auditory nerve. Neural Computation, 13(10):2273-2316.