The bionic ear in the second and third millennia [Abstract]

Abstract

The development of the Bionic Ear or cochlear implant required answers to a number of questions: I) the cochlear hair cells and their nerve connections were too complex and numerous to be replaced by a small number of electrodes for reproducing the coding of sound; 2) a cochlear implant would destroy the very nerves it was hoped to stimulate; 3) speech was too complex to be presented to the nervous system by electrical stimulation for speech understanding to occur; 4) there would not be enough residual hearing nerves in the cochlea after die back due to deafness to transmit essential speech information; 5) children born deaf would not develop the neural connections in the auditory pathways for hearing with electrical stimulation. The questions have been answered by basic and clinical studies. Research on the experimental animal showed that electrical stimulation could not reproduce the synchrony of neural responses or frequency discrimination above 300 pulses/s, (less than the 3kHz range for speech understanding). Therefore, a multiple-electrode system was proposed to reproduce place as well as temporal coding of frequency. Experimental studies showed that current could be localised in the cochlea for place coding, and biological research established that an intracochlear electrode would be safe. Initial implants on adults confirmed the limitations frequency and intensity coding seen in the experimental animal, and a speech processing strategy was developed that extracted the important elements of speech for transmission through the "electro-neural" bottle-neck to the brain. This enabled the patients to understand running speech, and with the further improvements in this speech processing strategy the results are comparable to severely deaf people using a hearing aid. Similar good results have been obtained in children born deaf. There are a number of challenges for the third millenium to further improve the performance of cochlear implants. These challenges are: I) better simulation of the way the brain codes sounds; 2) invisible cochlear implants; 3) the use of nerve growth factors to protect the auditory nerve from die back after deafness or result in its regeneration; and 4) the use of neurotrophins to re-establish the neural plasticity seen in young children to develop the appropriate neural connections for the coding of frequency.