With the aid of cochlear implants many deaf persons are able to understand running speech.Hearing impairments are widespread. Significant hearing loss affects one in three adults over the age of 60 and half of all adults over the age of 75. In addition, one out of every 1000 children is born deaf.  

The vast majority of these hearing impairments result from the loss of hair cell receptors in the inner ear. These hair cell receptors are located in the cochlea and convert sounds from the environment into neural signals. This information is transmitted by the acoustic nerve to the brain, where it is analyzed and interpreted as motor noises, the sound of a piano, or singing. Because these nerves remain functional even if the hair cell receptors are damaged, the brain can be stimulated with a cochlear implant, which is based on a special electrical microstimulation device.  

Cochlear implant systems for the treatment of acquired or congenital deafness have evolved rapidly over the past several decades from early experimental wires and posts to sophisticated multichannel devices that enable deaf patients to participate fully in the hearing world. The first electric stimulation of the acoustic nerve was already done in humans in the 1930s, followed by the first implants in patients in the late 1950s and early 1960s. However, the long-term benefit of these early devices was poor. It is only because of intensive research and animal studies that electrodes can now be implanted safely and remain functional for long periods of time.  

It is primarily thanks to basic animal research that we have been able to decipher the structure and function of the cochlea so well. And it was through electrophysiological studies in cats that the functional consequences of chronic electrical stimulation of the auditory nerve could be assessed. These experiments showed that the implant leads to long-term changes in the midbrain. Thus, we now know that the brain can adjust to improved hearing by restructuring and reorganizing important auditory areas, so that the signals of the implant are processed just like natural sensory information.  

Thanks to animal research, the first two multichannel processors could be implanted in humans the end of the 1970s. For the first time, patients were able to understand running language without reading from lips. Today, more than 60,000 persons worldwide (including children) have cochlear implants to restore functional hearing.  

Unfortunately, there is still a great deal of variability in performance. The average implant user can talk on the telephone in a quiet environment. But some patients fail completely to gain benefit from these devices. There is still room for improvement in the quality of the devices, and even the most modern systems cannot come close to restoring natural hearing ability. Animal experiments remain necessary to further improve cochlear implant technology.