Treatment of Deafness: Cochlear Implants
In 1800, Allesandro Volta, shortly after his invention of the electric batter, placed metal rods into ears and connected them to a 50-volt circuit. He described the unique experience as a jolt and hearing a noise “like a thick boiling soup”. This is probably the first discovery that electrical stimulation of the ear can produce sound perception.
Direct stimulation of an acoustic nerve with an electrode was demonstrated by the French-Algerian surgeons André Djourno and Charles Eyriès back in the 1950s. During an operation, they placed wires on exposed nerves and the patient reported hearing of sounds like "a roulette wheel" and "a cricket" when a current was applied.
Several years later, William House from the House Ear Institute in Los Angeles, Djourno and Eyries from France, as well as Robin Michelson from the University of California, San Francisco, collaborated to create and successfully implant cochlear devices in human volunteers. Robin Michelson is commonly credited with being the inventor of modern cochlear implant widely used today in the world.
How it works
Hair cell damage is the most common cause of deafness in humans. The auditory nerve usually remains intact, enabling the restoration of some hearing with a cochlear implant. It is a electronic device surgically implanted to provide a sense of sound to a person who has problems with hearing. Indeed, cochlear implant systems have revolutionized the treatment of inner ear damage in the past few decades.
Figure 1：Cochlear implant
Image courtesy of http://commons.wikimedia.org/wiki/File:Cochlear_implant.jpg This image is in the public domain and thus free of any copyright restrictions.
The device starts with an external headpiece containing a microphone that picks up sound and converts it to an electrical signal. The signal is then delivered to a battery-powered speech processor, which selectively filters sound to prioritise audible speech and sends the processed signal through a thin cable to the transmitter. The radio transmitter held in position by magnet behind the external ear transmits the digital code to an internal device that has been surgically implanted beneath the skin.
The internal device is a receiver and stimulator, which translates the code into electrical impulses that it sends to a thin, flexible bundle of wires that has been threaded into the cochlea. The wires build up an array of up to 22 separate electrodes that can stimulate the auditory nerve at various places along the cochlea, from the base toward the apex. This design allows the exploitation of the tonotopic arrangement of auditory nerve fibers, whereby stimulation close the base of the cochlea creates a perception of high-frequency sounds while stimulation near apex triggers low-frequency sounds.
Number of implant recipients
It has been estimated in 2004 that around 60,000 people worldwide have received cohlear implants, including about 20, 000 children in. By 2007, the total number of cochlear implant users has grown almost twice to an estimated 120,000 in the world. Despite the rapidly increasing number, the implantation of the device is still considerably a very expensive procedure.
People with cochlear implants can improve significantly in understanding conversational speech with training over a period of time. However, the success of implants in providing hearing capacity varies for reasons that are often unknown.
In general, the best candidates for implants are young children, as well as older children or adults who have experience with speech before acquiring deafness. For those who have not learned any speech preceding deafness, cochlear implants do not effectively provide distinguishable perception of sounds. Like other sensory systems in the brain, environmental stimuli (sounds in this case) during critical periods of brain development are important for central processing of hearing. In the absence of sound exposure early in life, the auditory system fails to develop properly for normal function despite hearing restoration in later years.