Electrical Stimulation of Hearing

Cochlear implants are surgically implanted electronic devices that provide a sense of sound to a person who is profoundly deaf or severely hard of hearing in both ears. Most otologists and audiologists know that these devices bypass the normal hearing process by the use of a microphone and some behind the ear electronics that are outside the skin. These electronics digest an acoustic signal into the electrical stimulation of an array of electrodes that have been placed in the cochlea, electronically stimulating the acoustic nerve .  Since those very first French experimental cochlear implants, designed by André Djourno and implanted by Charles Eyriès in 1957, experimentation with auditory stimulation has brought the miracle of hearing to thousands of deaf individuals around the world for well over 35 years.  

 Electronic Stimulation of the Ear

Interest in the electrical stimulation of hearing, however, goes back much further than the first cochlear implants.  It actually begins with Alessandro Volta (1745-1827) a famous physicist, chemist and a pioneer of electrical science.  In 1774, he became the professor of physics and the Royal School in Como, Italy. While there, he designed his first invention which was the electrophorus, this was a device that produced static electricity. For years at Como, he studied and experimented with atmospheric electricity by igniting static sparks. He has also made countless discoveries in electrostatics, meteorology and even pneumatics. In 1779, he was appointed professor of physics at the University of Pavia and it was there that he invented the electric battery.

The basic idea for the battery came from Luigi Galvani, a well known 18th century anatomist from the University of Bologna. As many of us have seen in 9th grade biology class, Galvani was dissecting a frog and the frog’s leg began to twitch. He thought was because of some type of electrical action in the vicinity, such as lightening. Volta knew of Galvani’s experiment and duplicated it on a clear day when there was no lightening in the area.  Since there was still electrical activity, Volta theorized that the two different metal objects holding the frog leg might be the source of the twitch. Over several years he worked out that the wet muscle tissue conducted a current between the two different types of metals. Volta modified this effect to produce the first continuous flow of electric current and in 1799 (1800 in some sources) he invented a wet battery called a Voltaic Pile. 

The First Electronic Stimulation of the Ear

Interest in the use of this newfound source of electricity to stimulate hearing had its beginnings  with Alessandro Volta himself. This crazy Italian hooked himself to his newly discovered battery, stimulating his ears, just for the heck of it to see what would happen.  He connected a battery of 30 or 40 ‘couples’ (approximately 50V) to two metal rods that were inserted into his ears.  According to Zeng (2004), Volta’s reaction to the stimulation was “. . . at the moment when the circuit was completed, I received a shock in the head, and some moments after I began to hear a sound, or rather noise in the ears, which I cannot well define: it was a kind of crackling with shocks, as if some paste or tenacious matter had been boiling . . . The disagreeable sensation, which I believe might be dangerous because of the shock in the brain, prevented me from repeating this experiment ”.  Others suggested that his reaction to putting two rods wired to a battery in each ear was  a ‘‘boom within the head” followed by a sensation of sound similar to that of ‘‘boiling, thick soup.”  Naturally, Volta’s reaction was to immediately terminate the experiment and did not repeat it.  Arguably, it is this first Volta “experiment” that could be considered the beginning of the electronic stimulation the auditory system.   In 1801, a German, Carl Grapengiesser published a chapter on using direct current to treat diseases of the ear, and his apparatus probably resembled that of Volta (pictured right).   Of course, the electrical unit of measure, ‘Volt’, is named after him and in 1810, Napoleon made him a Count for his contributions to science.

Other Early Studies

This was the first report of auditory percepts elicited with electrical stimulation, although it is not certain if the experiment was produced with direct electrical activation of auditory neurons or via electro-mechanical effects, such as those underlying electrophonic hearing.  While his experiment was the first, Volta’s observation sparked sporadic attempts to investigate the phenomenon over the next 50 years in Paris, Amsterdam, London and Berlin. Wilson & Dorman (2008) present that the sensation described by patients was always momentary and lacked tonal quality.  Since sound is an alternating disturbance in an elastic medium, it was soon realized that stimulating the auditory system with a direct current could not reproduce a satisfactory hearing sensation.

The next real step in electrical stimulation of hearing was taken by a pioneer in electrical stimulation, Duchenne of Boulogne who, in 1855, stimulated the ear with an alternating current that he produced by inserting a vibrator into a circuit containing a condenser and induction coil.  His result was recorded as a sound that resembled, “the beating of a fly’s wings between a pane of glass and a curtain”. While more successful than the Volta experiment, it was still a crude application of electrical stimulation.  Simmons (1966) indicates that it was not until Brenner in 1868 that a more extensive investigation published of these effects studying the effects of altering the polarity, rate and intensity of the stimulus, and placement of the electrodes, on the hearing sensation produced. Brenner’s results found that hearing was better with an electrical stimulus that created a negative polarity in the ear, and that correct placement of the electrodes could reduce the unpleasant side effects. Brenner used bipolar stimulation, meaning that one electrode was placed in saline in the external auditory meatus, and the other was placed on a more distant part of the body. This electrode is now referred to as the Brenner electrode. The initial optimism surrounding the bioelectrical approaches to cure deafness was followed by a period of skepticism as the applications appeared to be invasive and required ongoing critical evaluation. However, in the 1930s, interest was renewed in the problem of reproducing hearing artificially. This coincided with the introduction of the thermionic valve (vacuum tube), which allowed for the auditory system to be stimulated electrically with substantially greater precision.

The Renaissance in Electrical Stimulation of the Ear

The work of Wever and Bray (1930) demonstrated that the electrical response recorded form the vicinity of the auditory nerve of a cat was similar in frequency and amplitude to the sounds to which the ear had been exposed.  Meanwhile, the Russian investigators Gersuni and Volokhov in 1936 examined the effects of an alternating electrical stimulus on hearing. They found that hearing could persist, following the surgical removal of the tympanic membrane and ossicles and thus hypothesized that the cochlea was the site of stimulation.  Another set of researchers, Stevens and Jones (1939), thought that electrical stimulation could be transduced into sound vibrations before it reached the inner ear. Hearing induced in this way has been called the electrophonic effect. They were able to determine whether a linear or non-linear transducer was involved by the presence and strength of the overtones, which were detected when the subject heard beats. The studies by Stevens and Jones (1939), as well as Jones et al (1940) indicated that when the cochlea was stimulated electrically, there were three mechanisms, which produced hearing: 

1. The middle ear could act as a transducer, which obeys the ‘square law’ and convert alternations in the strength of an electrical field into the mechanical vibrations that produce sound.
2. Electrical energy could be converted into sound by a direct effect on the basilar membrane, which would then vibrate maximally at a point determined by the frequency and these vibrations would stimulate the hair cells
3. Direct stimulation of the auditory nerve produced a crude hearing sensation. 

Their conclusions were basically correct, although other body tissues have now been shown to act as transducers.

A wealth of research in the 1940s and 1950s into the mechanisms involved in electrophonic hearing indicated that hearing is produced by transducing electrical energy into sound vibrations and that residual cochlear function is also required.  It became apparent that total perception deafness could not be corrected by inducing a widespread electrical filed in the region of cochlea. Instead, a more localized stimulation of the auditory nerve fibers is required.