Graeme Clark (doctor) - Development of Cochlear Implants

Development of Cochlear Implants

Clark considered that the notion that hearing, particularly for speech, might be reproduced in people with deafness if the damaged or underdeveloped ear were bypassed and the auditory nerve electrically stimulated, to reproduce the coding of sound. His initial doctoral research at the University of Sydney investigated the effect of the rate of electrical simulation on single cells and groups of cells in the auditory brainstem response, the centre where frequency discrimination is first decoded.

Clark’s research demonstrated that an electrode bundle with 'graded stiffness' would pass without injury around the tightening spiral of the cochlea to the speech frequency region. Until this time he had difficulty identifying a way to place the electrode bundle in the cochlea without causing any damage. A breakthrough was achieved during a vacation at the beach – using a seashell to replicate the human cochlea and grass blades (which were flexible at the tip and gradually increasing in stiffness) to represent electrodes.

Clark showed that the electrode bundle had to be free-fitting and the wires terminated with circumferential bands to reduce friction against the outer wall of the cochlea, and so make it easier to pass the required distance. The bands had to be wide enough to minimize the charge density of the electrical current for safety, but narrow enough for localized stimulation of the nerve fibers for the place coding of frequency. In order to satisfy questions about the safety of the device Clark conducted experiments to show that there was a minimal risk of meningitis from a middle ear infection if a fibrous tissue sheath grew around the electrode bundle. The sheath was developed from a connective tissue graft from the person’s own body that was placed around the electrode bundle where it entered the cochlea. The ultimate question however was: could speech be coded with multi-channel stimulation so that it could be understood by a deaf person? and this could only be discovered by operating on a deaf person.

The first multi-channel cochlear implant operation was done at the Royal Victorian Eye and Ear Hospital in 1978 by Clark and Dr Brian Pyman. The first person to receive the implant was Rod Saunders who had lost his hearing aged 46.

After successfully completing the surgery Clark in 1978 discovered with his post-doctoral colleague Yit Chow Tong how multi-channel electrical stimulation of the brain could reproduce frequency and intensity as pitch and loudness in severely-to-profoundly deaf adults who originally had hearing before going deaf. Electrical stimulation at low rates of 50 pulses/s was perceived as a pitch of the same frequency, but at rates above 200 pulses/s what was heard was poorly discriminated and a much higher pitch. This discovery established that the timing of electrical stimuli was important for low pitch when this had been difficult to determine with sound. But discrimination of pitch up to 4000 Hertz is required for speech understanding, so Clark emphasized early in the development of the cochlear implant that place coding through multi-channel stimulation would have to be used for the important mid-to-high speech frequencies. Clark and Tong next discovered that place of stimulation, was experienced as timbre, but without a strong pitch sensation. The patient could identify separate sensations according to the site of stimulation in the cochlea.

Clark and Tong at the end of 1978 then made the ground-breaking discovery - the first speech processing strategy to give speech understanding to severely-to-profoundly deaf people using electrical stimulation alone, and in combination with lipreading. The speech processing strategy coded the second formant as place of stimulation along the cochlear array, the amplitude of the second formant as current level, and the voicing frequency as pulse rate across the formant channels.

Clark in December 1978 arranged that his audiologist present open-set words to his first patient, who was able to identify several correctly. Clark realized then that this was the breakthrough in providing speech understanding that everyone had been hoping for.“it was the moment I had been waiting for. I went into the adjoining room and cried for joy.”

This discovery was established by Clark with objective audiological tests in 1979. The open-set speech test results on this patient were the first time that speech recognition for electrical stimulation alone had been demonstrated, under standardized conditions. Previously single-channel strategies had only shown a small improvement when electrical stimulation was used as a lip reading aid, but no speech understanding for electrical stimulation alone.

As a result Clark went on to operate on a second patient who had been deaf for 17 years. He was able to show that the speech coding strategy was not unique to one person’s brain response patterns, and that the memory for speech sounds could persist for many years after the person became deaf.

The successful development of the cochlear implant was confirmed when it gained approval from the Food and Drug Administration (FDA) in the United States. In 1982 Clark supervised the initial clinical studies mandated by the agency, and in 1985 after a world trial the FDA granted approval for the cochlear implant for adults 18 and over who had hearing before going deaf. It thus became the first multi-channel cochlear system to be approved as safe and effective by the FDA or any health regulatory body for giving speech understanding both with lip reading and for electrical stimulation alone in people who had hearing before going deaf.

In 1985, Clark as the surgeon-in-charge performed the cochlear implant surgery on the first children along with Drs Pyman and Webb. The first child was 10 years old and the second was 5 years old. From 1985 to 1990 Clark and the members of his Cochlear Implant Clinic at the Eye and Ear Hospital in Melbourne, followed by other clinics world wide, found that the formant extraction speech coding strategies developed by Clark and team resulted in up to 60% of children being able to understand significant numbers of words and sentences with electrical stimulation alone without help from lipreading. With a strategy that also extracted a band of high frequencies there were increased numbers of children with improved speech perception, speech production and language scores.

In 1990 the FDA announced that the 22-channel cochlear implant was safe and effective in enabling