Contributed editorial appearing in
Scientific Computing & Instrumentation 19:1, December 2001, pg. 16.
Rush H. Limbaugh III is sometimes characterized as a"shock jock." However, the host of the most widely heard long format syndicated talk show astounded the more than 20 million members of his listening audience when he announced that he began to experience hearing loss in late May 2001, which progressed to near total deafness in four months. While miniature hearing aids are used successfully by millions who have difficulty hearing soft sounds, Limbaugh's condition is one wherein a component of the ear ceases to function and results in a condition of profound deafness.Our ability to detect sounds or pressure waves is obtained through an auditory sensor system consisting of the outer, middle, and inner ear. The outer ear serves as an antenna that channels acoustic pressure waves to the eardrum in the middle ear. This elastic membrane is attached to three small bones that transduce the pressure waves into mechanical vibrations. These vibrating bones serve as a power transformer between the air pressure waves sensed by the eardrum and traveling density pulsations created within the fluid of the inner ear. The fluid is contained within a spiral cavity known as the cochlea. The cochlea is lined with a flexible coating out of which grow microscopic hairs. The pulsations in the fluid bend these hairs and cause the auditory neurons located at their roots to inform the brain of the pulses detected at the particular location in the cochlea. The shape of the cochlea is such that different regions of the cochlea respond to different frequencies. Hair cells at the base or beginning of the cochlea respond to high frequency while those at the end or apex respond best to low frequency sounds. In this way, the cochlea encodes frequency as linear location along its inner surface and serves as the biological equivalent of a spectrum analyzer.
Most hearing loss is the result of damage to the tiny hairs in the cochlea. The neurons remain functional and connected to the auditory center of the brain, but without the hairs, are unable to sense the signals contained in the fluid. If the neurons can be induced to respond to electrical pulses generated by an electrode positioned against the inner wall of the cochlea, then the transduction can be restored. This is the goal of cochlear implant (CI) technology.
First approved by the FDA for adults in 1985, CI technology has benefited from advances in electronic miniaturization and digital signal processing. Commercial suppliers of CI systems are advancing the design and surgical implantation procedures of linear electrode arrays used to stimulate the neurons along the inner surface of the cochlea. Wireless communications permit the electrodes to be activated using a small transmitter warn behind the ear of the patient.
Professor Philipos Loizou and his colleagues at the Callier Advanced Hearing Research Center at the University of Texas at Dallas are among the leading researchers in the field of auditory processing and CI electronics. Initial CIs used one electrode to provide single-channel excitation at a specific location in the cochlea. although necessary for the characterization of early designs, the stimulation of neurons at a single frequency location along the cochlea prevented the patient from receiving the broad spectrum of frequencies produced by the consonants and vowels of human speech. Multi-channel CIs utilize an array of electrodes that are positioned along the entire frequency range of the cochlea. This advancement led investigators to explore the optimal number of electrodes and the appropriate type of information to be sent to each electrode. Simultaneous stimulation of multiple electrodes was found to suffer from electrical crosstalk. Researchers at the Research Triangle Institute (RTI) developed a popular frequency encoding strategy known as the Continuous Interleaved Sampling (CIS) approach, which uses non-simultaneous, interleaved pulses. The pulse modulation frequency ranging from 100 - 2500 pulses per second in addition to the electrode firing order are adjusted for optimal frequency reception by each patient. In this fashion, the CI can simulate the simultaneous reception of multiple sound frequencies while avoiding the interference limitations of the electrodes. Once the CI is installed, improved neuron stimulation and packaging technology can continue to evolve while patients are provided with upgraded transmitters.
