Cochlear Implant Systems

The cochlear implant devices available for implantation, as well as the speech processing strategies used, continue to undergo technologic improvements. Currently, three types of multichannel, multielectrode cochlear implant devices are commercially available for children in the United States. These devices have several characteristics in common. All have an electrode array that is surgically implanted into the cochlea and an external unit, consisting of a microphone that picks up sound energy and converts it to an electric signal, and a signal processor that modifies the signal, depending on the processing scheme in use. The processed signal is amplified and compressed to match the narrow electrical dynamic range of the ear. (The typical response range of the ear to electrical stimulation is on the order of only 10 to 20 dB, and even less in the high frequencies.) Transmission of the electrical signal across the skin from the external unit to the implanted electrode array is most commonly accomplished by the use of electromagnetic induction or radiofrequency transmission. The neural elements stimulated appear to be the spiral ganglion cells or axons. These devices use place coding to transfer frequency information in addition to providing temporal and amplitude information.

The Nucleus (Cochlear Corporation, Englewood, CA) family of cochlear implant systems (the 22-channel and 24-channel devices) are currently the most commonly used multichannel system. The Nucleus implantable electrode array consists of plat-inum-iridium band electrodes placed in a silastic carrier.28 Several generations of speech processors have been employed with the Nucleus multichannel cochlear implant. The initial Nucleus speech processors used a feature-extraction scheme in which selected key features of speech were presented through the implanted electrode array. An early speech processing strategy, the F0F1F2 strategy, primarily conveyed vowel information, including the first and second formant frequencies and their amplitudes, as well as voice pitch. A later coding scheme, the MULTIPEAK strategy, presented these acoustic features along with additional information from three high-frequency spectral bands to aid in consonant perception. One current Nucleus speech processing strategy is the Spectral Peak (SPEAK) strategy. This strategy uses a vocoder in which a filterbank consisting of 20 filters covering the center frequencies from 200 to 10,000 Hz is employed. Each filter is allocated to an active electrode in the array. The filter outputs are scanned and the electrodes that are stimulated represent filters that contain speech components with the highest amplitude. Depending on the acoustic input, the number of spectral maxima detected, and thus the number of electrodes stimulated, on each scan cycle can vary from one to ten, with an average of six per cycle. The rate at which the electrodes are stimulated varies adap-tively at 180 to 300 pulses per second.

The Clarion multichannel cochlear implant (Advanced Bionics, Sylmar, CA) has an eight-channel electrode array that uses a radial bipolar configuration through electrode pairs positioned adjacent to the osseous spiral lamina in a 90-degree orientation.29 The Clarion multichannel cochlear implant offers two types of speech-processing strategies: simultaneous analog stimulation (SAS) and continuous interleaved sampling (CIS). Both strategies represent the waveform or envelope of the speech signal.30 The Clarion SAS strategy first compresses the analog signal into the restricted range for electrically evoked hearing and then filters the signal into a maximum of eight channels for presentation to the corresponding electrodes. Speech information is conveyed via the relative amplitudes and the temporal details contained in each channel. The CIS strategy filters the incoming speech into eight bands, obtains the speech envelope, and compresses the signal for each channel. Stimulation consists of interleaved digital pulses that sweep rapidly through the channels at a rate of 833 pulses per second when using all eight channels for a maximum pulse rate of 6664 pulses per second (8 X 833 = 6,664). With the CIS strategy, rapid changes in the speech signal are tracked by rapid variations in pulse amplitude. The pulses are delivered to consecutive channels in sequence to avoid channel interaction.

The MED-EL COMBI 40-Cochlear Implant system (Medical Electronics, Innsbruck, Austria) uses the CIS (continuous interleaved sampling) strategy, which provides both spectral and temporal resolution. Up to eight active electrodes can be used. The electrode array used has the capability of deep insertion into the apical regions of the cochlea.31 The MED-EL has the capacity to provide the most rapid stimulation rate of any of the currently available implants (maximum of 12,000 biphasic pulses per second).32

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