Multichannel cochlear implants that deliver different temporal information to different parts of the cochlea (place or spectral information) have been the most successful in generating speech perception in postlingually deafened adults with profound hearing loss and in postlingually and prelingually deafened children (Cohen, Waltzman, and Fisher, 1993; Gantz, McCabe, and Tyler, 1988). Innovative methods of processing speech have been developed by many independent research programs and cochlear implant manufacturers.
Most cochlear implants today employ a band-pass filter system to separate the acoustic signal into discrete frequency bands that can be delivered to the appropriate frequency regions of the cochlea, providing spectral information about the speech signal. Temporal and intensity cues are delivered by varying the rate of stimulation and the amount of stimulating current. Multichannel implant systems use between 8 and 24 channels, depending on the implant manufacturer.
Current cochlear implant systems are designed to take advantage of the tonotopic organization of the cochlea. Thus, place coding is used to transfer spectral information in the speech signal as well as to encode durational and intensity cues (American Speech-Language-Hearing Association, 2004). The number of channels may not be as important as the speech processing algorithm. Most implant systems use a nonsimultaneous stimulation paradigm, which prevents stimulation of more than one channel at a time and eliminates electrical field interaction across electrodes; however, newer speech coding algorithms employing simultaneous stimulation are now in development. Electrical field interaction can induce excessive current loads causing increased loudness and distortion from overlapping signals.
One type of speech processing that has been shown to improve speech perception scores has been labeled continuous interleaved sampling (CIS) (Wilson, Lawson, Finley, and Wolford, 1991). CIS speech processing employs a rapid pulse rate (up to 25 microseconds/phase) to deliver envelope cues similar to an analog signal. The CIS processing is available in the Advanced Bionics Clarion, the Nucleus CI24M, and the MedEl implants.
The Nucleus implant has additional proprietary Spectral Peak (SPEAK) and Advanced Combination Encoders (ACE) speech coding algorithms. The ACE strategy claims to combine advantages of the SPEAK and the CIS strategies (Cochlear Corporation, 2004). The Clarion implant can be programmed in a pulsatile, compressed analog, or combination pulsatile/analog format. It is capable of stimulating at a rate of 82000 Hz, allowing presentation of more fine temporal information in their proprietary coding strategy, “Hi Res.” A new implant by the Cochlear Corporation (RP-8), now in feasibility trials by the Food and Drug Administration