Dewey T. Lawson

Multicenter U.S. bilateral MED-EL cochlear implantation study: Speech perception over the first year of use

Objective: Binaural hearing has been shown to support better speech perception in normal-hearing listeners than can be achieved with monaural stimulus presentation, particularly under noisy listening conditions. The purpose of this study was to evaluate whether bilateral electrical stimulation could confer similar benefits for cochlear implant listeners. Design: A total of 26 postlingually deafened adult patients with short duration of deafness were implanted at five centers and followed up for 1 yr. Subjects received MED-EL COMBI 40+ devices bilaterally; in all but one case, implantation was performed in a single-stage surgery. Speech perception testing included CNC words in quiet and CUNY sentences in noise. Target speech was presented at the midline (0 degrees), and masking noise, when present, was presented at one of three simulated source locations along the azimuth (−90, 0, and +90 degrees). Results: Benefits of bilateral electrical stimulation were observed under conditions in which the speech and masker were spatially coincident and conditions in which they were spatially separated. Both the “head shadow” and “summation” effects were evident from the outset. Benefits consistent with “binaural squelch” were not reliably observed until 1 yr after implantation. Conclusions: These results support a growing consensus that bilateral implantation provides functional benefits beyond those of unilateral implantation. Longitudinal data suggest that some aspects of binaural processing continue to develop up to 1 yr after implantation. The squelch effect, often reported as absent or rare in previous studies of bilateral cochlear implantation, was present for most subjects at the 1 yr measurement interval.

Two new directions in speech processor design for cochlear implants.

Two new approaches to the design of speech processors for cochlear implants are described. The first aims to represent “fine structure” or “fine frequency” information in a way that it can be perceived and used by patients, and the second aims to provide a closer mimicking than was previously possible of the signal processing that occurs in the normal cochlea.

Comparative studies of speech processing strategies for cochlear implants

A wide variety of speech processing strategies for multichannel auditory prostheses were compared in studies of two patients implanted with the UCSF electrode array. Each strategy was evaluated using tests of vowel and consonant confusions, with and without lipreading. Included among the strategies were the compressed analog processor of the present UCSF/Storz prosthesis and a group of interleaved pulses processors in which the amplitudes of nonsimultaneous pulses code the spectral variations of speech. For these patients, each with indications of poor nerve survival, test scores were significantly higher with the interleaved pulses processors. We believe this superior performance was a result of 1. the substantial release from channel interactions provided by nonsimultaneous stimuli and 2. a fast enough rotation among the channels to support adequate temporal and spectral resolution of perceived speech sounds.

Speech processors for cochlear prostheses

The authors review considerations in the design of speech processors for cochlear prostheses. To illustrate issues of processor design, two fundamentally different processing strategies are described in terms of the information that they can convey and in terms of how they might perform under various conditions on nerve survival. A summary is presented of clinical tests comparing these strategies in eight implant patients. Key findings from the comparison studies are that (a) one processor (the interleaved pulse processor) is clearly superior for patients with psychophysical signs of poor nerve survival, (b) another processor (the compressed analog processor) may be superior for patients with signs of good survival, and (c) different processing strategies can produce widely different outcomes for individual patients. Future directions in the future development of speech processors for cochlear prostheses are outlined. >

New processing strategies for multichannel cochlear prostheses

Various strategies for representing speech information with multichannel cochlear prostheses were compared in tests with implant patients. The strategies included the compressed analog (CA) approach of a standard clinical device, and alternative interleaved pulses (IP) and continuous interleaved sampling (CIS) strategies. CA and IP strategies had been compared in previous studies with a wide range of subjects. The present studies compared all three types in tests with one subject and CA and CIS strategies in tests with six additional subjects. Subjects for the present studies were selected for their excellent performance with the clinical CA processor, and the tests included closed-set identification of consonants and open-set recognition of words and sentences. For every test, every subject obtained his or her highest score, or repeated a score of 100% correct, using a CIS strategy. In the comparisons of all three approaches, IP processor scores were between those obtained with CA and CIS processors. The results are discussed in terms of their implications for processor design.

Design for an Inexpensive but Effective Cochlear Implant

Widespread application of cochlear implants is limited by cost, especially in developing countries. In this article we present a design for a low-cost but effective cochlear implant system. The system includes a speech processor, four pairs of transmitting and receiving coils, and an electrode array with four monopolar electrodes. All implanted components are passive, reducing to a minimum the complexity of manufacture and allowing high reliability. A four-channel continuous interleaved sampling strategy is used for the speech processor. The processor and transmission link have been evaluated in tests with a subject previously implanted with the Ineraid electrode array and percutaneous connector. A prototype of the link, consisting of four pairs of transmitting and external receiving coils, was used, with the outputs of the receiving coils directed to four intracochlear electrodes through the percutaneous connector. The subject achieved speech reception scores with the prototype system that were equivalent to those achieved with a standard laboratory implementation of a continuous interleaved sampling processor with current-controlled stimuli.

Representations of Speech Features with Cochlear Implants

The development and application of cochlear prostheses have improved the quality of life for many deaf individuals. Much work remains to be done, however, in order to achieve high levels of speech recognition in a majority of patients. In particular, we do not fully understand how different speech processor and implanted electrode designs affect the perception of speech and other sounds. Also, we have only fragmentary and primitive knowledge of how various differences among patients affect their outcomes.