Proefschrift_vd_Beek

SUMMARY Cochlear implants, which provide hearing for the deaf, have evolved in recent decades from single-channel implants to multichannel implants that are able to restore speech perception abilities for many. Cochlear implantation has eased communication with the hearing world and has greatly facilitated language development in children. However, considerable variation in performance exists among subjects, and speech perception in background noise continues to be troublesome for most, if not all cochlear implant recipients. Cochlear implants consist of external and internal parts. The external part contains a microphone to pick up the sound signal. The sound signal is then processed in a speech processor. Basically, the speech processor codes the auditory signal based on separate frequency bands. Subsequently, the coded signal is sent through the skin to the receiver of the internal part by a transmitter coil. The received signal is then passed to the electrode array, which is located in the scala tympani of the cochlea. The signal leaving the different electrode contacts stimulates the auditory nerve fibers present in that portion of the cochlea. Cochlear implants form an interface between an audio signal and the nerve fibers of the deaf ear. This thesis focuses on optimizing the way in which the incoming speech signal is transferred to the excitable neural elements in the cochlea. Chapter 2 describes a study that analyzes the potential benefit of preprocessing the incoming signal to increase the signal to noise ratio for cochlear implant recipients. For thirteen cochlear implant patients, speech perception using directional microphones was compared with speech perception using an omnidirectional microphone. To mimic real-life situations, speech in noise was presented in a specially designed environment with a diffuse noise field. With assistive directional microphones, speech recognition in background noise improved substantially, and speech recognition in quiet was not affected. At an SNR of 0 dB, the average CVC scores improved from 45% for the headpiece microphone to 67% and 62% for the TX3 Handymic and the Linkit directional microphones, respectively. The speech reception threshold (SRT) improved by 8.2 dB with the TX3 Handymic and 5.9 dB with the Linkit, compared with the headpiece. It is concluded that these assistive microphones will allow users to understand speech in noisy environments with greater ease. Chapter 3 studies several clinical aspects of the use of perimodiolar electrodes. It compares the data of 25 patients, who were implanted with a Clarion HiFocus 1 with a silastic positioner, with that of 20 patients in whom the same implant was used, but without positioner. After one year of implant use, the patients who were implanted with a positioner showed a significantly better speech perception (67 vs 45% words correct on CVC words in quiet, p < 0.01), while the pre-operative characteristics were comparable between the groups. CT scans showed that the positioner brought the electrode closer to the modiolus basally, whereas apically, no difference in distances from the modiolus was present. Additionally, the positioner Chapter 1 provides a general introduction to the matters discussed in this thesis. It gives a historical overview of the developmental steps of cochlear implants, and it presents the outline of the present thesis.

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