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14 | Chapter 1
The progression from single-channel to multichannel electrode arrays enabled the use of the tonotopic
organization of the neural fibers in the cochlea. This technique proved to be a crucial improvement that
made speech perception with cochlear implants possible [Mudry and Mills, 2013]. Although all current
cochlear implant systems provide a higher number of channels, speech perception does not improve with
the use of more than seven channels [Friesen et al., 2001]. Not all electrode contacts provide independent
spectral information. The spread of currents through the highly conductive fluid in the cochlea prevents
neuronal excitation in a restricted area. Various electrode arrays have been used to improve spectral
resolution. Electrode contacts medially positioned in the cochlea near the neural elements facilitate
excitation [Shepherd et al., 1993]. Hence, different cochlear implant manufacturers have developed medially
positioned electrode arrays. These so-called perimodiolar electrodes offer improved speech perception [van
der Beek et al., 2005a;Holden et al., 2013].
Furthermore, with the increased emphasis on preserving residual hearing, cochlear implants’ electrode
arrays are designed to induce as little trauma as possible [Lenarz et al., 2013;Tavora-Vieira and Rodrigues,
2013]. The result is short, thin and flexible electrodes that are less likely to damage vulnerable cochlear
microstructures. Moreover, when residual hearing is preserved, the combination of electric and acoustic
stimulation is feasible.
Finally, even an optimized electrode-neural interface should be adapted to the individual patient and
to specific circumstances at different locations in the individual cochlea. This individualized tuning is
performed during the fitting process, and numerous parameters can be set; however, the core parameters
involve defining the threshold and maximum levels along the array. Research data concerning the stimulation
levels that are useful in clinical practice primarily focus on speeding up the fitting process [Plant et al.,
2005;Smoorenburg, 2007;Pfingst and Xu, 2004], and only a few studies report fitting improvements that
would provide better speech perception [Gani et al., 2007;Zhou and Pfingst, 2014;Noble et al., 2014].
Outline of the present thesis
In this thesis, the parameters that influence the performance of cochlear implant users are analyzed.
Specifically, we analyze the signal-to-noise ratio at the input of the processor, the intracochlear position of
the electrode design, the spread of excitation (SOE) and settings of the clinically used levels. In
Chapter
2,
the effect of background noise on speech perception is assessed in a trial studying the improvement of
speech perception in noise using directional microphones versus an omnidirectional microphone. To mimic
real-life situations, speech-in-noise was presented in a specially designed set-up with a diffuse noise field. In
Chapter 3,
the effect of electrode design and intracochlear position is analyzed by comparing the speech
perception scores of 25 patients with cochlear implants that were forced into a perimodiolar position with a
silastic positioner and the speech perception scores of 20 patients in whom no positioner was used. The 20
no-positioner patients were further subdivided into superficially and deeply implanted subgroups, both of
which included 10 patients. The intrascalar position of the individual electrode contacts was analyzed using
HDCT scans, and stimulation thresholds, maximum comfort levels, and dynamic ranges were obtained.