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150 | Summary
led to deeper insertion. Although the intracochlear conductivity paths of the two groups did not show
significant differences, a basal current drain was seen for the shallowly inserted non-positioner patients. It
was concluded that a basally perimodiolar electrode design benefits speech perception. The combination
of decreased distance from the modiolus, increased insertion depth, and the insulating properties of the
electrode array have functional implications for the clinical outcomes associated with the perimodiolar
electrode design. Further research is needed to elucidate these factors’ individual contributions to those
outcomes.
Chapter 4
focuses on the application of the cochlear implant’s ability to record the electrically evoked
action potentials (eCAP) of the neurons in the cochlea to measure the effectiveness of the electrode-to-
neural interface. The study investigated the spread of excitation (SOE) profiles using eCAP measures and
analyzed the effects of various parameter settings. Measurements were performed intra-operatively in 31
users of the Advanced Bionics HiRes 90K cochlear implant. SOE was measured using the forward masking
technique (selectivity) as well as with a “fixed stimulus, variable recording” (scanning) technique. SOE
profiles were produced at three stimulus levels and at three sites along the array. Additionally, the effects
of the position of the recording electrodes and artefact rejection methods were studied in five subjects. All
data were analyzed using linear mixed models. The selectivity method produced narrower excitation profiles
than the scanning method, showing asymmetry along the array with broader SOE apically. Moreover,
the position of the recording electrode shifted the SOE curves towards the recording contact, enhancing
asymmetry. Neither significant effects of the current level nor artefact rejection methods were observed, nor
was any significant correlation with speech perception found.
Chapter 5
reports a study that analyzed the predictability of fitting levels for individual cochlear implant
recipients based on a review of cohort data. The data included the threshold levels (T-levels) and maximum
comfort levels (M-levels) of 151 adult subjects who used a CII/HiRes 90K cochlear implant with a HiFocus
1/1 J electrode. The 10th, 25th, 50th, 75th and 90th percentiles of the T- and M-levels are reported. The
subjects’ speech perception was measured using a HiRes speech coding strategy during routine clinical
follow-up. The T-levels for most subjects were between 20 and 35% of their M-levels and were rarely (<
1/50) below 10% of the M-levels (which is the manufacturer’s default). Furthermore, both the T- and
M-levels increased over the first year of follow-up. Interestingly, the levels expressed in linear charge units
showed a clear increase in dynamic range (DR) over 1 year (29.8 CU; SD 73.0), whereas the DR expressed
in decibels remained stable. The T-level and DR were the only fitting parameters for which a significant
correlation with speech perception (r = 0.34, p < 0.01, and r = 0.33, p < 0.01, respectively) could be
demonstrated. Additionally, the T- and M-level profiles expressed in decibels turned out to be independent
of the subjects’ across-site mean levels, as demonstrated with mixed linear models. Based on the data set
from 151 subjects, clinically applicable predictive models for the T- and M-levels of all separate electrode
contacts were obtained. These closed-set formulae allow the close approximation of individual recipients’
T- and M-levels based on just one psychophysical measurement. Additionally, the analyzed fitting level data
can serve as a reference for future patients.