Proefschrift_vd

74 | Chapter 4

cochlea. For this purpose, the scanning and selectivity methods were compared and the effects of several

recording param-eters were analysed, including (1) position of the probe electrode (apical, middle, and basal

electrodes) and (2) level of the stimulus. Additionally, in smaller subgroups the effects of (3) the position of

the recording electrode, (4) masker position fi xed and probe position variable versus probe position fi xed

and masker position variable, and (5) artefact rejection method (forward masking or alternating polarity)

were analysed. Furthermore, we tested for correlation between selectivity measures and the subjects ’ speech

perception abilities. All data were taken into account separately, using linear mixed models.

METHODOLOGY

Subject characteristics

Thirty-one subjects were included in the study. All subjects were unilaterally implanted with an Advanced

Bionics’ HiRes 90K cochlear implant with the HiFocus 1J electrode array, which has 16 contacts spaced 1.1

mm apart. Patient demographics are shown in Table 1, A.

Test procedure

Neural response imaging (NRI) was used to obtain eCAP measurements intraoperatively, during the

implantation surgery. Using the bionic ear data collection system (BEDCS) research software, test parameter

(electrode contact) sequences were pre-set for the recording of scanning and selectivity series (see below).

Parameters used were as follows: monopolar biphasic pulses, cathodic first; pulse duration: 32

s/phase;

masker-probe interval: 500

s, sampling rate: 56 kHz; gain: 300; 32 averages. If eCAPs could be observed

then no adjustments in settings were made during the measurement series. Five patients, in who no eCAPs

were seen in BEDCS intraoperatively, were not included in the study. It is important to notice that their

demographic characteristics, however, were not essentially different from the study group.

Scanning and selectivity measures

Scanning measures (Figure 1, A) were obtained by stimulating one electrode contact (masker and probe at

a single contact) and recording the eCAP response sequentially at all other locations along the array (Cohen

et al, 2004). Forward masking was used for artefact rejection, but for a subset of five subjects the alternating

polarity method was also used for artefact rejection at the stimulating electrode (raw waveforms with

forward masking and alternating polarity are shown in Figure 1, C). The numbers of subjects tested with

the different parameter settings for scanning and selectivity measurements are summarized in Table 1, B.

Selectivity measures (Figure 1, B) were obtained by using a traditional forward masking technique as

described previously (Cohen et al, 2003). All subjects were tested with the masker contact fixed and eCAP

amplitudes measured for different probe electrode positions, which were stimulated after a 500

s interval.