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76 | Chapter 4
Speech perception
Speech discrimination scores were obtained for all 19 adult cochlear implants users in the study group
during normal clinical follow-up at predetermined intervals, starting one week after initial fitting. The
data used for analysis in this study were the scores obtained at two years of follow-up. Four patients had
to be excluded from the analysis, one patient since he was deceased (of a natural cause) 1.5 years after
implantation, one prelingually deaf patient, and two patients for whom only scanning data and no selectivity
measures were available. All subjects used the HiRes processing strategy. The standard Dutch speech test of
the Dutch Society of Audiology, consisting of phonetically balanced monosyllabic (CVC) word lists, was
used (Bosman & Smoorenburg, 1995). To improve test reliability, four lists (44 words) per condition were
administered. Although this test is typically scored with phonemes in the Netherlands and Flanders, the
data are shown as word scores, which is a more common reporting method in Anglo-Saxon countries. All
testing was performed in a soundproof room, using a calibrated loudspeaker in frontal position at one metre
distance. Subjects were tested in quiet at speech levels of 65 dB SPL in a CI-only condition.
468 F. B. van der Beek et al.
Speech perception
Speech discrimination scores were obtained for all 19 adult cochlear
implants users in the study group during normal clinical follow-up
at predetermined intervals, starting one week after initial fitting. The
data used for analysis in this study were the scores obtained at two
years of follow-up. Four patients had to be excluded from the analy-
sis, one patient since he was deceased (of a natural cause) 1.5 years
after implantation, one prelingually deaf patient, and two patients for
whom only scanning data and no selectivity measures were avail-
able. All subjects used the HiRes processing strategy. The standard
Dutch speech test of the Dutch Society of Audiology, consisting
of phonetically balanced monosyllabic (CVC) word lists, was used
(Bosman & Smoorenburg, 1995). To improve test reliability, four
lists (44 words) per condition were administered. Although this test
is typically scored with phonemes in the Netherlands and Flanders,
the data are shown as word scores, which is a more common report-
ing method in A gl -Saxon countries. All testing was performed in
a soundproof room, using a calibrated loudspeaker in frontal position
at one metre distance. Subjects were tested in quiet at spe ch level
of 65 dB SPL in a CI-only condition.
Analysis
Signal processing was performed off-line using Matlab. eCAP
amplitudes were automatically detected using Matlab software
(as per Frijns et al, 2002) and plotted against the electrode positions
along the array. Curves that did not show eCAP amplitudes above
0.1 mV were not included in the analysis. This criterion was not
reached in 16% of the responses, mainly in the low current range.
The average of the peak amplitude, for both selectivity and scanning,
was 0.6 mV. The curves were normalized by taking the value at the
electrode contact of interest and dividing all values along the array
by this value. Next, both flanks of the selectivity and scanning curves
were fitted by a 4th order polynomial.
The width was defined as the number of electrode contacts
(spaced 1.1 mm apart) from the stimulated contact to the point at
which the normalized amplitude reduces to 0.6. For the middle con-
tact both the width in the apical (EM-A) as well as the basal direc-
tion (EM-B) were calculated. In cases where the minimum value
did not drop to 0.6 the width was set as the limit of the array in
the apical or basal direction (as per Abbas et al, 2004). In previous
studies both 50% and 75% of the peak mplitu e hav been used
as a measure of the width of the region of excitation (Cohen et al,
2003; Hu hes & Abbas, 2006 ). Fo this study, 60% of the peak
amplitude, determined on the basis of the fitted, normalized curves,
was selected as a trade-off between obtaining as many curves as
possible and being able to measure differences between distinctive
profiles along the array.
Figure 2 shows typical selectivity curves recorded in one sub-
ject. The figure shows the normalized eCAP amplitudes obtained
0 0.2 0.4 0.6 0.8 1 1.2 -1
0
1
2
3
4
5
6
7
8
Time from stimulus end (ms)
Recorded potentials (mV)
0 0.2 0.4 0.6 0.8 1 1.2 -2
-1
0
1
2
3
4
5
6
7
8
Time from stimulus end (ms)
Recorded potentials (mV)
0 0.2 0.4 0.6 0.8 1 1.2 -0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
Time from stimulus end (ms)
Recorded potentials (mV)
A
B
P M
Stim
D
C
0 0.2 0.4 0.6 0.8 1 1.2 -0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
Time from stimulus end (ms)
Recorded potentials (mV)
Artefact rejection:
Forward masking
Artefact rejection:
Alternating polarity
Scanning
Selectivity
Patient 1 Patient 2
R16
R15
R14
R13
R12
R11
R10
R9
R8
R7
R6
R5
R4
R2
R1
R16
R15
R14
R13
R16
R15
R12
R11
R10
R9
R8
R7
R6
R5
R4
R2
R1
P16
P15
P14
P13
P12
P11
P10
P9
P7
P6
P5
P4
P2
P1
P3
P16
P15
P14
P13
P12
P11
P10
P9
P7
P6
P5
P4
P2
P1
P3
Excited fibres
Recording Recording Recording
Refractory fibres
Masker (M)
Probe (P)
Masked Probe (MP)
M + P - MP
Figure 1. A: For scanning measures a single probe electrode contact is stimulated (Stim) and the response is recorded at all electrodes
individually. Recordings at electrodes further away from the stimulating electrode results in lower eCAP amplitudes (shown below electrode
array). B: Selectivity measures are made by stimulating a masker electrode contact (M) immediately before the probe contact (P). The neural
population stimulated by the masker will then be in a refractory state when the probe is activated, so that the response to the probe will be
reduced in proportion to the amount of overlap. When the measurements of masker alone and probe alone are summed and the measurement
of masker and probe combined is subtracted (M P MP) the eCAP measurement will show the overlap between the cochlear region excited
by M and P. C & D: Raw waveforms of individual patients are shown with detected peaks marked (N1, circles; P1, squares). C: waveforms of
an individual patients’ scanning data using the forward masking and the alternating polarity artefact rejection method. Each waveform labeled
with corresponding recording electrode (R1, R2, R3…). D: waveforms of selectivity data from two individual patients (as always in selectivity
measurements a forward masking technique was used). Each waveform labeled with the corresponding position of the probe (P1, P2, P3…).
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Figure 1. A: For scanning measures a single probe electrode contact is stimulated (Stim) and the response is recorded at all electrodes
individually. Recordings at electrodes further away from the stimulating electrode results in lower eCAP amplitudes (shown below
lectrode ar ay). B: Selectivity measures are made by sti ulating a masker electrode co tact (M) immediately b fore the probe contact
(P). The neural population stimulated by the masker will then be in a refractory state when the probe is activated, so t at the response to
the probe will be reduced in proportion to the amount of overlap. When the measurements of masker alone and probe alone are summed
and the measurement of masker and probe combined is subtracted (M + P - MP) the eCAP measurement will show the overlap between
the cochl ar region excited by M and P. C & D: Raw waveforms of individual patients are shown with detected peaks marked (N1,
circles; P1, squares). C: waveforms of an individual patients’ scanning data using the forward masking and the alternating polarity artefact
rejection method. Each waveform labeled with corresponding recording electrode (R1, R2, R3…). D: waveforms of selectivity data from
two individual patients (as always in selectivity measurements a forward masking technique was used). Each waveform labeled with the
corresponding position of the probe (P1, P2, P3…).