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82 | Chapter 4
Masker or probe fixed
In five subjects two sets of selectivity measures were performed, one with the position of the masker fixed
and one with the probe contact fixed. Apart from some outliers at EB-A, the widths along the array and
for different current levels showed similar profiles. Figure 4, C, illustrates the small differences in masker-
fixed versus probefixed curves across location and current level in a typical subject. The third linear mixed
model using the data of all five subjects is summarized in Table 4. Results showed very similar curve widths
for masker or probe position fixed for all electrode contacts (mean differences 0.3 to 0.2 spacing, not
significant).
Artefact rejection method
A linear mixedmodel was generated using the data from the five subjects in which the scanning measurements
were performed using the forward masking and the alternating polarity artefact rejection schemes. The
curves of those subjects showed the same shape for both methods, as illustrated by an example in Figure 4,
D, and differences did not reach significance levels.
ent stimulus levels. There were, however, some notable exceptions,
for the middle electrode. For this individual p
for the highest current level compared to th
However, across all subjects the second linea
that the curve widths of the medium and h
significantly different from those found at the
and
p
0.8 respectively).
Recording electrode
The third linear mixed model with data of
rized in Table 4) revealed significant effect
on the width of the selectivity measures.
tended to shift in the direction of the rec
the apical electrode (EA-B) the width wa
smaller when measured apically compared
basal electrode (EB-A) the width was 1.7
measured apically. The results from the el
of the array (EM) did not show significan
the recording contacts.
Table 3.
Significance (p-value) of difference in width of selectivity
measures along the array (t-statistic and degrees of freedom between
brackets).
Selectivity
EA-B
EM-A
EM-B
EB-A
EA-B
EM-A
p
0.01
(t 2.56;
dF 289.7)
EM-B
p
0.01
(t 5.43;
dF 289.7)
p
0.01
(t 7.95;
dF 289.3)
EB-A
p
0.01
(t 2.86;
dF 292.2)
p
0.01
(t 5.21;
dF 291.2)
p
0.03
(t 2.17;
dF 291.2)
472
F. B. van der Beek et al.
Masker or probe fixed
The finding that scanning produces wider
measures has been previously reported by C
Hughes & Stille (2010), and is in line with th
This can be explained as follows: in the sc
recordings are made at several points along t
current from the stimulating contact (i.e. sti
current generated by the nerve fibers (neural
easily through the fluid of the cochlea and res
recording electrode some distance from the st
selectivity method, however, using a fixed re
shows the spread of excitation of the stimulati
for scanning were evident at all positions an
tion of the middle contact measured in the
exception is likely a consequence of the met
that many curves for both methods did not r
The rationale for the choice of this 0.6 criter
methodology section. The limitation of the
introduction of arbitrary, fixed values at the
proposed by Abbas et al (2004) and complic
Table 4.
Mean differences between widths (contact spacing)
obtained for selectivity measures with the masker fixed or the probe
fixed, with significance (Sig.) levels (left columns). The right columns
list the mean differences (with significance levels) of the widths
obtained with the recording contact 2 apical or 2 basal relative to the
contact of interest.
Measure
Difference masker
vs. probe fixed
(contact spacing)
Sig.
Difference apical
vs. basal recording
(contact spacing)
Sig.
EA-B
0.3
0.62
1.5
0.01
EM-A
0.2
0.75
1.0
0.08
EM-B
0.2
0.69
0.01
0.99
EB-A
0.3
0.60
1.7
0.01
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