University of Zurich
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http://www.zora.uzh.ch
Year: 2009
TNRT profiles with the Nucleus Research Platform 8 system
Lai, W K; Dillier, N; Weber, B P; Lenarz, T; Battmer, R; Gantz, B; Brown, C; Cohen,
N; Waltzman, S; Skinner, M; Holden, L; Cowan, R; Busby, P; Killian, M
Lai, W K; Dillier, N; Weber, B P; Lenarz, T; Battmer, R; Gantz, B; Brown, C; Cohen, N; Waltzman, S; Skinner, M;
Holden, L; Cowan, R; Busby, P; Killian, M (2009). TNRT profiles with the Nucleus Research Platform 8 system.
International Journal of Audiology, (9:1-10):Epub ahead of print.
Postprint available at:
http://www.zora.uzh.ch
Posted at the Zurich Open Repository and Archive, University of Zurich.
http://www.zora.uzh.ch
Originally published at:
International Journal of Audiology 2009, (9:1-10):Epub ahead of print.
TNRT profiles with the Nucleus Research Platform 8 system
Abstract
This study investigates the effect of the Nucleus CI24RE implant's neural response telemetry (NRT)
system, which has less internal noise compared to its predecessor, the CI24M/R implant, on the NRT
threshold (TNRT) profile across the array. CI24M/R measurements were simulated by ignoring CI24RE
measurements with response amplitudes below 50 uV. Comparisons of the estimated TNRTs from the
CI24RE measurements and the CI24M/R simulations suggest that, apart from a constant level
difference, the TNRT profiles from the newer implant generally would not have differed very much
from those of its predecessor. This view was also reflected by principal component analysis (PCA)
results which revealed a 'shift' component similar to that reported by Smoorenburg et al (2002). On the
whole, there is no indication that current practices of using the TNRT profiles for assisting with speech
processor programming need to be revised for the CI24RE implant.
Lai et al.: TNRT profiles with the Nucleus RP8 system
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TNRT profiles with the Nucleus Research Platform 8 system
a
a
a
b
b
c
c
d
Lai WK , Dillier N , Weber BP , Lenarz T , Battmer R , Gantz B , Brown C , Cohen N ,
d
e
e
f
f
Waltzman S , Skinner M , Holden L , Cowan R , Busby P , Killian M
g
a. Dept. of Otorhinolaryngology, University Hospital, Zürich, Switzerland
b. Dept. of Otolaryngology, Medical University of Hannover, Germany
c. Dept. of Otolaryngology, Head and Neck Surgery, University of Iowa, Iowa City, IO, USA
d. Dept. of Otolaryngology, New York University Medical Center, NY, USA
e. Dept. of Otolaryngology-Head & Neck Surgery, Washington University School of Medicine,
St Louis, MO, USA
f. Co-operative Research Centre for Cochlear Implant & Hearing Aid Innovation, Melbourne and
Sydney, Australia
g. Cochlear Technology Center, Mechelen, Belgium
Keywords
Cochlear implant, CI24RE, Neural Response Telemetry, Evoked Compound Action Potential, TNRT
Profile, Principal Component Analysis
Acroynms & Abbreviations
ECAP Evoked Compound Action Potential
RP8
Research Platform 8
NPE
Nucleus Programming Enviroment
DSP
Digital Signal Processor
NRT
Neural Response Telemetry
TNRT Neural Response Telemetry Threshold
AGF
Amplitude Growth Function
PCA
Principal Component Analysis
Corresponding author:
Wai Kong Lai
ENT Department
University Hospital Zürich
Frauenklinikstr 24
CH-8091 Zürich
Switzerland
e-mail: waikong.lai@usz.ch
Lai et al.: TNRT profiles with the Nucleus RP8 system
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Abstract
This study investigates the effect of the Nucleus CI24RE implant's neural response telemetry (NRT)
system, which has less internal noise compared to its predecessor CI24M/R implant, on the NRT
Threshold (TNRT) profile across the array. CI24M/R measurements were simulated by ignoring
CI24RE measurements with response amplitudes below 50uV. Comparisons of the estimated TNRTs
from the CI24RE measurements and the CI24M/R simulations suggest that, apart from a constant
level difference, the TNRT profiles from the newer implant generally would not have differed very
much from those of its predecessor. This view was also reflected by principal component analysis
(PCA) results which revealed a "shift" component similar to that reported by Smoorenburg et al.
(2002). On the whole, there is no indication that current practices of using the TNRT profiles for
assisting with speech processor programming need to be revised for the CI24RE implant.
Lai et al.: TNRT profiles with the Nucleus RP8 system
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Introduction
The Nucleus CI24M cochlear implant introduced in 1996 features a self-contained system for
intracochlear measurements of the electrically evoked compound action potential (ECAP), whose
measurement data is sent via back-telemetry to the controlling Neural Response Telemetry (NRT)
software. Although the measurement system is easy and reliable to use, it is not without its
shortcomings. The measurement amplifier has a noise floor of around 30-40uV after averaging
(Brown et al. 2000), limiting the accuracy in determining the ECAP threshold. Additionally, the
amplifier gain can only be reduced from 60dB to 40dB when avoiding saturation that would otherwise
arise from excessively large inputs, at the cost of a correspondingly poorer signal to noise ratio (SNR).
The nominal temporal resolution of the measured neural response was increased by using a doublebuffer scheme which in turn introduced non-linearity complexities when combining the two lowresolution buffers (Lai 2004). The CI24R implant, introduced in 2000 and featuring a smaller
receiver/stimulator package as well as a perimodiolar (Contour) electrode array, used the same
telemetry circuitry for measuring the ECAP.
The Nucleus Research Platform 8 (RP8) system consisting of the CI24RE cochlear implant, the L34
body-worn speech processor, and the Nucleus Programming Environment (NPE) software (Battmer et
al. 2004), was developed specifically for research purposes, such as investigating the effect of an
improved measurement amplifier. The CI24RE implant's measurement amplifier has a lower noise
floor for the same amount of averaging (around 2uV (Patrick et al. 2006)), as well as improved linearity
which reduces the susceptibility to saturation. The difference in the measurement noise between the
CI24M/R and the CI24RE implants is illustrated in Figure 1. The lower noise floor results in faster
measurements, since less averaging is needed to obtain a clear recording of the neural response. In
addition, finer Gain adjustment settings allow better optimization of the measurements to maximize the
SNR. The sampling rate for NRT recordings has also been increased, allowing higher temporal
resolution for the measured neural response without having to resort to additional manipulation.
The neural response threshold (TNRT) is usually estimated using either linear extrapolation of the
amplitude growth function (AGF), or visual detection of the smallest measurable neural response. The
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reduced noise floor of the CI24RE implant allows neural responses with smaller amplitudes to be
recorded than otherwise possible with the CI24M/R implant. The result should be more accurate
estimates of TNRT from either extending the lower portion of the AGF, or a lower visual threshold.
Either way, the TNRTs derived from measurements using the CI24RE implant are expected to be
different compared to those from the previous CI24M/R implant.
Although the TNRT does not correspond directly to the perceptual threshold, modest correlations
between TNRT and map T & C values for the CI24M/R implant have been reported by various studies
(e.g. Brown et al. 2000, Hughes et al. 2000, Franck & Norton 2001, Smoorenburg et al. 2002,
Cafarelli-Dees et al. 2005, King et al. 2006, Potts et al. 2007). These have encouraged the
increasingly widespread use of TNRT profiles in assisting with speech processor programming,
especially for very young children (e.g. Hughes et al. 2000, Mason et al. 2001, Gordon et al. 2002,
Kaplan-Neeman et al. 2004) as well as the elderly (e.g. Pedley et al. 2007).
Differences in TNRT between the CI24RE and CI24M/R implants would thus lead to differences in
their corresponding TNRT profiles as well. Unless the changes in TNRT values were uniform across
the electrode array, resulting in only a level shift in the TNRT profiles, this could change the
correlations between the TNRT profiles and map T & C values. This, in turn, could then affect the way
the CI24RE implant's TNRT profiles are applied to speech processor programming. There is therefore
a clinical interest to know whether the relationship between TNRT and the map T & C values would be
affected by the (more accurate) TNRT values obtained with the CI24RE implant.
This study is therefore aimed at examining the relationship between the CI24RE implant's TNRT
values with corresponding map T & C values, and to investigate whether the CI24RE implant's TNRT
profiles differ from those of its predecessor CI24M/R.
Lai et al.: TNRT profiles with the Nucleus RP8 system
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Method
Amplitude growth functions (AGFs) from 5 electrodes (typically e20, e15, e10, e5 and e3) spread
across the intracochlear electrode array were obtained postoperatively for each participating CI user.
All NRT recordings were made using the Forward Masking artefact-cancellation paradigm, each
averaged over 50 sweeps. The measurement amplifier's Gain was kept high (60dB) whenever
possible (whilst avoiding amplifier saturation) in order to maximize the SNR. The recording site was
always located 2 electrodes basal to the stimulation site (e.g. stimulation on electrode 20, recording on
electrode 18). The Masker pulse was always 10 Current Level units larger than the Probe pulse. The
stimulation levels were reduced in 5 Current Level steps to generate the AGF, beginning at the
Loudest Acceptable Presentation Level (LAPL) as indicated subjectively by the CI user. A stimulation
rate of 80Hz was used by default, but this could be reduced to 35Hz when necessary to yield a higher
stimulation level without exceeding LAPL. This reduction was only necessary with 1 single subject.
The TNRT values were then estimated from the AGF by linear regression extrapolation.
The AGFs were collected at regular 4 week intervals after and including the initial tune-up session
(denoted as week 0). The data collection was terminated at Week 12 after tune-up.
The Map T & C level data for the same electrodes were also collected during the same sessions so as
to be able to compare them directly against their corresponding TNRT counterparts. Note that the
AGFs at Week 0 (tune-up) were indeed based on data collected on that same day rather than from
intraoperative data. ACE maps with a stimulation rate of 1200 pps per channel were used for the
entire duration of the study.
One of the aims of this study was to compare the TNRT profiles from the CI24RE implant against the
TNRT profiles from the predecessor CI24M/R implant. Ideally, such comparisons should be made
using data from the two different implants in the same ear, which is impossible in practice. However, it
is possible to use the recording data from the CI24RE implant to simulate the data as though it had
been recorded with the CI24M/R implant in the following manner:
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The main difference between NRT recordings from the two implants lies in the newer CI24RE
implant's ability to measure much smaller response amplitudes, compared to the older CI24M/R
implant which has a higher noise floor of typically 30uV to 40uV (Brown et al. 2000). NRT recordings
with the CI24M/R implant are typically made with 100 averages (NRT3.1 software default), while the
CI24RE recordings in this study involved only half this amount of averaging. To allow for the likelihood
of greater noise due to this difference in averaging, the noise floor threshold was rounded up to a
more conservative 50uV for the simulations in this study. Assuming that measurements with response
amplitudes above 50uV would be similar with either implant, the AGF from the newer CI24RE implant
can be truncated below 50uV to simulate the NRT recordings as though they had been made in the
same ear with the older CI24M/R implant. Such simulations would only be approximations as the
remaining (untruncated) recordings would still contain less noise than their real CI24M/R counterparts.
This will have to be kept in mind when interpreting the data later. Other differences between the two
implant generations' circuitry such as linearity and resolution could theoretically also affect the
measurements but these are assumed to have lesser impact than the noise floor.
The truncation process is illustrated in Figure 2. A typical AGF measured using the CI24RE implant is
shown in Figure 2a. Data points above and below 50uV are distinguished by solid and empty
diamonds respectively. The truncated AGF with only data points above 50uV is shown in Figure 2b.
Truncating the AGF in this manner generally results in the extrapolated TNRT value being shifted
higher, as is also illustrated in Figure 2. The exact amount of shift in the TNRT depends on the shape
and the rate of growth of the AGF. At higher response amplitudes, the AGF can be approximated by a
linear function, whereas at lower response amplitudes, the AGF often exhibits an asymptotic tail
towards the x-axis. Differences in the AGF from one site to another would cause the TNRTs from the
truncated AGFs to be shifted by different amounts for each site along the array, resulting in the TNRT
profiles computed from the complete and the truncated AGFs to be not parallel to one another. Note
also that the asymptotic tail at lower response amplitudes means that the slope becomes less steep
as responses to higher stimulation levels are omitted from the calculation of the regression line used
to extrapolate TNRT. If there were a smaller number of suprathreshold data points in the AGF, the
effect of truncating the AGF would be more pronounced (see Figure 2c and 2d).
Lai et al.: TNRT profiles with the Nucleus RP8 system
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Subjects
Complete TNRT and Map data (ACE1200) from a total of 17 subjects from 6 centers (Hannover, Iowa
City, Melbourne, New York, St Louis and Zürich) were compiled for this study. The subjects were all
postlingually deafened adults aged between 27 & 72 years, with less than 20 years of deafness. Their
aetiologies are summarized in Table 1.
Results
i) Truncating the AGF: Estimating the "simulated" CI24M/R TNRT value
As described in the Methods section above, AGFs for the CI24M/R implant were simulated by
truncating (ignoring) all response amplitude values below 50uV. The truncated AGF yields a different
TNRT value, denoted as TNRT50. This TNRT50 is typically higher than the corresponding TNRT,
particularly when the AGF is not linear down past the point of truncation to the abscissa, such as when
an asymptotic tail is present in the AGF at lower response amplitudes. This was generally the case,
as can be seen from Figure 3, which plots TNRT versus TNRT50 pooled over all 4 sessions. The
values before truncation (i.e. TNRT) and after truncation (i.e. TNRT50) showed a very good correlation
(R=0.98) with one another, suggesting that the truncation has in general not produced radically
different TNRT50 values.
Note that truncation in this manner is only applicable to AGFs with at least 2 data points with response
amplitudes above 50uV, otherwise, the AGF would no longer be defined after truncation and the
TNRT50 can not be computed. As the AGFs were all measured post-operatively with awake subjects,
it was not always possible to guarantee that the maximum response amplitude of an AGF would be
over 50uV. Consequently, only 56% of the AGFs remained after truncation. It could be argued that
using a lower truncation threshold (e.g. 30uV or 40uV) would have resulted in slightly more truncated
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AGFs. However, the present choice of 50uV is meant to account for the smaller amount of averaging
used with the CI24RE recordings.
ii a) Correlations with Map T & C levels
The most common form of comparison of TNRT values versus psychophysical Map T & C levels found
in current literature is simple correlations (e.g. Brown et al 2000, Hughes et al 2000, Franck & Norton
2001, Cafarelli Dees et al 2005, King et al 2006). However, level differences in the TNRT, T and C
values across subjects need to be taken into account before the data can be pooled together.
Otherwise, the resultant pooled data would have greater scatter than appropriate. To take into
account level differences across subjects, the following was done:
For each subject, the mean difference between the TNRT and its corresponding T value (referred to
as the T-offset) across the array was first computed. This mean T-offset was then added to the T
values, effectively correcting the mean level differences between the TNRT and T level profiles across
the array. This correction does not change the correlation between the TNRT and T values for a
particular subject.
Similarly, the mean difference between the TNRT and its corresponding C value (referred to as the Coffset) across the array was also computed. This mean C-offset was then subtracted from the C
values, effectively correcting the mean level differences between the TNRT and C level profiles across
the array. As with the T-offset correction, this does not change the correlation between the TNRT and
C values for a particular subject. The above corrections to the T and C values are similar to those
employed by Franck & Norton (2001), and were computed for both TNRT as well as TNRT50 in order
to examine the effect of truncating the AGFs on the correlations with map T & C values.
The offset-corrected map T & C levels yielded correlations of R=0.78 and 0.80 respectively when
compared with the corresponding TNRT values (pooled data over all 4 sessions) (Figure 4). The
same offset-corrected map T & C levels produced very similar results when compared with the
estimated TNRT50 values, with correlations of also R=0.78 and 0.80 respectively (Figure 5). The
similarity between the correlations for both TNRT and TNRT50 indicate that the presence of the
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smaller response amplitudes below 50uV measurable with the CI24RE implant has not affected the
correlations between TNRT and the corresponding Map T & C levels.
ii b) Changes over time
Figure 6 illustrates how the averaged TNRT, TNRT50, T and C profiles change over time. Over a
period of 12 weeks from tune-up, the average TNRT and TNRT50 values were quite stable, while the
average T and C levels typically showed a small but gradual increase (up to 20 Current Level units) as
the subjects adapted to the CI's stimulation.
A one-way analysis of variance showed that the mean differences between TNRT and the Map T & C
levels did not vary significantly between sessions (T-TNRT: p=0.004, C-TNRT: p=0.006). On the other
hand, the mean differences between TNRT50 and the Map T & C levels were not so consistent over
the four sessions (T-TNRT50: p=0.318, C-TNRT50: p=0.469).
iii) Principal component analysis
The data was also subjected to principal component analysis (PCA) to examine the similarities
between TNRT, T level and C level profiles across the array, in the same manner as reported by
Smoorenburg et al. (2002). The analysis was then repeated for the TNRT50 profiles, compared with
the same T level and C level data, in order to assess the difference between TNRT and TNRT50.
Typically, only components with eigenvalues > 1 are considered to be significant. It is important to
note at this point that Smoorenburg et al. (2002) used profiles consisting of 20 electrodes, while the
data here only involve 5 electrodes spread out across the array per profile, raising the significance
threshold from 5% to 20% of the total variance.
The first 2 components from PCA of the individual TNRT, TNRT50, T and C data are summarized in
Table 2, including also the results from Smoorenburg et al. (2002) for convenience. For the present
data, only component 1, which accounted for 70.3%, 67.0%, 84.2% and 77.3% of the total variance for
TNRT, TNRT50, T level and C level respectively, was found to be significant. As expected, given the
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smaller number of data points per profile, these values are less than those reported by Smoorenburg
et al. (2002).
Interpretation of the principal components involves plotting them as a function of the electrode
number, as illustrated by Figure 7. In all cases, component 1 is fairly constant across the array,
suggesting it could be interpreted as an "offset" or "shift" component. In Smoorenburg et al. (2002),
two main components were found for each of the TNRT, T and C data which were interpreted as a
"shift" and a "tilt" component respectively.
The individual correlations between TNRT (as well as TNRT50) and the corresponding T and C levels
were then examined for component 1, and the results are summarized in Table 3. The correlations
between TNRT versus the T & C levels were a modest 0.40 and 0.57 respectively. Similar
correlations (0.58 and 0.55 respectively) were obtained for TNRT50 versus the T & C levels. Both
sets of correlations compared reasonably well with those reported by Smoorenburg et al. (2002). The
correlation between TNRT and the C levels for the present data was statistically significant at the 95%
confidence level.
PCA was also performed on the collective {TNRT, T & C} data, as well as on the collective {TNRT50,
T & C} data in the same manner as Smoorenburg et al. (2002). Here, only a single component was
found which accounted for 70.4% and 70.7% of the total variance, compared to 94.1% for the first
component reported by Smoorenburg et al. (2002), as summarized in Table 4. The similarity between
the {TNRT, T & C} and the {TNRT50, T & C} data sets can also be seen in Figure 8, which also shows
that the significant component can clearly be interpreted as a "shift" component.
Discussion
The truncation process for simulating the NRT recordings from the CI24M/R implant yields only an
approximation, as there would still be less noise in the remaining (untruncated) CI24RE recordings
than in actual CI24M/R recordings. Consequently, the corresponding TNRT50 estimates, while
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producing less scatter due to the reduced noise, would at the same time be compromised by being
derived using a smaller number of remaining data points. The specific combined effect of the reduced
scatter and the reduced number of data points are not investigated further here. It is important to bear
in mind that TNRT50 is also approximate in nature.
The NRT profiles from the CI24RE implant have been analyzed using various methods which yield
different degrees of details.
With simple correlations, no distinction is made between different sites along the electrode array, nor
are inter-subject differences accounted for. Nevertheless, the high correlation found between TNRT
and TNRT50 suggest that the truncation of the AGF below 50uV response amplitude generally
resulted in little more than a level shift between TNRT and TNRT50. The observation that TNRT50
was generally larger than TNRT indicates that the truncated portion of the AGF either had an
asymptotic nature as the response amplitudes approached zero, or had at least a slope lesser than
that of the untruncated portion. Had the AGF been linear with the same slope all the way down to the
abscissa, there would have been very little difference between the extrapolated values for both TNRT
and TNRT50.
Correlations between TNRT and Map T & C levels were similar to those reported in previous studies
(e.g. Brown et al 2000, Hughes et al 2000, Frank & Norton 2001). The improved sensitivity of the NRT
measurement circuitry has therefore apparently not altered the correlations dramatically. This again
suggests that the CI24RE implant's TNRT profiles are merely lower in level, and that the changes in
TNRT along the electrode array may possibly be regarded as being uniform. This view is further
reinforced by the correlations between TNRT50 and the Map T & C levels, which are also similar to
those found with the predecessor CI24M implant. Bearing in mind that the TNRT50 values are only
approximate in nature, it would seem that truncating the AGF was a reasonable way to simulate the
neural response measurements as if they had they been made with the predecessor CI24M/R implant.
The TNRT being smaller than TNRT50 is a direct consequence of the improved measurement
sensitivity. However, TNRT remains higher than the behavioral threshold (Map T level). A one-to-one
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correspondence between TNRT and the behavioral threshold is unlikely to be achieved as
measurements of the ECAP necessitate a certain degree of synchrony in firing of the activated neural
population, whereas such synchronous activity is probably not required for the perceptual system to
detect the activity at the behavioral threshold. The relationship between TNRT and the Map C levels
is more difficult to determine as the latter is very much subjective in nature. Correlations between
TNRT and the Map C levels are indirectly an indicator of the relationship between corresponding Map
T and C levels.
The results over time showed that there were not very large changes in TNRT or TNRT50 over time,
suggesting that postoperative measurements of the ECAP are quite stable over time. The observed
increases in both T & C levels over the same period are typical of CI users as they adapt to the CI
stimulation and are gradually able to accept slightly higher levels of stimulation with time.
PCA provides more information than simple correlations because it also takes into account variations
along the electrode array. The overall results (see Table 4 and Figure 8) show that about 70.4% or
70.7% of the variance between either TNRT or TNRT50 and the T & C levels could be explained by a
single component, interpreted as a "shift" component. Given that the TNRT50 results need to be
interpreted with care because of its reduced data set after AGF truncation, the similarities between the
{TNRT, T & C} and the {TNRT50, T & C} data suggest that the missing TNRT50 data has not
significantly affected the principal components. Unlike Smoorenburg et al. (2002), only a single
significant component which accounted for a smaller percentage of the total variance was found here.
This is not surprising considering that the present data set is smaller, with only 5 instead of 20
electrodes to describe the profiles.
The TNRT and TNRT50 data differed slightly from one another in that TNRT yielded significant
correlations between its "shift" component and the corresponding C level, whereas the TNRT50 data,
similar to the Smoorenburg et al. (2002) data, were not significantly correlated (see Table 3) with
either T or C levels. One possible explanation for the improved correlations for the TNRT data could
be the greater sensitivity of the CI24RE implant's measurement circuitry, since these significant
correlations disappear when the lower response amplitudes were excluded (with TNRT50 as well as
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the Smoorenburg et al. (2002) data) from the analysis. Note however that the TNRT50 data should
not be equated with the Smoorenburg et al. (2002) data due to the smaller number of electrodes per
profile as well as the reduction in TNRT50 data after AGF truncation. This is possibly a reason also
for the lack of significant correlations for TNRT50.
Another difference between the Smoorenburg et al. (2002) data and the present study is that the
former exclusively involved the CI24M implant which had a straight array of full-ring banded
electrodes. In the present study, the CI24RE implant is equipped with the perimodiolar
Contour/Contour Advance electrode array, which is expected to provide more variation along the
length of the electrode array compared to the straight electrode array. For instance, Smoorenburg
(2007) more recently reported a "curvature" component in the Map data where the majority (127
versus 88) of the implants had a perimodiolar electrode array. On the other hand, Willeboer &
Smoorenburg (2006) involved Contour/Contour Advance electrode arrays but reported only significant
"shift" and "tilt" components, possibly due to the smaller population size of only 18 subjects.
Unfortunately, the present data with only 5 data points for representing each profile is insufficient for
revealing features more subtle than the "shift" component, particularly features that are related to
longitudinal variations such as "tilt" or "curvature". In fact, from Smoorenburg et al. (2002) it could
already be seen that the "tilt" component accounted for a substantially smaller percentage of the total
variations than the "shift" component.
On the whole, PCA results were similar to those found with the CI24M implant, again reinforcing the
view that the TNRT profiles with the CI24RE implant have not been significantly altered to provoke any
changes in the clinical application of these TNRT profile.
Conclusions
The aim of this study was to determine if the TNRT profiles have changed with the new CI24RE
implant compared to its predecessor CI24M/R implant. Truncation of the AGF was employed to
produce TNRT50 profiles in order to simulate the TNRT profiles of the earlier implant. Both TNRT and
Lai et al.: TNRT profiles with the Nucleus RP8 system
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TNRT50 data produced correlations similar to those reported for the CI24M implant, suggesting that,
apart from a level change, there are no great differences in the TNRT profiles between the two implant
generations.
Due to the reduction in the number of data points representing each profile, principal component
analysis was able to only reveal a single principal component, with similar significance to that found for
the predecessor implant. Significant correlations found with the Map C levels are likely to be related to
the CI24RE implant's improved measurement amplifier, but this has little impact on the general
observations above. Thus, it is not expected that the use of TNRT profiles in assisting with clinical
speech processor programming will be altered greatly for the CI24RE implant.
Acknowledgments
This study was supported by a research grant from Cochlear AG, Basel as well as a research grant
No. 320000-110043 from the Swiss National Science Foundation
The authors would also like to thank Bas van Dijk, Kerrie Plant & Sarah Ashburn Reed for part of the
data collection, as well as Professor Guido Smoorenburg for comments on the principal component
analysis.
Declaration of interest
The authors report no conflicts of interest. The authors alone are responsible for the content and
writing of the paper
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implant system. Audiol Neurootol, 10(2), 105-116.
Franck KH & Norton SJ. 2001. Estimation of psychophysical levels using the electrically evoked
compound action potential with the Neurel Response telemetry scapabilities of Cochlear Corporation's
CI24M device. Ear Hear, 22(4), 289-299.
Gordon KA, Ebinger KA, Gilden JE & Shapiro WH. 2002. Neural response telemetry in 12- to 24month.old children. Ann Otol Rhinol Laryngol, 189(Suppl), 42-48.
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with map levels in the Nucleus 24 cochlear implant: Data from children. Ear Hear, 21(2), 164-174.
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Muchnik C. 2004. NRT-based versus behavioral-based MAP: a comparison of parameters and speech
perception in young children. J Basic Clin Physiol Pharmacol, 15(1-2), 57-69.
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King JE, Polak M, Hodges AV, Payne S & Telischi FF. 2006. Use of neural response telemetry
measures to objectively set the levels in the Nucleus 24 cochlear implant. J Am Acad Audiol, 17(6),
pp413-431.
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3.1. Cochlear, Basel.
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electrically evoked auditory nerve action potentials in young children by use of neural response
telemetry with the Nucleus CI24M cochlear implant. Br J Audiol, 35(4), 225-235.
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NRT and behavioral measures for MAPping elederly cochlear implant users. Int J Audiol, 46(5), 254262.
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Response Telemetry Thresholds, T- and C-Levels and loudness judgements in 12 adult Nucleus 24
cochlear implant recipients. Ear Hear, 28(4), 495-511.
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Willeboer C & Smoorenburg GF. 2006. Comparing cochlear implant users' speech performance with
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Lai et al.: TNRT profiles with the Nucleus RP8 system
Page 18 of 32
List of Tables
Table 1: Subjects summary
Table 2: Principal components analysis results – Component 2 for the present data (black cells) are
not significant. The present data's significant components account for a smaller proportion of
the total variances compared to those from Smoorenburg et al. (2002).
Table 3: Correlations between TNRT and the corresponding map T & C levels for the CI24RE implant
(TNRT), the simulated CI24M/R implant (TNRT50) and the actual CI24M/R implant
(Smoorenburg et al. (2002)).
Table 4: Combining TNRT and Map levels, only a single significant principal component waas found
for all 3 instances. The overall data with either TNRT or TNRT50 from the present data
showed very similar results.
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Table 1
Age
(years)
Duration of
Deafness
(years)
27
2
Autoimmune
42
7
Unknown
43
5
Unknown
43
2
Measles
43
20
Familial
43
12
Unknown
53
0
Meniere's Disease
61
12
Unknown
63
4
Unknown
64
12
Unknown
67
0
Familial
67
2
Unknown
71
6
Trauma
72
8
Ototoxic Drugs
72
1
Familial
74
5
Unknown
74
1
Unknown
Aetiology
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Table 2
Percentage variance accounted for
Present Data
Smoorenburg et al. (2002)
TNRT
TNRT50
T
C
TNRT
T
C
Component 1
70.3
67.0
84.2
77.3
90
90.0
88.3
Component 2
15.0
18.5
10.6
17.0
6.4
6.7
7.5
Sum (significant)
70.3
67.0
84.2
77.3
96.4
96.7
95.8
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Table 3
TNRT
Component
Interpretation
Shift
Tilt
* 0.05
TNRT50
TNRT
(Smoorenburg et al. 2002)
T Level
C Level
T Level
C Level
T Level
C Level
0.40
0.57*
0.58
0.55
0.64
0.39
0.82
0.36
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Table 4
Percentage variance accounted for
Smoorenburg et al.
(2002)
Present Data
Component 1
TNRT, T & C
TNRT50, T & C
TNRT, T & C
70.4
70.7
94.1
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List of Figures:
Figure 1: NRT recordings from typical CI users with the CI24RE (left column) and the CI24M/R (right
column) implants for subthreshold as well as suprathreshold input signals. The examples
here were selected to have comparable suprathreshold response amplitudes. The
recordings were all made with the same 60dB amplifier gain setting, and involved 50
averages. The second row shows how a response of around 25uV peak-to-peak amplitude
is clearly discernable with the CI24RE implant but not at all with the CI24M/R implant.
Figure 2: The original AGF (a) with all data points as measured with the CI24RE implant yields an
extrapolated TNRT of 138 CL. The threshold TNRT50 is shifted to 145 CL when the AGF
is truncated below 50uV (b). The truncated AGF simulates the AGF as if it had been
measured with the CI24M/R implant. If the highest stimulation level in the untruncated AGF
were lower (resulting in less suprathreshold data points), truncating the AGF produces a
larger difference between TNRT and TNRT50 (c and d).
Figure 3: Correlations between original TNRT (untruncated AGFs) data and TNRT50 (AGFs
truncated below 50uV). Note that TNRT50 was generally larger than the corresponding
TNRT.
Figure 4: Correlations between original TNRT (untruncated AGFs) data with offset corrected (left) T
levels and (right) C levels.
Figure 5: Correlations between TNRT50 (truncated AGFs) data with offset corrected (left) T levels
and (right) C levels.
Figure 6: Average TNRT and TNRT50 profiles appear to be stable over the 12-week period of the
study, while average T- and C-Levels show a small but gradual increase over the same
period.
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Figure 7: Plots of the principal components across the array show how component1 can be
interpreted as an "offset/shift" component for the individual TNRT, TNRT50, T Level or C
Level data.
Figure 8: Plots of the single principal component of the collective data sets show very similar
component values, suggesting that the TNRT and TNRT50 data are very similar.
Lai et al.: TNRT profiles with the Nucleus RP8 system
Figure 1
Page 25 of 32
Lai et al.: TNRT profiles with the Nucleus RP8 system
Page 26 of 32
Figure 2
Only data points ≥ 50uV
All data points
300
300
Data ≥ 50uV
250
Data < 50uV
Response Amplitude uV
Response Amplitude uV
250
200
150
100
TNRT=138
50
200
150
100
TNRT50=145
50
0
0
130
140
150
160
170
180
190
200
210
130
140
150
Current Level CL
160
170
180
190
200
210
Current Level CL
a)
b)
Only data points ≥ 50uV (with less suprathreshold data)
All data points (with less suprathreshold data)
300
300
Data ≥ 50uV
250
Data < 50uV
Response Amplitude uV
Response Amplitude uV
250
200
150
100
TNRT=132
50
200
150
100
TNRT50=142
50
0
0
130
140
150
160
170
180
190
200
210
130
c)
140
150
160
170
180
Current Level CL
Current Level CL
d)
190
200
210
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Figure 3
220
200
R = 0.98
TNRT50
180
160
140
120
100
100
120
140
160
TNRT
180
200
220
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220
220
200
200
180
C (less mean offset)
T (plus mean offset)
Figure 4
R = 0.78
160
140
120
100
100
180
160
R = 0.80
140
120
120
140
160
TNRT
180
200
220
100
100
120
140
160
TNRT
180
200
220
Lai et al.: TNRT profiles with the Nucleus RP8 system
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220
220
200
200
C (less mean offset)
T (plus mean offset)
Figure 5
180
R = 0.78
160
140
120
100
100
180
R = 0.80
160
140
120
120
140
160
TNRT50
180
200
220
100
100
120
140
160
TNRT50
180
200
220
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Figure 6
Changes over time: Average C levels
180
170
160
170
160
150
140
130
120
110
100
90
80
TNRT w 0
TNRT w 4
Level (CL units)
Level (CL units)
Changes over time: Average TNRT
180
TNRT w 8
TNRT w 12
150
140
130
120
Cw4
90
80
C w 12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Electrode
Changes over time: Average T levels
Changes over time: Average TNRT50
180
160
150
TNRT50 w 0
110
100
TNRT50 w 4
90
80
TNRT50 w 12
TNRT50 w 8
Level (CL units)
Level (CL units)
Cw8
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Electrode
180
170
140
130
120
Cw0
110
100
170
160
Tw0
150
140
Tw8
130
Tw4
T w 12
120
110
100
90
80
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Electrode
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Electrode
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Figure 7
1
Component value
Component value
1
0
TNRT data
TNRT50 data
Comp 1
2
3
4
5
6 7
8
1
9 10 11 12 13 14 15 16 17 18 19 20 21 22
Electrode
2
3
4
5
6 7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22
Electrode
1
1
Component value
Component value
Comp 1
-1
-1
1
0
0
T Level data
0
C Level data
Comp 1
-1
Comp 1
-1
1
2
3
4 5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22
Electrode
1
2
3
4
5
6 7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22
Electrode
Lai et al.: TNRT profiles with the Nucleus RP8 system
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Figure 8
1
Component value
Component value
1
0
Combined TNRT, T & C data
0
Comp 1
-1
Combined TNRT50, T & C data
Comp 1
-1
1
2
3
4
5
6 7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22
Electrode
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22
Electrode