PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Assessment of adjustment to electrical threshold (T) level andelectrical stimulation rate on intensity discrimination and amplitude
modulation detection at soft presentation level in adult CI users
Terry B NUNN1 Tim GREEN2; Dan JIANG3; Patrick BOYLE4; Deborah A VICKERS5,2
1
2

Ear Institute, University College London, UK

Department of Speech Hearing and Phonetic Sciences, University College London, UK
3

Guy’s & St Thomas’ NHS Trust, UK
4

Advanced Bionics, Germany

5

Department Clinical Neuroscience, Cambridge University, UK

ABSTRACT
A pilot study was conducted to evaluate the influence of electrical stimulation rate and the setting of electricalthreshold (T) level on intensity difference limens (IDLs) and amplitude modulation (AM) perception.
Participants were ten adult experienced Advanced Bionics cochlear implant (CI) users. The frequency of
acoustic sinusoidal stimuli was set at 1076Hz with the intention of stimulating intra-cochlear electrode 7 of
the 16 electrode array. Nine experimental maps were created with T levels set at: the threshold of audibility; alevel perceived as ‘very soft’ and 10% of the level judged to produce ‘most comfortable loudness’. Rates ofelectrical stimulation were 905, 1811 and 2750 pulses-per-second (pps). Adaptive 2I-2AFC IDL and AM
perception tasks were presented via headphones at soft presentation levels (approximately 50dBSPL). The
influence of T Level and stimulation rate adjustments on IDL and AM perception have been explored with
the intention of understanding the impact of altering individual parameter settings to optimise detection of
acoustic cues at low presentation levels.
Keywords: Cochlear Implant, Amplitude Modulation

1. INTRODUCTION
Cochlear implant (CI) users often show poorer speech perception at low intensity levels (Donaldson et al.
2009; Boyle et al. 2013). Very little is understood about the influence of sound processor fitting parameterson the discrimination of low intensity acoustic cues.
CIs transduce an input acoustic signal to an output electrical signal delivered to the hearing nerve via an
intra-cochlear electrode. For the Advanced Bionics CI system, the intra-cochlear electrode has 16 electricalcontacts or channels, that encode low frequencies at the apex and high frequencies at the base of the cochlea,
analogous to the natural tonotopic organization. When all 16 individual channels are active, each has a fixedbandwidth, with a centre frequency at 383Hz for channel 1 up to 6665Hz for channel 16.
The extracted acoustic energy within each channel is rectified, and low-pass filtered extracting theenvelope information. The envelope is used to amplitude modulate the biphasic electrical pulse trains
1

Terry.nunn.15@ucl.ac.uk

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delivered to the associated electrical contact. For CI users this envelope information is crucial for deliveringspeech information (Smith et al. 2002) and it has been demonstrated that poor AM detection is associatedwith poor speech understanding (Garadat et al. 2013).
Based on patient loudness judgements each electrode is configured to have a maximum current level
equivalent to a perception of ‘most comfortable’ loudness, termed the (M) level, and a minimum current level,termed the threshold or (T) level, which by default for the Advanced Bionics device is set to 10% of M level.
This range is the electrical dynamic range (EDR).
This pilot study evaluated three clinical methods for setting T level and used three electrical stimulation
rates which span a range commonly offered within clinical practice. The pilot aimed to determine the best
approach for setting the T level and the effect of electrical stimulation rate on amplitude modulation (AM)perception and intensity difference limens (IDL). The effect of electrical stimulation rate upon speech
perception has been explored (Arora et al. 2011, Battmer et al. 2010, Bonnett et al. 2012, Buechner et al. 2010,
Shannon et al. 2011) with a consensus that optimal stimulation rate varies on an individual basis, for speechpresented between 60 to 70dBSPL. However, there is a limited understanding of the influence of stimulation
rate on cues for speech perception presented at soft intensity. AM was selected as this has been demonstrated
to correlate with speech perception (De Ruiter et al., 2015, Gnansia et al., 2014, Won et al., 2011) and AM
detection has been shown to improve at lower electrical stimulation rates (Fraser and McKay, 2012; Green etal. 2012; Pfingst et al. 2007) when tested at moderate intensity, 60 to 70dBSPL. The effects of stimulation
rate and different methods for setting T level on intensity discrimination and perception of AM were assessedusing acoustic stimuli presented at soft intensity.

2. METHODS
2.1 Participants
Ten post-lingually deafened adult CI users were recruited from St Thomas’ Hearing Implant Centre.
Ethical approval was obtained from Health Research Authority (IRAS number 236017). Participants
met the following criteria
Inclusion criteria:• Over 18 years of age• Full intra-cochlear electrode insertion• Users of the Hi-focus 90K or later Advanced Bionics implant system• Post implant activation experience of more than 9 months• Post lingual onset of severe/ profound bilateral hearing loss
Exclusion criteria:• Degenerative neurological diseases or other aetiologies that would limit attendance• English not first language• Unable to accurately make the loudness judgements used when setting the M and T levels,
standard to cochlear implant programming
Two bilateral users had both ears tested. Eight participants completed tests in one single session
lasting 2½ hours. Two required an additional ½ hour session. Participant demographic details are in
Table 1.
Table 1 Relevant information about study participantsParticipant

Ear

GenderAetiology

Age(years)DeviceSurgery(year)

30

HRHiFocus 1J

2005

15

HRHiFocus 1J

2008

Clinicalspeechstrategy

Clinical
IDR

Clinicalrate(pulsesper-sec
ond)

60

3712

60

3712

70

2062

S004

Right

FemaleUnknown

75

7

HRHiFocusMS

2014

HiResS
Fidelity120
HiResS
Fidelity120
HiRes
OptimaS

S005

Right

Male

CLL

60

1

HRHiFocusMS

2017

HiRes
OptimaS

60

3375

S006

Right

Male

Unknown

52

8

HRHiFocusMS

2015

HiRes
OptimaS

70

3535

Right
S001

Male

Otosclerosis

71

Left

6530

Duration
profounddeafness(years)Right
S007

FemaleLeft

UshersType III

76

11

HRHiFocus1J

2011

9

HRHiFocus1J

2009

HiResS
Fidelity120
HiResS
Fidelity120
HiRes
OptimaS

75

3712

75

3712

70

2855

S008

Left

FemaleUnknown

60

1

HRHiFocusMS

2014

S009

Right

Male

NIHL

69

4

HRHiFocusMS

2017

HiResS
Fidelity120

70

3712

S010

Right

Male

Unknown

42

3

HRHiFocusMS

2015

HiRes
OptimaS

70

2475

S011

Left

Male

Unknown

37

8

HRHIFocus1J

2011

HiResS
Fidelity120

70

2320

S012

Right

FemaleMeningitis

54

12

HRHiFocus1J

2007

HiResS
Fidelity120

60

1904

2.2 Equipment
Participants used a dedicated Naida Q90 speech processor during testing. It was programmed using
SoundWave 3.1 software (version 3.1.18) operating on a standard clinical fitting station. Nine
experimental maps were created, which comprised T levels set at: the threshold of audibility; a level
perceived as ‘very soft’, and 10% of the level judged to produce ‘most comfortable loudness’,factorially combined with three rates of electrical stimulation: 905, 1811 and 2750 pulses-per-second.
Standard psychophysical measurement of loudness, using a ten-step loudness scaling chart was
adopted; this was familiar to all participants as it is common clinical practice. All measurements werecompleted by stimulating channel 7, which occupies a mid-cochlear position in a fully inserted
electrode array with a centre frequency of 1076Hz.
M level was measured using an ascending electrical current presentation to achieve perceptual
response of ‘loud, but comfortable’ before descending to achieve a current level providing a perceptionof ‘most comfortable’ loudness. T level was set in a descending electrical current presentation andmarked as the last audible level before inaudibility. Finally, using an ascending presentation, a current
level providing a perception of ‘very soft’ loudness was recorded.
Stimulus presentation and response acquisition were controlled via a MATLAB script. An adaptive
2I-2AFC procedure with feedback for both AM and IDL measurements was used with sinusoids of1076Hz. Stimuli were presented through Sennheiser HD-414 headphones and all participants used a
medium T-mic input microphone, placed in the pinna. For the IDL measurements the respondents had
to say which of the two stimuli presented were ‘louder’ for a fixed reference level of 48dB SPL. A
2-down 1-up adaptive procedure tracking the 71% threshold was used. An initial step size of 4 dB was
reduced to 1 dB with a maximum of 6 reversals or 30 trials. IDL was calculated as the mean of levels
at the smallest step size. The AM perception task was adapted from a technique developed by Moore etal, (2017). Two successive carrier bursts each lasting 2 seconds were presented with one modulated at
an AM rate of 4 Hz and one modulated at 8 Hz AM rate. A 2-down 1-up adaptive procedure tracking
the 71% threshold was used. The threshold modulation depth at which the listener could discriminate
the difference in modulation rate was defined for the average of the last four reversals and calculated
as geometric mean.
Each participant completed two practice runs, before completing a single run for each of the nine test
conditions described in Table 2. For this pilot phase the test order was the same for all but two
participants. Experimental maps were created and loaded to a research sound processor in the
numerical bracketed order indicated in Table 2. The order in which different methods for setting T
level were implemented was different to control, to some extent, for order effects. In two participants(S11 and S12) the order of rate of stimulation was reversed. Participants had all parameter settingsother than T level setting and stimulation rate configured to those used within their standard clinicalsound processing map. Participants with ‘Clear Voice’ activated in their standard clinical soundprocessing map had this implemented in the test map. ‘Clear voice’ is an input noise cancellingalgorithm, however at the low presentation levels used, the algorithm would not have been active andconsequently would not have had an effect.

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Table 2 – Indicates the test order by number in brackets

For each participant, AM measurements were completed before the IDL tests with a short break in
between. The responses from three participants, S006, S008 and S009 have been excluded fromanalysis. Participant S006 and S009 already had their T levels set above 10% of M level within theirclinical program and consequently both found stimuli inaudible with T level at 10% of M level. S008
was unable to perform the AM task in any condition and more thorough review of the clinical history
and electrophysiological measurements revealed that the individual might have auditory neuropathy(AN). Participants S001 (right ear) and S005 were unable to complete the AM test sequence on the firsttest date and have been unable to attend for initial follow up dates.

3. RESULTS
Preliminary findings and trends for exploration in future work are reported. Figure 1 shows IDLs
for the three methods of T level setting and three stimulation rates. Responses from nine ears and sevenparticipants indicate marked variability with poorer IDLs as rate of stimulation increased from 905 to
1811pps, but not for 2750pps. T levels set at ‘very soft’ had the smallest EDRs and consequently
provided a higher current to represent low intensity acoustic inputs, which when coupled with thelowest rate of stimulation (905pps) resulted in reduced variability and improved IDL.
A two-way repeated measures ANOVA was performed on IDL data with factors of rate ofstimulation (3 rates) and T level setting (three approaches). . There was no significant effect of either
stimulation rate [F(2,16) = 1.065, p = 0.368, partial ƞ2 =0.12] or method of T level setting [F(2,16) =0.861, p = 0.413, partial ƞ 2=0.10], and there was no significant interaction [F(4,32) = 2.551, p = 0.94,
partial ƞ 2=0.25].

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Figure 1 - IDL for stimulation rate as a function of T level by stimulation rate. Boxes indicate 25th and 75th
quartile, whiskers indicate the range. Lower IDL values indicate improved ability.
Figure 2 shows AM responses for the three stimulation rates as a function of method of T level setting.
Responses from seven ears and six participants were used, and the results showed great variability with nodiscernable pattern for changing T level approach or stimulation rate. However, there could be a trend forhigher rates of stimulation to have a higher geometric mean score for the perceptual T levels and the ‘very
soft’ T level setting.
The ANOVA revealed no significant effect for stimulation rate [F(2,12) = 3.793, p = 0.064, partialƞ 2=0.39], no significant effect of method of T level setting [F(2,12) = 0.116, p = 0.862, partial ƞ 2=0.02], andno significant interaction [F(5,24) = 1.944, p = 0.18, partial ƞ 2=0.25].

Figure 2 – AM at 48dBSPL by setting of electrical thresholds T level and stimulation rate. Boxes indicate
25th and 75th quartile, whiskers indicate the range. Lower AM values indicates improved ability.
To prevent vetting participants within the main study arm who were observed to have difficultyproviding responses in the pilot test phase, the following exclusions within selection criteria will be
added.
(i)
Clinical indicators of AN(ii)
Only include individuals using T levels automatically set at 10% M level
For AM rate discrimination the effect of stimulation rate was a large effect size (ƞ 2=0.39), the effectof T level approach was a small effect size (ƞ 2=0.02), and the interaction was a large effect size(ƞ 2=0.25). For IDL the effect sizes were large for both factors (rate, ƞ 2=0.12; and T level approach,ƞ 2=0.10) and for the interaction (ƞ 2=0.24). Using ƞ 2=0.25 and a power of 0.80 for a two-way fixed
effects ANOVA results in a sample size of 33 participants for the main study.

4. DISCUSSION
The study performed was a pilot intended to establish the optimal study design and power
calculations based on IDL and AM measurements presented acoustically using ‘soft’ intensity6533

presentation levels. The low presentation levels were used here to determine if testing abilities at thelower end of the dynamic range is appropriate. Study participants generally reported that the IDL and
AM measures were challenging but were understandable and could be completed for the nine
experimental conditions in one session. In this pilot there was only one run per condition and in themain study all testing would be repeated to give two thresholds per condition. It was decided that allconditions would be retained in the main study although the intention had been to remove someconditions. Variability across participants made it difficult to determine the most critical trends to
explore going forwards and consequently all conditions will be retained. To balance the study design
across conditions and participants a latin square design will be used in the main study so mult iples of
9 participants will be required.
There were a few participants who could not perform the psychophysical measures. This has
resulted in the inclusion criteria being adjusted to include the following. Participants using a clinicalprogram in which the T level was not set at 10% M level, because that is the default for AB users andmight indicate that there were issues when setting the T levels. Participant notes will be carefully
checked and individuals with any suspicion of AN will be excluded.
The procedure for the IDL test was amended for the main study to track 2 down 1 up for the first andevery subsequent presentation, which in the pilot phase had been set as a 1 down 1 up until the firstreversal. The initial design rationale was to shorten the test time and track fewer reversals before
reaching threshold; however following review of a sample of response plots it was apparent that the
protocol was too brief for accurate measurement of thresholds.
There was a large variability across participants as expected and this obviously impacts on the
power analyses for the main study. For this pilot study there were no significant effects of approach forsetting T level or rate of stimulation. However, this was as expected due to the small sample size.
From Figure 1, there appears to be a trend towards improved IDLs when using a T level setting at
perceptually ‘very soft’, which has a higher stimulus current level and a slower stimulation rate. This
was a large sized effect and power calculations indicated that 33 participants would be required forstatistical significance. For the AM rate discrimination task, no trends were observed for either T levelsetting or stimulation rate.

5. CONCLUSIONS
Experienced adult users of the Advanced Bionics CI device were able to perform the IDL and AM
rate discrimination tasks that we used at low presentation levels. Notwithstanding the limitations due
to sample size of the pilot study there appeared to be trend toward improved IDL response for T levelsetting at a perceptual equivalence to ‘very soft’ with a slower electrical stimulation rate and a moremarginal trend is in the AM data for higher rates of stimulation to have a higher geometric mean score forthe perceptual T levels and the ‘very soft’ T level setting.
These elements will be explored further in a main study using refined measurement procedures anda fully balanced study design.

Intellectual Property
There is no requirement to obtain permissions from authors for the use of intellectual property
within the content of the manuscript as all figures and images are original.

ACKNOWLEDGEMENTS
Authors acknowledge support from colleagues at St Thomas’ Hearing Implant Centre and theassistance of staff from UCL Speech Hearing and Phonetic Sciences laboratory. Debi Vickers is
supported by a Medical Research Council Senior Fellowship (MR/2002537/1).

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