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

Spatial release from masking in bilateral cochlear implant users
listening to the Temporal Limits Encoder strategy
Alan KAN1; Qinglin MENG2
1
2

University of Wisconsin-Madison, USA

South China University of Technology, China

ABSTRACT
Binaural hearing benefits with bilateral cochlear implants (CI) are usually smaller than with normal hearing(NH) in the same tasks. This gap in performance has typically been attributed to a lack of coordinatedstimulation between ears and the high stimulation rates used in clinical processors. These factors hinder
sensitivity to interaural timing differences (ITDs); an important binaural cue for NH listeners. The Temporal
Limits Encoder (TLE) strategy was originally designed to encode unilateral low-frequency temporal finestructure pitch cues into the signal envelope of CIs. However, TLE also lowers the stimulation rate on somechannels which may potentially provide useable ITD cues. Here, we measured spatial release from masking(SRM) in bilateral CI users listening to TLE vs Advanced Combinational Encoder (ACE) strategy todetermine if TLE provides a benefit. The CCiMobile research platform was used for testing. Results fromeight listeners showed comparable word recognition performance in quiet and co-located conditions for TLE
and ACE, even with a short acclimatization period with TLE. In the spatially-separated condition,
performance across the group was more similar with TLE than ACE, and more listeners showed SRM. These
results indicate that TLE has the potential for improving binaural hearing benefits for CI users.
Keywords: Cochlear Implants, Sound Coding, Spatial Release from Masking

1. INTRODUCTION
Cochlear implants (CIs) have been largely successful in restoring speech understanding in patients
who are profoundly deaf in both ears, especially in quiet situations (1,2). However, in noisy situations,the ability to understand speech with one CI is very poor (1), and bilateral cochlear implantation is fast
becoming the standard of care (3). While speech understanding in noise has been shown to improvewith bilateral CIs (4), the benefits of having two ears for listening are still much smaller than that ofnormal hearing (NH) listeners (5,6). To assess the benefits of bilateral hearing on speech
understanding in noise with devices such as CIs or hearing aids, spatial release from masking (SRM) is
often measured (e.g., 4,7). SRM is defined as the difference in signal-to-noise (SNR) needed tounderstand speech in noise due to the spatial separation of a target signal from masking interferers.
The benefit of spatial separation is a combination of monaural and binaural effects, namely headshadow, squelch and summation (6). Head shadow is largely a monaural effect and arises because thehead acts as an acoustic shadow to improve the SNR in one ear. Squelch and summation are binauraleffects; squelch is dependent on the auditory systems ability to utilize binaural cues such as interauraltime and level differences (ITD and ILD, respectively), and summation is the benefit of having access
to two signals that can be combined to enhance the audibility of a target signal in front of the listener.
For CI users, the benefits of bilateral CIs have largely been due to the monaural head shadow effectand the binaural effects of squelch and summation have been much smaller than that of NH listeners(5). Smaller binaural benefits in bilateral CI users have largely been attributed to the lack of access to
ITDs because of the way sound is encoded by CI sound coding strategies (6). CIs encode an incoming
acoustic signal by filtering it into a small number (between 12 and 24 depending on manufacturer) of
bandlimited channels. The signal in each bandlimited channel can be decomposed into a slow-varying
envelope and a faster temporal fine structure (TFS). In CIs, only the envelope is used to
1
2

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ahkan@waisman.wisc.edu
mengqinglin@scut.edu.cn

amplitude-modulate a high rate electrical pulse train (≥900 pulses per second) while the TFS is often
discarded. At the low frequencies, the TFS provides important cues used by NH listeners for soundlocalization (8,9).
Attempts to improve speech-in-noise understanding for bilateral CIs have focused on developing
sound coding strategies that encode TFS (10–12). To encode TFS, the pulse rate of low frequency
channels is usually reduced and firing of electrical pulses are timed to certain features of the acoustic
TFS, such as the signal peak or zero crossing. The reduction in pulse rate at these channels is thought
to be necessary because psychophysical studies have shown the ITD sensitivity is better at rates around
100 pulses per second (13,14). However, these approaches have only led to small improvement inbinaural benefits (15).
Recently, the temporal limits encoder (TLE) strategy was proposed to try to enhance TFS pitch cuesin CIs (16,17). The TLE strategy is different to other TFS-encoding strategy in that it does not
explicitly lower the stimulation rate, nor does it rely on encoding a “feature” of the TFS such as a peakor zero crossing. Instead, the TLE strategy transposes each bandlimited channel to an intermediate,
lower frequency range (e.g., 150-350 Hz) which is within the range of good temporal sensitivity forboth pitch and ITD. The transposed signal maintains the overall envelope but also introduces a slower
modulation that is related to the TFS into the signal envelope. Experiments in NH listeners listening to
CI vocoder simulations of the TLE strategy has shown some advantages in pitch and unilateralspeech-in-noise tasks (16,17). Further, TLE provided a ~4 dB spatial unmasking benefits in NH
listeners in a task measuring binaural intelligibility level differences (18). In this work, we measured
SRM in bilateral CI users when listening with the TLE strategy vs the Advanced Combinational
Encoder (ACE) strategy.

2. METHODS
2.1 Listeners
Eight (7 females and 1 male), postlingually-deafened bilateral CI users with Cochlear Ltd implants
were tested. All listeners used ACE as their everyday sound coding strategy. Listeners were between
the ages of 51 and 78 (Mean age: 62 years old) and had at least 7 years of bilateral CI experience. All
listeners had prior experience with psychophysical testing and had measurable ITD sensitivity whenmeasured with direct electrical stimulation at low pulse rates (100 pulses per second). Listeners were
tested at the University of Wisconsin-Madison and were paid a daily stipend for their time.
Experimental procedures conformed to the regulations set by the National Institutes of Health andwere approved by the University of Wisconsin-Madison’s Health Science Institutional Review Board.
2.2 Strategy Implementation
Testing was conducted using the CCiMobile research platform (19) connected to a Windows
Surface Pro. The CCiMobile is the second generation of a bilaterally-synchronized, real-time researchplatform developed by UT-Dallas for Cochlear Ltd internal devices. A description of the hardware canbe found in (19). A real-time implementation of ACE written in MATLAB is provided by UT-Dallas(20). We adapted the MATLAB code to implement the TLE strategy for this experiment, as well as
creating a new user interface that allows us to easily switch between strategies.
Figure 1 shows the signal block diagram. The same processing is applied to the right and left ears.
The CCiMobile samples the incoming audio at a 16 kHz rate and buffers the audio for 8 ms before
passing to MATLAB. Overlap-add processing is applied in 128-point sample frames and frames are
shifted according to the stimulation rate set by the patients clinical MAP (usually 900 pulse per second,
two listeners had 1200). A 128-point Fast Fourier Transform (FFT) is applied to each frame. The FFT
bins are then combined into 22 channels according to the frequency allocation set in the patient MAP.
For ACE, the N highest maxima are chosen from the 22 channels in each ear, where N is set by thepatient MAP. All listeners had N=8 except one who had N=10. The N maxima were then compressed to
fit into the patient’s dynamic range and added to the stimulation buffer. Once 8 ms worth of pulses had
been processed, the stimulation buffer was then passed to the CCiMobile.
For TLE, a frequency transposition step is added prior to peak-picking. Frequency transposition isachieved in the FFT domain by multiplying the FFT bins within the range of 300 to 1500 Hz bye jπ(N-1-2ft), where N is the index of the FFT frequency bin, t is the time index, and f is thedown-modulator frequency. In this experiment, f was set to the lower frequency cutoff of the channel
minus 150 Hz. The down-modulated FFT bins are then combined into 22 channels the same way as that2237

of ACE.
The output of the two strategies are shown in Figure 2 for the word “Jane” which has been filteredby a head-related transfer function corresponding to a source 90° to the right-hand side. It can be seenthat stimulation on the lower frequency channels of the TLE strategy are sparser but the overall signal
envelope is maintained. Further, there is an ITD introduced into the lower frequency channels in the
TLE strategy (lower panels of Figure 2).

Figure 1 – Signal block diagram of ACE and TLE strategies

Figure 2 – Electrodogram of the word “Jane” spoken by a source located on the right-hand side. Each blue
and red line represents an electrical pulse in the left and right ear, respectively. Upper panels show the entire
electrodogram, while lower panels show a magnified portion of the stimulation on electrodes #21 and #22(low-frequency electrodes).

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2.3 Testing Procedure
Experiments were conducted in a single-walled soundproof booth. The booth had sound absorbing
foam attached to the inside walls to reduce reflections. Inside the booth was a semi-circular array ofloudspeakers (Cambridge SoundWorks). The loudspeakers used in this experiment were at 0° (front),-90° (left) and +90° (right) on the horizontal plane. The listener sat in the middle of the array and was
1.2 m from the loudspeakers.
For the experiment, the listener’s task was to identify the word presented from the loudspeaker infront of the listener. A choice of 50 words were shown on a touchscreen in front of the listener. The
listener indicated their response by choosing the button corresponding to the word they heard.
Three conditions were tested: (1) word recognition in quiet; (2) co-located target talker andinterferers – in this condition, both target and interferers were presented from the 0 ° loudspeaker; (3)spatially-separated target talker and interferers – in this condition, the target talker was presented from0°, and an interferer was presented from each of the left and right loudspeakers.
The target talker was a male voice speaking a mono-syllable word, while the interferer was a female
voice speaking sentences. In the quiet condition, words were presented at a fixed level of 60 dBA, andlisteners was tested on 50 words divided into two blocks of 25 words each. In each block of trials, thechoice of words shown on the screen was different. In the conditions with interferers, the interferer
was presented at a fixed level of 50 dBA and always started/ended 250 ms before/after the target word.
The presentation level of the target word was changed on each trial using a two -down, one-up adaptive
staircase with twelve turnarounds. The staircase started at a +5 dB SNR and moved in 3 dB steps forthe first three turnarounds. For the remaining turnarounds, the SNR changed in 2 dB steps.
Prior to testing, listeners were given at least 1 hour of practice listening with the TLE strategy.
During this time, the CI user wore the CCiMobile device and engaged in conversation with the firstauthor. No listener reported having difficulty with understanding the conversation when listening with
the TLE strategy, though all listeners reported a noticeable difference in voice quality (lower pitch).
After the 1 hour of practice, listeners would report that the first authors voice sounded more “normal”,
though there was still a noticeable difference in their own voice.
Word recognition in quiet was tested first and always began with the TLE strategy followed by the
ACE strategy. Once testing of word recognition in quiet was complete, listeners were tested in theconditions with interferers. The order of spatial configuration and strategy was randomized for eachlistener, and two adaptive tracks were collected for each strategy/s patial configuration combination.

3. RESULTS
Percent correct word recognition in quiet is shown in Figure 3. On average, word recognition wasquite comparable between the two strategies (86.5% vs 90.25% correct when listening with the TLE
and ACE strategies, respectively). A paired t-test conducted on the percent correct scores revealed nosignificant difference between the two strategies (t(7)=1.46, p=0.19).

Figure 3 – Word recognition in quiet

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For the word recognition in noise conditions, a 70.7%-correct speech reception threshold (SRT)
was obtained for each condition using a Bayesian estimation of the psychometric function (21). Figure
4 shows the SRTs obtained in each condition for the two strategies. Substantial inter-subject
differences can be observed. Given the resolution of the adaptive track is 2 dB, then in the co-locatedcondition, four listeners (S1, S6, S7, S8) had SRTs that were similar between the two strategies, two
had TLE SRTs that were more than 2 dB lower than ACE (S3, S4), and the remaining two had TLE
SRTs that were more than 2 dB higher than ACE (S2, S5). In the spatially-separated condition, four
listeners had TLE SRTs that were more than 2 dB lower than ACE (S1, S4, S6, S7), three had TLE
SRTs that were more than 2 dB higher than ACE (S2, S3, S5), and one listener had SRTs that were
within 2 dB (S8). As a group, mean SRTs in the co-located condition were 9.49 dB and 8.65 dB for TLE
and ACE, respectively. Mean SRTs in the separated condition were 5.75 dB and 6.18 dB for TLE and
ACE, respectively. Two-way, repeated-measures analysis of variance revealed a significant effect of
spatial configuration (F(1)=8.417, p=0.02) but not strategy (F(1)=0.026, p=0.88).
SRM computed from the SRTs of the co-located and separated conditions are shown in Figure 5.
Five listeners obtained an SRM greater than 2 dB when using the TLE strategy vs three listeners forACE. One listener found it more difficult to listen with the TLE strategy when target and interferers
were spatially separated. On average, SRM was 3.74 dB and 2.47 dB for TLE and ACE, respectively.

Figure 4 – Speech reception thresholds for the TLE and ACE strategies when tested with target-interferers
co-located and spatially-separated.

4. DISCUSSION
This work evaluated whether the TLE strategy could provide a binaural benefit for CI users whentrying to recognize words in a noisy environment. The TLE strategy was implemented on th e
CCiMobile research platform which allowed real-time live testing of the strategy. Informal comments
obtained from listeners indicated that the TLE strategy was perceptually different to ACE and lowered
the pitch of voices. This was somewhat to be expected given that the original goal of TLE was toconvey pitch cues more saliently by reduction of stimulation rate. Prior psychoacoustic work has
shown that rate of stimulation is able to convey a pitch cues (22,23). Most listeners reported that after
an hour of listening with the TLE strategy, voices began sounding more “normal”, suggesting thatgiven a longer duration of acclimatization, TLE can be an acceptable listening strategy. Formal testingof word recognition in quiet found no significant difference between TLE and ACE. This is a strong
positive because it shows that TLE can maintain important speech envelope cues despite the lowered
stimulation rate.

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Figure 5 – Spatial release from masking for each listener when listening with the TLE and ACE strategies.
When listening in noise, TLE appears to provide reduced the variability in performance acrossbilateral CI listeners compared to ACE when target and interferers were spatially separated (see Figure4). In addition, more listeners were able to benefit from SRM when listening with TLE compared toACE. These are promising results and demonstrates a potential benefit of the TLE strategy forenabling the binaural benefits of spatial unmasking in bilateral CI users. More work is needed tounderstand the impact of choice of TLE parameters; in particular, the down -modulator bandwidth.
Also, it is likely that if listeners were given a longer period of listening with TLE, a more consistentbenefit across the group may emerge.

5. CONCLUSIONSThe TLE strategy was evaluated for the first time in CI users. Results showed that TLE canmaintain speech understanding performance that is comparable to the ACE strategy and has thepotential to provide a binaural benefit for speech understanding in noisy environments in a greaternumber of bilateral CI users.

ACKNOWLEDGEMENTS
The authors would like to thank the listeners for traveling to the University of Wisconsin-Madison
to participate in this research. This work was funded by the National Institutes of Health -National
Institute on Deafness and Other Communication Disorders (R03DC015321 to Kan), and in part by the
National Institutes of Health-Eunice Kennedy Shriver National Institute of Child Health and Human
Development (U54HD090256 to Waisman Center) and the National Natural Science Foundation ofChina (Grant No. 11704129 and 61771320 to Meng).

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