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

Binaural processing in a new cochlear-implant paradigm
inserting extra pulses with short inter-pulse intervals
Martin LINDENBECK1; Piotr MAJDAK; Bernhard LABACK
Acoustics Research Institute, Austrian Academy of Sciences, Wohllebengasse 12-14, A-1040, Vienna

ABSTRACT
Cochlear-implant (CI) listeners face degraded selective hearing because they lack sensitivity to bothinteraural time difference (ITD) and pitch cues. One major reason is the replacement of the acoustic temporalfine structure by periodic high-rate pulse trains to avoid electric-field interactions. A second major reason is aperceptual limitation of CI listeners in the maximum pulse rate up to which ITD and pitch cues are salient.
Recently, a new approach relying on inserting extra pulses with short inter-pulse intervals (SIPI pulses) into
the high-rate carrier pulse trains has been shown to enhance ITD and pitch sensitivity while likely
maintaining speech understanding. In this contribution, a concept for a new binaural CI stimulation paradigm
which implements the SIPI approach is presented. In this paradigm, by applying binaural models of normal
hearing, the ITD is extracted from a binaural audio signal. That ITD, combined with the envelope modulation
information, defines the positioning of the SIPI pulses. This concept allows enhancing ITD cue precision
through binaural processing by integrating reliable ITD information across frequencies or electrodes,
respectively. In the contribution, various models and parameters are discussed. Further, strategies on how to
avoid conflicts in the across-electrode timing of the SIPI pulses are outlined.
Keywords: Binaural processing, cochlear implants, short inter-pulse intervals

1. INTRODUCTION
Selective hearing refers to the remarkable ability of the human auditory system to decompose a
mixture of concurrent sounds, arriving at the two ears, into the original sources and, subsequently, toselectively attend to the desired source while ignoring the others. This is also referred to as the cocktail
party problem. While selective hearing is remarkably powerful in normal hearing (NH), it is severely
degraded in electric hearing (1), i.e., hearing with cochlear implants (CIs). For speech-in- speech
understanding, two important aspects of selective hearing are the segregation of sound sources basedon the localization of target and interferers, predominantly based on ITD (2), and the segregation
based on monaural features. For speech, one important monaural feature is voice pitch (3), i.e., atalker’s fundamental frequency (F0).
In NH, ITD encoded in the rapidly varying temporal fine-structure (AFS) dominates horizontalplane sound localization for frequencies up to 1.4 kHz whereas above, ITD encoded in the slowly
varying temporal envelope and interaural level differences (ILDs) contribute (4). Reliable real-world
ITD cues are encoded in the rising portions (i.e., the onset) of the envelope (5).
Both AFS and envelope can encode temporal-pitch cues encoded via phase locking in the auditorynerve (AN) fibers, yet, AFS-based pitch has been shown to be more salient than envelope-based
pitch (6). Oxenham et al. (7) suggested that salient pitch might be encoded by place cues, encoded via
the place of the most firing AN sub-populations, rather than AFS cues. However, Graves and Oxenham(8) demonstrated above-chance discrimination of three simultaneous F0s based on the envelope only.
In electric hearing, the spectral resolution is severely limited due to structural limitations of the“system” CI. In particular, the CI electrode array is located in an electrically conductive fluid, theperilymph, with some distance to the AN. To overcome the gap between electrode and AN, current CIs
stimulate with rather high electric currents in the range of milliamperes. Due to the perilymph, theelectric fields spread and stimulate major parts of the AN. In multi-electrode stimulation, fields of theelectrodes interact with each other (9), limiting the number of independent spectral channels to about
eight (10), thus being lower than the number of implanted electrodes.
1

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mlindenbeck@kfs.oeaw.ac.atElectric stimulation of the AN is typically pulsatile rather than continuous. This approach reduces
across-electrode interactions (9) and was the major innovation in the first CI stimulation paradigm thatprovided sufficient spectral precision for good speech understanding (11). Because speech cues (inquiet) are primarily envelope-based, most clinically used stimulation paradigms rely on envelope
coding and use “high” pulse rates.
Given their low spectral resolution, CIs rely on precise encoding of temporal cues. Fortunately, CI
listeners are rather sensitive to temporal cues (12). A major determinant of a CI’s performance is thestimulation paradigm implemented in the sound processors. The paradigm defines how the acoustic
signals arriving at the two ears are converted into electric pulse sequences stimulating the CI listeners’
cochleae. Most clinically used stimulation paradigms provide good to very good speech understanding
in quiet, yet, most of them lack precise encoding of ITD and temporal-pitch cues (or sometimes viceversa). Beyond monaural limitations of temporal coding precision, most current stimulation paradigms
do not implement binaural processing to improve ITD perception because current sound processors are
typically not linked between ears. This results in a further deficit of binaural sensitivity with clinicalprocessors compared to what is possible with highly-controlled experimental research interfaces.
In this contribution, we present the concept of a novel stimulation paradigm that aims to provide
precise ITD and temporal-pitch cues (hereafter referred to as timing cues) while preserving speech
understanding to an extent as currently possible with clinically used stimulation paradigms. First, we
briefly summarize timing-cue perception with both direct electrical stimulation via research interfaces
and with clinical sound processors while introducing previously developed paradigms that aimed at
improved timing-cue perception. Second, we introduce the basic approach on which the paradigm isbased on. The approach is derived from recent single-electrode experiments and exploits the use of
extra pulses with short inter-pulse intervals (so-called SIPI pulses). Third, we introduce thestimulation paradigm and, finally, outline a concept for binaurally-linked processing and stimulation
while assuming that this is someday possible with clinical CI sound processors.

2. TIMING-CUE PERCEPTION IN ELECTRIC HEARING
2.1 Direct Stimulation
When directly stimulating a single interaural electrode pair, sensitivity to ITD encoded in the electric
temporal fine-structure (EFS), i.e., the timing of a CI pulse train, degrades for rates exceeding
100 pps (13,14). Even best CI-ITD sensitivity is much worse than best NH-ITD sensitivity (15).
Moreover, CI listeners are sensitive to ITD encoded in the temporal envelope of an electric pulse
train (14,16), yet, much less than NH listeners (16) and affected by a modulation-rate limitation
similar to that found for the EFS pulse rate (14). When multiple interaural electrode pairs arestimulated directly, ITD sensitivity is broadly similar as compared to single pairs (17,18). Mixed-rate
approaches, including one electrode with low-rate stimulation in non-apical cochlear regions amongfour electrodes with high-rate stimulation, yielded low-rate-like sensitivity (19).
Similar to ITD, temporal-pitch perception is severely degraded with CIs compared to NH. Although
CI listeners are generally able to perceive temporal pitch, their ability to discriminate EFS-based
single-electrode temporal pitch is limited to rates up to around 300 pps (20). The nature of this rate
limitation seems to be similar to the rate limitation observed in ITD perception (21), probably bothshowing a common mechanism. CI listeners are further sensitive to envelope-based pitch if the carrierrate is sufficiently high, with best sensitivity at modulation rates of 100 Hz (22), similar to envelope
ITD. For temporal pitch, slight benefits of multi-electrode stimulation compared to single-electrodeconfigurations were found (23,24), yet, those might partly be attributable to increased loudness (24).
2.2 Stimulation with Clinical CI Sound Processors
The stimulation paradigm implemented in a CI sound processor crucially determines a CI listener’s
binaural sensitivity. Widely used clinical stimulation paradigms are continuous interleaved sampling[CIS (11)] and the advanced combination encoder [ACE (25)]. Both minimize channel interactions bytightly interleaving the pulses across electrodes. To achieve such a rigorous pulse organization, the
AFS is replaced by the EFS in the form of periodic high-rate pulse trains, resulting in envelope coding.
The per-channel EFS rate is typically too high to provide salient EFS-ITD cues. Beyond that,current CI processors operate independently on unsynchronized clocks, which further degrades any
residual binaural sensitivity. Still, clinical processors should principally encode envelope ITD withsome fidelity. Yet, for speech stimuli, CI listeners’ envelope-ITD sensitivity is rather weak (26).

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Clinically used envelope-based stimulation paradigms generally lack low-rate AFS information.
Several stimulation strategies that aim to encode the AFS have been proposed, some of which are
implemented in newer clinical processors and some have remained experimental. The so-called finestructure processing [FSP (27)] presents AFS information in the EFS of up to three apical electrodes
and is also available as a version with four “EFS channels” (FS4) and simultaneous stimulation[FS4-p (28)]. Both peak-derived timing [PDT (29)] and fundamental asynchronous stimulus timing[FAST (30)] lock pulses to envelope peaks, but FAST employs generally lower rates and is thus
considered to be a low-rate paradigm. In general, published studies demonstrating significant binaural
benefits with those paradigms are lacking (15).
With clinical sound processors, CI listeners also struggle in pitch-related tasks (31,32). Thus, whilerather huge across-gender F0 differences might contribute to some extent to CI listeners’ selective
hearing (33), smaller within-gender differences in F0 likely do not. Previous attempts to improve
temporal-pitch coding in stimulation paradigms were based on the explicit enhancement of the
per-channel envelope shape. For example, sawtooth (34) or sinusoidal (35) amplitude modulation atfull depth was applied. While showing improvements on pitch-related tasks, the effects on speech
understanding were so far mixed, showing both degraded (36) as well as preserved (37) performance.
Still, none of those approaches is capable of more encoding than one F0 cue concurrently.
Consequently, their potential benefit for selective hearing is questionable.

3. INSERTING EXTRA PULSES WITH SHORT INTER-PULSE INTERVALS
There is a trade-off in pulse rate between encoding speech cues (in quiet) and timing cues because
envelope-based speech cues require high EFS rates whereas timing cues require low EFS rates.
One approach to improve single-electrode ITD sensitivity at rates sufficiently high to encode
speech is to apply binaurally coherent jitter to the inter-pulse intervals of the pulse trains (38). Thiswas supported by neurophysiological data, additionally showing that only the randomly generated
SIPI pulses in the jittered pulse trains actually caused improved ITD sensitivity (39).

Figure 1 – High-rate single-electrode pulse trains with F0 amplitude modulation. (A) Amplitude
modulation only, (B) SIPI pulses inserted at every second envelope peak (SIPI rate is half the F0).
Compared to the jitter approach, a SIPI approach might be better suited for interleaved CIS-likestimulation because it exploits the benefit of the SIPI pulses in a deterministic way. To this end,
single-electrode ITD sensitivity was studied with both unmodulated (40) and amplitude-modulated(41) pulse trains. Using unmodulated pulse trains, Srinivasan et al. (40) investigated the effect of the
SIPI parameters “rate” (cf. Figure 1B) and “fraction”. The SIPI fraction defines the time gap between
carrier pulses and SIPI pulses as a portion of the carrier inter-pulse interval. With a 1000-pps carrier,
SIPI pulses yielded jitter-like left/right-discrimination performance for SIPI rates up to 200 pps withSIPI fractions up to 10% (40). For those rates and fractions, EFS-like ITD sensitivity was completely
restored. The SIPI-pulse effect on ITD sensitivity was also confirmed neurophysiologically (42).
As a next step, the effect of SIPI pulses on single-electrode ITD sensitivity (41) and temporal-pitch
sensitivity (43) was studied with high-rate amplitude-modulated pulse trains as shown in Figure 1. In
conditions with SIPI pulses placed at envelope peaks (cf. Figure 1B), discrimination performance
improved for F0s representing both male and female talkers as compared to conditions without SIPI
pulses (cf. Figure 1A). In particular, ITD sensitivity improved at all modulation depths between 0.1
and 0.9, and temporal-pitch sensitivity improved particularly at a modulation depth of 0.1. The data
further revealed that the SIPI rate was affected by a limitation similar to that described in Section 2.
The above-described effects of the SIPI pulses highlight the fact that the stimulation paradigm
proposed in this contribution cannot consider binaural hearing in isolation without controlling formonaural effects on temporal pitch.

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4. STIMULATION PARADIGM
4.1 General Concept
Figure 2 shows the flow chart of the proposed stimulation paradigm. It will provide precise encodingof timing cues in addition to speech understanding by combining 1) low-rate AFS encoded via EFS at
the two most apical electrodes [“EFS channels”, cf. FSP (27)], 2) high-rate envelope coding [“ENVchannels”, cf. CIS (11)], 3) superimposed with SIPI pulses (40,41,43) up to the most basal channels,
and 4) interaurally-linked, binaural signal processing employing models of acoustic binaural hearing.

Figure 2 – Flow chart of the MissiSIPI stimulation paradigm. Steps relevant for binaural sensitivity in grey.
Compared to previous approaches, innovations concern the steps (c) and (d). In the end, thestimulation paradigm will generate Multi-channel, interaurally-linked stimulation sequences
incorporating SIPI pulses, hereafter referred to as MissiSIPI.
The following processing steps are planned:
1. Up-sampling of the acoustic signals to enhance MissiSIPI’s temporal precision.
2. Bandpass filter banks split the signal into twelve bands within short time windows. The filter
bandwidths and thus MissiSIPI’s frequency range orientate on FSP/FS4 (28).
3. For the EFS channels, envelope and phase of the analytic signal, required for pulse timing,
are extracted. Pulses of the EFS are triggered by positive zero crossings and the amplitudesof those pulses are determined by the envelope. EFS channels are not further processed prior
to amplitude mapping. Timing cues are encoded implicitly.
4. For the ENV channels, envelope-peak timing is extracted per channel. The extraction method
is derived from statistical timing analysis of per-band signals of English [TIMIT corpus (44)]and German [Kiel corpus (45)] speech signals of both male and female talkers.
5. Binaurally-linked processing can be activated to enhance binaural sensitivity. When active,
per interaural channel, envelope ITDs are estimated from the left and right-ear
envelope-peak vectors. If the ITDs are unreliable, more reliable EFS-channel ITDs are used
to refine the envelope ITDs. More details on binaural processing are provided in Section 4.2.
6. SIPI and carrier-pulse masks: Each window is classified to be either voiced or unvoiced (46).
Because the SIPI approach affects pitch perception due to its repetitive manner, SIPI pulses

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are only inserted in voiced windows. In those windows, SIPI-pulse pairs are placed at time
instances of (refined) envelope-peak vectors. The so-called “F0-coding option”, when
activated, implicitly defines the SIPI rate by classifying envelope peaks as corresponding to
the F0 in the current window. The SIPI fraction remains adjustable within a certain(low-value) range. ENV-channel carrier pulse masks, filling time instances with neither a
SIPI-pulse pair nor an EFS-channel pulse are generated, aiming for uniform envelope
sampling and a target carrier rate of 1000 pps per channel.
7. Amplitude mapping: A logarithmic map-law is applied. The per-channel pulse vectors aretransformed into biphasic pulse sequences and sent to the CI electrode arrays.
In this early stage of development, our main goal is to provide high binaural sensitivity without
considering the limitations of current CI sound processors. Hence, MissiSIPI will be developed for
direct stimulation with a laboratory research interface such as the Research Interface Box II (RIB2).
Compared to existing stimulation paradigms aiming at improved ITD perception, we expect
advantages of MissiSIPI because 1) it provides timing cues across the entire CI electrode array,2) timing cues are provided at rates below the rate limitations and should, thus, be salient, and 3) it stilluses high-rate “speech-friendly” stimulation on all-but-two available electrodes.
Compared to existing stimulation paradigms aiming at improved temporal-pitch perception, we
expect advantages of MissiSIPI because 1) we aim for preservation of spectral cues and 2) we assume
sufficient temporal flexibility of the multi-electrode SIPI approach to encode F0 (and ITD) cues ofmore than one source.
4.2 Binaural Processing
The NH auditory system is exceptionally good at localizing concurrently active sound sources while
suppressing interferences such as reflections. It does so by evaluating AFS information only during therising portion of each envelope cycle where ITD is least affected by reverberation (5). Yet, in electric
hearing, ITD is most saliently encoded at envelope peaks (41,47).
Motivated by the Jeffress model for neural ITD coding which consists of several coincidence
detectors optimized for a certain interaural delay, interaural cross-correlation (ICC) is often used toestimate the direction of arrival of a sound source (48). Yet, ICC alone does not take into account thereliability of an ITD estimate, which in turn could lead to undetected errors.
One elegant way to consider the reliability is to also use the ICC estimate as an indicator of its own
reliability. Faller and Merimaa (49) proposed to use short-term integration of the interaural coherence(IC), i.e., the maximum of the ICC, to label time instances as reliable or not in case the IC exceeds acertain threshold. In addition to only having to perform one computation for both ITD and IC, NH
listeners seems to be exceptionally sensitivity to changes in IC (50).
For MissiSIPI, the results from Dietz et al. (5) will be considered to identify reliable per-electrodetiming events to place SIPI pulses. In addition to that, the use of SIPI pulses will depend on the
per-electrode IC to avoid erroneous SIPI-ITD cues. In case only some electrode signals have a high IC,
reliable ITD estimates can be integrated across electrodes to improve binaural sensitivity by providing
reliable and salient ITD cues across the entire CI electrode array. In particular, given that low-rate ITD
is perceived most saliently, reliable EFS-ITD estimates as determined by IC will be used to refine theenvelope ITD in non-apical channels. Yet, the effects of some important parameters of this method still
have to be evaluated to parameterize MissiSIPI for arbitrary acoustic signals.
4.3 Conflicts in Across-Electrode Timing
With arbitrary acoustic inputs from arbitrary azimuthal locations simultaneous SIPI stimulation acrosselectrodes is likely to be demanded occasionally. Because MissiSIPI will employ interleaved
stimulation across electrodes, routines to solve such pulse conflicts have to be implemented.
Currently, three candidates seem to be most promising. First, parallel stimulation resolves als
conflicts, however, it will result in severe electric-field interactions (9). Thus, conditions that apply
parallel stimulation have to be carefully selected. Second, individual SIPI pulses can be removed. A
recent study by Egger et al. (51) could aid in determining the maximum amount of (interaurally
uncoordinated) pulse removal. Assuming similar ITD sensitivity with low-rate EFS stimulation andhigh-rate SIPI stimulation (40), up to about 8% of pulses could be removed without significantly
degrading ITD sensitivity (51). Third, conflicting pulses could occasionally be delayed, resulting in aform of jitter of the SIPI rate with potential consequences on pitch sensitivity.
All routines will consider a trade-off between ITD and pitch coding, with ITD coding having
priority because of higher binaural than monaural timing sensitivity.

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5. SUMMARY
Current CI stimulation paradigms lack precise encoding of timing cues in addition to speech cues
because a pulse-rate trade-off specific to electric hearing remains unsolved. A single-electrodeapproach, relying on the insertion of SIPI pulses, has been shown to enhance timing cues. A novel
stimulation paradigm (MissiSIPI) was proposed, employing a multi-electrode SIPI approach for
non-apical electrodes with CIS-like stimulation (ENV channels) while using FSP-like stimulation at
apical electrodes. The paradigm uses models of binaural hearing in an interaurally-linked processing
stage to refine envelope-ITD encoding by combining reliable ITD information across electrodes.

ACKNOWLEDGMENTS
This work was supported in part by the National Institutes of Health, grant no. R01 DC 005775.

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