Comparison of sound perception using CIS and ACE sound coding strategies in cochlear implants

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Introduction

The history of coding a sound signal by means of an electrical impulse dates back to 1957, when André Djourno and Charles Eyriès first succeeded in stimulating the auditory nerve with an alternating current frequency of 100 Hz with stimuli at a rate of 15-20 times per minute [1]. Further development of such a stimulation system led to the creation of cochlear implants and a significant complication of the stimulation algorithm, which made it possible to achieve great success in the rehabilitation of patients with profound hearing loss [2]. The basic strategy for stimulating the auditory nerve for most cochlear implantation systems (Cochlear, Advanced Bionics, Med El, Nurotron) was the CIS (continuous interleaved sampling) strategy described by Blake S. Wilson in 1991 [3]. This strategy made it possible to solve the problem of channel interaction using asynchronous alternating pulses and to increase the stimulation rate per channel, which had a significant impact on speech recognition [4, 5]. Since 2002, Cochlear has adopted the Advanced Combination Encoder (ACE) strategy in the Nucleus 24 implant and the Freedom speech processor. The ACE combines more processed bands with a faster stimulation rate by having 22 electrodes, which gives better frequency resolution. In the ACE strategy, a channel is generated by one implanted electrode, and the original spectrum is reproduced by 8-10 fixed channels [6].

Currently, both CIS and ACE strategies are relevant, used by modern Cochlear cochlear implantation systems, which include the CI24RE series implants and the Nucleus CP810 / 910 series speech processors. Each strategy has its own characteristics of the speed of stimulation per channel, the width and amplitude of the stimulus, which affects sound perception. Cochlear Corporation's cochlear implant systems currently have a suite of audio coding strategies including CIS, SPEAK, ACE, MP3000. Our research focused on two strategies, CIS and ACE. Both strategies are based on the principles of the Vocoder VODER (Voice Operating Demonstrator) Homer W. Dudley (Bell Laboratories), presented in 1939 at the World's Fair in New York and San Francisco [7]. The transformation of sound by the indicated strategies is based on the vocoder principle and explains the impressions of patients after cochlear implantation, when the human voice became similar to the voice of a robot. In the CIS and ACE strategies, the signal is converted into a small number of bands (16-22) through a fast Fourier transform or through a set of bandpass filters, the signal envelopes are extracted from each band. Envelopes are used to modulate the biphasic pulses that are required to stimulate the electrodes. The number of envelopes and the number of electrode groups selected for stimulation in each cycle differ between the CIS and ACE strategies. In the ACE strategy, only a subset of n (n = 8–10) of the 22 envelopes are selected and used for stimulation in each cycle, and all 22 electrode sites are used for stimulation. In the CIS strategy, the number of envelopes is fixed (8-10) and only the corresponding electrode sites (8-10) are used for stimulation. However, the rate of stimulation per channel in these strategies is in the same range, from 250 to 3500 stimuli per channel per second (pps). The influence of existing differences in the ACE strategy was carried out by a number of foreign authors [8, 9] and they showed that users of Nucleus-24M more often preferred ACE than CIS, including in different acoustic conditions or due to slightly higher speech intelligibility [10] ... Later, the introduction of the new Nucleus Freedom system based on the Nucleus CI24RE implant made it possible to increase the total stimulation frequency to 32 kHz and the stimulation frequency per channel to 3500 pps / channel [11]. However, studies have shown that high stimulation rates per channel (around 3500 pps) tend to give lower results, however, word intelligibility in patients who prefer this stimulation rate was on average no worse than those who prefer low speed of stimulation [12]. Technical improvement of the cochlear implantation system at different levels, including the speech processor and cochlear implant, as well as the principles of rehabilitation, the presence of peculiarities of the sound of the Russian language, led to the study of the influence of modern sound coding strategies on its perception [13].

The aim of the study was to study the effect of ACE and CIS sound coding strategies on sound perception in patients with the Cochlear cochlear implantation system. to 2019. Patients aged 7 years and older were selected who received a multi-channel cochlear implant (Nucleus Freedom, model CI24RE) and used a Nucleus CP810 or CP910 speech processor. The group consisted of 50 people of various ages, the inclusion criteria for the study included the systematic use of the speech processor (more than 8 hours a day), regular training sessions for 5 years, the ability to conduct speech tests. The study group included patients with clearly recorded parameters of the telemetry of the nervous response by the main electrodes (1, 6, 11, 16, 22) and the interelectrode resistance corresponding to the permissible range (from 0.7 kΩ to 20 kΩ). Correction of the stimulation program (MAP) was carried out by establishing a connection between the speech processor and the software interface on the computer using a programmer. All received information on the functioning of the CI system was analyzed using the Cochlear Custom Sound 5.0 software (Cochlear Ltd., Australia). All subjects underwent measurement of the interelectrode resistance and telemetry of the action potential of the auditory nerve before starting the correction of the stimulation program. The research algorithm included the study of sound perception with a standard stimulation strategy set by default, and with a stimulation strategy set by a specialist during the speech processor programming during rehabilitation. To assess changes in the perception of a sound signal and speech, the standard technique of tonal threshold audiometry in a free sound field and speech audiometry, also in a free sound field, using the speech tables of Greenberg (for adults) and Osherovich (for children) was used. The patients were divided into 2 groups, the first (main), which included 25 people with the CIS strategy, and the control group (25 people) with the ACE strategy. Each group consisted of 15 children and 10 adults. The study was conducted 2 months after the change from the default ACE strategy to CIS. The results obtained were entered into spreadsheets in MS Excel and subjected to statistical analysis.

RESULTS AND DISCUSSION Based on the results of threshold tone audiometry in a free sound field, audiogram curves were obtained for each patient at the main speech frequencies (500, 1000, 2000, 4000 Hz). The obtained thresholds results were summarized for each frequency. Based on the results obtained, an average audiogram was created for the main group (CIS) and the control group (ACE) of patients (Table 1). When comparing the hearing thresholds for the ACE and CIS strategies, no significant differences can be noted. The perception of the tone by these strategies is not significant.

 

Стратегия (strategy)	Частота тона (Tone frequency), Hz
	500 Hz	1000 Hz	2000 Hz	4000 Hz
ACE	30	35	40	40
CIS	30	35	40	45

 

Table 1. Averaged data for tonal threshold audiometry in a free sound field. Table 1. Averaged tonal threshold audiometry data in a free sound field. The percentage of speech intelligibility calculated using the Greenberg and Osherovich speech tables during free sound field audiometry using the indicated coding strategies showed more pronounced differences (Table 2).  

 

Стратегия (strategy)	Разборчивость (discrimination), %
	Взрослые (adults)	Дети (childrens)	Среднее значение (average)
ACE	71,25	64,11	66,4
CIS	68,12	59,68	62,2

Table 2. Speech intelligibility in a free sound field. Table 2. Speech intelligibility in a free sound field. Patients in the control group with the ACE strategy showed a high level of speech intelligibility for this group of patients (66.4%), while the level of intelligibility in children was 7.14% lower than adults. Patients from the main group who changed the coding strategy of ACE to CIS, on average, showed slightly lower results 2 months after correction - on average 62.2%, which is 4.2% lower than the main group. The difference between children and adults was 8.44%, which is slightly higher than in the control group. It should be noted that the decrease in speech intelligibility in adults was, on average, less than in children (3.13% versus 4.43%). Among the subjects from the main group, in a number of patients, an increase in speech intelligibility was noted when changing the strategy from ACE to CIS within 5%, this concerned more often adult patients and patients who had in their experience using hearing aids for a long time. Speech intelligibility indices in the main and control groups ranged from 45 to 90%, the lowest results were shown by patients with meningitis, CNS diseases, psychoemotional disorders and other diseases. The results of our study showed the importance of understanding the differences in the strategies of stimulation of the auditory nerve, the presence of advantages specific strategies for certain patient groups, the importance of using rehabilitation aids in considering the history of the anamnesis Conclusions The use of the modern ACE audio coding strategy in the Cochlear cochlear implant system yields slightly better results on speech understanding tests and comparable results on threshold tone audiometry compared to the CIS strategy. The use of the CIS strategy in patients who have lost their hearing after speech formation and have extensive experience with hearing aids is justified in view of the increase in speech intelligibility in some cases. The selection of a coding strategy for an audio signal requires an individual approach and monitoring the results for a long time by an audiologist and a deaf teacher. The effectiveness of using the cochlear implantation system largely depends not so much on the technical capabilities of the system as on the intellectual abilities of the patient and the state of his health.

About the authors

Oleg V. Kolokolov

The National Medical Research Center for Otorhinolaryngology of the Federal Medico-Biological Agency

Author for correspondence.
Email: oleg_kolokolov@mail.ru
ORCID iD: 0000-0002-7155-9544

external PhD student, Head of the polyclinic department

Russian Federation, 2 Tatishcheva st., Astrakhan, 414056

Aleksandr O. Kuznetsov

The National Medical Research Center for Otorhinolaryngology of the Federal Medico-Biological Agency; Pirogov Russian National Research Medical University

Email: aokuznet@mail.ru
ORCID iD: 0000-0001-6499-7506

PhD, Chief Physician, Associate professor, Department of otorhinolaryngology

Russian Federation, Moscow; Moscow

Anton S. Machalov

The National Medical Research Center for Otorhinolaryngology of the Federal Medico-Biological Agency; Pirogov Russian National Research Medical University

Email: anton-machalov@mail.ru
ORCID iD: 0000-0002-5706-7893

PhD, Head of scientific-clinical Department of audiology, hearing aid and audio-verbal rehabilitation; physician- audiologist-otorhinolaryngologist; Associate professor of the Department of otorhinolaryngology, faculty of additional professional education

Russian Federation, Moscow; Moscow

Alla A. Grigoreva

The National Medical Research Center for Otorhinolaryngology of the Federal Medico-Biological Agency; Astrakhan State Medical University

Email: agrigoryeva@mail.ru
ORCID iD: 0000-0003-2994-2555

PhD, deputy Chief Physician, Associate professor, Department of otorhinolaryngology and ophthalmology

Russian Federation, 2 Tatishcheva st., Astrakhan, 414056; Astrakhan

References

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