**1. Introduction**

The effects of music on the brain extend far beyond hearing [1,2] and positively affect quality of life [3]. The stimulating properties of music not only promote the development of the auditory system in children [4,5] and increase the capacity of speech discrimination in noise [6], but also reinforce many cognitive capacities involved in communication skills and social integration [7,8]. For the hard of hearing, music is a valuable tool for training [7–9] and for exploring the hearing loss in a complementary manner to conventional audiometry [10]. It is a source of joy even in patients with profound hearing loss and a cochlear implant (CI), and this may explain their motivation to engage in musical rehabilitation programs [11].

In adult CI patients, music perception is severely deteriorated [12]. Recognizing melodies remains a difficult task and shows high interindividual variability (25% success versus 88% in normal hearing patients) [13]. This handicap is mainly attributed to the limitations inherent to CI sound coding and processing strategies [14–16]. In addition, auditory nerve survival in the implanted ears, which can be suboptimal, is directly related to the number of functional channels eliciting different auditory sensations, the electrical dynamic range, and also the capacity of benefiting from high rates of stimulations [17].

**Citation:** Leterme, G.; Guigou, C.; Guenser, G.; Bigand, E.; Bozorg Grayeli, A. Effect of Sound Coding Strategies on Music Perception with a Cochlear Implant. *J. Clin. Med.* **2022**, *11*, 4425. https://doi.org/10.3390/ jcm11154425

Academic Editors: Nicolas Guevara and Adrien Eshraghi

Received: 25 May 2022 Accepted: 26 July 2022 Published: 29 July 2022

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Alterations in central sound processing mechanisms caused by auditory deprivation may additionally contribute to this poor perception [18]. Despite these limitations, many implantees enjoy music [11,19], and some can also perform well, especially after training [20]. When evaluating CI patients for basic music characteristics such as rhythm, melody, and timbre, typically, poor pitch discrimination and melody recognition are described but a near-normal performance in rhythm perception is reported [21,22]. The existence of a few star patients and the effect of training on timbre perception and melody recognition suggest that some patients can extract spectral cues to compensate for a lack of pitch resolution, that central auditory processing is probably subject to plasticity in this field, and finally, that patients learn to enjoy music based on cues different from those used by normal hearing individuals [23]. This idea is supported by the observation that recognizing a melody is influenced by the timbre of the instrument in CI users [24]. These spectral cues depend largely on coding and the sound processing strategies [13,24]. Pitch resolution refers to the smallest pitch interval detectable by the patients, which is coded by the place (electrode position) and time (pulse rate and pattern) cues for each electrode and is related to the number of functional channels in the CI [17]. Spectral differences can generate different activation patterns across several electrodes, and their distinction requires complex peripheral and central mechanisms [25,26].

To improve sound quality delivered by CI, several interconnected issues should be tackled. Alteration in pitch perception severely deteriorates harmonies and musical lines [27]. This phenomenon is largely due to the modified cochlear tonotopy after CI [28] and the drastic reduction in functional channels (number of electrodes eliciting a distinctive pitch) entailing a significant loss of frequency resolution [29]. Attempts to increase the number of functional channels by current steering (simultaneous current delivery by adjacent electrodes with variable ponderation) have shown some improvement in speech performance [30] but cannot compensate for the reduced number of nerve endings in the cochlea.

Another issue in music listening with CI is the loss of spectral information. Better encoding the sound envelope and providing the temporal fine structure have shown their efficacy in enhancing bass frequency discrimination and higher musical sound quality [31]. However, these relatively new coding strategies encounter a pathophysiological barrier, which is the channel interaction and overload [18]. Indeed, delivering electrical pluses at a higher rate on a larger number of electrodes requires performant and numerous functional channels [32] that many patients do not have [33]. These channel interactions are largely responsible for inter-individual performance variability [33]. Improvement in the acoustic dynamic range is another paramount obstacle not only for understanding speech in noise, but also for enjoying music [34]. Indeed, delivering sound intensity nuances of daily life or music while disposing of a restricted range of tolerable sound intensities is problematic in many patients with a long history of hearing deprivation. With the increasing processing capacity of hearing aids, new sound processing algorithms such as nonlinear frequency compression and adaptative dynamic range optimization have been developed in the field of hearing aids [35–37], and some of these solutions have been more recently implemented in CI technology [34,38]. CrystalisXDP strategy (Oticon Medical, Vallauris, France) focuses on rendering the spectral details of the entering signal with a lower distortion than the standard "main peak interleaved sampling" (MPIS) strategy. In addition, it provides possibly more comfortable listening through an adjustable compression system [38]. In a previous study, this sound processing strategy seemed to enhance speech perception in quiet and noise [38]. Its effect on music perception has not been evaluated to our knowledge.

The present study focuses on the emotional response to music, which is the most important aspect of everyday life music experience. Tempo and mode were found to be the most robust factors inducing joy and sadness in listeners [39]. A given musical piece will be perceived to be happier when played faster, and in major rather than in minor mode. Although the perception of tempo raises no difficulty in CI, the perception of mode remains a challenging issue. In contrast to the major mode, minor music contains intervals such as minor third intervals, which induce significant roughness or dissonance in the auditory filter [40]. Accordingly, we hypothesized that, with a poor pitch resolution, CI patients would have difficulty distinguishing happy from sad music using spectral cues, but that reducing the spectral distortion would enhance this capacity. The goal of this study was to evaluate the effect of reducing spectral distortion with the CrystalisXDP sound processing program on the ability of CI patients to distinguish happy from sad music based on rhythmical and/or modal cues, and to confront this performance to their subjective musical experience.
