Study of Music and Speech Perception in New Cochlear Implanted Subjects Using or Not a Tonotopy Based Fitting

Introduction: Cochlear implantation allows the rehabilitation of profound bilateral deafness, restoring speech perception and verbal communication when the traditional hearing aid no longer provides satisfactory hearing gain (Nimmons et al.).

A cochlear implant includes an electrode array and its functioning is based on the principle of cochlear tonotopy: each electrode encodes a frequency spectrum according to its position in the cochlea (high frequencies are assigned to the basal electrodes and low frequencies to the apical electrodes).

The cochlear implant thus breaks down the frequency spectrum into a number of frequency bands via bandpass filters corresponding to the number of electrodes in the implant. During the fitting these bands can be modified by the audiologist.

The fitting software developed by the manufacturers proposed a default fitting with a lower limit between 100 and 250 Hz according to the brands and an upper limit of about 8500 Hz. The frequency bands assigned to each electrode follow a logarithmic scale with the high frequencies for the basal electrodes and the low frequencies for the apical electrodes. This distribution takes into account the number of active electrodes but does not take into account the anatomy and the natural cochlear tonotopy specific to each patient.

Several studies have analyzed the anatomical variations of the cochlear dimensions: size of the cochlea and the ratio between the contact surfaces of the electrodes with the cochlea are variable from one patient to another (Stakhovskaya O et al., P. Pelliccia et al.).

The insertion depth during surgery is also variable due to parameters related to the patients as well as to the operator, which seems to impact the understanding of speech in noise (Deep electrode insertion and sound coding in cochlear implants - Ingeborg Hochmair et al.).

Mathematical algorithms have recently been developed to estimate the cochlear tonotopy of each patient from a CT scan assessment (Jiam et al., Sridhar et al.). CT imaging of the implanted ear combined with 3D reconstruction software, provides cochlear length measurements (Cochlear length determination using Cone Beam Computed Tomography in a clinical setting - Würfel et al .) Using this approach it is possible to measure the position of each electrode relative to the cochlear apex. These measurements are applied to the modified Greenwood equation to obtain the tonotopic frequency for each electrode and to determine for each patient a fitting based on the tonotopy of each electrode.

Main objective:

Show the superiority of tonotopy based fitting strategy compared to default fitting strategy on the perception speech in noise.

Secondary objectives:

Show the superiority of tonotopy based fitting strategy compared to default fitting strategy on the perception of musical elements (contour test).

Show the non inferiority of tonotopy based fitting strategy compared to default fitting strategy on the perception of speech elements in quiet.

Show the superiority of tonotopy based fitting strategy compared to default fitting strategy on the qualitative preference for the listening of musical pieces.

Plan of the study:

It is a prospective open monocentric randomized crossover study: measures will be done on the patient at 6 weeks and 12 weeks post-activation.

Inclusion criteria

• Adult patient (>= 18 years old) speaking French
• Patient who fulfils the criteria for cochlear implantation

Exclusion criteria

• retro-cochlear pathology: auditory neuropathy, vestibular schwannoma
• patient with residual hearing < 60 dB HL at 250 Hz and < 80 dB HL at 500 Hz