Cochlear Implants as diagnostic probes
In case of residual hearing (electroacoustic stimulation), the cochlear implant located in the inner ear can be used to record signals generated during acoustic stimulation. Such signals can be used for several purposes: on one hand they can provide information on the capacity of residual hearing, on the other hand they can be used to identify the location of the implant relative to the hearing portion of the cochlea. This latter approach can be used during surgeries to quantify the risk of cochlear damage during implantation.
In our labs we initiated projects to better understand the signals recorded from the implant. These can have different origins: the cochlear microphonics is believed to be generated by the hair cells, the compound action potential (CAP) is generated by the auditory nerve, and the summating potential is likely generated by the hair cells, too. 
In an animal experiment the standout advantage is the possibility to implant a normal hearing cochlea with limited cochlear trauma, thus having the potential to use a nearly normal hearing ear for experimentation. In previous studies we could provide evidence of this (Sato et al., 2016; 2017). Here (Helmstaedter et al., 2018) we recorded a set of animals with significant residual hearing and identified these signals recorded through the cochlear implant. By the animal experiment we could also identify the exact location of the implant in the cochlea using the post-mortem histology (microgrinding technique or µCT combined with the Greenwood function, see below). The compound action potential, generated within the auditory nerve, did not provide any usable positional information in this study. We identified the summating potential as a signal that provides a reliable marker of the position of the recorded contact within the cochlea with a remarkable precision of ± 1/4 octave in the tonotopic field. The data furthermore significantly extend the previous studies on the origin of the cochlear signals and provide information on the distribution of these signals within the cochlea.  Bipolar recordings further improved the technique and allowed the suggestion of new surgical approach for implantation of residually hearing cochlea (Baumhoff et al., 2022).
This project aims as determining the potential to use cochlear implants not only for therapeutical, but also for diagnostic purposes beyond the ability to determine cochlear nerve stimulation thresholds, thus to use it as a theragnostic probe.    Teragnostic_probe_files/J_Neurosci_2016.pdfTeragnostic_probe_files/Otol_Neurotol_2017.pdfTeragnostic_probe_files/EANDH-D-16-00356_epub.pdfTeragnostic_probe_files/EANDH-D-16-00356_epub.pdfTeragnostic_probe_files/Ear_Hear_2022_EPub.pdfshapeimage_2_link_0shapeimage_2_link_1shapeimage_2_link_2shapeimage_2_link_3shapeimage_2_link_4
Recordings of acoustically-evoked signals via different electrodes of the cochlear implant in a case of pantonal residual hearing in a guinea pig. Note that SP with large amplitude, marked by the asterisk, shift from apical electrodes (1) to basal electrodes (6) with increasing stimulation frequency. Data from  (Helmstaedter et al., 2018).

CAP - compound action potential; CM - cochlear microphonics; SP - summating potential.Teragnostic_probe_files/EANDH-D-16-00356_epub_1.pdfshapeimage_3_link_0

Approach for reconstruction of the position of the cochlear implant and relative to the tonotopic field. Using µCT (here, top left) or microgrinding technique first the implant is localized within the cochlea and the individual contacts are identified. Using data from previous studies, the guinea pig Greenwood function (bottom) is used to assign the position to the characteristic frequency at the corresponding cochlear position (top right). These data can then be compared to the data obtained by summating potential recordings.

This approach clearly highlights the advantage of animal experiments for CI research: 1) it is possible to implant a normal hearing ear, providing data that are not available in humans; 2) it is possible to reconstruct the position of the implant in the cochlea post mortem, with very high precision not available in humans.

Speech preprocessing in CIsSpeech_processing_in_CI.htmlshapeimage_4_link_0
Additional information:
Cochlear anatomy in CIsCochlea.htmlshapeimage_6_link_0
Laser light stimulation studiesLaser_prosthesis.htmlshapeimage_7_link_0
Overview of existing neuroprosthesesNeuroprostheses.htmlshapeimage_8_link_0
Cochlear health effectsCochlear_Health.htmlshapeimage_9_link_0