Place-Based Mapping in Electric-Acoustic Stimulation Listeners

Abstract Cochlear implant (CI) recipients with acoustic hearing in the implanted ear are fit with a hearing aid and CI in the same ear, a configuration described as electric-acoustic stimulation (EAS). While EAS supports better performance than a CI alone, the benefit varies widely across individuals.

One possible source of individual differences is variability in the CI frequency-to-place mismatch, which is a discrepancy between the electric filter frequency assigned to a particular electrode and the normal cochlear place frequency at the location of that electrode.

While prior studies have shown that tonotopic mismatch impacts speech perception in CI-alone users, the impact of mismatch on EAS performance is poorly understood.

Mismatch in EAS occurs because current default mapping procedures use the patient?s unaided hearing thresholds in the implanted ear to determine the lowest frequency filter assigned to electric stimulation and distributes higher-frequency information logarithmically across the remaining electrodes irrespective of their location within the cochlea.

Mismatch is prevalent in EAS users due to wide variability in angular insertion depth (AID) across and within electrode arrays, and variability in residual hearing.

Mismatch occurs in two scenarios: 1) apical electrodes lie basal to the acoustic cut-off frequency and deliver frequency information that is lower than their associated place frequency, or 2) apical electrodes lie apical to the cut-off frequency and deliver frequency information that is higher than their associated place frequency.

In both scenarios, performance with EAS is likely hindered by the mismatch between place of stimulation and natural tonotopicity of the cochlea. Performance in the latter scenario could be further compromised by peripheral electric-on-acoustic masking.

However, these problems can be avoided if a patient?s acoustic hearing and electrode array AID are incorporated into EAS mapping using a novel place-based mapping procedure.

The proposed experiments evaluate a place-based mapping procedure that uses postoperative computed tomography (CT) to calculate the AID of individual electrodes to determine their cochlear place frequency.

The electric filters are assigned to align with the associated cochlear place frequency, resulting in a patient-specific map that eliminates frequency-to-place mismatch and minimizes peripheral masking.

We hypothesize that a place-based map will support better monaural hearing (e.g. speech perception) in EAS users by limiting the need to acclimate to spectrally-shifted electric information while the acoustic information is transduced in the natural place.

Further, we expect that it will support better binaural hearing (e.g. spatial release from masking) by providing an interaural match of frequency information between ears.

Benefits of a place-based map over a default map will likely be most apparent during the initial months of EAS use when acclimatization to mismatch is occurring in the default case, and may persist longer-term for those with the greatest degrees of mismatch.

This project is expected to lay the groundwork for a novel and clinically feasible mapping procedure to maximize performance for EAS users.