2019 Conference on Implantable Auditory Prostheses

14-19 July 2019

Granlibakken, Lake Tahoe

Page 274

TH2: CHARACTERIZATION OF EAS INTERACTION THROUGH

ECOCHG AMPLITUDE GROWTH FUNCTIONS

Benjamin Krueger, Andreas Buechner, Waldo Nogueira

Medical University Hannover, Cluster of Excellence “Hearing4all”, Hannover

, DEU

Introduction: Cochlear implantation is increasingly being used in hearing loss treatment for subjects with

residual hearing to obtain combined electric acoustic stimulation (EAS). Previous psychoacoustic studies

have shown that electric stimulation interacts with acoustic stimulation at threshold level (Krüger et al.

2017; Imsiecke et al. 2018). In EAS users, intracochlear electrocochleography (ECochG) can provide

insights about the pathophysiology of the peripheral auditory. In response to acoustic stimuli the cochlear

microphonics (CM/DIF) and the auditory nerve neurophonics (ANN/SUM) can be derived from ECochG

recordings. CM/DIF and ANN/SUM recordings can be used to assess whether the EAS interaction

origins in the peripheral auditory system.

Objective: This work characterizes electrophysiological EAS interaction from threshold to most

comfortable level (MCL) by measuring ECochG amplitude growth functions (AGF) to assess the relation

between electrophysiological and psychoacoustic EAS interaction and to give a more detailed insight

about the origin of EAS interaction.

Methods: The backward telemetry capability of the CI was used to record ECochG AGFs in ten

Advanced Bionics EAS users. CM/DIF and ANN/SUM responses were derived and compared to post-

operative hearing thresholds and MCLs. In seven of Advanced Bionics EAS users, the AGFs were used

to characterize the interaction between electric and acoustic stimulation for various acoustic stimulation

levels. This interaction was compared with psychoacoustic masking measurements.

Electrophysiologically, ECochG AGFs were recorded for an acoustic tone (A), an electric pulse train (E)

and simultaneously presented acoustic and electric stimuli (A+E). The effect of electric stimulation on an

acoustic tone was determined by subtracting the E from A+E response. Psychoacoustic threshold

elevation of an acoustic probe caused by the presence of an electric masker was measured using a 3

interval-forced-choice task. An electric masker stimulus was presented at MCL at the most apical

electrode while the level of an acoustic probe was adapted to the detection threshold.

Results: The results show significant amplitude responses for acoustic stimulation at MCL in 9 of 10

subjects for CM/DIF recordings and in 8 of 10 subjects for ANN/SUM recordings. The mean CM/DIF

response was 8.76 μV and the mean ANN/SUM response was 0.98 μV. Significant CM/DIF amplitudes

at the psychoacoustic detection threshold could be recorded for 8 of the 10 subjects. The CM/DIF

threshold (defined as CM/DIF amplitude significant above noise floor) was in mean -17.5 dB below the

psychoacoustic detection threshold. No significant ANN/SUM amplitude responses could be recorded at

psychoacoustic detection thresholds.The results demonstrate EAS interaction using both, ECochG

recordings and psychoacoustic measurements. A mean threshold elevation of 2.4 dB was observed from

the psychoacoustic masking experiment. The CM/DIF measures show a mean interaction of 1.15 dB.

The CM/DIF interaction at detection threshold was 1.5 dB and therefore lager than the interaction at MCL

(1.0 dB). However, no significant effect of acoustic probe stimulation level could be observed. The results

suggest a constant effect of the electric masker independent of the acoustic probe stimulus.

Conclusions: From these results we conclude that intracochlear ECochG can be used to estimate

psychoacoustic detection threshold and that some aspects of EAS interaction can be measured

electrophysiologically. However, the sensitivity of this measure underestimates the threshold elevations

observed psychophysically and therefore EAS interaction may not only occur in the periphery but also

more centrally along the auditory pathway.

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) Cluster of Excellence EXC 1077/1

“Hearing4all”and DFG

- Projekt number 396932747 (PI: Waldo Nogueira). Moreover, this project received funding from

Advanced Bionics.