2019 Conference on Implantable Auditory Prostheses

14-19 July 2019

Granlibakken, Lake Tahoe

Page 119

M28: INVESTIGATION OF AUDITORY NERVE FIBERS FACILITATION EFFECT IN

COCHLEAR IMPLANT RECIPIENTS

Sonia Tabibi, Andrea Kegel, Waikong Lai, Norbert Dillier

ETHZ, Department of Information Technology and Electrical Engineering, Zurich, CHE

ENT Department, University Hospital, Zurich, CHE

Sydney Cochlear Implant Center, RIDBC, Sydney, AUS

University of Zurich, ENT Department, Zurich, CHE

Electrical stimulation of auditory nerve fibers (ANFs) in animals with pairs of pulses has shown facilitation

which is defined as an increase in nerve excitability caused by subthreshold stimulation in short intervals

(Dynes, 1996; Cartee et al., 2000; Heffer et al., 2010). Apart from animal studies, the facilitation effect has

also been observed in human cochlear implant (CI) recipients (Cohen, 2009; Hey et al., 2017). Facilitation

may occur when the neuron does not respond to the first pulse, but if the membrane potential remains near

the threshold long enough, the second pulse can produce a response (Boulet et al., 2016). Although

facilitation has been observed in animals and human CI recipients, no study has modeled this in human CI

users until now. Thus, the aim of this study is to model the facilitation effect measured via electrically evoked

compound action potential (ECAP) at short masker probe intervals (MPIs) and small probe current levels

(CLs) and explore the effect of masker and probe levels on this phenomenon.

11 CI recipients implanted with the Nucleus® implants participated in the study and individual

amplitude growth functions (AGFs) were recorded for three test electrodes (E6, E12 and E18) with a masker

offset of +10 CL. The ECAPs were measured with the modified forward masking method by Miller et al. (2000)

and the probe level was set to a visually detected threshold of the ECAP (vT-ECAP) and a current level that

yielded 100 μV ECAP response amplitude. 10 masker offsets from +10 CL to the masker level equal to zero

as a baseline were measured and the MPIs varied from 13 μs (the shortest MPI possible with the clinical

Custom Sound EP software) to CI recipient

’s individual absolute refractory period.

A decaying exponential function was fitted to facilitation data by finding the minimum residual sum of

squares and the same function used for +10 CL masker offset condition could be applied to all other masker

offsets measurements. Facilitation time constant and amplitude showed no dependency on the probe level.

However, facilitation effect was stronger for the masker level at or around vT-ECAP. There was a positive

correlation between facilitation amplitude and AGF slope, which indicates that CI subjects with better

peripheral neural survival have stronger facilitation effect.

Although facilitation time constant for most of the CI recipients in this study was in the range, which

was reported by Cartee et al. (2000), one CI subject had larger facilitation time constant. We further examined

ECAP data recorded from this subject and found an enhancement of recovery function at the MPI around

1000 μs. The larger facilitation time constant might lead to this elevation while the facilitation effect did not

disappear completely when the recovery function started to rise and these two phenomena had a larger

overlap in comparison to the other CI subjects participated in the study. Further investigations should be

conducted to explore the range of facilitation time constant and possible correlation with other parameters.

Acknowledgments: This work was supported by the People Programme (Marie Curie Actions) of the European

Union’s Seventh Framework Programme FP7/2007

-2013/ under REA grant agreement n° PITN-GA-2012-31752.

Funding for the completion of first author’s PhD thesis was provided by the University of Zürich and the Truus und

Gerrit van Riemsdijk Foundation.

References: Boulet J, White M, Bruce IC. (2016). Temporal considerations for stimulating spiral ganglion neurons

with cochlear implants. Journal of the Association for Research in Otolaryngology, 17(1): 1-17.

Cartee LA, van den Honert C, Finley CC, Miller RL. (2000). Evaluation of a model of the cochlear neural

membrane. I. Physiological measurement of membrane characteristics in response to intrameatal electrical

stimulation. Hearing research, 146(1): 143-152.

Cohen LT (2009). Practical model description of peripheral neural excitation in cochlear implant recipients: 5.

refractory recovery and facilitation. Hearing research, 248(1): 1-14.

Hey M, Müller-Deile J, Hessel M, Killian M (2017). Facilitation and refractoriness of the electrcially evoked

compound action potential. Hearing research, 355: 14-22.

Miller CA, Abbas PJ, Brown CJ. (2000). An improved method of reducing stimulus artifact in the electrically evoked

whole-nerve potential. Ear and hearing, 21(4): 280-290.