Abstract
Direct acoustic cochlear stimulation uses piston motion to substitute for stapes footplate (SFP) motion. The ratio of piston to stapes footplate motion amplitude, to generate the same loudness percept, is an indicator of stimulation efficiency. We determined the relationship between piston displacement to perceived loudness, the achieved maximum power output and investigated stapes fixation and obliteration as confounding factors. The electro-mechanical transfer function of the actuator was determined preoperatively on the bench and intraoperatively by laser Doppler vibrometry. Clinically, perceived loudness as a function of actuator input voltage was calculated from bone conduction thresholds and direct thresholds via the implant. The displacement of a 0.4 mm diameter piston required for a perception equivalent to 94 dB SPL at the tympanic membrane compared to normal SFP piston displacement was 27.6–35.9 dB larger, consistent with the hypothesis that the ratio between areas is responsible for stimulation efficiency. Actuator output was 110 ± 10 eq dB SPLFF @1Vrms ≤ 3 kHz and decreased to 100 eq dB SPLFF at 10 kHz. Output was significantly higher for mobile SFPs but independent from obliteration. Our findings from clinical data strongly support the assumption of a geometrical dependency on piston diameter at the biological interface to the cochlea.
Highlights
Direct acoustic cochlear stimulation uses piston motion to substitute for stapes footplate (SFP) motion
The signal processing and amplification are similar to hearing aids, the conversion of the output voltage to sound at the inner ear cannot be determined
Using clinical audiological data from a large cohort of patients, the displacement amplitude to evoke the same loudness percept was calculated for speech relevant frequencies (0.25–6.0 kHz)
Summary
Direct acoustic cochlear stimulation uses piston motion to substitute for stapes footplate (SFP) motion. It was recently shown that results in the temporal bone model correspond well with clinical outcomes in patients[2] This method cannot be used in direct acoustic stimulation of the cochlea, because the middle ear—required for the use of the ASTM standard—is circumvented, and the cochlea fluids are stimulated directly by an actuator. The ratio of the stimulation efficiency between naturally conducted sound via the stapes footplate and an actuator driven piston is crucial for the specification of future direct stimulation devices. We investigated the stimulation efficiency of an actuator driven piston in comparison to normal physiological sound transmission For this purpose, we developed a new approach to determine the transfer function from piston (0.4 mm Fisch Teflon) displacement to perceived loudness from clinical patient data and investigated possible factors contributing to stimulation output levels
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