Optoacoustic effect is responsible for laser-induced cochlear responses

N Kallweit,A Kral,P Baumhoff,H Maier,A Krueger,T Ripken,N Tinne

doi:10.1038/srep28141

Abstract

Optical stimulation of the cochlea with laser light has been suggested as an alternative to conventional treatment of sensorineural hearing loss with cochlear implants. The underlying mechanisms are controversially discussed: The stimulation can either be based on a direct excitation of neurons, or it is a result of an optoacoustic pressure wave acting on the basilar membrane. Animal studies comparing the intra-cochlear optical stimulation of hearing and deafened guinea pigs have indicated that the stimulation requires intact hair cells. Therefore, optoacoustic stimulation seems to be the underlying mechanism. The present study investigates optoacoustic characteristics using pulsed laser stimulation for in vivo experiments on hearing guinea pigs and pressure measurements in water. As a result, in vivo as well as pressure measurements showed corresponding signal shapes. The amplitude of the signal for both measurements depended on the absorption coefficient and on the maximum of the first time-derivative of laser pulse power (velocity of heat deposition). In conclusion, the pressure measurements directly demonstrated that laser light generates acoustic waves, with amplitudes suitable for stimulating the (partially) intact cochlea. These findings corroborate optoacoustic as the basic mechanism of optical intra-cochlear stimulation.

Highlights

Today, direct electrical stimulation of spiral ganglion neurons (SGNs) using cochlear implants (CIs) is the standard treatment for profound sensorineural hearing loss[1]
Based on the theoretical considerations about optoacoustic signal generation we formed the hypothesis that the compound action potential (CAP) response is most likely of optoacoustic nature if it shows similar time dependent characteristics with an onset and an offset signal corresponding to the slope of the laser power
One possible explanation discussed in Schultz et al.[22] is that the form of the heated volume functioning as the source term for the optoacoustic effect might change from a far reaching linear/elliptic type to a short ranging acoustic point source confined to the direct vicinity of the fiber tip due to the reduced light penetration depth

Summary

Introduction

Direct electrical stimulation of spiral ganglion neurons (SGNs) using cochlear implants (CIs) is the standard treatment for profound sensorineural hearing loss[1]. Optical stimulation is being discussed as a potentially higher resolved and artifact-free alternative to conventional CI-treatment It is not yet clear whether the mechanism of intra-cochlear optical stimulation is based on direct activation of SGNs13–18 or whether it is the optoacoustic effect (see materials and methods section, mathematical description of the optoacoustic effect), which deflects the basilar membrane and activates the inner hair cells[19,20,21,22,23]. The characteristic dependency of CAP response amplitude of hearing guinea pigs on the absorption coefficient and laser parameters and the lack of auditory responses of completely deaf animals led to the assumption of an optoacoustic stimulation mechanism This hypothesis is supported by observation of basilar membrane vibrations caused by laser pulses[29,30,31]. In order to verify (or falsify) this hypothesis we searched for the typical features of optoacoustic signals described above by comparison of in vivo CAP measurements with pressure measurements using the same laser parameters

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Methods

Results

Discussion

Conclusion