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Excitation patterns in the Inferior colliculus point to an opto-acoustic mechanism of intra-cochlear
infrared laser stimulation
Baumhoff P. , Schultz M. , Kallweit N. , Sato M. , Krüger A. , Kral A.
Medizinische Hochschule Hannover, Institute of Audioneurotechnology (VIANNA), Hannover, Germany, 2Laser Zentrum Hannover,
Hannover, Germany
Laser stimulation of the cochlea has attracted attention as a potential, more focused, alternative for conventional
cochlear implants or hearing aids. The absorption of pulsed laser energy also induces sound waves in the
absorber, an effect e.g. applied in optoacoustic imaging. We utilized two pulsed laser sources (variable
wavelength source: 434 - 1961 nm, 5 ns, 6µJ; constant wavelength source: 1860nm, 20µs - 20 ms, 6 - 500 µJ) to
perform in-vivo and ex-vivo experiments in stress and thermal confinement. Multiunit responses in the inferior
colliculus (IC) were recorded in-vivo from normal hearing, ketamine anesthetized guinea pigs. An optical fiber
was positioned into a cochleostomy of the basal turn for intra-cochlear stimulation. A 32-channel Neuronexusprobe was stereotactically inserted into the IC along the tonotopic axis and characteristic frequencies were
determined with tonal stimulation prior laser stimulation. Additionally, acoustic resonances caused by the
absorption of pulsed laser light in a closed small volume (tympanic bulla) were investigated ex-vivo. An optical
fiber was inserted into the bulla of a macerated skull through a bullotomy. The laser beam was directed towards
the basal cochlear turn. A calibrated Bruel&Kjaer microphone placed on the outer ear canal was used to record
the sound generated during laser pulse emission. Spectral analyses of the opto-acoustic effect recorded ex-vivo
indicated resonance maxima at frequencies of 4.5 kHz (resonance of the bulla) and to a lesser extend at 8 and
12.5 kHz. Similar frequency characteristics could be shown for all wavelengths. The IC activation by inner ear
laser stimulation was strong at units with characteristic frequencies mostly below 10 kHz for intra cochlear laser
stimulation. The general activation pattern in the IC did not depend on wavelength, pulse duration or intracochlear fiber orientation. Yet each of these parameters had a modulatory effect on the response strength. No
responses to laser stimulation could be recorded from the IC of completely deafened animals, but the IC
remained responsive to electric intra cochlear stimulation at normal thresholds in all cases tested.
The results suggest an opto-acoustic effect causing a sound activation of the cochlea rather than a direct
neuronal or hair cell excitation. We propose residual hearing as the most likely explanation for any response of
the auditory system to laser pulses in the cochlea. Hearing status has to be precisely controlled for in laser
stimulation experiments, particularly at low frequencies. Laser induced sound within auditory structures seems to
have strong low frequency content due to resonance, even though the short pulse duration could suggest
Supported by Deutsche Forschungsgemeinschaft (Cluster of Excellence Hearing4all), EU grant ACTION and
MedEl Comp, Innsbruck, Austria.