Intensity modulation of frequency-specific optoacoustic stimulation at the peripheral hearing level

Laser light applied to the tympanic membrane (TM) induces mechanical vibrations that activate the auditory system. For controlled optoacoustic stimulation, a quantitative characterization of the mechanical properties of the TM and their relation to stimulation intensity is required. We therefore investigated which optical stimulation parameters induce vibratory amplitudes comparable to those caused by acoustic stimulation. TM vibration velocity was recorded ex vivo using a scanning laser Doppler vibrometer. Mechanical parameters of the TM, including effective compliance and exponential decay constants (τ), were determined from single-pulse laser stimulation. In addition, vibration amplitudes were quantified in response to sinusoidally amplitude-modulated laser pulse trains at different laser modulation rates (LMR). Laser modulation rates of 1, 2, 4, 8 and 10 kHz, and laser pulse repetition rates (LPR) of 32 kHz and 50 kHz were applied. Ex vivo measurements were compared with frequency-specific in vivo recordings from the central nucleus of the inferior colliculus (ICC). The extracted mechanical parameters, including compliance and τ, were consistent across stimulation conditions. Ex vivo measurements showed that the maximal displacement amplitude at the umbo decreased with increasing LMR and approached values comparable to those obtained with pure-tone acoustic stimulation up to 60 dB SPL. These intensity-dependent mechanical responses were reflected by corresponding neural activity at the ICC level in vivo, demonstrating consistent optoacoustic coupling across mechanical and physiological domains.