Does the distance from a laser source to the ear influence the optoacoustic activation of the auditory system?

Hearing loss is the most frequent sensory deficit in humans with approximately 436 million affected people worldwide requiring treatment. If untreated, it can lead to social isolation and reduced quality of life for many individuals. Currently available auditory prostheses based on mechanical and electrical energy are not sufficiently supplying the hard of hearing patients, thus making novel stimulation strategies necessary. A new generation of hearing aids based on optoacoustic effects provide promising results for the future. For this purpose, we proposed to assess the effect of the distance between the laser source and the vibratory site at three different anatomical structures of the peripheral hearing organ. Therefore, the auditory threshold of anesthetized female albino guinea pigs was determined using click-evoked brainstem potentials of 0–80 dB SPL. 10 ns laser pulses at 532 nm wavelength and 10 Hz repetition rate with increasing intensity were then used to irradiate the tympanic membrane, the round window, and the otic capsule. For this, the laser fiber, with a 365 µm diameter, was positioned at 0.1, 2.6, 5.1 and 10.1 mm distance from the three anatomical sites. To assess the neuronal activity, optically induced acoustic brainstem potentials were recorded. The auditory threshold for each distance was estimated by the identification of the first recorded Jewett complex. The optical stimulation using a laser with 532 nm wavelength and 10 Hz repetition rate induced a progressively decreasing signal, the further the distance between the source and the target. The further the laser was placed from the target, the higher was the intensity of the laser pulse required to achieve an equivalent auditory activation threshold. In our experiment this effect was consistent at all the measured distances and for all the three targeted anatomical sites. As expected, the maximal activation was achieved in the position most proximal to the irradiated anatomical site. The distance between the laser source and the targeted anatomical structure demonstrated, therefore, an inverse correlation to the amplitude of the induced optoacoustic activation. The use of this result in new developments could allow for adjustment of the laser source in order to adapt to the anatomical characteristics and the specific pathology present in each case.