Acoustic events and "optophonic" cochlear responses induced by pulsed near-infrared laser.

Optical stimulation of neural tissue within the cochlea was described as a possible alternative to electrical stimulation. Most optical stimulation was performed with pulsed lasers operating with near-infrared (NIR) light and in thermal confinement. Under these conditions, the coexistence of laser-induced optoacoustic stimulation of the cochlea ("optophony") has not been analyzed yet. This study demonstrates that pulsed 1850-nm laser light used for neural stimulation also results in sound pressure levels up to 62 dB peak-to-peak equivalent sound pressure level (SPL) in air. The sound field was confined to a small volume along the laser beam. In dry nitrogen, laser-induced acoustic events disappeared. Hydrophone measurements demonstrated pressure waves for laser fibers immersed in water. In hearing rats, laser-evoked signals were recorded from the cochlea without targeting neural tissue. The signals showed a two-domain response differing in amplitude and latency functions, as well as sensitivity to white-noise masking. The first component had characteristics of a cochlear microphonic potential, and the second component was characteristic for a compound action potential. The present data demonstrate that laser-evoked acoustic events can stimulate a hearing cochlea. Whenever optical stimulation is used, care must be taken to distinguish between such "optophony" and the true optoneural response.

Optical stimulation of the facial nerve: a new monitoring technique?

OBJECTIVES/ HYPOTHESIS: One sequela of skull base surgery is iatrogenic damage to cranial nerves, which can be prevented if the nerve is identified. Devices that stimulate nerves with electric current assist in nerve identification. Contemporary devices have two main limitations: 1) the physical contact of the stimulating electrode and (2) the spread of the current through the tissue. In contrast to electrical stimulation, pulsed infrared optical radiation can be used to safely and selectively stimulate neural tissue and might be valuable for screening. METHODS: The gerbil facial nerve was exposed to 250 microsecond pulses of 2.12 microm radiation delivered via a 600-microm-diameter optical fiber at a repetition rate of 2 Hz. With use of 27 GA, 12-mm intradermal electrodes, muscle action potentials were recorded. Nerve samples were examined for possible tissue damage. RESULTS: Eight facial nerves were stimulated with radiant exposures between 0.71 and 1.77 J/cm, resulting in compound muscle action potentials (CmAPs) that were simultaneously measured at the m. orbicularis oculi, m. levator nasolabialis, and m. orbicularis oris. Resulting CmAP amplitudes were 0.3 to 0.4 mV, 0.15 to 1.4 mV, and 0.3 to 2.3 mV, respectively, depending on the radial location of the optical fiber and the radiant exposure. Individual nerve branches were also stimulated, resulting in CmAP amplitudes between 0.2 and 1.6 mV. Histology revealed tissue damage at radiant exposures of 2.2 J/cm but no apparent damage at radiant exposures of 2.0 J/cm. CONCLUSIONS: The experiments showed that selective muscle action potentials can be evoked optically in the gerbil facial nerve without direct physical contact.

The hemicochlea preparation of the guinea pig and other mammalian cochleae.

The hemicochlea and its slice preparation is a novel method that allows access to various cochlear structures without the physical distortion that typically occurs from tissue dehydration during the embedding process. Therefore, the hemicochlea preparation provides an excellent model to study (1) cochlear morphology during cochlear development, (2) malformation caused by genetic defects, (3) changes related to diseases, (4) sensory physiology, (5) cochlear micromechanics, and (6) the expression of proteins by immunohistochemistry. This paper describes in detail the method of slicing hemicochleae for different mammalian species, including mice, rats, gerbils, guinea pigs, pigs, and human temporal bones. Furthermore, guinea pig cochleae are used as an example to provide cochlear dimensions of important anatomical structures. The values obtained in eight guinea pig hemicochleae are compared to published values, and upon review, discrepancies do exist. For example, gelatinous structures, such as the tectorial membrane, appear larger in the hemicochlea when compared to traditional embedding. Dimensions obtained for selected cochlear structures at different locations along the guinea pig cochleae provide an improved basis for cochlear models.