Minimally invasive surgical method to detect sound processing in the cochlear apex by optical coherence tomography

Abstract

Sound processing in the inner ear involves separation of the constituent frequencies along the length of the cochlea. Frequencies relevant to human speech (100 to 500 Hz) are processed in the apex region. Among mammals, the guinea pig cochlear apex processes similar frequencies and is thus relevant for the study of speech processing in the cochlea. However, the requirement for extensive surgery has challenged the optical accessibility of this area to investigate cochlear processing of signals without significant intrusion. A simple method is developed to provide optical access to the guinea pig cochlear apex in two directions with minimal surgery. Furthermore, all prior vibration measurements in the guinea pig apex involved opening an observation hole in the otic capsule, which has been questioned on the basis of the resulting changes to cochlear hydrodynamics. Here, this limitation is overcome by measuring the vibrations through the unopened otic capsule using phase-sensitive Fourier domain optical coherence tomography. The optically and surgically advanced method described here lays the foundation to perform minimally invasive investigation of speech-related signal processing in the cochlea.

Fig. 1

Schematic of the PhS-FDOCT system used in this study to measure in vivo vibrations in guinea pig cochlear apex through the otic capsule. SLD with center wavelength of 840 nm.

Fig. 2

The guinea pig skull (top) shows the location of the cochlea, and the expanded schematic (bottom) shows the orientation of the apex. Direct optical access to the cochlear apex in the axial direction required in earlier experiments that much of the jaw and neck tissues be removed. Such extensive surgery involved significant blood loss and posed high risk for loss in hearing sensitivity.

Fig. 3

The method used to measure axial and radial vibrations of the guinea pig cochlear apex. (a) To measure the vibrations in the axial direction, the OCT light from the sample arm is deflected by 90 deg using a mirror device with a reflective front surface mirror placed inside the bulla of the live guinea pig. This step significantly reduces the surgery needed to measure the axial vibrations in the guinea pig apex. (b) The vibrations in the radial direction are measured by direct placement of the animal with the appropriate head orientation in the sample arm, without the use of the mirror device. (c) The mirror device is created by attaching a mirror to the 45-deg surface of a three-dimensional printed prism-shaped plastic part. In this image, the mirror is located on the bottom side. Scale bar 2.5 mm. (d) Mirror device placed inside the bulla in a guinea pig skull for axial direction measurements.

Fig. 4

(a) Maximum displacement that can be measured using PhS-FDOCT with center wavelength of 840 nm and for camera sampling rates $S$ indicated in the legend. Note that a quarter wavelength of the optical source is 210 nm. The sound-induced displacements in the guinea pig apex can approach several hundred nanometers for 90 dB SPL stimulus level, as shown in (b). In the sample axial frequency response shown in (b), the maximum displacement at 94 dB SPL (or 1 Pascal) is 300 nm at 300 Hz. This value can be higher in some animals.

Fig. 5

PhS-FDOCT measurement of the guinea pig apex in the axial direction. (a) Reflectance or structural image of the apex turn viewed in the axial direction using the method described in Fig. 3(a). Otic capsule (not shown) is at the top of the image. A schematic is shown in (c) for comparison (schematic image from web, courtesy of Britta Flock). Axial and radial directions are marked in the schematic. RM, Reissner’s membrane; TM, tectorial membrane; BM, basilar membrane; OoC, organ of Corti. (b) Displacement in nanometers in the axial direction for a 90 dB SPL sound stimulus at 200 Hz.

Fig. 6

PhS-FDOCT measurement of the guinea pig apex in the radial direction. (a) Reflectance or structural image of the apex turn viewed in the radial direction using the method described in Fig. 3(b). The schematic is shown in (c) (schematic image from web, courtesy of Britta Flock). RM, Reissner’s membrane; BM, basilar membrane; OoC, organ of Corti. (b) Displacement in nanometers in the radial direction for sound stimulus at 150 Hz, 90 dB SPL.