Fangyi Chen

A differentially amplified motion in the ear for near-threshold sound detection

The ear is a remarkably sensitive pressure fluctuation detector. In guinea pigs, behavioral measurements indicate a minimum detectable sound pressure of ∼20 μPa at 16 kHz. Such faint sounds produce 0.1-nm basilar membrane displacements, a distance smaller than conformational transitions in ion channels. It seems that noise within the auditory system would swamp such tiny motions, making weak sounds imperceptible. Here we propose a new mechanism contributing to a resolution of this problem and validate it through direct measurement. We hypothesized that vibration at the apical side of hair cells is enhanced compared with that at the commonly measured basilar membrane side. Using in vivo optical coherence tomography, we demonstrated that apical-side vibrations peaked at a higher frequency, had different timing and were enhanced compared with those at the basilar membrane. These effects depend nonlinearly on the stimulus sound pressure level. The timing difference and enhancement of vibrations are important for explaining how the noise problem is circumvented.

Low coherence interferometry of the cochlear partition

Interferometric measurement of the vibration of the organ of Corti in the isolated guinea pig cochlea was conducted using low-coherence light (1310+/-47 nm wavelength) from a superluminescent diode. The short coherence length of the light source localized measurements along the axial direction to within a approximately 10-microm window (in tissue), even when using a low numerical-aperture lens. The ability to accomplish this is important because measurement of the vibration of the basal-turn organ of Corti is generally done via a small hole in the bone of the cochlea, which effectively limits the numerical aperture. The axial localization, combined with the inherent sensitivity of the method, allowed distinct measurements of the basilar membrane (BM) and the putative reticular lamina (RL) vibration using only the native tissue reflectance, that is without requiring the use of reflective particles. The system was first operated in a scanning mode as an optical coherence tomography (OCT) system to yield an image of the organ of Corti. The reflectance of intensity from the BM and RL was 8x10(-5) and 8x10(-6), respectively. The internal structure between the BM and RL presented a variable reflectivity of about 10(-7). A mirror would define a reflectance of 1.00. Then the instrument was operated as a homodyne interferometer to measure the displacement of either the BM or RL. Vibration at 16 kHz was induced by a piezoelectric actuator, causing whole movement of a dissected cochlea. After calibration of the system, we demonstrated clear measurement of mechanically driven vibration for both the BM and RL of 0.30 nm above a noise floor equivalent to 0.03 nm. OCT interferometry, when adapted for in vivo organ of Corti measurements, appears suitable to determine the micromechanical vibration of cells and tissue elements of the organ.

Quantification of vestibular-induced eye movements in zebrafish larvae

BackgroundVestibular reflexes coordinate movements or sensory input with changes in body or head position. Vestibular-evoked responses that involve the extraocular muscles include the vestibulo-ocular reflex (VOR), a compensatory eye movement to stabilize retinal images. Although an angular VOR attributable to semicircular canal stimulation was reported to be absent in free-swimming zebrafish larvae, recent studies reveal that vestibular-induced eye movements can be evoked in zebrafish larvae by both static tilts and dynamic rotations that tilt the head with respect to gravity.ResultsWe have determined herein the basis of sensitivity of the larval eye movements with respect to vestibular stimulus, developmental stage, and sensory receptors of the inner ear. For our experiments, video recordings of larvae rotated sinusoidally at 0.25 Hz were analyzed to quantitate eye movements under infrared illumination. We observed a robust response that appeared as early as 72 hours post fertilization (hpf), which increased in amplitude over time. Unlike rotation about an earth horizontal axis, rotation about an earth vertical axis at 0.25 Hz did not evoke eye movements. Moreover, vestibular-induced responses were absent in mutant cdh23 larvae and larvae lacking anterior otoliths.ConclusionsOur results provide evidence for a functional vestibulo-oculomotor circuit in 72 hpf zebrafish larvae that relies upon sensory input from anterior/utricular otolith organs.

A hydromechanical biomimetic cochlea: Experiments and models

The construction, measurement, and modeling of an artificial cochlea (ACochlea) are presented in this paper. An artificial basilar membrane (ABM) was made by depositing discrete Cu beams on a piezomembrane substrate. Rather than two fluid channels, as in the mammalian cochlea, a single fluid channel was implemented on one side of the ABM, facilitating the use of a laser to detect the ABM vibration on the other side. Measurements were performed on both the ABM and the ACochlea. The measurement results on the ABM show that the longitudinal coupling on the ABM is very strong. Reduced longitudinal coupling was achieved by cutting the membrane between adjacent beams using a laser. The measured results from the ACochlea with a laser-cut ABM demonstrate cochlear-like features, including traveling waves, sharp high-frequency rolloffs, and place-specific frequency selectivity. Companion computational models of the mechanical devices were formulated and implemented using a circuit simulator. Experimental data were compared with simulation results. The simulation results from the computational models of the ABM and the ACochlea are similar to their experimental counterparts.

In vivo outer hair cell length changes expose the active process in the cochlea

Background Mammalian hearing is refined by amplification of the sound-evoked vibration of the cochlear partition. This amplification is at least partly due to forces produced by protein motors residing in the cylindrical body of the outer hair cell. To transmit power to the cochlear partition, it is required that the outer hair cells dynamically change their length, in addition to generating force. These length changes, which have not previously been measured in vivo, must be correctly timed with the acoustic stimulus to produce amplification. Methodology/Principal Findings Using in vivo optical coherence tomography, we demonstrate that outer hair cells in living guinea pigs have length changes with unexpected timing and magnitudes that depend on the stimulus level in the sensitive cochlea. Conclusions/Significance The level-dependent length change is a necessary condition for directly validating that power is expended by the active process presumed to underlie normal hearing.

In vivo imaging and low-coherence interferometry of organ of Corti vibration

An optical coherence tomography (OCT) system is built to acquire in vivo both images and vibration measurements of the organ of Corti of the guinea pig. The organ of Corti is viewed through a approximately 300-microm-diam hole in the bony wall of the cochlea at the scala tympani of the first cochlear turn. In imaging mode, the image is acquired as reflectance R(x,z). In vibration mode, the basilar membrane (BM) or reticular lamina (RL) are selected by the investigator interactively from the R(x,z) image. Under software control, the system moves the scanning mirrors to bring the sensing volume of the measurement to the desired membrane location. In vivo images of the organ of Corti are presented, indicating reflectance signals from the BM, RL, tectorial membrane, and Reissners membrane. The tunnel of Corti and the inner sulcus are also visible in the images. Vibrations of +/-2 and +/-22 nm are recorded in the BM in response to low and high sound levels at 14 kHz above a noise floor of 0.2 nm.

Filtering of Acoustic Signals within the Hearing Organ

The detection of sound by the mammalian hearing organ involves a complex mechanical interplay among different cell types. The inner hair cells, which are the primary sensory receptors, are stimulated by the structural vibrations of the entire organ of Corti. The outer hair cells are thought to modulate these sound-evoked vibrations to enhance hearing sensitivity and frequency resolution, but it remains unclear whether other structures also contribute to frequency tuning. In the current study, sound-evoked vibrations were measured at the stereociliary side of inner and outer hair cells and their surrounding supporting cells, using optical coherence tomography interferometry in living anesthetized guinea pigs. Our measurements demonstrate the presence of multiple vibration modes as well as significant differences in frequency tuning and response phase among different cell types. In particular, the frequency tuning at the inner hair cells differs from other cell types, causing the locus of maximum inner hair cell activation to be shifted toward the apex of the cochlea compared with the outer hair cells. These observations show that additional processing and filtering of acoustic signals occur within the organ of Corti before inner hair cell excitation, representing a departure from established theories.

Volumetric Imaging of Blood Flow Within Cochlea in Gerbil In Vivo

Changes in blood flow to the inner ear are thought to influence a number of cochlear diseases, including noise-induced hearing loss, sudden hearing loss, and Menieres disease. Advances have been made in the areas of vital microscopic studies of microcirculation, and the laser Doppler flowmetry. But none of these techniques can provide in vivo 3-D mapping of microvascular perfusion within the cochlea. To overcome this limitation, we have developed and used a method of optical microangiography (OMAG) that can generate 3-D angiograms within millimeter of tissue depths by analyzing the endogenous optical scattering signal obtained from an illuminated sample. We used OMAG to visualize the cochlear microcirculation of adult living gerbil through the intact cochlea, which would be difficult, if not impossible, by use of any other current techniques.

Persistence of Past Stimulations: Storing Sounds within the Inner Ear

Tones cause vibrations within the hearing organ. Conventionally, these vibrations are thought to reflect the input and therefore end with the stimulus. However, previous recordings of otoacoustic emissions and cochlear microphonic potentials suggest that the organ of Corti does continue to move after the end of a tone. These after-vibrations are characterized here through recordings of basilar membrane motion and hair cell extracellular receptor potentials in living anesthetized guinea pigs. We show that after-vibrations depend on the level and frequency of the stimulus, as well as on the sensitivity of the ear. Even a minor loss of hearing sensitivity caused a sharp reduction in after-vibration amplitude and duration. Mathematical models suggest that after-vibrations are driven by energy added into organ of Corti motion after the end of an acoustic stimulus. The possible importance of after-vibrations for psychophysical phenomena such as forward masking and gap detection are discussed.

Depth-resolved dual-beamlet vibrometry based on Fourier domain low coherence interferometry

Abstract. We present an optical vibrometer based on delay-encoded, dual-beamlet phase-sensitive Fourier domain interferometric system to provide depth-resolved subnanometer scale vibration information from scattering biological specimens. System characterization, calibration, and preliminary vibrometry with biological specimens were performed. The proposed system has the potential to provide both amplitude and direction of vibration of tissue microstructures on a single two-dimensional plane.