Modeling the utility of binaural cues for underwater sound localization.

The binaural cues used by terrestrial animals for sound localization in azimuth may not always suffice for accurate sound localization underwater. The purpose of this research was to examine the theoretical limits of interaural timing and level differences available underwater using computational and physical models. A paired-hydrophone system was used to record sounds transmitted underwater and recordings were analyzed using neural networks calibrated to reflect the auditory capabilities of terrestrial mammals. Estimates of source direction based on temporal differences were most accurate for frequencies between 0.5 and 1.75 kHz, with greater resolution toward the midline (2°), and lower resolution toward the periphery (9°). Level cues also changed systematically with source azimuth, even at lower frequencies than expected from theoretical calculations, suggesting that binaural mechanical coupling (e.g., through bone conduction) might, in principle, facilitate underwater sound localization. Overall, the relatively limited ability of the model to estimate source position using temporal and level difference cues underwater suggests that animals such as whales may use additional cues to accurately localize conspecifics and predators at long distances.

Probing cochlear tuning and tonotopy in the tiger using otoacoustic emissions.

Otoacoustic emissions (sound emitted from the ear) allow cochlear function to be probed noninvasively. The emissions evoked by pure tones, known as stimulus-frequency emissions (SFOAEs), have been shown to provide reliable estimates of peripheral frequency tuning in a variety of mammalian and non-mammalian species. Here, we apply the same methodology to explore peripheral auditory function in the largest member of the cat family, the tiger (Panthera tigris). We measured SFOAEs in 9 unique ears of 5 anesthetized tigers. The tigers, housed at the Henry Doorly Zoo (Omaha, NE), were of both sexes and ranged in age from 3 to 10 years. SFOAE phase-gradient delays are significantly longer in tigers--by approximately a factor of two above 2 kHz and even more at lower frequencies--than in domestic cats (Felis catus), a species commonly used in auditory studies. Based on correlations between tuning and delay established in other species, our results imply that cochlear tuning in the tiger is significantly sharper than in domestic cat and appears comparable to that of humans. Furthermore, the SFOAE data indicate that tigers have a larger tonotopic mapping constant (mm/octave) than domestic cats. A larger mapping constant in tiger is consistent both with auditory brainstem response thresholds (that suggest a lower upper frequency limit of hearing for the tiger than domestic cat) and with measurements of basilar-membrane length (about 1.5 times longer in the tiger than domestic cat).

A system to generate patient-specific stimuli for use with the auditory brainstem response test.

The short-pulsed "click" stimuli most commonly used to evoke an Auditory Brainstem Response (ABR) do not account for temporal shifts along the basilar membrane and do not produce ABRs of maximal amplitude. This paper describes a system that maps these temporal shifts and generates patient-specific stimuli to compensate. This is of interest both to enhance the ABR and as a potential way to map the physiology of the basilar membrane.

Basilar membrane vibration in the basal turn of the sensitive gerbil cochlea.

The basal membrane (BM) velocity responses to pure tones were measured using a newly developed laser interferometer microscope that does not require placing a reflecting object on the BM. It was demonstrated that the instrument is able to measure sub-nanometer vibration from the cochlear partition in the basal turn of the gerbil. The overall shape of the amplitude spectra shows typical tuning features. The 'best' frequencies (BFs) for the BM locations studied were between 14 kHz and 27 kHz, depending on the longitudinal position. For a given BM location, tuning sharpness was input level dependent, indicated by the Q(10dB), which varied from approximately 3 at low stimulus levels to near 1.5 at high input levels. At frequencies below BF, parallel amplitude/frequency curves across stimulus levels indicate a linear growth function. However, at frequencies near BF, the velocity increased linearly at low levels (<40 dB SPL) and became compressed between 40 and 50 dB SPL. Although the velocity gain for the frequency range below BF was a function of frequency, for a given frequency the gains were approximately constant across different levels. At frequencies near BF, the velocity gain at low sound pressure level was greater than that at a high sound pressure level, indicating a nonlinear negative relationship to stimulus level. The data also showed that the BF shifts toward the low frequencies with stimulus intensity increase. The phase spectra showed two important features: (1) at frequencies about half octave below the BF, phase slope is very small, indicating an extremely short delay; (2) the greatest phase lag occurs at frequencies near the BF, indicating a significant delay near this frequency range.

Vibration of reflective beads placed on the basilar membrane.

Most investigators place reflective beads on the basilar membrane to measure its vibration with optical methods. It is therefore important to find out if the beads faithfully follow the motion of the structures on which they are placed. Vibration of the beads on the basilar membrane and basilar membrane adjacent to the beads are measured in the third turn of the guinea pig cochlea in a temporal bone preparation. It is shown that the beads do not follow the motion of the organ. The mechanism by which this departure may occur is investigated by modeling the motion of the beads on the Claudius' cells.

Anatomical correlates of the passive properties underlying the developmental shift in the frequency map of the mammalian cochlea.

As the cochlea develops, the cells in the basal cochlea become sensitive to progressively higher frequencies. To identify features of cochlear morphology that may underlie the place code shift, measurements of infant and adult gerbil cochleas were made at both the light and electron microscopic levels. The measurements included areas of the cochlear duct, basilar membrane, and organ of Corti, height and width of the basilar membrane, thickness of the tympanic cover layer, thickness of the upper and lower basilar membrane fiber bands, and optical density of the basilar membrane. The results indicated that basilar membrane dimensions do not change as the place code shifts and that regions that code for the roughly the same frequency (e.g., approximately 11.2 kHz) at different ages can have basilar membranes of very different dimensions. In contrast, the size of the organ of Corti and the thickness of fiber bands inside the basilar membrane do change in ways consistent with the shift in the frequency map.

Quantitative anatomical basis for a model of micromechanical frequency tuning in the Tokay gecko, Gekko gecko.

The basilar papilla of the Tokay gecko was studied with standard light- and scanning electron microscopy methods. Several parameters thought to be of particular importance for the mechanical response properties of the system were quantitatively measured, separately for the three different hair-cell areas that are typical for this lizard family. In the basal third, papillar structure was very uniform. The apical two-thirds are subdivided into two hair-cell areas running parallel to each other along the papilla and covered by very different types of tectorial material. Both of those areas showed prominent gradients in hair-cell bundle morphology, i.e., in the height of the stereovillar bundles and the number of stereovilli per bundle, as well as in hair cell density and the size of their respective tectorial covering. Based on the direction of the observed anatomical gradients, a 'reverse' tonotopic organization is suggested, with the highest frequencies represented at the apical end.

Tuned hair cells for hearing, but tuned basilar membrane for overload protection: evidence from dolphins, bats, and desert rodents.

A cochlear model is presented suggesting that the organ of Corti (OC) and the basilar membrane (BM) are both tuned resonant systems, but have different functions. The OC provides frequency filtering and amplification by means of tuned outer hair cells. The BM provides resonant absorption of excessive vibrational energy as an overload protection for vulnerable elements in the OC. Evidence supporting this model is demonstrated in dolphins, bats, and desert rodents. Specialized auditory capabilities correlate with cochlear deviations, some of them dramatically changing BM compliance. In characteristic regions along the cochlea there are BM thickenings and, on both sides of the OC, hypertrophied supporting cells. Structures of striking similarity have evolved independently across orders or families, revealing multiple events of convergent evolution. In all cases, the locations of deviating structures rule out a BM function in auditory frequency selectivity but support one in resonant absorption. Cochlear microphonics and BM responses demonstrate strongest high-level absorption in the frequency bands most vital for the tested species. The assumed cause is increased internal damping in the enlarged structures during BM motion. Species with intermediate specializations supply further evidence that resonant absorption is universally the genuine function of BM mechanics in mammals, providing complementary high-level protection of low-level sensitivity.

Measurement of cochlear basilar membrane traveling wave velocity by derived ABR.

Auditory brainstem response (ABR) can be used to measure the basilar membrane traveling wave velocity (TWV). Traveling wave velocity was calculated from the latency difference between wave V of different derived ABR and the cochlear location distance between the appropriate derived band center frequency. The latency of wave V of derived ABR produced by 6 noise-masked ABR using high pass filtered noise and the location of the corresponding cochlear partition (distance from the stapes foot-plate) were measured, and five traveling wave velocities were estimated based on this parameter. Ten subjects with normal hearing, 7 patients with Meniere's disease, and 8 patients with sensorineural hearing loss were used in this study. The traveling wave velocity in the sensorineural hearing loss group was within normal limits at all frequencies, whereas the traveling wave velocity at 8 kHz in the Meniere group greatly exceeded that of the normal and sensorineural hearing loss group.