Edward H. Overstreet

Development of wide-band middle ear transmission in the Mongolian gerbil

UNLABELLED Stapes vibrations were measured in deeply anesthetized adult and neonatal (ages: 14 to 20 days) Mongolian gerbils. In adult gerbils, the velocity magnitude of stapes responses to tones was approximately constant over the entire frequency range of measurements, 1 to 40 kHz. Response phases referred to pressure near the tympanic membrane varied approximately linearly as a function of increasing stimulus frequency, with a slope corresponding to a group delay of 30 micros. In neonatal gerbils, the sensitivity of stapes responses to tones was lower than in adults, especially at mid-frequencies (e.g., by about 15 dB at 10-20 kHz in gerbils aged 14 days). The input impedance of the adult gerbil cochlea, calculated from stapes vibrations and published measurements of pressure in scala vestibuli near the oval window [E. Olson, J. Acoust. Soc. Am. 103, 3445-3463 (1998)], is principally dissipative at frequencies lower than 10 kHz. CONCLUSIONS (a) middle-ear vibrations in adult gerbils do not limit the input to the cochlea up to at least 40 kHz, i.e., within 0.5 oct of the high-frequency cutoff of the behavioral audiogram; and (b) the results in both adult and neonatal gerbils are inconsistent with the hypothesis that mass reactance controls high-frequency ossicular vibrations and support the idea that the middle ear functions as a transmission line.

Basilar Membrane Vibrations Near the Round Window of the Gerbil Cochlea

Using a laser velocimeter, responses to tones were measured at a basilar membrane site located about 1.2 mm from the extreme basal end of the gerbil cochlea. In two exceptional cochleae in which function was only moderately disrupted by surgical preparations, basilar membrane responses had characteristic frequencies (CFs) of 34–37 kHz and exhibited a CF-specific compressive nonlinearity: Sensitivity near the CF decreased systematically and the response peaks shifted toward lower frequencies with increasing stimulus level. Response phases also changed with increases in stimulus level, exhibiting small relative lags and leads at frequencies just lower and higher than CF, respectively. Basilar membrane responses to low-level CF tones exceeded the magnitude of stapes vibrations by 54–56 dB. Response phases led stapes vibrations by about 90° at low stimulus frequencies; at higher frequencies, basilar membrane responses increasingly lagged stapes vibration, accumulating 1.5 periods of phase lag at CF. Postmortem, nonlinearities were abolished and responses peaked at ~0.5 octave below CF, with phases which lagged and led in vivo responses at frequencies lower and higher than CF, respectively. In conclusion, basilar membrane responses near the round window of the gerbil cochlea closely resemble those for other basal cochlear sites in gerbil and other species.

Enhanced methods for determining iso-loudness contours for fitting cochlear implant sound processors

Methods are taught to simplify the cochlear implant fitting process for various cochlear prostheses and stimulation strategies, including high rate stimulation strategies. For instance, patient self-programming is made possible. In addition, auto-fitting is made possible (particularly useful for very young patients and other patients for whom it is challenging to obtain feedback) using iso-neural response contours which can be linearly transposed to arrive at iso-loudness contours. Furthermore, M iso-loudness contours (or iso-neural contours) can be linearly transposed to determine T iso-loudness contours. In addition, wider pulse widths can be used to generate an iso-loudness contour whose shape can be used (via linear transposition) to program high-rate, narrow pulse width stimulation.

Method and system for obtaining stapedial reflexes in cochlear implant users using multiband stimuli

A method and system for fitting a multichannel cochlear implant system to a patient increases the percentage of patients for which stapedial reflexes can be obtained, and increases the accuracy of predicting the “live speech” comfort levels of the patients fitting programs from the stapedial reflex. Electrical stimuli are applied on multiple electrodes at “live speech” pulse rates. The neural excitation patterns elicited from such stimulation more closely resemble that which occurs when the system is subjected to normal speech patterns. By progressively setting threshold levels in bands, e.g., groups of electrodes, either overlapping or non-overlapping, as well as with a final check by globally adjusting the band obtained contour to the stapedial reflex, such values more closely resemble actual “live speech” program levels than those obtained with traditional methods. Further, broader excitation patterns produced by the activation of multiple electrodes increases the probability of obtaining reflex measurements where single electrode stimulation fails due to sparse neural survival.

Passive basilar membrane vibrations in gerbil neonates: mechanical bases of cochlear maturation

Using a laser velocimeter, basilar membrane (BM) responses to tones were measured in neonatal gerbils at a site near the round window of the cochlea. In adult gerbils, ‘active’ BM responses at this site are most sensitive at 34–37 kHz and exhibit a compressive non‐linearity. Postmortem, BM responses in adults become ‘passive’, i.e. linear and insensitive, and the best frequency (BF) shifts downwards by about 0.5 octaves. At 14 and 16 days after birth (DAB), BM responses in neonatal gerbils were passive but otherwise very different from postmortem responses in adult gerbils: BF was more than an octave lower, the steep slopes of the phase vs. frequency curves were shifted downwards in frequency by nearly 1 octave, and the maximum phase lags amounted to only 180 deg relative to stapes. BFs and phase lags increased systematically between 14 and 20 DAB, implying drastic alterations of the passive material properties of cochlear tissues and accounting for a large part of the shift in BF that characterizes maturation of auditory nerve responses during the same period.

High-frequency sensitivity of the mature gerbil cochlea and its development

The thresholds of compound action potentials evoked by tone pips were measured in the cochleae of anesthetized gerbils, both in adults and in neonates aged 14, 16, 18, 20 and 30 days, using round-window electrodes. Stapes vibrations were also measured, using a laser velocimeter, in many of the same ears of adults and neonates aged 14, 16, 18 and 20 days to assess cochlear sensitivity in isolation from middle ear effects and to circumvent problems associated with calibration of acoustic stimuli at high frequencies. Whether referenced to sound pressure level in the ear canal or stapes vibration velocity, thresholds in adults were roughly uniform in the entire range of tested frequencies, 1.25–38.5 kHz. In neonates, thresholds decreased systematically as a function of age, with the largest reductions occurring at the highest frequencies. Thresholds remained slightly immature at all frequencies 30 days after birth. The results for adult gerbils are consistent with the recent finding that basilar-membrane responses to characteristic frequency tones normalized to stapes vibrations are as sensitive at sites near the round window as at more apical sites. The results for neonates confirm that the extreme basal region of the cochlea is the last to approach maturity, with substantial development occurring between 20 and 30 days after birth.

Bionic ear implant

A bionic ear cochlear stimulation system has the capability to stimulate fast enough to induce stochastic neural firing, thereby acting to restore “spontaneous” neural activity. Such neurostimulation involves the use of a high rate pulsitile stimulation signal that is amplitude modulated with sound information. Advantageously, by using such neurostimulation, a fitting system may be utilized that does not normally require T-level threshold measurements. T-level threshold measurements are not required in most instances because the high-rate pulsitile stimulation, even though at levels that would normally be a sub-threshold electrical stimulus, is able to modulate neural firing patterns in a perceptible way.

System and method for fitting a cochlear implant sound processor using alternative signals

Alternative stimuli, i.e., stimuli other than the constant amplitude stimuli used in prior fitting schemes, are used to set the parameters of a cochlear implant system. The use of such alternative stimuli allows the entire fitting process to be completed in a very short time period, and generally eliminates the need for secondary adjustments. In one preferred embodiment, the alternative stimuli comprise white noise that is internally generated within the speech processor.

A NEW AND IMPROVED MIDDLE EAR

The present review argues that the performance of the middle ear of amniotic vertebrates approaches that of an ideal pressure transformer over a wide range of frequencies. Firstly, we note that sensitivity at ultrasonic frequencies comparable to sensitivity at lower frequencies has been demonstrated in the gerbil ear for stapes vibration, pressure in scala vestibuli near the stapes, basilarmembrane responses near the round window and compound action potential thresholds. Secondly, we show that the bandwidth of stapes velocity magnitude is greater than that of behavioral thresholds in gerbil, turtle, pigeon, guinea pig, bat and chinchilla. Thirdly, we question the validity of ossicular vibration data from cadaveric temporal bones and suggest that the middle ears of living humans are not exceptionally narrow-band among those of amniotic vertebrates and actually provide an input to the cochlea with bandwidth commensurate to, or exceeding, that of the audiogram. Fourthly, we show that phase-vs.-frequency curves of pressure in scala vestibuli in gerbil, stapes vibration in gerbil and chinchilla have roughly constant slopes, consistent with the idea that middleear mechanisms involve spatially distributed components arranged as a transmission line. Such transmission line components apparently act to selectively boost high-frequency vibrations at the incudo-stapedial joint (as shown in chinchilla and guinea pig) and/or at the malleus-incus complex (as seen in cat). The same components may convert the complex motions of the tympanic membrane into the simple, piston-like motion of the stapes.

System and method for fitting a hearing prosthesis sound processor using alternative signals

Alternative stimuli, i.e., stimuli other than the constant amplitude stimuli used in prior fitting schemes, are used to set the parameters of a hearing prosthesis, such as a cochlear implant system. The use of such alternative stimuli allows the entire fitting process to be completed in a very short time period, and generally eliminates the need for secondary adjustments. In one preferred embodiment, the alternative stimuli comprise white noise that is internally generated within the speech processor.