Biomedical subjects

W T Peake

Publications and source records attributed to W T Peake.

36 records · Page 2Linked to original sources

PubMed1991

Impedance matching, optimum velocity, and ideal middle ears.

One way to assess an ear's performance as a receiver of acoustic power is to consider impedance matching at the tympanic membrane. Assumptions about some of the impedances involved have lead to the idea of an optimum velocity magnitude (per unit pressure), which has been used as a test of middle-ear performance. We show that this approach is not a realistic way to assess effectiveness of power absorption at the tympanic membrane. More generally, we suggest that, if the performance of the combined external-and-middle ear in collecting acoustic power and delivering it to the inner ear is considered, the external- and middle-ear power-transfer efficiencies, as well as impedance matching, are involved in relating performance to an ideal.

Acoustic Impedance Tests↗

PubMed1988

The radiation impedance of the external ear of cat: measurements and applications.

The configuration of external ears varies dramatically among mammalian species. In order to relate these structural differences to acoustic performance, it is useful to determine the "output" (radiation) impedance of the external ear. Measurements were made of the radiation impedance ZE of the cat external ear looking out from the location of the tympanic membrane. Freshly excised external ears were coupled to a calibrated sound source at the tympanic ring, and the resulting sound pressure at the source was measured. The ZE calculated from these measurements is masslike at frequencies below 2 kHz and approximately resistive above 4 kHz. The contributions of anatomically distinct sections of the external ear to ZE were assessed by measuring the impedance before and after surgical removal of the pinna flange and of the concha. Mean measurements of the lengths and cross-sectional areas of components of the external ear are used in a simple model that consists of a uniform tube and an exponential horn; the radiation impedance of the model shows many of the features of the measured ZE's. Measurements of the input impedance of the middle ear are combined with ZE to infer the diffuse-field absorption cross section ADF, which is a measure of the ear's performance as a coupler of acoustic power. It is suggested that ADF is useful for across-species comparisons of the performance of external and middle ears.

Acoustics↗

PubMed1985

A model for signal transmission in an ear having hair cells with free-standing stereocilia. I. Empirical basis for model structure.

No adequate theory for the signal-transmission properties of the peripheral auditory system exists for any vertebrate ear. Because the mammalian ear seems to pose conceptual and technical problems that complicate the development of an adequate theory, it is worthwhile to investigate simpler ears. The ear of the alligator lizard is simpler than mammalian ears in several respects: the motion of the basilar membrane is approximately independent of longitudinal position and is approximately linearly related to the sound pressure at the tympanic membrane; in a large region of the cochlea the hair cells have free-standing stereocilia that are not in contact with a tectorial membrane; the receptor potential of these hair cells is related to the sound pressure at the tympanic membrane in a relatively simple manner; the cochlear-nerve fiber responses from this region do not exhibit two-tone rate suppression. Also, the relative accessibility of this ear has enabled measurement of several response variables: tympanic-membrane volume velocity, extracolumella velocity, basilar-membrane velocity, hair-cell stereociliary displacement, hair-cell receptor potentials, and cochlear-nerve-fiber discharges. A model is developed to represent these results in terms of underlying anatomical structures and physiological mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Air Pressure↗

PubMed1985

A model for signal transmission in an ear having hair cells with free-standing stereocilia. II. Macromechanical stage.

A model of the signal-processing properties of the macromechanical system of the alligator-lizard middle and inner ear is developed. The model is based on measurements (as a function of tone frequency) of ossicular and basilar-membrane motion and of acoustic input-admittance at the tympanic membrane before and after alterations of middle-ear and inner-ear structures. From the structure of the ear, we formulate an equivalent electric network consisting of six admittance blocks and two transformers. The admittance blocks represent the mechanical properties of particular structures, e.g. the tympanic membrane (TM), the ossicle, the TM-ossicular joint, the vestibule, the basilar membrane and the helicotrema. The transformers represent the conversions from acoustical to mechanical variables performed by the TM-ossicular system and by the columella footplate. The block admittances and transformer ratios are inferred from the measurements, and each admittance is approximated by a network with a few simple elements. The resulting model fits the experimental results satisfactorily, allows correlations between specific structures and response behavior, and predicts the macromechanical transfer function. This transfer function acts as the first stage of a model of the alligator-lizard auditory periphery (Weiss et al., Weiss and Leong, 1985, Hearing Res. 20, 131-138, 157-174, 175-195). In the model the frequency dependence of basilar-membrane motion (1) is determined primarily by the tympanic membrane and extracolumella in the middle frequencies and (2) is affected by the helicotrema at low frequencies and the TM-ossicular joint at high frequencies.

Acoustics↗

PubMed1984

Cochlear nonlinearities inferred from two-tone distortion products in the ear canal of the alligator lizard.

Distortion products ( DPs ) evoked by two-tone stimuli at frequencies F1 and F2 were measured in the ear-canal sound pressure of the alligator lizard. The largest sound pressures measured, other than those at F1 and F2, where at the cubic difference frequencies 2F1-F2 and 2F2-F1. All cubic DPs were greatly reduced by destruction of the basilar membrane, which suggests that its nonlinear properties are the source of the DPs . Measurements following acoustic overstimulation show a complex relationship between the magnitude of DPs and cochlear state, as assessed by measurements of cochlear potential, and indicate the existence of multiple nonlinear sources within the inner ear. Relative magnitudes of the DPs and their dependence on stimulus level suggest that the inner-ear DP sources are cubic nonlinearities. The DPs are not highly sensitive to either average stimulus frequency or stimulus frequency separation, suggesting that the nonlinear processes are within the macromechanical processes of the inner ear. Contrary to some interpretations of ear-canal DP measurements in mammals, we conclude that DPs need not be associated with hair-cell processes and are not particularly useful indicators of cochlear health.

Acoustic Stimulation↗

PubMed1984

Acoustic input-admittance of the alligator-lizard ear: nonlinear features.

The acoustic input-admittance at the alligator lizard's tympanic membrane varies with stimulus level; the magnitude of the variation can be as much as a factor of three. At 1.6 kHz, the frequency of maximum admittance magnitude, the admittance varies when the stimulus level exceeds 65 dB SPL. At frequencies above or below 1.6 kHz, larger SPLs are needed to produce admittance changes. With stimulus frequencies below 0.3 kHz or above 4.0 kHz the admittance is virtually constant for stimulus levels up to at least 100 dB SPL. The nonlinear behavior (a) is greatly reduced when the cochlear partition is destroyed, (b) does not return when the mechanical load of the partition is replaced, (c) is decreased by the introduction of proteolytic enzymes into the inner ear, and (d) is not affected by some manipulations that greatly reduce cochlear potentials. The results suggest that the mechanical properties of the cochlear partition are the source of the nonlinear admittance. Parallels between this phenomenon and two-tone distortion products in the ear canal (Rosowski et al. (1984): Hearing Res. 13, 141-158) suggest that the same nonlinear mechanical source that generates the level-dependent admittance also produces two-tone distortion products in the lizard ear canal. Published demonstrations of level-dependent admittance in mammalian ears, although rather different from these results, do not rule out the presence of a similar mechanism in the mammalian cochlea.

Acoustic Impedance Tests↗

PubMed1982

Input impedance of the cochlea in cat.

Tones were delivered directly to the stapes in anesthetized cats after removal of the tympanic membrane, malleus, and incus. Measurements were made of the complex amplitudes of the sound pressure on the stapes PS, stapes velocity VS, and sound pressure in the vestibule PV. From these data, acoustic impedance of the stapes and cochlea ZSC delta equal to PS/US, and of the cochlea alone ZC delta equal PV/US were computed (US delta equal to volume velocity of the stapes = VS X area of the stapes footplate). Some measurements were made on modified preparations in which (1) holes were drilled into the vestibule and scala tympani, (2) the basal end of the basilar membrane was destroyed, (3) cochlear fluid was removed, or (4) static pressure was applied to the stapes. For frequencies between 0.5 and 5 kHz, ZSC approximately equal to ZC; this impedance is primarily resistive ([ZC] approximately equal to 1.2 X 10(6) dyn-s/cm5) and is determined by the basilar membrane and cochlear fluids. For frequencies below 0.3 kHz, [ZSC] greater than [ZC] and ZSC is primarily determined by the stiffness of the annular ligament; drying of the ligament or changes in the static pressure difference across the footplate can produce large changes in [ZSC]. For frequencies below 30 Hz, ZC is apparently controlled by the stiffness of the round-window membrane. All of the results can be represented by an network of eight lumped elements in which some of the elements can be associated with specific anatomical structures. Computations indicate that for the cat the sound pressure at the input to the cochlea at behavioral threshold is constant between 1 and 8 kHz, but increases as frequency is decreased below 1 kHz. Apparently, mechanisms within the chochlea (or more centrally) have an important influence on the frequency dependence of behavioral threshold at low frequencies.

Acoustic Impedance Tests↗

PubMed1980

Basilar-membrane motion in the alligator lizard: its relation to tonotopic organization and frequency selectivity.

In the alligator lizard the entire basilar membrane is accessible for measurements of its velocity by the Mössbauer method. Tests of the method indicate (1) the Mössbauer source can be placed on the basilar membrane without altering the signal-transmission properties of the cochlea, and (2) the source adheres to the basilar membrane. Isovelocity curves (IVCs) were constructed by plotting (as a function of tone frequency) the sound-pressure level at the tympanic membrane required to produce a specified velocity amplitude. IVCs from 21 lizards for source locations spanning the length of the basilar membrane indicate that basilar-membrane velocity does not vary systematically with longitudinal location as it does in mammalian cochleas. Measurements of velocity waveforms in two lizards do not indicate substantial nonlinearity in the inner-ear mechanical system. The frequency dependence of the basilar-membrane velocity is similar to that of the extrastapes velocity over the range 0.4 to 2 kHz. Thus, the tonotopic organization and frequency selectivity, which have been previously demonstrated in this species in responses of both auditory-nerve fibers and cells of the receptor organ, are apparently not primarily determined by basilar-membrane motion.

Animals↗

PubMed1974