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Biomedical subjects

G Zweig

Publications and source records attributed to G Zweig.

At least 19 recordsLinked to original sources
PubMed1992

Middle-ear phenomenology: the view from the three windows.

To provide a common ground for the comparison between theory and experiment, this paper presents a framework for the phenomenological description of middle-ear mechanics. The framework defines those measurements sufficient to characterize the transduction properties of the middle ear and its components. Phenomenological equations are represented in the form of an equivalent electrical circuit that can be used to deduce testable relations among measurable quantities. Two applications are then discussed. First, the classical concept of the middle-ear transformer ratio is generalized to include any effects of eardrum flexion or nonrotational ossicular motion. Middle-ear models predict that the resulting transformer ratios vary considerably with frequency. Second, the conditions under which the topology of existing circuit analogs satisfactorily approximates middle-ear mechanics are given. Most middle-ear models cannot be used to correctly predict the absolute pressures in the cochlea.

Animals↗
PubMed1992

Analyzing reverse middle-ear transmission: noninvasive Gedankenexperiments.

The phenomenological framework outlined in the companion paper [C. A. Shera and G. Zweig, J. Acoust. Soc. Am. 92, 1356-1370 (1992)] characterizes both forward and reverse transmission through the middle ear. This paper illustrates its use in the analysis of noninvasive measurements of middle-ear and cochlear mechanics. A cochlear scattering framework is developed for the analysis of combination-tone and other experiments in which acoustic distortion products are used to drive the middle ear "in reverse." The framework is illustrated with a simple psychophysical Gedankenexperiment analogous to the neurophysiological experiments of P. F. Fahey and J. B. Allen [J. Acoust. Soc. Am. 77, 599-612 (1985)].

Cochlea↗
PubMed1992

An empirical bound on the compressibility of the cochlea.

Effects of a possible inner-ear compressibility on middle-ear transfer functions are explored and a small upper bound on the magnitude of that compressibility established. Consequently. the traditional two-port representation of middle-ear mechanics remains valid to within a few percent. If the compressibility of the cochlea is small but finite, a simple phenomenological model of that compressibility correctly predicts hearing thresholds in the "middleless" ear at low frequencies. Experiments to establish the value of cochlear compressibility and to explore further its possible contributions to residual hearing in patients with missing or disarticulated middle-ear ossicles are suggested.

Animals↗
PubMed1991

A symmetry suppresses the cochlear catastrophe.

When the independent spatial variable is defined appropriately, the empirical finding that the phase of the cochlear input impedance is small [Lynch et al., J. Acoust. Soc. Am. 72, 108-130 (1982)] is shown to imply that the wavelength of the pressure wave in the cochlea changes slowly with position near the stapes. As a result, waves traveling in either direction through the basal turn undergo little reflection, and the transfer of energy between the middle and inner ears remains efficient at low frequencies. The slow variation of the wavelength implies that the series impedance Z and shunt admittance Y of the cochlear transmission line are approximately proportional at low frequencies and thus requires that the width of the basilar membrane and the cross-sectional areas of the cochlear scalae taper in opposite directions. Maintenance of the symmetry between Z and Y is both necessary and sufficient to ensure that the spatial derivative of the wavelength, and hence the phase of the cochlear input impedance, remains small. Although introduced in another context, the model of Zweig ["Finding the impedance of the organ of Corti," J. Acoust. Soc. Am. 89, 1229-1254 (1991)] manifests the symmetry between Z and Y. In other transmission-line models of cochlear mechanics, however, that symmetry is absent, and the spatial derivative of the wavelength diverges at low frequencies--the "cochlear catastrophe." Those models therefore contradict the impedance measurements and predict little transfer of energy between the middle and inner ears.

Acoustic Stimulation↗
PubMed1991

Finding the impedance of the organ of Corti.

Measurements of the nonlinear response of the basilar membrane to a pure tone are shown to have a simple form for moderate membrane velocities: V(x,f;Vu)/Vu approximately [V(x,f)/Vu]v(x,f), f less than or equal to fc(x), where the response V is the velocity of the membrane at measurement position x, Vu is the umbo velocity, f is the frequency of the stimulus, and fc(x) is the local characteristic frequency. The frequency dependence of the functions v(x,f) and V(x,f) is determined from the data, and v(x,f) and ln V(x,f) are shown to be analytic functions in the lower half of the complex frequency plane, with Re [v(x,f)] a monotonically increasing function of f at fixed x. The linear limit of basilar membrane motion is characterized by a transfer function T(x,f) = (V/V1)v/(1-v), estimated by extrapolating V(x,f;Vu)/Vu to a small membrane velocity V1.T(x,f) and ln T(x,f) are shown to be analytic functions in the lower half of the complex frequency plane. The inverse of the amplitude of the transfer function, which has both a deep dip at f approximately fc(x) and a broad shoulder at lower frequencies, bears a striking resemblance to the neural threshold tuning curve. The functional form of T(x,f) is used to deduce the equation governing the motion of a section of the organ of Corti. Each section acts like a negatively damped harmonic oscillator stabilized at time t by a feedback force proportional to the velocity at the previous time t-tau. The time delay tau is proportional to the oscillator period [tau approximately 1.75/fc(x)]. Like a laser, the organ of Corti pumps energy into harmonic traveling waves. Unlike the laser, the direction of energy flow abruptly reverses as the traveling wave approaches the point of maximum membrane velocity [fc(x) approximately f]. All accumulated wave energy is then pumped back into a small section of the organ of Corti where transduction presumably occurs. Outer hair cells are conjectured to be active elements contributing to the negative damping and feedback of the cochlear amplifier.

Animals↗
PubMed1991

Reflection of retrograde waves within the cochlea and at the stapes.

A number of authors [de Boer and Viergever, Hear. Res. 13, 101-112 (1984); de Boer et al., in Peripheral Auditory Mechanisms (Springer-Verlag, Berlin, 1986); Hear. Res. 23, 1-7 (1986); Viergever, in Auditory Frequency Selectivity (Plenum, New York, 1986), pp. 31-38; Kaernbach et al., J. Acoust. Soc. Am. 81, 408-411 (1987)] have argued that backward-traveling waves, in striking contrast to waves traveling forward towards the helicotrema, suffer appreciable reflection as they move through the basal turns of the cochlea. Such reflection, if present, would have important consequences for understanding the nature and strength of otoacoustic emissions. The apparent asymmetry in reflection of cochlear waves is shown, however, to be an artifact of the boundary condition those authors impose at the stapes: conventional cochlear models are found not to generate reflections of waves traveling in either direction even when the wavelength changes rapidly and the WKB approximation breaks down. Although backward-traveling waves are not reflected by the secular variation of the geometrical and mechanical characteristics of the cochlea, they are reflected when they reach the stapes. The magnitude of that boundary reflection is computed for the cat and shown to be a large, rapidly varying function of frequency.

Acoustics↗
PubMed1991

Phenomenological characterization of eardrum transduction.

A phenomenological description of the transduction effected by the eardrum is presented. That description is provided by a transfer matrix, whose elements define those measurements sufficient to characterize eardrum transduction. Causality provides constraints on the matrix elements. In addition, measurements of the matrix elements can determine whether they satisfy constraints imposed by minimum-phase behavior and the principle of reciprocity. Those constraints may be used either to reduce the number of measurements necessary to characterize the eardrum or to check the consistency of measurements that overdetermine the system. Within its region of validity, the transfer matrix of the eardrum provides a common ground for the comparison between theory and experiment. As an example, a simple model for the transduction characteristics of the eardrum, defined completely in terms of measurable quantities, is presented.

Animals↗
PubMed1985

Pesticides.

Explore the source record for details and available documents.

Carbamates↗
PubMed1985

The relationship between dermal pesticide exposure by fruit harvesters and dislodgeable foliar residues.

Dermal pesticide exposure rates, expressed in mg/hr, by strawberry and blueberry harvesters and dislodgeable foliar pesticide residues were determined in 7 separate field experiments during 1981-1983 in California and Oregon. The pesticides which were studied included captan, vinclozolin, carbaryl, and methiocarb. A positive correlation between these two parameters was found and compared with literature values involving different pesticides and tree crops. The ratio between dermal exposure rate and dislodgeable foliar residues, the units of which are area/time, may have a possible use as an empirical factor for a first approximation of dermal exposure rates by fruit harvesters without the involvement of human subjects.

Adolescent↗
PubMed1984

Percutaneous absorption of triadimefon in the adult and young male and female rat.

The percutaneous absorption of 14C-phenoxy ring labeled triadimefon was studied in adult and young male and female Sprague-Dawley rats. Triadimefon was applied (41.1 to 46.4 micrograms/cm2) in 0.2 ml of acetone to areas comprising 3% of the body surface (7.0 to 14.5 cm2). Thirty-six animals were treated at the initiation of each study. Groups of three animals were subsequently killed at 1, 4, 8, 12, 24, 48, 72, 96, 120, 144, 168, and 192 hr after treatment. Skin from the treated area as well as blood, heart, liver, kidneys, remaining carcass, urine, and feces were analyzed for 14C by scintillation counting techniques. Based on 14C counts, triadimefon was lost more rapidly from the skin of young animals (t 1/2, 20 to 25 hr) than from the skin of adult animals (t 1/2, 29 to 53 hr). Recovery studies indicated that adult males, adult females, young males, and young females, respectively, absorbed 53, 82, 57, and 52% of the dose. The rest of the dose based on material balance was presumably lost by evaporation. Approximately 2.5 to 3.9% of the dose penetrated the skin in one hour and was available for absorption. The rate of entry triadimefon into blood was 2 to 2.5 times faster for young than that observed in adult animals. Elimination of it from blood was faster in the case of the young animals. Triadimefon was absorbed through the skins of the adult male, adult female, young male, and young female rats, respectively, at rates of 0.20, 0.50, 0.58, and 0.48 micrograms/hr/cm2 of skin.

Absorption↗
PubMed1984

Percutaneous absorption and dermal dose-cholinesterase response studies with parathion and carbaryl in the rat.

Dermal dose-ChE response (toxicity) and percutaneous absorption of parathion and carbaryl were studied in the rat. Parathion was as toxic to young animals as it was to adults of the same sex and more toxic to adult females than to adult males. Carbaryl was nontoxic at administered dosages. In each percutaneous absorption study, [14C]parathion or [14C]carbaryl was applied to back skin (12.5-13.8 cm2) at 43.5 to 48 micrograms/cm2. Thirty-six rats were treated and three were killed at time intervals between 0.5 and 168 hr. The area of the skin which had been treated, plasma, heart, liver, kidneys, urine, feces, and the remaining carcass were analyzed for 14C. Recovery studies indicated that adult male and female rats, respectively, absorbed 59.2 and 57.0% of the applied parathion, while adult males absorbed 57.7% of the applied carbaryl. Parathion was lost from skin (t1/2, 28.6 hr) more rapidly than carbaryl (t1/2, 40.6 hr). Approximately 1.4 and 5.8%, respectively, of the applied parathion and carbaryl penetrated the skin within 1 hr and was available for absorption. Parathion was absorbed through skin of adult male and female rats, respectively, at rates of 0.33 and 0.49 micrograms hr-1 cm-2. Carbaryl was absorbed by male rats at the rate of 0.18 micrograms hr-1 cm-2. The half-lives for absorption of parathion by blood ranged between 0.38 and 2.1 hr, while elimination half-lives ranged between 28.6 and 39.5 hr. Carbaryl absorption and elimination half-lives were 1.26 and 67 hr, respectively.

Animals↗
PubMed1978

Analytical determination of N-nitroso compounds in pesticides by the United States Environmental Protection Agency--a preliminary study.

Lijinsky, Eisenbrand and others have demonstrated the formation of N-nitroso derivatives of certain secondary amine pesticides under specified experimental conditions. Partly based on these studies and on reports of the industrial discharge of N-nitrosos compounds from several chemical manufacturing facilities the Environmental Protection Agency (EPA) has issued several reports on the nitroso problem, including a report by a special ad hoc committee under the Administrator's Science Advisory Board (1976). Unexpectedly high N-nitrosoalkylamine residues found in several pesticide products (Fine et al., 1976) have prompted EPA to conduct a survey, initially of approximately 70-80 pesticides that potentially were representative of the following three routes of contamination: 1. industrial process formation and carryover; 2. in situ (container) nitrosation and 3. utilization of contaminated amines in the manufacturing process. Environmental formation was not studied in this survey. Duplicate samples were analysed by two different laboratories using GC and HPLC interfaced with Hall, UV and chemiluminescent (TEA) detectors. Results of some analyses of this continuing survey are presented. Based partly on the results of this study, the Agency has decided to conduct risk-benefit analyses on pesticides suspected of containing N-nitroso contaminants, with special consideration given to oncogenic and mutagenic potential. The analytical results of the study are discussed, as is the cooperative effort within the Agency.

Chromatography, Gas↗