Source of level dependent minima in rabbit distortion product otoacoustic emissions.

Sharp level dependent minima (commonly called nulls or notches) in the distortion product otoacoustic emissions (DPOAEs) have been postulated to be due to two different mechanisms. It is shown here that the level dependent nulls in rabbit 2f(1)-f(2) DPOAEs carry the signature of the mixing of a third order nonlinear term with a fifth order nonlinear term. This suggests that the minima are not due to the mixing of signals from two different physical sites of origin, but rather are due to the nature of the nonlinearity itself. Model simulations show that null production is indifferent to several properties of nonlinear input/output functions.

Mechanism for bandpass frequency characteristic in distortion product otoacoustic emission generation.

It is commonly observed that the levels of the 2f1-f2 and the other mf1-nf2 (m = n + 1 = integer) distortion product otoacoustic emissions (DPOAEs) initially increase in level for fixed f2 as fl -->f2, starting at f1

Nonlinear interactions that could explain distortion product interference response areas.

Suppression and/or enhancement of third- and fifth-order distortion products by a third tone that can have a frequency more than an octave above and a level more than 40 dB below the primary tones have recently been measured by Martin et al. [Hear. Res. 136, 105-123 (1999)]. Contours of iso-suppression and iso-enhancement that are plotted as a function of third-tone frequency and level are called interference response areas. After ruling out order aliasing, two possible mechanisms for this effect have been developed, a harmonic mechanism and a catalyst mechanism. The harmonic mechanism produces distortion products by mixing a harmonic of one of the primary tones with the other primary tone. The catalyst mechanism produces distortion products by mixing one or more intermediate distortion products that are produced by the third tone with one or more of the input tones. The harmonic mechanism does not need a third tone and the catalyst mechanism does. Because the basilar membrane frequency response is predicted to affect each of these mechanisms differently, it is concluded that the catalyst mechanism will be dominant in the high-frequency regions of the cochlea and the harmonic mechanism will have significant strength in the low-frequency regions of the cochlea. The mechanisms are dependent on the existence of both even- and odd-order distortion, and significant even- and odd-order distortion have been measured in the experimental animals. Furthermore, the nonlinear part of the cochlear mechanical response must be well into saturation when input tones are 50 or more dB SPL.

Measurement of distortion product phase in the ear canal of the cat.

Amplitudes of odd order distortion products (DPs) that are detected in animal ear canals have been used to probe cochlear health, to search for cochlear amplification, and to measure aspects of cochlear mechanical frequency response. Like the DP amplitude, DP phase is also an important measure of the cochlear mechanical response. Reported here are measurements of DP phase in the ear canal of the cat. The phase data show frequency-dependent time delays. One of these delays is a function of f2, the frequency of the higher-frequency primary. Hence the DP phase phi d is of the form phi d = phi 0 + omega d tau, where omega d is the DP angular frequency and tau is a fixed time delay. Our results show that phi d is independent of input level a2 as long as the ratio a2/a1 < or = 2, where a2 and a1 are the amplitudes of the input tones. As a2/a1 becomes greater than two, the fixed time delays increase for DPs whose frequencies are less than the frequencies of the input tones. When both levels are varied together the delay increases as the levels decrease. There can be phase changes as large as pi associated with deep nulls in the DP magnitude for the two lower-frequency DPs. Features of the nulls may be modeled assuming that there is partial reflection of the DP wave from the DP place. The assumption of energy remitted from the DP place also explains amplitude-ratio-dependent time delays and 2 pi level-dependent bifurcations in phase. The DP phase shows different dependencies for f2 < 1 kHz compared to f2 > 2 kHz.

A second cochlear-frequency map that correlates distortion product and neural tuning measurements.

Acoustic intermodulation distortion products (DPs) are generated by the nonlinear motion of the basilar membrane (BM) in the cochlea, and propagate back to the ear canal where they may be measured. One common method of measuring these distortion products is to hold the higher-primary frequency f2 fixed while varying the lower-primary frequency f1. When doing this, it is well known that the ear canal distortion product is maximum for a particular value of f2/f1, usually between 1.1 and 1.4. In fact all odd order distortion products of the form fd(n) = f1 -n(f2-f1), n = 1,2,3,... are maximum at the same fd(n), independent of order n, but dependent on f2 which determines the place of DP generation. In this paper, it is argued that this maximum must result from filtering by micromechanical resonances within the cochlea. In fact the frequency where the neural tuning curve "tip" meets the "tail" is the same as the frequency where the distortion products are maximum. This suggests that each section of the basilar membrane must consist of two resonant impedances. The first is the usual series basilar membrane resonant impedance that gives rise to the characteristic frequency (CF). The second resonant impedance must be tuned to a frequency that is lower than the CF and must act as a shunt across the inner hair-cells, since it acts to reduce the forward transmission to the neuron, while, at the same time, it maximally couples all the distortion products back into the cochlear fluids, giving them a frequency dependent increase at its resonant frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

Using acoustic distortion products to measure the cochlear amplifier gain on the basilar membrane.

Most models of the cochlea developed during the last decade have explained frequency selectivity and sensitivity of the cochlea at threshold by the use of power amplification of the acoustic wave on the basilar membrane. This power amplification has been referred to as the cochlear amplifier (CA). In this paper, a method to measure the cochlear amplifier gain as a function of position along the basilar membrane is derived from a simple model. Next, experimental evidence is presented that strongly restricts the properties of these proposed cochlear amplifier models. Specifically, it is shown that small signals generated by mechanical nonlinearities in the basilar membrane motion are not amplified during basilar membrane propagation, contrary to what would be expected from the cochlear amplifier hypotheses. This paper describes a method of measuring the cochlear power gain as a function of frequency and position, from the stapes to within 2 mm of the place corresponding to the frequency being measured. Experimental results in the cat indicate that the total gain of the cochlear amplifier, over the range of positions measured, must be less than 10 dB. The simplest interpretation of the experimental results is that there is no cochlear amplifier. The results suggest that the cochlea must achieve its frequency selectivity by some other means.

Nonlinear phenomena as observed in the ear canal and at the auditory nerve.

We report here several measures of nonlinear effects in the mammalian ear made in the external auditory meatus and in single neurons of the auditory nerve. We have measured the 2f1-f2 and the f2-f1 distortion products and we have found that the neural distortion product threshold curve for 2f1-f2 mirrors the low-frequency side of the frequency threshold curve, when the neural distortion product threshold curve of 2f1-f2 is plotted versus log(f2/f1) its slope is about 50 dB/oct and its intercept is 10-20 dB above the frequency threshold at the characteristic frequency CF, substantial 2f1-f2 distortion was seen in all animals studied while the f2-f1 distortion product was only rarely found at substantial levels, and the distortion product pressure observed in the ear canal was at a level equal to that detected at threshold by the neural units under study. We have also made measurements of two-tone rate suppression thresholds using two new and consistent threshold paradigms. We find that for high and intermediate characteristic frequency neural units the suppression threshold is independent of frequency and at a level of about 70 dB SPL, the suppression above CF is much less than below CF, and the tip of the frequency tuning curve can be suppressed by up to 40 dB by a low-frequency suppressor.

Lateral diffusion in phospholipid bilayer membranes and multilamellar liquid crystals.

The lateral-diffusion coefficients (D) of a fluorescent lipid analogue, 3,3'-dioctadecylindotricarbocyanine (diI), have been measured in black lipid membranes (BLMs), in large (20--50-micron diameter) solvent-free bilayer and multilayer membrane vesicles, and in multilamellar liquid crystals of dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine, and egg lecithin. They show that D changes by several orders of magnitude at the liquid-crystal transformations of the solvent-free bilayers and multilayer. In all BLMs, D approximately 10(-7)cm2/s with only weak temperature dependence even near the putative phase-transformation temperatures, Tt. In the corresponding liquid crystals and large vesicles, D approximately 10(-8)cm2/s above Tt, decreasing by about two orders of magnitude to D approximately 10(-10)cm2/s below Tt. The changes of D in bilayer vesicles suggest that the expected liquid-crystal phase transitions from smectic A to a higher ordered state do persist in isolated bilayers. Retained solvent in black lipid membranes formed by both Mueller--Rudin and Montal--Mueller methods appears to enhance lateral diffusion. A simple method of forming small numbers of large solvent-free vesicles is described.

Lateral diffusion in planar lipid bilayers.

Direct measurements by fluorescence correlation spectroscopy of lateral diffusion coefficients of fluorescent lipid analogs in lipid bilaryer membranes indicate self-diffusion coefficients D greater than 10(-7) square centimeters per second for various lipid systems above their reported transition temperatures. Cholesterol in egg lecithin at mole ratio of 1 : 2 reduces D by about twofold, while retained hydrocarbon solvent can increase it by two- to threefold.

Effect of pressure on the apparent specific volume of proteins.

The magnetic densimeter has been employed to measure the densities and apparent specific volumes of certain proteins in aqueous solutions as a function of pressure. The method gave values in satisfactory agreement with those found in the literature for aqueous electrolyte solutions. A change in apparent specific volume of the monomeric proteins, ribonuclease and turnip yellow mosaic virus and its capsid protein, at pressures up to 400 atmospheres at 20 degrees C was not observed within the precision of the measurements. Also, no change in the apparent specific volume of tobacco mosaic virus protein was observed as a function of these pressures whether the protein was predominantly in the polymerized or unpolymerized state. The magnetic densimeter was found to be a convenient instrument for measuring compressibilities of very small samples of solutions.