Low-frequency Bias Research Articles

Enhancement of electrically evoked oto-acoustic emissions associated with low-frequency stimulus bias of the basilar membrane towards scala vestibuli.

Electrically evoked oto-acoustic emissions (EEOAEs) are sounds present in the ear canal when ac current is passed into the cochlea. EEOAEs are attributed to the activation of fast electromotile responses in outer hair cells (OHCs). An interesting property of EEOAEs is the phenomenon of "acoustic enhancement," where the emission amplitude is increased by moderate-level sound [D. C. Mountain and A. E. Hubbard, Hear. Res. 42, 195-202 (1989)]. In this report a form of enhancement is described which occurs with displacements of the basilar membrane toward scala vestibuli, during amplitude modulation of the EEOAE waveform by low-frequency tones. This "SV-bias enhancement" possibly consists of two components: (i) a low-level component induced by sound at levels which produce nonlinear growth of the cochlear microphonic and which may be equivalent to the "acoustic enhancement" described previously, and (ii) a high-level component which occurs at sound levels well above those which cause saturation of the cochlear microphonic. The low-level component could be explained by either an increased access of the extrinsically applied current to a membrane-based source of OHC motility, perhaps coupled with a reduction in negative feedback, or an increase in electromotile output during scala vestibuli displacements, but the origin of the high-level component is obscure.

Halogen based copper RIE—influence of the material characteristics and deposition processes

As a consequence of the well-known difficulties in reactive dry Cu patterning only a small number of processes have been published world-wide. Using the low frequency biasing, as an alternative approach including an intense ion bombardment, the formation of thick sidewall films during etching (as proposed by other groups) is not necessary. As will be shown, the dimensional accuracy of the Cu pattern is strongly influenced by the metal deposition processes (PVD or CVD), barrier materials and interfaces. Also the electrical behaviour of Cu is influenced by material issues as could be proven for the line resistance.

Effects of electron temperature in high-density Cl2 plasma for precise etching processes

Silicon etching characteristics are investigated by using radio-frequency (rf) biased ultrahigh frequency (UHF) and other conventional plasmas (electron cycotron resonance plasma, inductive coupled plasma, surface wave plasma) determined by using a Cl2 etchant. The silicon etching rate and its pattern dependence are significantly improved by decreasing the electron temperature when supplying a 600-kHz rf bias. In particular, use of the UHF plasma allows high-rate and microloading-free silicon trench etching. It is suggested that a larger number of negative ions are generated in the UHF plasma because of the extremely low electron temperature. The low-frequency bias accelerates the negative and positive ions alternately to the substrate surface. As a result, the low-frequency biased UHF plasma reduces the charge accumulation on the substrate.

Charge-free etching process using positive and negative ions in pulse-time modulated electron cyclotron resonance plasma with low-frequency bias

Charge build-up on a substrate is greatly reduced by using a pulse-time modulated electron cyclotron resonance generated plasma of more than 50 μs with 600 kHz radio frequency (RF) substrate bias. Negative ions are generated in the afterglow plasma due to decay in electron temperature. Low-frequency RF bias directly accelerates both negative and positive ions from the plasma onto the substrate and reduces self-bias voltage, thus reducing charge accumulation on the substrate. This method shows promise for charge-free etching processes in the production of ultra large scale integrated circuits.

Effects of the biasing frequency on RIE of Cu in a Cl 2-based discharge

The reactive ion etching of copper films is examined in several Cl 2 containing gas mixtures at different discharge frequencies. Low (< 1 MHz) and high (13.56 MHz) frequency biasing was used to study the influence of the ion bombardment on the etch profile. Using the low excitation frequency anisotropic profiles have been obtained even without the formation of a thick sidewall protecting film in contrast to the high frequency discharge. Despite the enhanced ion bombardment using the low frequency discharge, the etch selectivity with respect to masking and bottom materials, respectively, is high enough for its application in multilevel metallization systems. To reduce any residues in the etch ground the addition of CF 4 to the Cl 2 Ar mixture was found to be advantageous for low Cl 2 partial pressures. Both PVD-Cu and CVD-Cu layers have been patterned with dimensions of about 1 μm.

Analysis of the LOS Gravity Data Set from Cycle 4 of the Magellan Probe around Venus

The new line-of-sight gravity data set from cycle 4 of the Magellan spacecraft around Venus is analyzed, and its usefulness for geodetic and geophysical purposes is assessed. These data contain large low-frequency biases, which are estimated by comparison with synthetic data computed from a harmonic model of the gravity field. With the corrected data set, the noncentral part of the radial gravity field at the surface over Eistla Regio and the west part of Aphrodite Terra, with a 1° × 1° resolution, is calculated.

Osmotically induced pressure difference in the cochlea and its effect on cochlear potentials

The electrophysiological effects observed during scala tympani displacements in low-frequency biasing experiments, an increase of the summating potential (SP) together with a decrease of the compound action potential (CAP), correlate well with the effects found in guinea pigs with evoked endolymphatic hydrops. This contributes to the hypothesis that displacement of the basilar membrane underlies the changes found in endolymphatic hydrops. A major difference between both experimental situations is that in low-frequency biasing the basilar membrane is continuously moving, whereas in hydrops the hypothesized displacement would be static. To evaluate the importance of this difference, experiments were performed which attempted to evoke a static displacement of the basilar membrane by perfusing the perilymphatic spaces with perfusates of various osmolalities. Perfusion with hypotonic perfusate (183 mOsm/kg) increased the SP and decreased the CAP (4 kHz stimulation) whereas perfusion with a hypertonic perfusate (397 mOsm/ kg) decreased both these potentials. The cochlear microphonics were hardly affected. These data demonstrate that both experimental situations (biasing, i.e. dynamic displacement and osmotic pressure, i.e. static displacement) cause similar changes in the SP and the CAP and the data support the hypothesis that basilar membrane displacement towards scala tympani is an important contributing factor to the electrophysiologic changes in endolymphatic hydrops.

Projections of physiologically characterized spherical bushy cell axons from the cochlear nucleus of the cat: Evidence for delay lines to the medial superior olive

Bushy cells in the anteroventral cochlear nucleus (AVCN) receive their principal excitatory input from the auditory nerve and are the primary source of excitatory input to more centrally located brainstem auditory nuclei. Despite this pivotal position in the auditory pathway, details of the basic physiological information being carried by axons of these cells and their projections to more central auditory nuclei have not been fully explored. In an attempt to clarify these details, we have physiologically characterized and anatomically labeled individual axons of the spherical bushy cell (SBC) class of the cat AVCN. The characteristic frequencies (CFs) of our injected SBC population are low, all less than 12 kHz and primarily (83%) less than 3 kHz, while their spontaneous activity is comparatively high (mean of 59 spikes/sec). In response to short tone bursts at CF, low CF (< 1 kHz) SBC units can phase-lock better than auditory nerve fibers. SBCs with CFs above 1 kHz have primary-like responses at all stimulus levels and can show robust phase-locking to an off-CF, 500 Hz tone. When compared with our previously reported population of labeled globular bushy cells (GBC; Smith et al., 1991, J. Comp. Neurol. 304:387-407), some similarities and differences are apparent in both physiological response properties and axonal projection pattern. GBCs show no low frequency bias in CFs, have lower spontaneous rates, and the high CF units exhibit a primary-like-with-notch response at high stimulus levels as a consequence of a very well timed onset component. Low CF, GBC short tone responses are indistinguishable from those of SBCs. Anatomically, the axons of SBCs cross the midline in the dorsal component of the trapezoid body and typically innervate the medial superior olive (MSO) on both sides, the ipsilateral lateral superior olive (LSO), and the contralateral ventral nucleus of the lateral lemniscus (VNLL). The projections to the contralateral, but not the ipsilateral MSO, show a rostral to caudal delay line configuration, similar to the scheme first proposed by Jeffress (1948, J. Comp. Psychol. 41:35-39). The form of this delay line is consistent with the topographic map of interaural time delays reported by Yin and Chan (1990, J. Neurophysiol. 64:465-488). Projections to the ipsilateral LSO often take an indirect route. In contrast, GBC axons travel in the ventral component of the trapezoid body, never innervate the MSO, rarely innervate the ipsilateral LSO, and always innervate the contralateral medial nucleus of the trapezoid body. The terminal specializations of both SBC and GBC axons contain round vesicles.

Auditory word recognition: Extrinsic and intrinsic effects of word frequency.

Two experiments investigated the influence of word frequency in a phoneme identification task. Speech voicing continua were constructed so that one endpoint was a high-frequency word and the other endpoint was a low-frequency word (e.g., best-pest). Experiment 1 demonstrated that ambiguous tokens were labeled such that a high-frequency word was formed (intrinsic frequency effect). Experiment 2 manipulated the frequency composition of the list (extrinsic frequency effect). A high-frequency list bias produced an exaggerated influence of frequency; a low-frequency list bias showed a reverse frequency effect. Reaction time effects were discussed in terms of activation and postaccess decision models of frequency coding. The results support a late use of frequency in auditory word recognition.

Additivity of threshold elevations produced by disruption of outer hair cell function

We present a simple model describing the additivity of hearing loss in the mammalian cochlea produced by disruption of the outer hair cell transduction processes. The validity of this model has been tested experimentally in the guinea-pig by inducing threshold elevations using two simultaneous cochlear manipulations, including acoustic overstimulation, two-tone suppression, low-frequency acoustic biasing of the cochlear partition and electrical stimulation of the medial olivo-cochlear system of efferent fibres. The results of these experiments suggest that the model presented is an adequate description, within the measurement error of our experiments, of the hearing losses produced.

Modulation of hair cell voltage responses to tones by low-frequency biasing of the basilar membrane in the guinea pig cochlea

Inner (IHC) and outer (OHC) hair cell receptor potentials were recorded during stimulation with combinations of high-frequency (HF) tones and a 100 Hz tone burst of constant level (80 dB SPL). For frequencies at and below characteristic frequency (CF), OHC AC receptor potentials were suppressed by the 100 Hz tone at levels of the HF tone below about 70 dB SPL (the initial steep part of the AC/level function) and at levels that were frequency specific for frequencies above CF. Suppression was associated with a phase lead for frequencies at and close to the CF. For frequencies above CF, the OHC AC response was either suppressed or augmented at levels of the HF tone both below and above 70 dB SPL, depending on the frequency. The action of the 100 Hz tone on the AC response/level functions was to change nonmonotonic functions into monotonic functions or vice versa. IHC DC receptor potentials were suppressed maximally at the CF and at levels and frequencies where suppression of the OHC AC response and the appearance of the IHC DC response overlapped. For levels of the HF tone above 70 dB SPL, the amplitude of the responses of both IHCs and OHCs to the 100 Hz tone are suppressed and become more symmetrical through selective attenuation of the depolarizing phase of the IHC response and the hyperpolarizing phase of the OHC response. In IHCs from insensitive preparations, the response to the 100 Hz tone is augmented in the presence of the HF tone, which may indicate a shift in the operating point of transduction. At frequencies about one-half an octave below the CF, the phase of the 100 Hz voltage response of OHCs but not IHCs is inverted for levels of the HF tone above about 90 dB SPL. It is proposed that amplitude and phase changes in the response to the HF tone due to the presence of the 100 Hz tone are the result of changes in OHC feedback processes and in the mode of movement of the interface between OHC stereocilia and tectorial membrane. The interaction between the responses to HF and 100 Hz tones indicates that feedback contributes significantly to the voltage responses of OHCs throughout the frequency response range.

Low-frequency acoustic biasing and its effect on middle and inner ear pathology.

It has been shown that a continuous low-frequency sound can modulate threshold detection in psychoacoustic measurements and that the low-frequency acoustic biasing technique enables one to differentiate between a hydropic and a normal cochlea. The purpose of the present study was to establish differences in CAP, CM, and SP modulation for the normal cochlea, the cochlea with endolymphatic hydrops, and sensorineural and conductive hearing loss in the guinea-pig model. Six guinea pigs were operated on according to the method of Kimura and Schuknecht by obliterating the endolymphatic duct and destroying the endolymphatic sac. In addition six animals were treated with gentamycin (75 mg/kg) for 4 weeks and in six guinea pigs a conductive hearing loss was created by disconnecting the ossicular chain. Each group has a control collective of equal size. The degree of hearing loss was monitored using ECoG recording on the round window. A low-frequency masker of 52 Hz (90–110 dB SPL) was used to modulate CAP, CM, and SP elicited by tone bursts (1–8 kHz, 80–100 dB SPL). Following the measurements all temporal bones were histologically processed. In a normal cochlea, the CAP amplitude was modulated according to the change of phase of the low-frequency masker. In addition a pronounced modulation was registered for the SP amplitude, whereas the CM responses were suppressed. The CAP amplitude modulation of the hydropic cochlea and an ear with a conductive hearing loss was less biased, whereas a cochlea with gentamycine intoxication was found in between. For the SP modulation there was a significant phase shift in the hydropic cochlea compared to other pathologists and the normal group. A valid criteria to differentiate between a hydropic cochlea and other pathologists was the range of CAP modulation. A CAP modulation of less than 50 μV throughout the measurements was pathognomonique for endolymphatic hydrops. Frequency specific changes and the effect of the biasing technique on cochlea micromechanics will be discussed in detail. [Work supported by DFG Grant No. HE-477/8-2.]

Harmonic generation by current modulation of the microwave transmission through granular high- Tc superconducting films

We report on microwave transmission through thin superconducting films of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O in the presence of a low-frequency bias current I 0 = IDC + IAC cos 2 τft in the plane of the film. We found that the alternating current can modulate the microwave transmission with the generation of many even harmonics (2 f, 4 f, 6 f…) for I AC larger or equal to the critical current, I c, and for samples which contain a significant amount of weak links. Additional odd harmonics are observed upon applying a DC current I DC. The presence of weak links was determined from the variation of the I– V curves upon microwave irradiation. The harmonic generation is attributed to the nonlinear response of the microwave complex resistivity of the Josephson medium to the bias current. A model based on a single Josephson junction can account qualitatively for the experimental data.

Cochlear potentials and their modulation by low-frequency sound in early endolymphatic hydrops

Seventeen guinea pigs were unilaterally operated to produce endolymphatic hydrops. After 2 wk (9 animals) or 4 wk (8 animals), extracochlear electrophysiological responses to tone bursts of several frequencies were recorded in both the operated and non-operated ears. In addition, modulation by low-frequency (29 Hz sinusoidal bias) sound of the responses to 8 kHz tonebursts was measured. After the electrophysiological measurements, the animals were killed and examined histologically. Four weeks after the operation, cochlear microphonics in response to a 500 Hz tone burst and to the 29 Hz bias were significantly smaller in the operated ears. The summating potential showed a tendency to be larger in the operated ears. The compound action potential input-output curves for 2 kHz probes showed a small threshold shift accompanied by steep slopes, reminiscent of recruitment. Modulation of summating potentials by the low-frequency bias was smaller on the operated side. In most cochleae an endolymphatic hydrops was observed. Three cochleae showed a collapse of Reissner's membrane.

Suprathreshold stereo-depth matches as a function of contrast and spatial frequency.

Thresholds for stereoscopic-depth perception increase with decreasing spatial frequency below 2.5 cycles deg-1. Despite this variation of stereo threshold, suprathreshold stereoscopic-depth perception is independent of spatial frequency down to 0.5 cycle deg-1. Below this frequency the perceived depth of crossed disparities is less than that stimulated by higher spatial frequencies which subtend the same disparities. We have investigated the effects of contrast fading upon this breakdown of stereo-depth invariance at low spatial frequencies. Suprathreshold stereopsis was investigated with spatially filtered vertical bars (difference of Gaussian luminance distribution, or DOG functions) tuned narrowly over a broad range of spatial frequencies (0.15-9.6 cycles deg-1). Disparity subtended by variable width DOGs whose physical contrast ranged from 10-100% was adjusted to match the perceived depth of a standard suprathreshold disparity (5 min visual angle) subtended by a thin black line. Greater amounts of crossed disparity were required to match broad than narrow DOGs to the apparent depth of the standard black line. The matched disparity was greater at low than at high contrast levels. When perceived contrast of all the DOGs was matched to standard contrasts ranging from 5-72%, disparity for depth matches became similar for narrow and broad DOGs. 200 ms pulsed presentations of DOGs with equal perceived contrast further reduced the disparity of low-contrast broad DOGs needed to match the standard depth. A perceived-depth bias in the uncrossed direction at low spatial frequencies was noted in these experiments. This was most pronounced for low-contrast low-spatial-frequency targets, which actually needed crossed disparities to make a depth match to an uncrossed standard. This bias was investigated further by making depth matches to a zero-disparity standard (ie the apparent fronto-parallel plane). Broad DOGs, which are composed of low spatial frequencies, were perceived behind the fixation plane when they actually subtended zero disparity. The magnitude of this low-frequency depth bias increased as contrast was reduced. The distal depth bias was also perceived monocularly, however, it was always greater when viewed binocularly. This investigation indicates that contrast fading of low-spatial-frequency stimuli changes their perceived depth and enhances a depth bias in the uncrossed direction. The depth bias has both a monocular and a binocular component.

Attenuation Stress Hysteresis Loops and Low‐Frequency Internal Friction Associated with Thermomechanical Breakaway of Dislocations

AbstractMeasurements of ultrasonic attenuation and velocity changes are performed on 5N aluminium during the application of a low frequency bias stress. The characteristics of the attenuation‐stress Δα = f(σ) loops are studied as a function of the repetition of loading, amplitude σm of loading stress, temperature, and pretreatment of the sample (prestrain, ageing time, or temperature). The results show that at least two kinds of dislocation‐point defect breakaway mechanisms are involved in the experiments. Then a model is developed in order to calculate from the attenuation‐stress hysteretic loop a low frequency internal friction decrement δα: peak events versus amplitude (δα = f(σm)) are thus observed, in good accordance with classical models of internal friction associated with breakaway of dislocations; the activation energy for depinning and the space between pinning points are deduced from these peaks and their evolutions are followed as a function of the pretreatment of the sample. In addition, although the internal friction decrements δα and δ, obtained directly from torsion pendulum, have some similar characteristics, a noticeable difference appears in the decrement levels which is especially explained by the characters of the solicitation in bias stress and torsion pendulum experiments, respectively, one sided and alternative; their effects are discussed in terms of dislocation mechanisms.

Modulation at the guinea pig round window of summating potentials and compound action potentials by low-frequency sound.

Low-frequency sound was used to modulate responses to short single-frequency tone bursts at the guinea pig round window. Summating potentials (SP) increase (reach higher positive values) during the negative half-cycle of the low-frequency cochlear microphonic (LFCM) and decrease during the positive half-cycle of the LFCM. The compound action potential (AP) amplitude decreases during the negative half-cycle of the LFCM. The negative half-cycle of the LFCM can be identified with scala tympani displacement. SP modulation depth is defined as the difference between the highest and the lowest SP value found for tone burst stimulation at different phases of the low-frequency sound while the sound levels of the tone burst and the low-frequency bias are kept constant. When normalized with respect to the SP amplitude found without bias, the SP modulation depth is independent of the sound level of the tone burst in the range from 48 to 68 dB SPL. The normalized AP suppression tends to increase with decreasing tone burst sound level. A dynamic nonlinear mechanism which might explain these results is discussed. This mechanism is based on voltage-sensitive changes.

Reducing Ferrite Tuner Power Loss by Bias Field Rotation

It has been suggested that ferrite tuners for rf cavities with the magnetic bias field perpendicular to the rf magnetic field would have greatly reduced rf losses. Recent measurements at Los Alamos National Laboratory appear to confirm this effect. A simple model proposed here allows the calculation of tuning characteristics for a variety of bias schemes. The model shows that the perpendicular bias scheme mentioned above requires very much larger bias levels than does the parallel bias scheme in order to achieve the same tuning range with a particular ferrite tuner. However, further investigation with the model has led to the discovery that the use of perpendicular bias at low frequency and parallel bias at high frequency requires only a modest increase in the bias field. In effect, the ferrite is kept highly magnetized, reducing ferrite losses, and is tuned primarily by rotating the bias field direction with respect to the rf field direction. The resulting reduction in dissipation can significantly reduce the amount of ferrite required per cavity.

Effects of low-frequency biasing on auditory-nerve activity.

Intensity functions of single fibers in the auditory nerve of gerbils were obtained to tone bursts alone and tone bursts superimposed on a low-frequency biasing signal. The bias was a triangular sound-pressure waveform with a period of 100 ms and was usually presented at 90 dB SPL. Cochlear microphonics (CM) recorded in scala media of the basal and second turns of the cochlea were trapezoidal in shape in response to the triangular waveform of sound pressure. Assuming the CM waveform reflects basilar-membrane displacement at very low frequencies, it is concluded that the input impedance of the gerbil cochlea is substantially resistive at frequencies as low as 10 HZ. Intensity functions of single fibers with characteristic frequencies (CFs) below about 12 kHZ exhibited enhanced thresholds to CF tones associated with basilar-membrane displacement toward scala tympani; conversely, thresholds to CF tones were suppressed during displacement toward scala vestibuli. Tone-alone thresholds were usually between the two biased conditions. The bias had little effect on responses of tones placed below or above CF and on the activity of fibers with CFs greater than about 12 kHZ. These physiological results are compatible with corresponding psychophysical measures obtained from human observers.

Expedient method of obtaining interface state properties from MIS conductance measurements

In obtaining accurate values of interface state density and majority carrier capture cross section for SiSiO 2 interfaces from MIS capacitor admittance measurements, it has been necessary to analyze considerable data. The reason for this is a new effect described previously, namely the large dispersion of interface state time constant caused by fluctuations of surface potential. A computer program briefly outlined here has been developed which calculates interface state properties from a minimum of measured input data. The data required are: (i) low frequency capacitance vs. bias to get surface potential, (ii) admittance vs. frequency at fixed bias to get standard deviation of surface potential, and (iii) admittance vs. bias at two frequencies to get interface state density and majority carrier capture cross section vs. energy. A listing of the computer program and a description of how to use it can be obtained on request.