Low-frequency Sensitivity Research Articles

Cell Sensitivity to Magnetic Fields

Attempts to establish extremely low-frequency (ELF) threshold sensitivity limits in biological systems are presently based on estimates of thermal noise in the cell membrane. The Weaver-Astumian (Science 247:459–462, 1990) threshold (8 × 10−3 V/m) should in principle also apply to electric fields produced by Faraday induction. However, the 60-Hz magnetic field required to induce an electric field of 8 × 10−3 V/m is improbably large and at variance with the experimental facts, implying either that Faraday induction is not the mode of weak ELF magnetic field biointeractions or that such interactions have nothing to do with the cell membrane, which constitutes only 1 % of the cell volume. We explore the possibility that magnetic field interactions are connected to the periodic changes in free calcium concentration associated with the cellular Ca2+ oscillator (CaO). Estimates of the free energy associated with the CaO reveal cyclic voltage changes of the order of 20 mV, suggesting that already existing electr...

Post-retinal processing of background luminance

It is generally thought that mean luminance and low spatial frequency information in a visual image are sharply attenuated at the retina, due to processes of light adaptation and the spatial filtering effects of lateral inhibition. Our results from interocular luminance masking suggest, however, that cortical masking effects play a primary role in the attenuation of low frequency sensitivity. Results also revealed that interocular luminance masking saturates and that semisaturation occurs where left and right eye luminances are equal, implying that the test luminance limits the effectiveness of the mask through interocular gating.

Calculation of playback signals from MFM images using transfer functions

Magnetic force microscopy has proven to be a suitable tool for analysis of high-density magnetic recording materials. Comparison of the MFM image of a written signal with the actual read-back signal of the recording system can give valuable insight in the recording properties of both heads and media. In a first order approach one can calculate a `signal’ by plotting the line integral over the track width along the track direction (Glijer et al., IEEE Trans. Magn. 32 (1996) 3557). The method however does not take into account the spatial frequency dependence of the transfer functions of both the MFM and the readback system. For instance the gap width of the head (limiting the high frequency signals) and the finite length of the MFM tip (limiting the sensitivity for low frequencies) are completely disregarded (Porthun et al., J. Magn. Magn. Mater. 182 (1998) 238). This type of problem involving spatial frequencies can be very elegantly solved in the Fourier space. The response of the MFM is described by the force transfer function (FTF) as introduced by (Porthun et al. (J. Magn. Magn. Mater. 182 (1998) 238) and Hug et al. (J. Appl. Phys. 83 (1998) 5609), which describes the relation between the MFM signal and the sample stray field at the height of the tip. From this stray field an `effective surface charge distribution’ can be calculated, by means of the field transfer function (HTF). The same function HTF can be used to calculate the stray field at the height of the head. From this stray field the playback voltage can be calculated, resulting in the playback transfer function (PTF). In order to do this the Karlquist model had to be extended to three dimensions.

Sensitivity measurements of piezoelectric polymer hydrophones from 0.2–2 MHz using a broadband-pulse technique

It is widely recognized that the sensitivity of hydrophones used to measure medical diagnostic ultrasound fields should be uniform over several octaves above the center frequency (i.e., above the mean of the upper and lower −3 dB frequencies in the transmitted acoustic-pressure spectrum). However, a bandwidth extending to at least ten times below the diagnostic pulse-center frequency is needed for accurate (error ≈5%) measurement of the peak rarefactional pressure. Since at present it is not common for manufacturers of medical-use hydrophones to provide sensitivity information below 1–2 MHz, a study was undertaken to determine these low-frequency sensitivities. The technique uses broadband, plane-wave pressure pulses generated by electrical short-pulse excitation of a thick piezoelectric ceramic disk. The hydrophone response is calculated from measurements of the source transducer and hydrophone-voltage waveforms. The frequency responses of both needle-type and spot-poled membrane polymer hydrophones were measured using this technique. The spot-poled membrane hydrophones had −3-dB bandwidths extending below 0.2 MHz, the lower limit for the calibration technique. The needle-type hydrophones studied, however, all exhibited a response roll-off of greater than 3 dB in the frequency range studied. Therefore, given the above bandwidth criterion as a function of diagnostic pulse-center frequency, the sensitivity to at least 0.2 MHz should be established for diagnostic-use hydrophones, because a uniform response below 1 MHz cannot be assumed.

An Application for Tank System and its Analysis in Frequency Domain of Two-Degree-of-Freedom Optimal Type-1 Servo System

In this paper, we do an experimental study of two-degree-of-freedom optimal type-1 servo system based on the LQ (linear quadratic) method using a tank system. First, the experiments for the state feedback system which takes into account of a trade-off between low frequency sensitivity and guaranteed stability margin by using an integral gain are carried out. Next, we extend to the output feedback case and carry out the experiments to show the effectiveness of the LTR (loop transfer recovery) method or the integral gain. As the results of these experiments and its analysis in frequency domain, the influence of an observation noise which appears in the control input, the effect of disturbance rejection and robust stability for a plant perturbation are clarified.

Speech Audiometry in Noise-exposed Workers: The SRT-PTA Relationship Revisited

The speech recognition threshold (SRT) is believed to be related primarily to the pure-tone average (PTA) and the steepness of the hearing loss. However, there are indications that it may also be influenced by perceptual or cognitive-linguistic factors, or both, such as meaningfulness of the speech stimuli. The purpose of the present study was to ascertain the correspondence between SRT and PTA in noise-exposed workers with various degrees of speech recognition threshold shift in noise. To this end, a total of 807 SRTs and PTAs collected from fluent speakers of Quebec French noise-exposed workers were compared. Measurements of context effects on speech recognition were taken based on a general hypothesis that they should be facilitating phoneme or word restoration in conditions of high stimulus uncertainty as present in SRT assessment, thus acting to confound the SRT PTA relationship. Using principal components analysis, we found a significant effect not only of low-frequency hearing sensitivity but language context effects on SRT. After a correction was introduced to partial out these linguistic context effects, correlations between SRT and PTA increased but they were lower than predicted. In a related treatment analysis, we found a large number of observations (230 out of 807) where SRTs were more sensitive than PTAs by a factor of 8 to 16 dB. This was the case even though correlations between the two measurements were within the range commonly advocated in the field of clinical audiology (0.85-0.95). This was interpreted as a sign of phonological and lexical context effects on the speech recognition task actually used by individual subjects to facilitate speech understanding, to the point perhaps of making it as simple as the detection of pure tones.

Coherent terahertz radiation detection: Direct comparison between free-space electro-optic sampling and antenna detection

We compare the use of free-space electro-optic sampling (FSEOS) with photoconducting antennas to detect terahertz (THz) radiation in the range of 0.1–3 THz. For the same average THz power and low-frequency modulation, signal-to-noise ratio and sensitivity are better with antenna detection at frequencies smaller than 3 THz. When the modulation frequency is increased to more than 1 MHz in FSEOS, both detection schemes have comparable performance. Using a singular-electric-field THz emitter, we demonstrate the feasibility of a THz imaging system using real-time delay scanning in FSEOS and only 20 mW of laser power.

Effects of noise on inferior colliculus evoked potentials and cochlear anatomy in young and aged chinchillas

Like many aging humans, the aging chinchilla tends to lose high-frequency sensitivity at a faster rate than low-frequency sensitivity. This feature, combined with its excellent low-frequency hearing, makes the chinchilla attractive as an animal model for studying the relationship between noise-induced hearing loss (NIHL) and age-related hearing loss (AHL). In the present study, we examined susceptibility to noise in 15 aged (10–15 years old) and 15 young chinchillas. Two levels of noise were used, with the aim of determining whether age-related differences exist in the magnitude and rate of recovery from temporary threshold shifts produced by a moderate-level (95 dB) noise exposure, or in susceptibility to permanent threshold shifts and cochlear damage caused by a high-level (106 dB) noise exposure. Thresholds and response amplitudes at 0.5, 1, 2, 4, 8 and 16 kHz were determined from evoked potentials recorded from the inferior colliculus. Cochlear histology was performed on animals exposed to high-level noise. The results suggest that older animals are equally vulnerable to moderate-level noise, but may be slightly more vulnerable to high-level noise. For moderate-level exposures, there appears to be a simple additive relationship (in dB) between AHL and NIHL. For high-level exposures, the relationship may be more complex.

LISA - an ESA cornerstone mission for a gravitational wave observatory

The European Space Agency has selected LISA, a gravitational wave observatory, as a cornerstone mission in its future science program Horizons 2000. This observatory will complement the development of ground-based gravitational wave detectors currently under construction. A spaceborne detector will enable the observation of low-frequency gravitational waves in a frequency range from to which is totally inaccessible to ground-based experiments. This frequency range is unique in that it is expected to contain signals from massive black holes, galactive binary stars, as well as the most violent events in the Universe. LISA will attain this low-frequency sensitivity by employing laser interferometric distance measurements over a very long baseline of . Three of these baselines form an equilateral triangle with spacecraft at each vertex. The cluster of spacecraft is in an Earth-like orbit around the Sun trailing the Earth by . The spacecraft contain infrared light-emitting Nd:YAG lasers and freely floating test masses made from a special platinum - gold alloy with vanishing magnetic susceptibility. The spacecraft are being kept centred on their test masses by using drag-free technology and field-emission electric propulsion, thus letting the test masses follow purely inertial orbits.

Prototypes of composite aluminum/PZT shear transducers

A composite aluminum/PZT shear transducer has been tested as a sensor during prototype development. The mechanically amplified flexi-distortional system may improve the dhgh figure of merit by transferring the compressional stress on the device to a lateral shear on the electroceramic element. This type of amplified system is mechanically classified as distortional and, therefore, without volume change. By transforming the hydrostatic stress to transverse strain, as in the flexi-extensional devices, transverse forces can be used to develop shear strain and activate the d15 coefficient. Flexi-distortional devices are tested in order to check for their low frequency sensitivity. Initial Berlincourt tests indicate performance of up to 1200 pC/N for the driver configuration.

Can spectral cues contribute to species separation in closely related grasshoppers?

The acridid grasshoppers Chorthippus biguttulus and Ch. mollis, which are closely related and often sympatric species, were compared intra- and interspecifically with regard to the spectra of their calling and courtship songs and of the sound-induced vibrations of the tympanal membrane, as well as the threshold curves of the tympanal nerve. In the low-frequency range but not in the ultrasound region, the maxima of these various curves fall at distinctly different frequencies in the two species. It is shown that the low-frequency sensitivity of the auditory system in both species, especially in females, is well matched to the conspecific song spectra but not to those of the heterospecific songs. Whether these characteristics actually contribute to species discrimination remains to be determined by behavioural tests.

Sound and vibration sensitivity of VIIIth nerve fibers in the grassfrog, Rana temporaria.

We have studied the sound and vibration sensitivity of 164 amphibian papilla fibers in the VIIIth nerve of the grassfrog, Rana temporaria. The VIIIth nerve was exposed using a dorsal approach. The frogs were placed in a natural sitting posture and stimulated by free-field sound. Furthermore, the animals were stimulated with dorso-ventral vibrations, and the sound-induced vertical vibrations in the setup could be canceled by emitting vibrations in antiphase from the vibration exciter. All low-frequency fibers responded to both sound and vibration with sound thresholds from 23 dB SPL and vibration thresholds from 0.02 cm/s2. The sound and vibration sensitivity was compared for each fiber using the offset between the rate-level curves for sound and vibration stimulation as a measure of relative vibration sensitivity. When measured in this way relative vibration sensitivity decreases with frequency from 42 dB at 100 Hz to 25 dB at 400 Hz. Since sound thresholds decrease from 72 dB SPL at 100 Hz to 50 dB SPL at 400 Hz the decrease in relative vibration sensitivity reflects an increase in sound sensitivity with frequency, probably due to enhanced tympanic sensitivity at higher frequencies. In contrast, absolute vibration sensitivity is constant in most of the frequency range studied. Only small effects result from the cancellation of sound-induced vibrations. The reason for this probably is that the maximal induced vibrations in the present setup are 6-10 dB below the fibers' vibration threshold at the threshold for sound. However, these results are only valid for the present physical configuration of the setup and the high vibration-sensitivities of the fibers warrant caution whenever the auditory fibers are stimulated with free-field sound. Thus, the experiments suggest that the low-frequency sound sensitivity is not caused by sound-induced vertical vibrations. Instead, the low-frequency sound sensitivity is either tympanic or mediated through bone conduction or sound-induced pulsations of the lungs.

Derived-Band Auditory Brain-Stem Response Estimates of Traveling Wave Velocity in Humans: II. Subjects With Noise-Induced Hearing Loss and Meniere’s Disease

Estimates of cochlear traveling wave velocity (TWV) were computed from derived-band auditory brain-stem response (ABR) latencies in subjects with noise-induced hearing loss (NIHL) or Meniére's disease (MD). ABR wave V latencies were determined for each of six derived frequency bands (unmasked-8 kHz, 8-4 kHz, 4-2 kHz, 2-1 kHz, 1 kHz-500 Hz, and 500-250 Hz). Representative frequencies were assigned to the derived bands by estimating their energy midpoints, and cochlear positions corresponding to these frequencies were determined using Greenwood's (1961) place-frequency function for humans. An exponential function of the form I = A + BeCd was fitted to each subject's latency-by-distance data using a least-squares algorithm, and a TWV function was generated by taking the reciprocal of the derivative of the latency function with respect to distance [v = 1/(BCeCd)]. Expected values for subjects' TWV functions were compared to normative data from Donaldson and Ruth (1993) at five cochlear loci. NIHL subjects' TWV estimates fell within normal limits at all cochlear loci, and no relation between severity of high-frequency hearing loss and TWV could be discerned. MD subjects with good low-frequency hearing sensitivity generally yielded normal TWV estimates, whereas MD subjects with low-frequency hearing loss yielded either normal or elevated TWVs. MD subjects' data generally support the hypothesis that endolymphatic hydrops results in increased TWV or, alternatively, a basalward shift in the peak of the traveling wave, in cochleas with presumed normal basilar membrane elasticity.

Selective damage to either the "M" or "P" cell pathway in human retinal disease: implications for visual processing.

Studies in primates have shown there to be 2 main visual afferent pathways (magno and parvo) operating in parallel with different functional roles. Disease state may selectively affect one or other of these, as shown by particular sensory losses. In cases of treated Cuban tropical "neuropathy" (a retinopathy also occurs) visual acuity and achromatic contrast sensitivity are preserved although colour vision is severely affected indicating selective preservation of the "magno" pathway. In Melanoma Associated Retinopathy, red-green colour vision is preserved although achromatic contrast sensitivity for low spatial frequencies, and the detection of motion is damaged suggesting preferential damage to the "magno" pathway. Our studies illuminate two controversial aspects of the P and M systems: the role of type II parvocellular cells in colour vision and the role of M cells in detecting stimuli of low spatial and low temporal frequency.

Dyslexia: the link with visual deficits

Some research reports suggest that visual anomalies may have a causative role in dyslexia, and on this basis certain forms of therapy have been proposed. Recently, we have published the initial results of a matched group study which found dyslexia to be associated with binocular instability, reduced amplitude of accommodation, and reduced contrast sensitivity for both low spatial frequencies and uniform field flicker. The binocular instability was best identified by measuring the vergence amplitude: a modified Dunlop test failed to differentiate reliably between the two groups. Here, we report a significant correlation between flicker threshold and binocular instability, thus linking sensory and motor visual correlates of dyslexia. We also present further new analyses on the interaction between optometric variables and the psychometric measurement of coding skills. The results of these analyses disagree with a recent claim that binocular vision anomalies might cause poor performance at coding tasks. Our studies indicate that visual characteristics are not the major aetiological factors in specific reading difficulty.

Dyslexia: the link with visual deficits

Some research reports suggest that visual anomalies may have a causative role in dyslexia, and on this basis certain forms of therapy have been proposed. Recently, we have published the initial results of a matched group study which found dyslexia to be associated with binocular instability, reduced amplitude of accommodation, and reduced contrast sensitivity for both low spatial frequencies and uniform field flicker. The binocular instability was best identified by measuring the vergence amplitude: a modified Dunlop test faired to differentiate reliably between the two groups. Here, we report a significant correlation between flicker threshold and binocular instability, thus linking sensory and motor visual correlates of dyslexia. We also present further new analyses on the interaction between optometric variables and the psychometric measurement of coding skills. The results of these analyses disagree with a recent claim that binocular vision anomalies might cause poor performance at coding tasks. Our studies indicate that visual characteristics are not the major aetiological factors in specific reading difficulty.

Eddy current technique with high temperature SQUID for non-destructive evaluation of non-magnetic metallic structures

For the first time, a laboratory test system for non-destructive evaluation of metals using the eddy current technique in conjunction with a high temperature superconductor (HTS) SQUID gradiometer has been developed and set up in a magnetically unshielded laboratory environment. The first preliminary experiments prove the advantages of the technique with respect to low frequency sensitivity (large penetration depths), thus showing the potential of HTS SQUID eddy current systems.

Broadband hydrophone calibration below 1 MHz

Recent calibration efforts for miniature hydrophones used to measure medical diagnostic ultrasound fields have been devoted to increasing the upper frequency range of calibration (≳10–15 MHz). However, a bandwidth extending to at least 10 times below the diagnostic pulse center frequency is needed for accurate (error ≊5%) measurement of the peak rarefactional pressure and mechanical index, both important quantities. Since at present no commercial hydrophones for medical ultrasound use provide sensitivity information below 1 MHz, a study was undertaken to determine these low frequency sensitivities. The technique uses broadband, plane‐wave pressure pulses generated by electrical shock excitation of a thick piezoceramic disk. The hydrophone response is calculated from measurements of the source transducer and hydrophone voltage waveforms. The frequency responses of both needle and membrane polymer hydrophones were measured using this technique. The membrane hydrophones studied had bandwidths extending below...

Prenatal monocular enucleation induces a selective loss of low-spatial-frequency cortical responses to the remaining eye.

During early development, interactions between the two eyes are critical in the formation of eye-specific domains within the lateral geniculate nucleus and the visual cortex. When monocular enucleation is done early in prenatal life, it induces remarkable anatomical and functional reorganizations of the visual pathways. Behavioral data have shown a loss in sensitivity to low-spatial-frequency gratings in cats. To correlate the behavioral observations with a possible change in the analysis of contrast at the level of primary visual areas we recorded visual evoked potentials at the 17/18 border in two cats enucleated prenatally (gestational age at enucleation, 39-42 days), three neonatal, two control animals, and one animal with a surgical removal of Y-ganglion fibers. Our results show a strong attenuation in the amplitude of response at all contrast values for gratings of low spatial frequency in prenatally enucleated cats, whereas neonatally enucleated and control animals present responses of comparable amplitude. We conclude that the behavioral results reflect the reduced sensitivity for low frequencies of visual cortical neurons. In addition, we define a critical period for the development of the contrast-sensitivity function that seems to be limited to the prenatal gestation period. We suggest that the prenatal interruption of binocular interactions leads to a functional elimination of the Y-ganglion system.

Modulation of synaptic transfer between retinal cones and horizontal cells by spatial contrast.

We studied the influence of steady annular light on the kinetics and sensitivity of horizontal cell (HC) responses to modulation of the intensity of small concentric spots in the turtle retina. As shown by previous investigators, when the intensity of the annulus was equal to the mean spot intensity, spot response kinetics were the same as those for the modulation of spatially uniform light. Turning off the annulus attenuated dramatically high-frequency flicker sensitivity and enhanced somewhat low-frequency sensitivity. This phenomenon reflects a modulation of synaptic transfer between cones and second-order neurons that is mediated by cones, and it will be referred to as cone-mediated surround enhancement (CMSE). Our main results are as follows: (a) The change in test-spot response sensitivity and kinetics upon dimming a steady surrounding annulus is a consequence of the change in spatial contrast rather than change in overall light level. (b) Introduction of moderate contrast between the mean spot intensity and steady surrounding light intensity causes a marked change in spot response kinetics. (c) The dependence of spot response kinetics on surrounding light can be described by a phenomenological model in which the steady state gain and the time constant of one or two single-stage, low-pass filters increase with decreasing annular light intensity (d) The effect of surrounding light on spot responses of a given HC is not determined by change in the steady component of the membrane potential of that cell. (e) Light outside the receptive field of an HC can affect that cell's spot response kinetics. (f) In an expanding annulus experiment, the distance over which steady annular light affects spot response kinetics varies among HCs and can be quite different even between two cells with closely matched receptive field sizes. (g) The degree of CMSE is correlated with HC receptive field size. This correlation suggests that part of the enhancement mechanism is located in the HC. Taken together, our results suggest the involvement of the inner retina in CMSE.