Low-frequency Tuning Research Articles

Lower frequency companions for the Advanced LIGO gravitational wave interferometric detectors: an observational opportunity?

Recent x-ray and optical observations provide evidence for a population of intermediate mass black holes with masses of tens to thousands of solar masses. Dynamical braking in high stellar density regions may ‘catalyze’ the inspiral of heavy mass objects down to the million-year time scale. Black-hole binaries, with the masses implied by the observations, will plunge below 100 Hz.It may be technologically possible to build ground-based low frequency gravitational wave interferometric detectors optimized to detect these events and install them next to Advanced LIGO (AdL), within the existing LIGO facilities. This additional interferometer, operated coherently with AdL and Virgo, would greatly enhance the effectiveness of the existing interferometers by generating a wealth of triggers for potentially frequent but otherwise undetectable heavy mass inspirals. AdL would study, at higher frequency, the triggered, ultra-relativistic phases (merging and ringdown) of these inspirals.Comparisons are made between the expected detection performances of AdL in its proposed wide band tuning, as well as AdL in its best low frequency tuning, with a low frequency gravitational wave interferometric detector that is mechanically and optically optimized for operation at the lowest possible frequency. Finally, the synergies of tandem operation of AdL and the proposed low frequency interferometer have been considered.

Modeling Hair Cell Tuning by Expression Gradients of Potassium Channel β Subunits

The receptor potential of sensory hair cells arises from the gating of mechanosensitive cation channels, but its amplitude and time course also depend on the number and kinetics of voltage-gated ion channels in each cell. Prominent among these are “BK” potassium channels encoded by the slo gene that support electrical tuning in some hair cells. Hair cells tuned to low frequencies have slowly gating BK channels, whereas those of higher-frequency hair cells gate more rapidly. Alternative splicing of the slo gene mRNA that encodes the pore-forming α subunit can alter BK channel kinetics, and gating is dramatically slowed by coexpression with modulatory β subunits. The effect of the β subunit is consistent with low-frequency tuning, and β mRNA is expressed at highest levels in the low frequency apex of the bird’s auditory epithelium. How might an expression gradient of β subunits contribute to hair cell tuning? The present work uses a computational model of hair cell-tuning based on the functional properties of BK channels expressed from hair cell α and β slo cDNA. The model reveals that a limited tonotopic gradient could be achieved simply by altering the fraction of BK channels in each hair cell that are combined with β subunits. However, complete coverage of the tuning spectrum requires kinetic variants in addition to those modeled here.

The growth of hearing loss in neonatal chicks exposed to intense pure tones

One-day-old chicks were exposed to an intense pure tone (0.9 kHz, 120 dB SPD and assigned to one of eight groups based on continuous exposure durations from 1 to 200 h. As each animal was removed from the exposure, it was anesthetized and an electrode was placed in the nucleus magnocellularis. Sound-evoked potentials were used to measure absolute thresholds and frequency selectivity. Thresholds were measured at 10 frequencies between 0.1 and 4.5 kHz while frequency selectivity was assessed by a simultaneous masking tuning-curve procedure at five probe-tone frequencies between 0.3 and 2.5 kHz. Threshold shift was greatest at 1.3 kHz and reached a maximum loss of approximately 57 dB between 48 and 200 h of exposure. The shape of the threshold-shift curve as a function of exposure duration (for frequencies between 0.9 and 2.5 KHz) suggested the presence of an early and late effect. The loss in tuning sharpness was evaluated by comparing the values of Q 10 dB for control and exposed tuning curves and expressing the difference between them as a percent change. Probe-tone tuning curves above 0.9 kHz became less selective as exposure duration increased. A maximum decrease in tuning of about 54 percent was reached by 48 h and this remained constant to 200 h. The low-frequency tuning curves (below 0.9 kHz) did not show any systematic loss in selectivity. The changes in sensitivity and selectivity are discussed in relation to the patterns of cochlear injury that occurred on the basilar papilla as exposure duration lengthened.

Low-frequency detection and discrimination following apical hair cell destruction

This study assessed the effects of apical hair cell destruction on the detection and discrimination of low-frequency stimuli. Monauralized chinchillas were trained using operant conditioning and positive reinforcement to respond to pure-tone stimuli in the absence and the presence of a high-pass noise masker. Following the collection of the baseline absolute thresholds, psychophysical tuning curves (PTCs) also were determined at low and high frequencies. Apical hair cells in the experimental ear of each subject then were destroyed by applying a liquid-nitrogen-cooled miniature cryoprobe to the bony wall of the cochlea. Post-cryosurgery, unmasked and masked absolute thresholds and psychophysical tuning curves were re-evaluated. Following cryosurgery, low-frequency absolute thresholds increased by 30–50 dB. High-pass masking data suggested that receptors that were unaffected by the masking noise were responsible for the remaining low-frequency hearing. Low-frequency tuning, monitored by assessing changes in PTCs, was significantly altered following apical receptor cell loss, with the most effective maskers located several octaves above the test tone frequency. Following these determinations, one control and two experimental subjects then participated in a third experiment assessing low-frequency discrimination acuity. Some discrimination ability was retained after the cryosurgery; however, these post-lesion difference limens increased when a high-pass noise masker was added to the test environment. At the termination of the behavioral experiment, subjects were euthanized and their cochleae dissected to correlate behavioral and histopathological data. The data suggest that receptors located in frequency regions of the cochlea normally responsive to middle and high frequencies may be responsible for detection and discrimination of low-frequency stimuli in apically damaged cochleae. These data are consistent with other reports which indicate that redundant mechanisms are available for the detection of low-frequency stimuli, and they provide new information regarding how low-frequency stimuli are discriminated throughout the cochlea.

Sliding backshorts for planar circuits

The Superconductor-Insulator-Superconductor (SIS) tunnel junction is an extremely sensitive heterodyne detector at millimeter and submillimeter wavelengths. The large inherent capacitance associated with this device results in a substantial impedance mismatch with typical antennas and, therefore, requires a tuning circuit for optimum results. At frequencies where waveguide dimensions are realizable, impedance matching can be accomplished by embedding the detector in a waveguide circuit with adjustable waveguide backshorts. At higher frequencies, where waveguide dimensions become prohibitively small, a planar transmission line embedding circuit provides a reasonable alternative. Typically, such planar circuits offer no post-fabrication adjustability, resulting in demanding materials and design requirements. An adjustable planar embedding circuit based on coplanar transmission lines with movable noncontacting shorting elements has been developed. The shorting elements each consist of a thin metallic plate with an optimized arrangement of rectangular holes, placed along the insulated metallic transmission line to provide a periodic variation of the line impedance. A scadel (1–5 GHz) has shown that a large reflection coefficient, |s11|≥−0.5 dB, can be achieved with these sliding elements. A low frequency tuning circuit incorporating these shorting elements has been tested to demonstrate practical tuning ranges.

Hearing in a primitive ensiferan: the auditory system of Cyphoderris monstrosa (Orthoptera: Haglidae)

1. Insects of the family Haglidae are relicts of primitive Ensifera. Members of this group display acoustic behaviour in the typical ensiferan pattern, with males producing stereotyped acoustic signals by tegminal stridulation. They are closely related to the major groups of acoustic Ensifera, but their auditory system has not been previously studied. I present here a description of the morphology, and physiology of the peripheral auditory system of Cyphoderris monstrosa. 2. The tympana of C. monstrosa are similar to those of Tettigoniidae, but less well-differentiated from normal leg cuticle. The acoustic trachea is well-developed, but does not form a single, continuous tube in the leg proximal to the tympana. The organization of the auditory organ is similar to the Tettigoniidae, with the tibial organ comprising the subgenual organ, intermediate organ, and crista acustica. Projections of the tibial organ sensory cells to the prothoracic ganglion are also typical of acoustic Orthoptera. The meso- and metathoracic legs possess tibial organs similar to that of the prothoracic leg, although lacking tympana and well-developed acoustic tracheae. Central projections of the mesothoracic tibial organ are similar to those of the prothoracic. 3. Auditory whole nerve responses are not tuned to the frequency of the species calling song. Auditory best frequency (BF) is 2 kHz; calling song frequency is 12 kHz. Threshold intensity levels at BF are typical for insect hearing organs (35–45 dB SPL). The response of the auditory organ is not strongly directional at the calling song frequency. Occlusion of the acoustic trachea has no effect on the sensitivity, tuning, or directionality of the auditory system. Occlusion of either of the tympana alone also has no effect on auditory responses, but occlusion of both tympana leads to a marked decrease in sensitivity. Tuning is not affected. The tibial organ of the mesothoracic leg is sensitive to airborne sound at low frequencies (BF = 1 kHz) with thresholds comparable to the ear. 4. The auditory system of C. monstrosa is a pressure receiver, adapted to low frequency sensitivity. This low frequency tuning, in contrast to the calling song frequency, suggests an auditory system adapted to another function in addition to intraspecific communication.

Phase channels and the square-wave illusion

We have utilized a phase-sensitive effect, the square-wave illusion, to investigate the presence and tuning of phase-sensitive mechanisms. Subjects viewed a 1 c/deg triangular-wave grating and reported the presence or absence of the illusion, before and after adaptation to square-wave gratings of various spatial frequencies. Illusion strength declined with adaptation and was a function of spatial frequency. This function may reflect the spatial frequency tuning of a 'phase channel'. We have also measured by adaptation a contrast sensitivity function, maximally sensitive to a frequency of 1 c/deg. When normalized to the same maximal depression from baseline sensitivity, the phase channel and spatial-frequency channel shared the same low frequency tuning, but the phase channel was broader, with extended sensitivity to higher frequencies. These results can be understood if the phase channel is constructed from combinations of phase sensitive spatial-frequency channels.

Independence of spatial and temporal tuning in the NP I cutaneous mechanoreceptor system

Results from a previous study that measured tactile detection thresholds for spatiotemporal sinusoids suggest that the spatial and temporal tuning of the P and NP I cutaneous mechanoreceptor systems are independent. In other words, provided that the spatial configuration of a vibratory stimulus is independent of temporal frequency, changing the spatial configuration should not change the shape of the temporal tuning curve of either system. To test this hypothesis for the NP I system, vibrotactile thresholds were measured as a function of temporal frequency (10, 12, 15, 20, 25, 30, 40, 55, 80, 100, 175, and 250 Hz) for 200-ms stimuli delivered to the thenar eminence via a 0.75-cm2 circular contactor, with and without a rigid surround. At frequencies where the threshold was mediated exclusively by the NP I system, the presence of a rigid surround did not significantly alter the shape of the temporal tuning curve, but did change the overall sensitivity. That is, NP I thresholds were uniformly higher without the surround than with the surround. Similar results were obtained when the experiment was repeated on the forearm. The observed invariance in the shape of the low-frequency tuning curve is consistent with the spatiotemporal independence hypothesis, but is consistent with conclusions from prior studies. The apparent discrepancy will be discussed.

Middle ear and cochlear receptors in the subterranean mole-rat, Spalax ehrenbergi

The eardrum and the auditory ossicles of the middle ear, and the population of cochlear receptors in subterranean mole-rats of Spalax ehrenbergi complex were qualitatively and quantitatively analyzed. The area of the eardrum was 10.2 ( S. D. = 0.9) mm 2, that of the stapedial footplate 0.6 ( S. D. = 0.07) mm 2, the malleus: incus lever ratio was 1.25:1. The malleus is simply built, not attached to the tympanic ring. The incus is large; its long process is almost parallel to the manubrium of the malleus. The short process of the incus is provided with a peculiar bony cap, a structure thus far undescribed in mammalian auditory ossicles. Firmness of mutual connection of the ossicles may vary among different forms of Spalax. The size and form of the stapes are characterized by a large variability as well. The length of the basilar membrane was 12.6 ( S. D. = 0.7) mm. The mean density of inner hair cells amounted to 93.3 that of outer hair cells to 386.4 cells per mm of the basilar membrane length. In general, the density increased from the base towards the apex. On average there were 415 outer, 100 inner hair cells in the organ of Corti; the ratio being constant along the basal 65% of the length and decreasing in the apical part of the organ of Corti. The radial width of the triad of outer hair cells increased and remained constant along the apical half of the Corti organ. Based on available findings of comparative and functional morphology, most of the studied features may be considered correlates of low-frequency tuning of the hearing organ in mole-rats.

Clinical precision of the Rinne test.

The precision of the Rinne test at 128-2048 Hz was studied in 100 ears with confirmed conductive pathologies. The number of negative (diagnostic) Rinnes decreased significantly with fall in frequency. While there were significantly more negative than positive Rinnes at 128 and 256 Hz, there were significantly more positive results at higher frequencies; indicating that the Rinne is not reliably diagnostic above 256 Hz. However, the possibility of vibrotactile responses must be remembered with low frequency tuning forks. Also, conductive lesions manifested as high tone air-bone gaps will not be identified with low frequency Rinne tests. Air-bone gaps of 25 to 40 dB, depending on frequency, are necessary for the Rinne to identify the presence of conductive components in most cases. Gaps of 25-30 dB for 128 Hz; 35-40 dB for 256 Hz; 55-60 dB for 512 Hz; and 45-50 dB for 1024 Hz, are necessary for the Rinne to meet a 75% correct detection criterion. The 2o48 Hz Rinne does not attain even chance detection. The Rinne test cannot be validly employed as a criterion against which other measures can be evaluated.