High Frequency Amplification Research Articles

Rapid Word-Learning in Normal-Hearing and Hearing-Impaired Children: Effects of Age, Receptive Vocabulary, and High-Frequency Amplification

This study examined rapid word-learning in 5- to 14-year-old children with normal and impaired hearing. The effects of age and receptive vocabulary were examined as well as those of high-frequency amplification. Novel words were low-pass filtered at 4 kHz (typical of current amplification devices) and at 9 kHz. It was hypothesized that (1) the children with normal hearing would learn more words than the children with hearing loss, (2) word-learning would increase with age and receptive vocabulary for both groups, and (3) both groups would benefit from a broader frequency bandwidth. Sixty children with normal hearing and 37 children with moderate sensorineural hearing losses participated in this study. Each child viewed a 4-minute animated slideshow containing 8 nonsense words created using the 24 English consonant phonemes (3 consonants per word). Each word was repeated 3 times. Half of the 8 words were low-pass filtered at 4 kHz and half were filtered at 9 kHz. After viewing the story twice, each child was asked to identify the words from among pictures in the slide show. Before testing, a measure of current receptive vocabulary was obtained using the Peabody Picture Vocabulary Test (PPVT-III). The PPVT-III scores of the hearing-impaired children were consistently poorer than those of the normal-hearing children across the age range tested. A similar pattern of results was observed for word-learning in that the performance of the hearing-impaired children was significantly poorer than that of the normal-hearing children. Further analysis of the PPVT and word-learning scores suggested that although word-learning was reduced in the hearing-impaired children, their performance was consistent with their receptive vocabularies. Additionally, no correlation was found between overall performance and the age of identification, age of amplification, or years of amplification in the children with hearing loss. Results also revealed a small increase in performance for both groups in the extended bandwidth condition but the difference was not significant at the traditional p = 0.05 level. The ability to learn words rapidly appears to be poorer in children with hearing loss over a wide range of ages. These results coincide with the consistently poorer receptive vocabularies for these children. Neither the word-learning or receptive-vocabulary measures were related to the amplification histories of these children. Finally, providing an extended high-frequency bandwidth did not significantly improve rapid word-learning for either group with these stimuli.

Cl- flux through a non-selective, stretch-sensitive conductance influences the outer hair cell motor of the guinea-pig.

Outer hair cells underlie high frequency cochlear amplification in mammals. Fast somatic motility can be driven by voltage-dependent conformational changes in the motor protein, prestin, which resides exclusively within lateral plasma membrane of the cell. Yet, how a voltage-driven motor could contribute to high frequency amplification, despite the low-pass membrane filter of the cell, remains an enigma. The recent identification of prestin's Cl- sensitivity revealed an alternative mechanism in which intracellular Cl- fluctuations near prestin could influence the motor. We report the existence of a stretch-sensitive conductance within the lateral membrane that passes anions and cations and is gated at acoustic rates. The resultant intracellular Cl- oscillations near prestin may drive motor protein transitions, as evidenced by pronounced shifts in prestin's state-probability function along the voltage axis. The sensitivity of prestin's state probability to intracellular Cl- levels betokens a more complicated role for Cl- than a simple extrinsic voltage sensor. Instead, we suggest an allosteric modulation of prestin by Cl- and other anions. Finally, we hypothesize that prestin sensitivity to anion flux through the mechanically activated lateral membrane can provide a driving force that circumvents the membrane's low-pass filter, thus permitting amplification at high acoustic frequencies.

Psychophysical evaluation of cochlear hair cell damage due to the A3243G mitochondrial DNA mutation.

Mitochondrial dysfunction is an important cause of human deafness, implicated in genetic deafness, toxin and noise damage. We assessed the mechanism of cochlear dysfunction in a population of 11 subjects with a specific mitochondrial disorder caused by the A3243G mitochondrial DNA mutation. Psychophysical tests were carried out to assess the inner and outer hair cell functions in vivo. Inner hair cell function was assessed using a measure of hearing threshold in the presence of "threshold-equalizing noise" which can indicate "dead regions" where the transduction mechanism fails. Outer hair cell function was assessed by using the notched-noise method to measure auditory filter width, dependent on active mechanisms in the outer hair cell. The results support the conclusion that this mitochondrial disorder causes both inner and outer hair cell dysfunctions. Evidence of inner hair cell dysfunction was found mainly in basal (high frequency) regions of the cochlea and occurred even in some subjects with only mild hearing loss. Evidence of outer hair cell dysfunction was found in some instances where pure tone threshold was at or close to normal. The common occurrence of dead regions in the basal cochlea has treatment implication for this form of deafness; such people may not be helped by amplification of high frequencies.

A generalized- α method for integrating the filtered Navier–Stokes equations with a stabilized finite element method

A generalized- α method is developed and analyzed for linear, first-order systems. The method is then extended to the filtered Navier–Stokes equations within the context of a stabilized finite element method. The formulation is studied through the application to laminar flow past a circular cylinder and turbulent flow past a long, transverse groove. The method is formulated to obtain a second-order accurate family of time integrators whose high frequency amplification factor is the sole free parameter. Such an approach allows the replication of midpoint rule (zero damping), Gear's method (maximal damping), or anything in between.

AZT induces high frequency, rapid amplification of centromeric DNA.

The reverse transcriptase inhibitor 3'-azido-deoxythymidine (AZT) has previously been shown to be incorporated into specific regions near the telomeres and centromeres of Chinese hamster ovary cell chromosomes. Our investigation of the effects of AZT on chromosome stability has led to the discovery of a high frequency amplification of telomere-like centromeric DNA. The amplified structures, when analyzed cytogenetically, appear as tandem arrays of tightly clustered blocks of centromeric repeats containing telomeric sequences (TTAGGG)n. There were 5-13 blocks of amplified DNA per structure. These structures form rapidly within one or two cell cycles and can be observed with an incidence as high as 2%. Because the amplification was so rapid, we tested whether the amplification structures could be the result of aberrant overreplication by analyzing BrdU incorporation. Our results indicate that the amplified DNA does not undergo abnormal replication during its formation, but appears to form from existing centromeric regions. We propose a model that involves the excision of multiple centromeric DNA regions from other chromosomes and their relocalization to a new site.

The effects of hearing aid frequency responses on speech discrimination score

The effects of the frequency responses of hearing aids on speech discrimination scores were studied in 100 patients with sensori-neural hearing loss. Consonant discrimination scores of /s, z, m/ in frequency responses with high frequency amplification were better than those with low frequency amplification. On the other hand, consonant discrimination scores of /g/ were better in frequency responses with low frequency amplification. The discrimination scores of the other consonants were not affected by the frequency responses. In the fitting of the frequency response of a hearing aid, low frequency amplification is effective for improving sound quality through the hearing aid. However, there is a risk of decreasing the discrimination scores of /s, z, m/.

Presynaptic enhancement of signal transients in photoreceptor terminals in the compound eye

Intracellular recordings show that photoreceptor axons in the first optic ganglion of the blowfly have characteristic electrical properties not seen in the soma. In the axon, a fast depolarizing transient (FDT) occurs on the rising phase of the photoresponse. The FDT is elicited by depolarizing current injections, can follow a sudden relaxation from hyperpolarization, has a distinct voltage threshold, and is accompanied by a rapid transient increase in membrane conductance. Blocking voltage-sensitive potassium conductances in photoreceptors with TEA unmasks spontaneous, long-lasting (ca. 50-150 ms) action potentials. These voltage-sensitive effects and measurements of membrane resistance suggest that the FDT is caused by a voltage-dependent increase in conductance, localized in the photoreceptor axon terminals. The transient amplifies the peak presynaptic response at least twofold, and appears to generate the fast, initial component of the transient postsynaptic response. Thus the FDT contributes to the synaptic amplification and boosting of high frequencies that enhance signals at the visual system's first synapse.

Amplification and deletion of a nod-nif region in the symbiotic plasmid of Rhizobium phaseoli

One remarkable characteristic of the genomes of some Rhizobium species is the frequent occurrence of rearrangements. In some instances these rearrangements alter the symbiotic properties of the strains. However, no detailed molecular mechanisms have been proposed for the generation of these rearrangements. To understand the mechanisms involved in the formation of rearrangements in the genome of Rhizobium phaseoli, we have designed a system which allows the positive selection for amplification and deletion events. We have applied this system to investigate the stability of the symbiotic plasmid of R. phaseoli. High-frequency amplification events were detected which increase the copy number of a 120-kb region carrying nodulation and nitrogen fixation genes two to eight times. Deletion events that affect the same region were also found, albeit at a lower frequency. Both kinds of rearrangements are generated by recombination between reiterated nitrogenase (nifHDK) operons flanking the 120-kb region.

High-Frequency Amplification with ITC-HA and BTE-HAA Comparative Investigation

The purpose of the present investigation was to evaluate the magnitude of selective high-frequency amplification obtained by an in-the-canal hearing aid (ITC-HA), and to compare it with that obtained with a conventional behind-the-ear hearing aid (BTE-HA) with an open mould. 22 subjects (20 males, 2 females) at a median age of 66 years (range 46-84) were included in the investigation. All suffered from a bilateral high-frequency hearing loss with normal or near-normal hearing thresholds at 0.5 and 1 kHz. Insertion gain measurements demonstrated that the ITC-HA gives a significantly higher amplification in the high-frequency area, when compared with the BTE-HA, though neither of the hearing aids amplified as much as indicated by the POGO strategy. Although a better amplification in the high-frequency area was obtained with the ITC-HA, it did not result in significantly better speech recognition when compared with the BTE-HA. At the hearing aid fitting, all subjects preferred the ITC-HA, and after an observation period of one year, 68% of those fitted used the aid continuously. Based on the investigation it is concluded that hearing-impaired persons can be fitted with an ITC-HA, even if they have pronounced high-frequency hearing loss, and that this type of aid is preferred by the hearing-impaired to the conventional BTE-HA.

High frequency gallium arsenide current mirror

A new negative current mirror using n-channel GaAs MESFETs is proposed. The circuit combines the advantages of linearity and good high frequency performance. Applications are as a general high speed circuit building block and for high frequency amplification.

Resonant peak control

A hearing aid having a microphone, an amplifier and a receiver for delivering the amplified sound to a user's ear, in which an adjustable resonant peak control is used to shift the frequency response of the amplifier to a lower frequency range in order to reduce feedback without substantial reduction in high frequency amplification or saturation levels.

High-frequency amplification and generation in charge injection devices

Room-temperature high-frequency measurements on a three-terminal amplifying device based on real-space hot-electron transfer between two conducting layers separated by a potential barrier have been performed. The devices grown by organometallic chemical vapor deposition utilize a novel undoped GaAs/AlGaAs heterostructure which, through its dramatically reduced parasitic leakage compared to previous structures, permits, for the first time, operation as a charge injection transistor. The charge injection transistor exhibits true three-terminal amplification due to real-space hot-electron transfer controlled by electron temperature in a high mobility channel. The device demonstrates power and current gains with cut-off frequencies of 9.8 and 29 GHz, respectively, with maximum current gain of 39 dB. In the negative resistance transistor mode, the same device was found capable of microwave generation up to 7.7 GHz. The frequency response of our device is shown to be limited by RC in the output circuit and methods for improvement of the cut-off frequency are discussed.

High frequency amplification in quantum well oscillators

We have calculated the frequency limit of negative differential resistance and a.c. amplification of quantum well oscillators. From the time development of resonant tunneling, negative resistance is expected to occur at very high frequencies. However, the large device impedance results in a resistive cut-off frequency that is several orders of magnitude below that predicted from transit time considerations. This cut-off frequency is estimated from an equivalent circuit model, and the power of an oscillator as a function of frequency and negative resistance is found from a transmission line model.

Performance of High Frequency Impaired Listeners with Conventional and Extended High Frequency Amplification

This study sought to determine if a high-pass hearing aid can provide increased improvement in word recognition and consonant discrimination over that of a conventional high frequency emphasis hearing aid in listeners with hearing loss limited to frequencies above 1 000 Hz. Word and consonant discrimination were assessed in quiet and in the presence of 12 talker speech babble for 10 subjects under three listening conditions: (1) unaided; (2) wearing a conventional high frequency emphasis hearing aid, and (3) wearing an experimental high-pass instrument. The speech testing materials included: (1) Northwestern University Auditory Test No. 6; (2) California Consonant Test, and (3) eight voiceless English consonants. Results suggested that both instruments provided similar benefit in quiet for improving word recognition and resolving consonant errors. For the noise condition, however, the experimental high-pass aid provided a considerable advantage in both word recognition and consonant identification and was particularly sensitive to reducing within- and between-manner voiceless consonant confusions. Furthermore, measurements of real-ear gain revealed that the high-pass aid afforded considerably greater acoustic gain above 4 000 Hz than that shown for the conventional high frequency emphasis hearing aid,20

Measurements of High Frequency Amplification with Shielded-Grid Pliotrons

Tests of cascade high frequency amplification are described, using specially constructed tubes in which the control grid is electrostatically shielded from the plate. These tubes are free from internal feed-back, so that any number may be operated in series. The voltage amplification per stage depends only on the circuit impedance, and was found to be 200 at 50 kilocycles, 40 at 1000 k.c., and 7 at 10,000 k.c. Total amplifications as high as 2,000,000 in voltage were obtained and measured at 1000 k.c. It is shown that this is the largest amplification that can be usefully employed at any frequency. At 10,000 k.c. a total amplification of 10,000 was obtained. Examples are given of other applications, such as the amplification of small photo-electric currents, where shielding is advantageous.