SEARCH · PubMed Health

Results for “frequency”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 235 records · Page 13Linked to original sources

PubMed1992

Responses of broiler chickens to high-frequency and low-frequency fluorescent light.

1. The influence of the flicker frequency on physical activity and energy expenditure of broilers was studied using commercially available high-frequency (HF) and low-frequency (LF) fluorescent lamps in a 23L:1D lighting schedule. 2. Broilers were reared under and adapted to HF. They were alternately subjected to HF and LF during measurement of activity and energy expenditure. 3. In comparison with HF, LF inhibited activity (number and intensity of movements), but did not influence energy expenditure. 4. It was concluded that the 100 Hz flickering of low-frequency light is detected by broilers and has measurable behavioural effects upon them. 5. The results were discussed in relation to current knowledge of human and birds' critical flicker frequency and perception.

Animals↗

PubMed2004

Interaction of excitatory and inhibitory frequency-receptive fields in determining fundamental frequency sensitivity of primary auditory cortex neurons in awake cats.

Harmonic complex tones produce pitch-height perception corresponding to the fundamental frequency (F0). This study investigates how the spectral cue of F0 is processed in neurons of the primary auditory cortex (A1) with sustained-response properties. We found F0-sensitive and -insensitive cells: the former discriminated between harmonics and noise, while the latter did not. F0-sensitive cells preferred F0s corresponding to the best frequency (BF) and 0.5 x BF. The F0-sensitivity to F0=0.5 x BF was preserved for missing F0, but abolished by eliminating both F0 and the second harmonic. The inhibitory subfield of the frequency-receptive field was restricted to the spectral region between the preferred harmonics in F0-sensitive cells, while it was frequency unspecific in F0-insensitive cells. We conclude that (i) A1 is well organized for discrimination between harmonics and noise; (ii) pitch-height is represented along with the tonotopic axis; (iii) all aspects of the sustained neural responses to harmonic and noise stimuli are consequences of spectral filtering; and (iv) although the observed cell behavior explains some psychophysical pitch perception behaviors, such as pitch-chroma (helical pitch perception with frequency elevation), pitch-level tolerance and adaptive behavior, F0-encoding in A1 remains at the incomplete perceptual level (dominance of the third to fifth harmonics for pitch strength is unexplainable by the cell behavior).

Acoustic Stimulation↗

PubMed1985

Frequency and percent inspiratory time for high-frequency jet ventilation.

A variety of frequencies and percent inspiratory times (%TI) may be used for high-frequency jet ventilation (HFJV). Five physiologic criteria were used to evaluate various combinations of frequency and %TI: mean airway pressure (Paw), cardiac output, PaCO2, PaO2, and intrapulmonary shunt (Qsp/Qt). At a constant drive pressure, the effects of frequencies of 100, 200, 300, 450, 600, 750, and 900 cycle/min at %TI values of 20%, 30%, and 40% of the respiratory cycle were evaluated and compared with the effects of controlled mechanical ventilation (CMV) at 8 to 12 breath/min. Only at 200 cycle/min and 20% TI, were Paw, cardiac output, PaCO2, PaO2, and Qsp/Qt all the same as the CMV values. At 100 cycle/min and 20% TI, CO2 elimination increased without significantly affecting Paw, cardiac output, PaO2, or Qsp/Qt. These data suggest that HFJV might compromise one or more physiologic variables at certain combinations of frequency and %TI. Therefore, at a fixed drive pressure, there appears to be a narrow range of HFJV ventilator settings that should be considered.

Animals↗

PubMed2000

Accuracy comparison of absolute optical frequency measurement between harmonic-generation synthesis and a frequency-division femtosecond comb

Using an iodine-stabilized He-Ne laser as a transfer oscillator, we compare absolute measurements of the optical frequency from a traditional frequency synthesis chain based on harmonic generation and from the frequency division technique of an ultrawide bandwidth femtosecond frequency comb. The agreement between these two measurements, both linked to the Cs standard, is 220+/-770 Hz, yielding a measurement accuracy of 1.6x10(-12). We report 473 612 353 604.8+/-1.2 kHz as a preliminary updated value of the absolute frequency of the " f" component for the He-Ne laser international standard at 633 nm.

Journal Article↗

PubMed2001

Stabilization of femtosecond lasers for optical frequency metrology and direct optical to radio frequency synthesis.

We have actively stabilized the comb of frequencies from a mode-locked femtosecond laser using a Fabry-Perot reference cavity. This technique offers the ability to synthesize a comb of highly stable radio frequencies directly from optical transitions. The measured fractional frequency instability of the components of the frequency comb relative to the reference cavity was <5x10(-13) in 0.1 sec. The variation of the optical mode spacing versus frequency of the reference cavity was also directly measured using this technique.

Journal Article↗

PubMed2005

Power dependence of distributed cavity phase-induced frequency biases in atomic fountain frequency standards.

We discuss the implications of using high-power microwave tests in a fountain frequency standard to measure the frequency bias resulting from distributed cavity-phase shifts. We develop a theory which shows that the frequency bias from distributed cavity phase depends on the amplitude of the microwave field within the cavity. The dependence leads to the conclusion that the frequency bias associated with the distributed cavity phase is typically both misestimated and counted twice within the error budget of fountain frequency standards.

Journal Article↗

PubMed1973

Synaptic frequency response: the influence of sinusoidal changes in stimulation frequency on the amplitude of the end-plate potential.

1. The synaptic response to frequency modulated trains of stimuli was investigated at the neuromuscular junction of the frog. The frequency of stimulation was usually varied sinusoidally about a carrier frequency of ca. 25 impulses per sec (Hz).2. The post-synaptic response to sinusoidally modulated trains of nerve stimuli was found to grow almost linearly when the variations in input frequency were increased up to 60% of the carrier frequency.3. The synaptic frequency response (gain and phase) was reminiscent of that of a low-pass filter, best transmitting slow variations in input frequency.4. The synaptic filter characteristics could not be explained by the post-synaptic cable properties, but were attributed to presynaptic facilitation processes.

Animals↗

PubMed2000

Frequency selectivity as a function of level and frequency measured with uniformly exciting notched noise.

Thresholds for detecting sinusoidal signals were measured as a function of the spectral width of a notch in a noise masker. The notch was positioned both symmetrically and asymmetrically around the signal frequency. The noise was designed to create equal excitation per ERB within its passbands (uniformly exciting noise), after allowing for the transfer function of the headphone and the middle ear. For a signal frequency of 250 Hz, the level per ERB ranged from 35 to 80 dB in 15-dB steps. For signal frequencies of 500, 1,000, 2,000, and 4,000 Hz, the level per ERB ranged from 40 to 70 dB per ERB in 15-dB steps. Auditory filter shapes were derived from the data by modeling the auditory filter as the sum of a sharply tuned tip filter and a broader tail filter. The gain of the tip filter was assumed to be a function of level. The shape of the tip filter and the gain and shape of the tail filter were assumed to be level independent. The data for all levels were fitted simultaneously. The data were fitted best when the gain of the tip filter was assumed to be a function of the signal level (as opposed to the masker level per ERB). The filter shapes showed a level dependence that qualitatively resembled the level dependence of filtering on the basilar membrane. The maximum gain of the tip filter tended to increase with increasing center frequency up to 1 kHz, but to remain roughly constant for higher frequencies.

Adult↗

PubMed2003

Optimization of acoustic scattering from dual-frequency driven microbubbles at the difference frequency.

The second harmonic radiation of acoustically driven bubbles is a useful discriminant for their presence in clinical ultrasound applications. It is useful because the scatter from a bubble at a frequency different from the driving can have a contrast-to-tissue ratio better than at the drive frequency. In this work a technique is developed to optimize the scattering from a microbubble at a frequency different from the driving. This is accomplished by adjusting the relative phase and amplitudes of the components of a dual-frequency incident ultrasound wave form. The investigation is focused primarily on the example of dual-mode driving at frequencies of 1 MHz and 3 MHz, with the scattering optimized at 2 MHz. Bubble radii of primary interest are 0.5 to 2 microm and driving amplitudes to 0.5 atm. Bubbles in this size range are sensitive to modulation of driving. It is shown that an optimal forcing scheme can increase the target response eightfold or more. This suggests new applications in imaging and in bubble detection.

Animals↗

PubMed2003

Time-frequency analyses of transient-evoked stimulus-frequency and distortion-product otoacoustic emissions: testing cochlear model predictions.

Time-frequency representations (TFRs) of otoacoustic emissions (OAEs) provide information simultaneously in time and frequency that may be obscured in waveform or spectral analyses. TFRs were applied to transient-evoked stimulus-frequency (SF) and distortion-product (DP) OAEs to test cochlear model predictions. SFOAEs and DPOAEs were elicited in 18 normal-hearing subjects using gated tones and tone pips. Synchronous spontaneous (SS) OAEs were measured to assess their contributions to SFOAEs and DPOAEs. A common form of TFR of measured OAEs was a collection of frequency-specific components often aligned with SSOAE sites, with each component characterized by one or more brief segments or a single long-duration segment. The spectral envelope of evoked OAEs differed from that of the evoking stimulus. Strong emission regions or cochlear "hot spots" were detected, and sometimes accounted for OAE energy observed outside the stimulus bandwidth. Contributions of hot spots and multiple internal reflections to the OAE, and differences between measured and predicted OAE spectra, increased as stimulus level decreased, consistent with level-dependent changes in the estimated cochlear reflectance. Suppression and frequency-pulling effects between components were observed. A recursive formulation was described for the linear coherent reflection emission theory [Zweig and Shera, J. Acoust. Soc. Am. 98, 2018-2047 (1995)] that is well suited for time-domain calculations.

Acoustic Stimulation↗

PubMed1982

The cochlear frequency map for the cat: labeling auditory-nerve fibers of known characteristic frequency.

Iontophoresis of horseradish peroxidase was used to label single auditory nerve fibers after determination of threshold tuning curves and rates of spontaneous discharge. The relation between characteristic frequency (CF) and cochlear longitudinal location is reconstructed from 52 labeled neurons in 16 cochleas. The length of the organ of Corti allotted to an octave of stimulus frequency increases steadily from low to high frequencies. Thus there is not a simple linear-distance-to-log frequency conversion. When comparing cochleas of different total length, the best predictor of CF at a given location is the distance from base or apex expressed as a percentage of the total length. The cochlear frequency map derived from these single-neuron labeling experiments is compared to maps derived by a number of different physiological and psychophysical techniques, and the significance of the similarities and differences is discussed.

Action Potentials↗

PubMed1983

A general equation describing frequency discrimination as a function of frequency and sensation level.

Frequency-discrimination thresholds, for a wide range of stimulus frequencies and stimulus levels, were obtained from three normal-hearing listeners. Linear regression analyses of the present data, and of data from two previous studies, indicate that an SL-1 transformation of stimulus level and a square root F transformation of stimulus frequency yield linear dimensions that allow accurate predictions of frequency-discrimination thresholds from normal-hearing listeners over a wide range of stimulus frequencies (125-8000 Hz) and stimulus levels (5-8 dB SL) with a single prediction equation.

Humans↗

PubMed1985

Discrimination and identification of frequency-lowered speech in listeners with high-frequency hearing impairment.

The effects of frequency lowering on consonant perception were studied in four listeners with high-frequency sensorineural loss. Frequency lowering, accomplished by pitch-invariant nonuniform compression of the short-term spectral envelope, included lowering to bandwidths of 2500 and 1250 Hz. Performance on frequency lowering was compared to that obtained with linear amplification using uniform or high-frequency emphasis. Results of pairwise discrimination tests indicated that performance on lowering to 1250 Hz was inferior to that obtained with linear amplification. After training, performance on consonant identification with lowering to bandwidths of 2500 or 1250 Hz was equivalent or inferior to that obtained with linear amplification, depending on the subject. In most cases, the performance of the impaired subjects on a given lowering condition was inferior to that obtained by normal subjects, except for one impaired subject whose performance on two lowering conditions was similar to normal.

Hearing Loss, Sensorineural↗

PubMed1986

The role of frequency versus informational cues in uncertain frequency detection.

The role of frequency and informational cues in the detection of tones of uncertain frequency was investigated using the probe-signal method [G.Z. Greenberg and W.D. Larkin, J. Acoust. Soc. Am. 44, 1513-1523 (1968)] with auditory and visual test patterns. Patterns consisted of a sequence of events, either tones or visual stimuli, preceding the tone to be detected. Both frequency and informational cues were available in the auditory patterns, whereas only informational cues were available in the visual patterns. Results indicated that observers in the auditory condition displayed trial-by-trial selective attention to one or another frequency band as a function of the cues provided by earlier pattern components. In contrast, listeners in the visual condition displayed simultaneous attention to the two separate frequency bands that could possibly contain the tone, regardless of the information provided by the cues. Results are discussed in terms of single- and multiple-band models of attention.

Adult↗

PubMed1993

Effect of low-frequency "conditioning" on hearing loss from high-frequency exposure.

Recent research has revealed that repeated exposures to a low-frequency noise results in a progressive reduction in threshold shifts (TS). This reduction in TS is not restricted to the exposure frequency, but can be observed at frequencies up to 3 or 4 octaves higher. Such "conditioning" exposures have also been observed to protect the auditory system against hearing loss from exposures to the same noise at higher levels. The aim of this study was to determine if "conditioning" using low-frequency exposures protects the auditory system against hearing loss from a high-frequency exposure. Monaural chinchillas were exposed to a 0.5-kHz octave band noise (OBN) at 95 dB SPL for 6 h a day. The animals were allowed to recover for 5 days, following which they were exposed to a 4-kHz OBN at 100 dB SPL for 48 h. Hearing thresholds determined using evoked potential recordings, indicated significantly greater permanent threshold shifts in this group of animals when compared to a control group exposed only to the 4-kHz OBN. These results were confirmed by histological examination which revealed greater hair cell loss in the experimental group.

Acoustic Stimulation↗

PubMed1994

The effect of nonsimultaneous on-frequency and off-frequency cues on the detection of a tonal signal masked by narrow-band noise.

Listeners' detection thresholds were measured for a 125-ms, 1-kHz tonal signal masked by a similarly gated 50-Hz-wide band of noise. A two-interval, adaptive, forced-choice procedure either with or without temporally surrounding cuing intervals containing 50-Hz-wide bands of noise was employed. When the cues were present, their center frequency was either 1 kHz (on-frequency) or 900 or 700 Hz (off-frequency). In the conditions of principal interest, the envelopes and phase modulations of the bands of noise were "frozen" across the four intervals that defined a trial, but were chosen randomly across trials. Thresholds were lowest with cues centered at 1 kHz and increased substantially when the center frequency of the cues was changed to 900 or 700 Hz. With cues centered at 700 Hz, performance was equivalent to that obtained without cues and with the masking noise "frozen" across the two intervals that defined a trial. A similar pattern of results was obtained with high-frequency stimuli, where sensitivity to fine-structure information is greatly reduced. Roving the level of the stimuli over a 40-dB range generally reduced sensitivity but did not greatly affect the overall pattern of the data. Thresholds obtained in the two-interval task with masking waveforms chosen randomly were compared with thresholds obtained when the masking waveform was "frozen" within, but not across trials. Differences in threshold appeared to be accounted for by the listeners' use of changes in the mean slope of the envelope of the noise produced by adding the tonal signal.

Acoustic Stimulation↗

PubMed1995

The perception of frequency peaks and troughs in wide frequency modulations. III. Complex carriers.

In widely frequency-modulated (FM) sine tones, local frequency maxima are perceived more accurately than local frequency minima [L. Demany and K. I. McAnally, J. Acoust. Soc. Am. 96, 706-715 (1994); L. Demany and S. Clément, J. Acoust. Soc. Am. 97, 2454-2459 (1995)]. The aim of the present work was to determine if a similar perceptual asymmetry exists for nonsinusoidal FM carriers. Within each stimulus, the logarithm of instantaneous frequency followed one cycle of a (2.5- or 5-Hz) cosine function, starting at phase pi in the "peak" condition and phase 0 in the "trough" condition. In each condition, subjects had to detect shifts in the frequency apex occurring at the temporal center of the stimuli. In experiment 1, the FM functions were imposed on complex tones consisting of a series of consecutive harmonics. Some of the stimuli were bandpass filtered in a 1-oct window with fixed edges. The measured thresholds were about four times lower in the peak condition than in the trough condition, which suggests that the asymmetry previously observed for "spectral" pitches also exists for "virtual" pitches. In experiment 2, the FM carriers were Shepard tones. With such carriers, the standard peak and trough stimuli could be made identical at both the apex and the end points. In spite of these local identities, the results were similar to those of experiment 1, which suggests that the perceptual asymmetry is not determined by local differences between the stimuli and is instead a genuine "motion" effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory Perception↗

PubMed1998

Modeling interaural-delay sensitivity to frequency modulation at high frequencies.

Interaural-delay sensitivity to high-frequency (> or = 3 kHz) sinusoidal-frequency-modulated (SFM) tones is examined for rates from 25 to 800 Hz and depths of -12 to 18 dB. Comparison is made to thresholds obtained for sinusoidal-amplitude-modulated (SAM) tones for the same observers and modulation rates. Both SAM and SFM threshold-by-rate functions are U-shaped with optimum sensitivity to SFM tones occurring at higher rates (fm = 200-400 Hz) compared to those for SAM tones (fm = 100-200 Hz). Effects of modulation depth were examined for rates from 50 to 300 Hz. In all cases thresholds improved considerably with increasing modulation depth. It is also shown that a hybrid dichotic signal composed of an SFM tone presented to one ear and an SAM tone to the other, can perceptually fuse and be lateralized, with the contingency that both stimuli have equal modulation rates but not necessarily equal carrier frequencies. Using bandpass noise to restrict off-frequency listening, it was shown that for this stimulus, observers can use information from filters either below or above the carrier frequency. Consistent with FM-to-AM conversion from cochlear bandpass filtering, several important differences between the SAM- and SFM-tone data can be predicted from a nonstationary stochastic model of binaural interaction whose parameters are uniquely determined from the SAM-tone data.

Auditory Perception↗