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PubMed2003

Modulation masking produced by complex tone modulators.

Thresholds were measured for detecting sinusoidal amplitude modulation in the presence of a complex-tone masker modulation. Both modulations were applied to the same sinusoidal carrier. Two different masker modulations were used: (i) a pair of components beating at the difference frequency and (ii) a three-tone complex producing a sinusoidal amplitude modulation of the modulation depth at the difference frequency between adjacent components. Both maskers show a periodicity in the waveform that is not contained in the envelope spectrum itself but can be observed when the envelope of the envelope, referred to as the "venelope" [Ewert et al., J. Acoust. Soc. Am. 112, 2921-2931 (2002)], is calculated. For a signal frequency equal to the masker-venelope periodicity, modulation depth at threshold was measured as a function of the signal phase relative to the phase of the masker-venelope component. Signal frequencies of 5, 30, and 90 Hz were used. It was found that masking was phase dependent for all three signal frequencies. Thresholds were lower for the in-phase condition, where maxima in the signal waveform coincided with maxima in the masker-venelope waveform, than for the antiphase condition. The maximum threshold difference was 15 dB. The results are in contrast to recent data [Moore et al., J. Acoust. Soc. Am. 106, 908-918 (1999)], where lowest thresholds were reported for the antiphase condition in a similar experiment. The present data are in line with the idea that a nonlinearity prior to a modulation filterbank extracts the venelope of the masker modulator. However, a compressive nonlinearity such as that associated with the processing on the basilar membrane cannot account for the empirical findings, since it predicts the opposite phase effect.

Adult↗
PubMed2003

Improved temporal coding of sinusoids in electric stimulation of the auditory nerve using desynchronizing pulse trains.

Rubinstein et al. [Hearing Res. 127, 108-118 (1999)] suggested that the representation of electric stimulus waveforms in the temporal discharge patterns of auditory-nerve fiber (ANF) might be improved by introducing an ongoing, high-rate, desynchronizing pulse train (DPT). To test this hypothesis, activity of ANFs was studied in acutely deafened, anesthetized cats in response to 10-min-long, 5-kpps electric pulse trains that were sinusoidally modulated for 400 ms every second. Two classes of responses to sinusoidal modulations of the DPT were observed. Fibers that only responded transiently to the unmodulated DPT showed hyper synchronization and narrow dynamic ranges to sinusoidal modulators, much as responses to electric sinusoids presented without a DPT. In contrast, fibers that exhibited sustained responses to the DPT were sensitive to modulation depths as low as 0.25% for a modulation frequency of 417 Hz. Over a 20-dB range of modulation depths, responses of these fibers resembled responses to tones in a healthy ear in both discharge rate and synchronization index. This range is much wider than the dynamic range typically found with electrical stimulation without a DPT, and comparable to the dynamic range for acoustic stimulation. These results suggest that a stimulation strategy that uses small signals superimposed upon a large DPT to encode sounds may evoke temporal discharge patterns in some ANFs that resemble responses to sound in a healthy ear.

Animals↗
PubMed2003

Improved neural representation of vowels in electric stimulation using desynchronizing pulse trains.

Current cochlear implant processors poorly represent sound waveforms in the temporal discharge patterns of auditory-nerve fibers (ANFs). A previous study [Litvak et al., J. Acoust. Soc. Am. 114, 2079-2098 (2003)] showed that the temporal representation of sinusoidal stimuli can be improved in a majority of ANFs by encoding the stimuli as small modulations of a sustained, high-rate (5 kpps), desynchronizing pulse train (DPT). Here, these findings are extended to more complex stimuli by recording ANF responses to pulse trains modulated by bandpass filtered vowels. Responses to vowel modulators depended strongly on the discharge pattern evoked by the unmodulated DPT. ANFs that gave sustained responses to the DPT had period histograms that resembled the modulator waveform for low (< 5%) modulation depths. Spectra of period histograms contained peaks near the formant frequencies. In contrast, ANFs that gave a transient (< 1 min) response to the DPT poorly represented the formant frequencies. A model incorporating a linear modulation filter, a noisy threshold, and neural refractoriness predicts the shapes of period histograms for both types of fibers. These results suggest that a DPT-enhanced strategy may achieve good representation of the stimulus fine structure in the temporal discharge patterns of ANFs for frequencies up to 1000 Hz. It remains to be seen whether these temporal discharge patterns can be utilized by cochlear implant subjects.

Animals↗
PubMed2004

Nonoccupational noise: exposures associated with routine activities.

Efforts to characterize nonoccupational noise exposures have focused primarily on infrequent, episodic events. Few studies have assessed noise levels resulting from routine daily activities. In the current study, 112 construction workers wore datalogging noise dosimeters and simultaneously completed activity logs during two phases of data collection. The 81 subjects monitored in phase 1 received logs listing numerous preselected occupational and nonoccupational activities, while the 31 subjects monitored in phase 2 used free-field logs and reported nonoccupational activities in greater detail. Nearly all of the 221,439 1-min intervals of nonoccupational L(eq) level and activity reporting were below 70 dBA; only a small percentage exceeded 80 dBA. The primary contributor to nonoccupational noise exposure was traveling in a car or bus, while time at home contributed the least. One hundred seventy 24-h L(eq) levels were computed from the 1-min noise level data. The percentage of phase 2 workday L(eq(24)) levels which exceeded 80 dBA was higher than that of the nonworkday levels. The mean L(eq(24)) level of phase 2 workdays was higher than that of nonworkdays, and the difference was statistically significant. Routine nonoccupational noise exposures contributed much less to total noise dose than occupational exposures in the subjects evaluated.

Adult↗
PubMed2003

Effects of fundamental frequency and vocal-tract length changes on attention to one of two simultaneous talkers.

Three experiments used the Coordinated Response Measure task to examine the roles that differences in F0 and differences in vocal-tract length have on the ability to attend to one of two simultaneous speech signals. The first experiment asked how increases in the natural F0 difference between two sentences (originally spoken by the same talker) affected listeners' ability to attend to one of the sentences. The second experiment used differences in vocal-tract length, and the third used both F0 and vocal-tract length differences. Differences in F0 greater than 2 semitones produced systematic improvements in performance. Differences in vocal-tract length produced systematic improvements in performance when the ratio of lengths was 1.08 or greater, particularly when the shorter vocal tract belonged to the target talker. Neither of these manipulations produced improvements in performance as great as those produced by a different-sex talker. Systematic changes in both F0 and vocal-tract length that simulated an incremental shift in gender produced substantially larger improvements in performance than did differences in F0 or vocal-tract length alone. In general, shifting one of two utterances spoken by a female voice towards a male voice produces a greater improvement in performance than shifting male towards female. The increase in performance varied with the intonation patterns of individual talkers, being smallest for those talkers who showed most variability in their intonation patterns between different utterances.

Attention↗
PubMed2003

Factors affecting the duration effect in pitch perception for unresolved complex tones.

Previous research has shown that fundamental frequency (F0) discrimination thresholds for complex tones containing unresolved harmonics decrease as the duration of the tone increases [White and Plack, J. Acoust. Soc. Am. 103, 2051-2063 (1998)]. In this paper F0 discrimination was measured as a function of duration for complexes with F0s of 62.5, 125, and 250 Hz, bandpass filtered into two spectral regions (2750-3750 and 5500-7500 Hz). The harmonics were summed either in sine phase (SINE) or with alternating sine-cosine phase (ALT), which affects the envelope of the waveform and the pitch of the complex. Tone duration was 20, 40, 80, and 160 ms. The improvement in F0 discrimination with duration increased with decreasing F0. When harmonics where spectrally filtered between 2750 and 3750 Hz, for complexes with an F0 of 62.5 Hz, F0 discrimination thresholds decreased from approximately 30% for a 20-ms tone to approximately 3% for a 160-ms tone. For complexes with an F0 of 250 Hz, thresholds decreased from 3% for a 20-ms tone to 1% for a 160-ms tone: a lower envelope repetition rate led to a larger change in performance with increasing duration. The phase manipulation also affected the size of the duration effect, in that the effect was less for an ALT complex compared to a SINE complex with the same F0, consistent with the change in envelope repetition rate. Overall, the results suggest that for unresolved complex tones it is primarily envelope repetition rate, not spectral region, that determines both the F0 discrimination threshold and the size of the duration effect.

Adult↗
PubMed2003

High-rate conditioning pulse trains in cochlear implants: dynamic range measures with sinusoidal stimuli.

The addition of a continuous, unmodulated, high-rate pulse train to the electrical signals of cochlear implant recipients results in statistically significant increases in psychophysical dynamic range (41 out of 46 electrode pairs tested). The observed increases in dynamic range are thought to result from nerve conditioning by appropriate levels of high-rate pulse train. Five dynamic range profiles are characterized, defining the different responses of dynamic range observed with increasing levels of the conditioner. Four of the five profiles demonstrate increases in dynamic range, with three showing behavior consistent with stochastic resonance. One profile depicts evidence of adaptation in response to higher levels of the conditioner, with a recovery period lasting throughout the duration (on the scale of tens of minutes) of experimentation. Dynamic range profiles are shown to be similar across sinusoidal frequencies (202, 515, and 1031 Hz) but potentially different across electrode pairs (electrodes 1-2, 7-8, and 15-16). Correlation analysis does not reveal any predictors of optimal conditioner level or amount of dynamic range increase with the conditioner.

Adult↗
PubMed2003

Acoustic variability and distinguishability among mouse ultrasound vocalizations.

Auditory neurobiology has benefited significantly from ethological approaches using acoustic communication signals. Developing an ethological model in a genetically manipulable system such as the mouse would enhance the ability to investigate the processing, learning, and recognition of sounds. Characterizing the basic acoustic structure of mouse vocalizations would help lay a foundation for such a future study. Towards this goal, ultrasound vocalizations emitted by isolated mouse pups and pairs of adult males and females have been digitally recorded and examined. Previous work suggests that these calls may have communicative significance. An analysis of the natural variability in their spectral content, median frequency, duration, and repetition period reveals acoustic structure that could be used for recognizing the calls. Other parameters, like the rate of frequency modulation, may also be informative, but have not been examined. Pup isolation calls develop systematically between postnatal day 5 and 12 towards a more stereotyped vocalization--contracting from a wide range of values into narrower clusters of frequency and duration, and shifting from longer to shorter repetition periods. Most significantly, pup isolation and adult encounter calls fall into two distinct spectral and temporal categories, making it possible for a receiver to acoustically distinguish between them, and to potentially categorically perceive them along those dimensions.

Age Factors↗
PubMed2003

Temporal masking of multidimensional tactual stimuli.

Experiments were performed to examine the temporal masking properties of multidimensional tactual stimulation patterns delivered to the left index finger. The stimuli consisted of fixed-frequency sinusoidal motions in the kinesthetic (2 or 4 Hz), midfrequency (30 Hz), and cutaneous (300 Hz) frequency ranges. Seven stimuli composed of one, two, or three spectral components were constructed at each of two signal durations (125 or 250 ms). Subjects identified target signals under three different masking paradigms: forward masking, backward masking, and sandwiched masking (in which the target is presented between two maskers). Target identification was studied as a function of interstimulus interval (ISI) in the range 0 to 640 ms. For both signal durations, percent-correct scores increased with ISI for each of the three masking paradigms. Scores with forward and backward masking were similar and significantly higher than scores obtained with sandwiched masking. Analyses of error trials revealed that subjects showed a tendency to respond, more often than chance, with the masker, the composite of the masker and target, or the combination of the target and a component of the masker. The current results are compared to those obtained in previous studies of tactual recognition masking with brief cutaneous spatial patterns. The results are also discussed in terms of estimates of information transfer (IT) and IT rate, are compared to previous studies with multidimensional tactual signals, and are related to research on the development of tactual aids for the deaf.

Deafness↗
PubMed2004

Performance of time- and frequency-domain binaural beamformers based on recorded signals from real rooms.

Extraction of a target sound source amidst multiple interfering sound sources is difficult when there are fewer sensors than sources, as is the case for human listeners in the classic cocktail-party situation. This study compares the signal extraction performance of five algorithms using recordings of speech sources made with three different two-microphone arrays in three rooms of varying reverberation time. Test signals, consisting of two to five speech sources, were constructed for each room and array. The signals were processed with each algorithm, and the signal extraction performance was quantified by calculating the signal-to-noise ratio of the output. A frequency-domain minimum-variance distortionless-response beamformer outperformed the time-domain based Frost beamformer and generalized sidelobe canceler for all tests with two or more interfering sound sources, and performed comparably or better than the time-domain algorithms for tests with one interfering sound source. The frequency-domain minimum-variance algorithm offered performance comparable to that of the Peissig-Kollmeier binaural frequency-domain algorithm, but with much less distortion of the target signal. Comparisons were also made to a simple beamformer. In addition, computer simulations illustrate that, when processing speech signals, the chosen implementation of the frequency-domain minimum-variance technique adapts more quickly and accurately than time-domain techniques.

Algorithms↗
PubMed2003

Modulation spectra of natural sounds and ethological theories of auditory processing.

The modulation statistics of natural sound ensembles were analyzed by calculating the probability distributions of the amplitude envelope of the sounds and their time-frequency correlations given by the modulation spectra. These modulation spectra were obtained by calculating the two-dimensional Fourier transform of the autocorrelation matrix of the sound stimulus in its spectrographic representation. Since temporal bandwidth and spectral bandwidth are conjugate variables, it is shown that the joint modulation spectrum of sound occupies a restricted space: sounds cannot have rapid temporal and spectral modulations simultaneously. Within this restricted space, it is shown that natural sounds have a characteristic signature. Natural sounds, in general, are low-passed, showing most of their modulation energy for low temporal and spectral modulations. Animal vocalizations and human speech are further characterized by the fact that most of the spectral modulation power is found only for low temporal modulation. Similarly, the distribution of the amplitude envelopes also exhibits characteristic shapes for natural sounds, reflecting the high probability of epochs with no sound, systematic differences across frequencies, and a relatively uniform distribution for the log of the amplitudes for vocalizations. It is postulated that the auditory system as well as engineering applications may exploit these statistical properties to obtain an efficient representation of behaviorally relevant sounds. To test such a hypothesis we show how to create synthetic sounds with first and second order envelope statistics identical to those found in natural sounds.

Animals↗
PubMed2003

Impedance measurements of ex vivo rat lung at different volumes of inflation.

A previous study [J. Acoust. Soc. Am. 111, 1102-1109 (2002)] showed that the occurrence of ultrasonically induced lung hemorrhage in rats was directly correlated to the level of lung inflation. In that study, it was hypothesized that the lung could be modeled as two components consisting of air and parenchyma (contiguous tissue [pleura and septa]). The speed of sound and lung impedance would then depend on the fractional volume of air in the lung. According to that model, an inflated lung should act like a pressure-release surface for sound incident from tissue onto a tissue-lung boundary. A deflated lung containing less air should allow more acoustic energy into the lung tissue because the impedance was more closely matched to the contiguous tissues. In the study reported herein, a measurement technique was devised to calculate the impedance of seven rat lungs, ex vivo, under deflation (atmospheric pressure) and three volumes of inflation pressure (7-cm H2O, 10-cm H2O, and 15-cm H2O). Lungs were dissected from rats and immediately scanned in a tank of degassed 37 degrees C water. The frequency-dependent acoustic pressure reflection coefficient was measured over a frequency range of 3.5 to 10 MHz. From the reflection coefficient, the frequency-dependent lung impedance was calculated with values ranging from an average of 1 Mrayls in deflated lungs to 0.2 Mrayls for fully inflated lungs. Lung impedance calculations showed that deflated lungs had an impedance closer to water (1.52 Mrayls) than inflated lungs. At all volumes of inflation, the lungs acted as pressure-release surfaces relative to the water. The average of the four lung impedance values (deflated, 7-cm H2O, 10-cm H2O, and 15-cm H2O) at each level of inflation was statistically different (p<0.0001).

Acoustic Impedance Tests↗
PubMed2003

Binaural models and the strength of dichotic pitches.

Modern physiologically based models of the binaural system incorporate internal delay lines in the pathways from left and right peripheries to central processing nuclei. Different binaural models for the formation of dichotic pitch employ these delay lines in different ways. Consequently, the different models make different predictions for the relative strengths of dichotic pitches made with particular phase conditions. The differences are magnified for dichotic pitches at low frequencies where especially long delay lines may be required. Data from four low-frequency pitch strength experiments on pure-tone-like dichotic pitches (two on Huggins pitch and two on binaural coherence edge pitch) are consistent with models of the equalization-cancellation type and not consistent with the central activity pattern model.

Adult↗
PubMed2003

A longitudinal study of distortion product otoacoustic emission ipsilateral suppression and input/output characteristics in human neonates.

Past work has shown that distortion product otoacoustic emission (DPOAE) (2f1-f2) ipsilateral suppression and input/output (I/O) characteristics are not adult-like in prematurely born neonates [Abdala, J. Acoust. Soc. Am. 110, 1465-1476 (2001)]. These age differences are most pronounced at f2 = 6000 Hz and have been interpreted to indicate a subtle immaturity in human cochlear function prior to term birth. It is still not clear, however, whether term-born neonates are completely adult-like in cochlear function. To study this question, DPOAE suppression and I/O functions for f2 = 6000 Hz were measured in a group of prematurely born neonates at weekly intervals over a period of 7-8-weeks, a group of normal-hearing adults, and during a one-time test session in a group of term-born neonates. Results show that there was no significant change in suppression tuning, suppression growth, and various I/O characteristics across test session for premature neonates, but there was an age-group effect; even once prematurely born neonates reached the equivalence of term-like status (38-40-weeks postconceptional weeks), they continued to show narrower suppression tuning than adults, shallower suppression growth for low-frequency side suppressor tones, and an elevated amplitude saturation plateau on the I/O function. Term-born neonates showed DPOAE results that were comparable to those measured from premature neonates and unlike adult findings. These results suggest that a subtle immaturity in cochlear function persists into the postnatal period.

Adult↗
PubMed2003

Influence of hearing sensitivity on mechano-electric transduction.

This study examined the relation between the extent of permanent hearing loss and the change in a third-order polynomial transducer function (PTF) representing mechano-electric transduction (MET). Mongolian gerbils were exposed to noise for 1 to 128 h. A control group received no exposure. The cochlear microphonic (CM) was recorded from a round-window electrode and stapes velocity was recorded with a laser Doppler vibrometer in response to Gaussian noise. A nonlinear systems identification procedure provided the frequency-domain coefficients of the PTF and their associated coherence functions. In the control group, the PTF in the high frequencies was dominated by linear and cubic terms. In noise-exposed animals, the magnitude of these terms decreased with increasing threshold, suggesting a progressive decrease in the receptor currents through basal hair cells. Moreover, the linear coherence increased and the cubic coherence decreased, indicating that MET in the cochlear base became linear. In the low frequencies, noise exposure altered the group delay of the CM, demonstrating a redistribution of hair-cell currents. The low-frequency PTF was characterized by an increase in the contribution in the quadratic term. With increasing threshold, the slope of the PTF decreased and the saturation for positive CM was eliminated.

Action Potentials↗