Low-pass Noise Research Articles

Case reopened: A temporal basis for harmonic pitch templates in the early auditory system?a).

A fundamental assumption of rate-place models of pitch is the existence of harmonic templates in the central nervous system (CNS). Shamma and Klein [(2000). J. Acoust. Soc. Am. 107, 2631-2644] hypothesized that these templates have a temporal basis. Coincidences in the temporal fine-structure of neural spike trains, even in response to nonharmonic, stochastic stimuli, would be sufficient for the development of harmonic templates. The physiological plausibility of this hypothesis is tested. Responses to pure tones, low-pass noise, and broadband noise from auditory nerve fibers and brainstem "high-sync" neurons are studied. Responses to tones simulate the output of fibers with infinitely sharp filters: for these responses, harmonic structure in a coincidence matrix comparing pairs of spike trains is indeed found. However, harmonic template structure is not observed in coincidences across responses to broadband noise, which are obtained from nerve fibers or neurons with enhanced synchronization. Using a computer model based on that of Shamma and Klein, it is shown that harmonic templates only emerge when consecutive processing steps (cochlear filtering, lateral inhibition, and temporal enhancement) are implemented in extreme, physiologically implausible form. It is concluded that current physiological knowledge does not support the hypothesis of Shamma and Klein (2000).

2D-dwell-time analysis with simulations of ion-channel gating using high-performance computing

Single-channel patch-clamp recordings allow observing the action of a single protein complex in real time and hence the deduction of the underlying conformational changes in the ion-channel protein. Commonly, recordings are modeled using hidden Markov chains, connecting open and closed states in the experimental data with protein conformations. The rates between states denote transition probabilities that could be modified by membrane voltage or ligand binding. Modeling algorithms have to deal with limited recording bandwidth and a very noisy background. It was previously shown that the fit of two-dimensional (2D)-dwell-time histograms with simulations is very robust in that regard. Errors introduced by the low-pass filter or noise cancel out to a certain degree when comparing experimental and simulated data. In addition, the topology of models (that is, the chain of open and closed states) could be inferred from 2D-histograms. However, the 2D-fit was never applied to its full potential. A major reason may be the extremely time-consuming and often unreliable fitting process, due to the stochastic variability in the simulations. We have now solved these issues by introducing a message-passing interface (MPI) allowing massive parallel computing on a high-performance computing (HPC) cluster and obtaining ensemble solutions. With ensembles, we have demonstrated how important ranked solutions are for difficult tasks related to a noisy background, fast gating events beyond the corner frequency of the low-pass filter, and topology estimation of the underlying Markov model. Finally, we have shown that, by combining the objective function of the 2D-fit with the deviation of the current amplitude distributions, automatic determination of the current level of the conducting state is possible, even with an apparent current reduction due to low-pass filtering. Making use of an HPC cluster, the power of 2D-dwell-time analysis can be used to its fullest with minor input of the experimenter.

Testing the plausibility of harmonic templates based on temporal fine-structure

The existence of harmonic templates in the central nervous system is a basic tenet of place models of pitch. Shamma & Klein (2000) propose that coincidences in temporal fine-structure across auditory nerve fibers in response to broadband stimuli suffice to set up harmonic templates for all fundamental frequencies in the phase-locking range. However, their computational model uses cochlear filters with unphysiologically steep slopes, calling into question the plausibility of the proposed scheme. We obtained responses from chinchilla auditory nerve fibers and “high-sync” neurons to pure tones, low-pass noise, and broadband noise, including “warped” noise (Heinz, 2006; Cedolin & Delgutte, 2010). Coincidences were computed using shuffled auto- or crosscorrelograms. We find that the maximum number of coincidences in response to tones of different frequencies, simulating the output of fibers with infinite sharp filters, indeed shows the template structure modeled by Shamma and Klein. This was not the case for responses to low-pass or broadband noise. However, if coincidences were only counted at zero delay, higher numbers of coincidences were indeed found for harmonically-related characteristic frequencies, even in response to noise. We conclude that steep-sloped cochlear filtering is not a necessary condition to obtain harmonic templates based on fine-structure.

Document image binarization using difference of concatenated convolutions

Binarization is the most important stage in historical document image processing. Efficient working of character and word recognition algorithms depend on effective segmentation methods. Segmentation algorithms in turn depend on images free of noises and degradations. Most of these historical documents are illegible with degradations like bleeding through degradation, faded ink or faint characters, uneven illumination, contrast variation, etc. For effective processing of these document images, efficient binarization algorithms should be devised. Here a simple modified version of the Convolutional Neural Network (CNN) is proposed for historical document binarization. AOD-Net architecture for generating dehazed images from hazed images is modified to create the proposed network.The new CNN model is created by incorporating Difference of Concatenation layer (DOC), Enhancement layer (EN) and Thresholding layer into AOD-Net to make it suitable for binarization of highly degraded document images. The DOC layer and EN layer work effectively in solving degradation that exists in the form of low pass noises. The complexity of working of the proposed model is reduced by decreasing the number of layers and by introducing filters in convolution layers that work with low inter-pixel dependency. This modified version of CNN works effectively with a variety of highly degraded documents when tested with the benchmark historical datasets. The main highlight of the proposed network is that it works efficiently in a generalized manner for any type of document images without further parameter tuning. Another important highlight of this method is that it can handle most of the degradation categories present in document images. In this work, the performance of the proposed model is compared with Otsu, Sauvola, and three recent Deep Learning-based models.

Temporal Correlates to Monaural Edge Pitch in the Distribution of Interspike Interval Statistics in the Auditory Nerve.

Pitch is a perceptual attribute enabling perception of melody. There is no consensus regarding the fundamental nature of pitch and its underlying neural code. A stimulus which has received much interest in psychophysical and computational studies is noise with a sharp spectral edge. High-pass (HP) or low-pass (LP) noise gives rise to a pitch near the edge frequency (monaural edge pitch; MEP). The simplicity of this stimulus, combined with its spectral and autocorrelation properties, make it an interesting stimulus to examine spectral versus temporal cues that could underly its pitch. We recorded responses of single auditory nerve (AN) fibers in chinchilla to MEP-stimuli varying in edge frequency. Temporal cues were examined with shuffled autocorrelogram (SAC) analysis. Correspondence between the population’s dominant interspike interval and reported pitch estimates was poor. A fuller analysis of the population interspike interval distribution, which incorporates not only the dominant but all intervals, results in good matches with behavioral results, but not for the entire range of edge frequencies that generates pitch. Finally, we also examined temporal structure over a slower time scale, intermediate between average firing rate and interspike intervals, by studying the SAC envelope. We found that, in response to a given MEP stimulus, this feature also systematically varies with edge frequency, across fibers with different characteristic frequency (CF). Because neural mechanisms to extract envelope cues are well established, and because this cue is not limited by coding of stimulus fine-structure, this newly identified slower temporal cue is a more plausible basis for pitch than cues based on fine-structure.

A wavelet-based multi-dimensional temporal recurrent neural network for stencil printing performance prediction

The solder paste printing (SPP) is a critical procedure in a surface mount technology (SMT) based assembly line, which is one of the major attributes to the defect of the printed circuit boards (PCBs). The quality of SPP is influenced by multiple factors, such as the squeegee speed, pressure, the stencil separation speed, cleaning frequency, and cleaning profile. During printing, the printer environment is dynamically varying due to the physical change of solder paste, which can result in a dynamic variation of the relationships between the printing results and the influential factors. To reduce the printing defects, it is critical to understand such dynamic relationships. This research focuses on determining the printing performance during printing by implementing a wavelet filtering-based temporal recurrent neural network. To reduce the noise factor in the solder paste inspection (SPI) data, this research applies a three-dimensional dual-tree complex wavelet transformation for low-pass noise filtering and signal reconstruction. A recurrent neural network is utilized to model the performance prediction with low noise interference. Both printing sequence and process setting information are considered in the proposed recurrent network model. The proposed approach is validated using practical dataset and compared with other commonly used data mining approaches. The results show that the proposed wavelet-based multi-dimensional temporal recurrent neural network can effectively predict the printing process performance and can be a high potential approach in reducing the defects and controlling cleaning frequency. The proposed model is expected to advance the current research in the application of smart manufacturing in surface mount technology.

Sensitivity to interaural time differences and localization accuracy in cochlear implant users with combined electric-acoustic stimulation.

ObjectivesIn this study, localization accuracy and sensitivity to acoustic interaural time differences (ITDs) in subjects using cochlear implants with combined electric-acoustic stimulation (EAS) were assessed and compared with the results of a normal hearing control group.MethodsEight CI users with EAS (2 bilaterally implanted, 6 unilaterally implanted) and symmetric binaural acoustic hearing and 24 normal hearing subjects participated in the study. The first experiment determined mean localization error (MLE) for different angles of sound incidence between ± 60° (frontal and dorsal presentation). The stimuli were either low-pass, high-pass or broadband noise bursts. In a second experiment, just noticeable differences (JND) of ITDs were measured for pure tones of 125 Hz, 250 Hz and 500 Hz (headphone presentation).ResultsExperiment 1: MLE of EAS subjects was 8.5°, 14.3° and 14.7°, (low-, high-pass and broadband stimuli respectively). In the control group, MLE was 1.8° (broadband stimuli). In the differentiation between sound incidence from front and back, EAS subjects performed on chance level. Experiment 2: The JND-ITDs were 88.7 μs for 125 Hz, 48.8 μs for 250 Hz and 52.9 μs for 500 Hz (EAS subjects). Compared to the control group, JND-ITD for 125 Hz was on the same level of performance. No statistically significant correlation was found between MLE and JND-ITD in the EAS cohort.ConclusionsNear to normal ITD sensitivity in the lower frequency acoustic hearing was demonstrated in a cohort of EAS users. However, in an acoustic localization task, the majority of the subjects did not reached the level of accuracy of normal hearing. Presumably, signal processing time delay differences between devices used on both sides are deteriorating the transfer of precise binaural timing cues.

Improved Sensitivity of Digits-in-Noise Test to High-Frequency Hearing Loss.

Hearing loss is most commonly observed at high frequencies. High-frequency hearing loss (HFHL) precedes and predicts hearing loss at lower frequencies. It was previously shown that an automated, self-administered digits-in-noise (DIN) test can be sensitized for detection of HFHL by low-pass filtering the speech-shaped masking noise at 1.5 kHz. This study was designed to investigate whether sensitivity of the DIN to HFHL can be enhanced further using low-pass noise filters with higher cutoff frequencies. The US-English digits 0 to 9, homogenized for audibility, were binaurally presented in different noise maskers including one broadband and three low-pass (cutoff at 2, 4, and 8 kHz) filtered speech-shaped noises. DIN-speech reception thresholds (SRTs) were obtained from 60 normal hearing (NH), and 40 mildly hearing impaired listeners with bilateral symmetric sensorineural hearing loss. Standard and extended high-frequency audiometric pure-tone averages (PTAs) were compared with the DIN-SRTs. Narrower masking noise bandwidth generally produced better (more sensitive) mean DIN-SRTs. There were strong and significant correlations between SRT and PTA in the hearing impaired group. Lower frequency PTALF 0.5,1, 2, 4 kHz had the highest correlation and the steepest slope with SRTs obtained from the 2-kHz filter. Higher frequency PTAHF 4,8,10,12.5 kHz correlated best with SRTs obtained from 4- and 8-kHz filtered noise. The 4-kHz low-pass filter also had the highest sensitivity (92%) and equally highest (with the 8-kHz filter) specificity (90%) for detecting an average PTAHF of 20 dB or more. Of the filters used, DIN sensitivity to higher frequency hearing loss was greatest using the 4-kHz low-pass filter. These results suggest that low-pass filtered noise may be usefully substituted for broadband noise to improve earlier detection of HFHL using DIN.

Assessment of the difference limens for frequency, monosyllable articulation and word intelligibility by distantly-presented bone-conducted ultrasound.

Ultrasound can be clearly perceived by bone-conduction, and this "bone-conducted ultrasound (BCU)" can transmit speech information by using amplitude modulation (AM). Further, BCU can be perceived not only on the head but also on the distal parts of the body like the neck, trunk and arms. This "distantly-presented BCU" can be applied to the novel interface that can transmit sound information selectively to specific users who touches the vibrator. However, the ability to transmit sound information of distantly-presented BCU is unclear. First, to assess frequency discrimination ability, difference limens for frequency (DLFs) of the distantly-presented AM-BCU were measured with/without a low-pass masking noise that masked the self-demodulated components generated by the nonlinearity of biological tissues. DLFs comparable to that of air-conducted sounds were observed, whereas DLFs significantly increased above 1 kHz under the masking condition. These results suggest that practical frequency discrimination ability can be obtained even when BCUs were presented to distal body parts. Additionally, it is indicated that the demodulated components may contribute to transmitting frequency information above 1 kHz. Second, monosyllable articulation and word intelligibility tests were conducted in Japanese. The intelligibility and articulation at the neck were 55% and 38% respectively, whereas they decreased as the stimulus placement gets farther from the head. The results suggest the distantly-presented BCU device can be applied to transmission of speech information.

Noise edge pitch and models of pitch perception.

Monaural noise edge pitch (NEP) is evoked by a broadband noise with a sharp falling edge in the power spectrum. The pitch is heard near the spectral edge frequency but shifted slightly into the frequency region of the noise. Thus, the pitch of a lowpass (LP) noise is matched by a pure tone typically 2%–5% below the edge, whereas the pitch of highpass (HP) noise is matched a comparable amount above the edge. Musically trained listeners can recognize musical intervals between NEPs. The pitches can be understood from a temporal pattern-matching model of pitch perception based on the peaks of a simplified autocorrelation function. The pitch shifts arise from limits on the autocorrelation window duration. An alternative place-theory approach explains the pitch shifts as the result of lateral inhibition. Psychophysical experiments using edge frequencies of 100 Hz and below find that LP-noise pitches exist but HP-noise pitches do not. The result is consistent with a temporal analysis in tonotopic regions outside the noise band. LP and HP experiments with high-frequency edges find that pitch tends to disappear as the edge frequency approaches 5000 Hz, as expected from a timing theory, though exceptional listeners can go an octave higher.

Monaural edge pitch and models of pitch perception

Noise with a sharp spectral edge produces a definite, perceived pitch. The pitch of lowpass noise lies slightly below the edge and the pitch of highpass noise slightly above it. The pitch shifts away from the edge, expressed in semitones, increase with the decreasing edge frequency. A neural timing model based on positive peaks of the autocorrelation function accounts for the broad features of the shifts. So, does a place model based on the peak of an excitation pattern as enhanced by lateral inhibition. As the edge frequency decreases below 150 Hz, the pitch persists for a lowpass noise but disappears for highpass noise. This result is consistent with the timing model but not with the place model. For high edge frequencies, the pitch is stronger for highpass noise than for lowpass noise—consistent with both timing and place models. As the edge frequency approaches 5000 Hz, the pitch disappears for most listeners, but for some listeners, a pitch persists for edge frequencies beyond 8000 Hz. The latter res...

Comparison between the auditory lateralization ability of normal hearing elderly and youth with filtered noise

Background and Aim: Aging affects the audi­tory lateralization function that is achieved thr­ough processing binaural cues. One of the most important benefits of this process, along with getting informed of sound location, is increasing signal to noise ratio and improvement of speech comprehension in crowded environments, which is one of the most common hearing complaints in the elderly. This study aimed to compare the performance of the lateralization function under the headphone conditions between the elderly and the youth, with a filtered noise stimulus. Methods: This study was performed between 22 elderly aged 60−80 and 22 young people aged 20−30. The auditory threshold was less than 25 dB HL in 250 to 4000 Hz frequencies in both groups. By applying a time delay of −880 to +880 microsecond and the intensity differe­nce of −10 to +10 dB between the two ears, and with high-pass and low-pass noise stimulus, the lateralization function was examined. For descr­iption of the lateralization function, scatter diag­ram and in order to compare the results, paired t-test and independent t-test were used. Results: Findings showed that the elderly's err­ors were increased in all tests compared to those of the youth group. There was a significant difference (p < 0.05) between the two groups in lateralization by time clues with low-pass and high-pass noise. Conclusion: Results confirm the impairment of the lateralization and processing of binaural cues in the elderly differently.

Masking Differentially Affects Envelope-following Responses in Young and Aged Animals

Age-related hearing decline typically includes threshold shifts as well as reduced wave I auditory brainstem response (ABR) amplitudes due to cochlear synaptopathy/neuropathy, which may compromise precise coding of suprathreshold speech envelopes. This is supported by findings with older listeners, who have difficulties in envelope and speech processing, especially in noise. However, separating the effects of threshold elevation, synaptopathy, and degradation by noise on physiological representations may be difficult. In the present study, the effects of notched, low- and high-pass noise on envelope-following responses (EFRs) in aging were compared when sound levels (aged: 85-dB SPL; young: 60- to 80-dB SPL) were matched between groups peripherally, by matching wave I ABR amplitudes, or centrally by matching EFR amplitudes. Low-level notched noise reduced EFRs to sinusoidally amplitude-modulated (SAM) tones in young animals for notch widths up to 2 octaves. High-pass noise above the carrier frequency reduced EFRs. Young animals showed EFR reductions at lower noise levels. Low-pass noise did not reduce EFRs in either young or aged animals. High-pass noise may affect EFR amplitudes in young animals more than aged by reducing the contributions of high-frequency-sensitive inputs. EFRs to SAM tones in modulated noise (NAM) suggest that neurons of young animals can synchronize to NAM at lower sound levels and maintain dual AM representations better than older animals. The overall results show that EFR amplitudes are strongly influenced by aging and the presence of a competing sound that likely reduces or shifts the pool of responsive neurons.

Role of expectancy in front-back confusions during sound source localization

Front-back confusions (FBCs) often occur in laboratory experiments. Stimulus complexity, especially for high-frequency sounds, and head motion significantly reduce FBCs, so that in the everyday world FBCs probably do not often influence sound source localization. There is a small literature suggesting that listeners “expectancies” (based on experience or other sensory inputs) for where sound sources might be also affect FBCs. A low-pass noise that produces a large number of FBCs and a high-pass noise that does not were used. Listeners indicated the location of sound sources on the azimuth plane. The study aimed to investigate FBCs and not sound source localization accuracy. For one condition, listeners were told that the noises would all be coming from in front, that in another condition from in back, and in a third condition from the full 360° azimuth circle. In the first (front) and second (back) conditions, most of the sounds came from the front or back respectfully, but 20% did not. The expectancy that sounds would be in front or in back had a large influence on FBCs as compared to when sounds were presented from the entire azimuth circle. (Work supported by grants from NIDCD and Oculus VR, LLC.)

The effects of diffuse noise and artificial reverberation on listener weighting of interaural cues in sound localization

The reliability of interaural time and level difference (ITD, ILD) sound location cues can be degraded by noise or reverberation. In this study, we determined how weighting of ITD and ILD varied with signal-to-noise ratio (SNR) in the presence of interaurally uncorrelated background noise and with direct-to-reverberant ratio (DRR) in the presence of artificial reverberation (generated by convolving the target signal with an interaurally uncorrelated pair of impulse responses created by multiplying Gaussian noise with a decaying exponential, RT60 = 500 ms). Wideband (0.5-16 kHz) 100-ms noise-burst targets were presented over headphones using individual head-related transfer functions. ITD and ILD were manipulated by attenuating or delaying the sound at one ear (by up to 300 μs or 10 dB), and cue weighting was computed by comparing localization response bias to imposed cue bias. Wideband (0.5-16 kHz) and low-pass (0.5-2 kHz) noise and reverberation and SNRs and DRRs from -5 to + 20 dB were used. ITD dominat...

The perception of musical consonance beyond the traditional existence region of pitch

The traditional notion that pitch perception breaks down for frequency components above 5 kHz was challenged by a study [Oxenham et al., Proc. Natl. Acad. Sci. U. S. A. 108, 7629-7634 (2011)] showing that complex tones high-pass filtered at 6 kHz convey the perception of melodic intervals. We investigated whether complex tones high-pass filtered above 6 kHz can also convey the perception of harmonic intervals. Eleven listeners rated the pleasantness of consonant and dissonant dyads (two-note chords) consisting of harmonic complex tones band-pass filtered either in a low-frequency (1-6 kHz) or in a high-frequency (7-12 kHz) region. The two tones in each dyad were presented to opposite ears. In the high-frequency condition the dyads were presented with a low-pass noise to mask combination tones. Pleasantness ratings were significantly higher for consonant than for dissonant intervals in both frequency regions. This preference for consonant dyads in the high-frequency region was not observed when the tones were made inharmonic by shifting all their components by a fixed offset so as to preserve envelope periodicity cues. The results indicate that the perception of consonance is preserved for tones composed entirely of components above the traditional existence region of pitch.

On the impact of smoothing and noise on robustness of CT and CBCT radiomics features for patients with head and neck cancers.

We investigated the characteristics of radiomics features extracted from planning CT (pCT) and cone beam CT (CBCT) image datasets acquired for 18 oropharyngeal cancer patients treated with fractionated radiation therapy. Images were subjected to smoothing, sharpening, and noise to evaluate changes in features relative to baseline datasets. Textural features were extracted from tumor volumes, contoured on pCT and CBCT images, according to the following eight different classes: intensity based histogram features (IBHF), gray level run length (GLRL), law's textural information (LAWS), discrete orthonormal stockwell transform (DOST), local binary pattern (LBP), two-dimensional wavelet transform (2DWT), Two dimensional Gabor filter (2DGF), and gray level co-occurrence matrix (GLCM). A total of 165 radiomics features were extracted. Images were post-processed prior to feature extraction using a Gaussian noise model with different signal-to-noise-ratios (SNR=5, 10, 15, 20, 25, 35, 50, 75, 100, and 150). Gaussian filters with different cut off frequencies (varied discreetly from 0.0458 to 0.7321 cycles-mm-1 ) were applied to image datasets. Effect of noise and smoothing on each extracted feature was quantified using mean absolute percent change (MAPC) between the respective values on post-processed and baseline images. The Fisher method for combining Welch P-values was used for tests of significance. Three comparisons were investigated: (a) Baseline pCT versus modified pCT (with given filter applied); (b) Baseline CBCT versus modified CBCT, and (c) Baseline and modified pCT versus baseline and modified CBCT. Features extracted from CT and CBCT image datasets were robust to low-pass filtering (MAPC=17.5%, pvalFisher¯=0.93 for CBCT and MAPC=7.5%, pvalFisher¯=0.98 for pCT) and noise (MAPC=27.1%, pvalFisher¯= 0.89 for CBCT, and MAPC=34.6%, pvalFisher¯=0.61 for pCT). Extracted features were significantly impacted (MAPC=187.7%, pvalFisher¯<0.0001 for CBCT, and MAPC=180.6%, pvalFisher¯<0.01 for pCT) by LOG which is classified as a high-pass filter. Features most impacted by low pass filtering were LAWS (MAPC=11.2%, pvalFisher¯=0.44), GLRL (MAPC=9.7%, pvalFisher¯=0.70) and IBHF (MAPC=21.7%, pvalFisher¯=0.83), for the pCT datasets, and LAWS (MAPC=20.2%, pvalFisher¯=0.24), GLRL (MAPC=14.5%, pvalFisher¯=0.44), and 2DGF (MAPC=16.3%, pvalFisher¯=0.52), for CBCT image datasets. For pCT datasets, features most impacted by noise were GLRL (MAPC=29.7%, pvalFisher¯=0.06), LAWS (MAPC=96.6%, pvalFisher¯=0.42), and GLCM (MAPC=36.2%, pvalFisher¯=0.48), while the LBPF (MAPC=5.2%, pvalFisher¯=0.99) was found to be relatively insensitive to noise. For CBCT datasets, GLRL (MAPC=8.9%, pvalFisher¯=0.80) and LAWS (MAPC=89.3%, pvalFisher¯=0.81) features were impacted by noise, while the LBPF (MAPC=2.2%, pvalFisher¯=0.99) and DOST (MAPC=13.7%, pvalFisher¯=0.98) features were noise insensitive. Apart from 15 features, no significant differences were observed for the remaining 150 textural features extracted from baseline pCT and CBCT image datasets (MAPC=90.1%, pvalFisher¯=0.26). Radiomics features extracted from planning CT and daily CBCT image datasets for head/neck cancer patients were robust to low-power Gaussian noise and low-pass filtering, but were impacted by high-pass filtering. Textural features extracted from CBCT and pCT image datasets were similar, suggesting interchangeability of pCT and CBCT for investigating radiomics features as possible biomarkers for outcome.

Tinnitus in Normal-Hearing Participants after Exposure to Intense Low-Frequency Sound and in Ménière's Disease Patients.

Tinnitus is one of the three classical symptoms of Ménière’s disease (MD), an inner ear disease that is often accompanied by endolymphatic hydrops. Previous studies indicate that tinnitus in MD patients is dominated by low frequencies, whereas tinnitus in non-hydropic pathologies is typically higher in frequency. Tinnitus of rather low-frequency (LF) quality was also reported to occur for about 90 s in normal-hearing participants after presentation of intense, LF sound (120 dB SPL, 30 Hz, 90 s). LF sound has been demonstrated to also cause temporary endolymphatic hydrops in animal models. Here, we quantify tinnitus in two study groups with chronic (MD patients) and presumably transient endolymphatic hydrops (normal-hearing participants after LF exposure) with a psychophysical procedure. Participants matched their tinnitus either with a pure tone of adjustable frequency and level or with a noise of adjustable spectral shape and level. Sensation levels of matching stimuli were lower for MD patients (mean: 8 dB SL) than for normal-hearing participants (mean: 15 dB SL). Transient tinnitus after LF-exposure occurred in all normal-hearing participants (N = 28). About half of the normal-hearing participants matched noise to their tinnitus, the other half chose a pure tone with frequencies below 2 kHz. MD patients matched their tinnitus with either high-frequency pure tones, mainly above 3 kHz, or with a noise. Despite a significant proportion of MD patients matching low-pass (roaring) noises to their tinnitus, the range of matched stimuli was more heterogeneous than previous data suggested. We propose that in those participants with noise-like tinnitus, the percept is probably generated by increased spontaneous activity of auditory nerve fibers with a broad range of characteristic frequencies, due to an impaired ion balance in the cochlea. For tonal tinnitus, additional mechanisms are conceivable: focal hair cell loss can result in decreased auditory nerve firing and a central auditory overcompensation. Also, normal-hearing participants after LF-exposure experience alterations in spontaneous otoacoustic emissions, which may contribute to a transient tonal tinnitus.

Evaluation of a method for enhancing interaural level differences at low frequencies.

A method (called binaural enhancement) for enhancing interaural level differences at low frequencies, based on estimates of interaural time differences, was developed and evaluated. Five conditions were compared, all using simulated hearing-aid processing: (1) Linear amplification with frequency-response shaping; (2) binaural enhancement combined with linear amplification and frequency-response shaping; (3) slow-acting four-channel amplitude compression with independent compression at the two ears (AGC4CH); (4) binaural enhancement combined with four-channel compression (BE-AGC4CH); and (5) four-channel compression but with the compression gains synchronized across ears. Ten hearing-impaired listeners were tested, and gains and compression ratios for each listener were set to match targets prescribed by the CAM2 fitting method. Stimuli were presented via headphones, using virtualization methods to simulate listening in a moderately reverberant room. The intelligibility of speech at ±60° azimuth in the presence of competing speech on the opposite side of the head at ±60° azimuth was not affected by the binaural enhancement processing. Sound localization was significantly better for condition BE-AGC4CH than for condition AGC4CH for a sentence, but not for broadband noise, lowpass noise, or lowpass amplitude-modulated noise. The results suggest that the binaural enhancement processing can improve localization for sounds with distinct envelope fluctuations.

More about monaural noise edge pitch

Lowpass noise with a sharp spectral cutoff generates a monaural noise edge pitch (MNEP) that listeners match with a sine tone having a frequency somewhat below the noise cutoff frequency (negative pitch shift). If the MNEP is a proper musical pitch, listeners should be able to identify musical intervals made using only the MNEP stimulus. Open-set MNEP experiments in the edge-frequency range of 600-2400 Hz found that four out of four listeners with musical training correctly identified octaves, fifths, fourths, and major thirds, proving that the MNEP is a musical pitch. The MNEP can be heard for edge frequencies at least as low as 100 Hz, but the negative pitch shift becomes larger than a semitone, consistent with a timing model of the MNEP having a fixed integration time. If the MNEP reflects the expected quasi-periodic timing response of the auditory system to a sharp spectral edge, the pitch should become inaudible if the edge frequency becomes too high for neural synchrony—expected to be about 5000 Hz. Sine tone pitch matching experiments showed that such a limit clearly applies for most listeners, but less clearly for others. [Work supported by the USAFOSR.]