Sinusoidally Amplitude-modulated Noise Research Articles

The fluctuating masker benefit for normal-hearing and hearing-impaired listeners with equal audibility at a fixed signal-to-noise ratio.

Normal-hearing (NH) listeners can extract and integrate speech fragments from momentary dips in the level of a fluctuating masker, yielding a fluctuating-masker benefit (FMB) for speech understanding relative to a stationary-noise masker. Hearing-impaired (HI) listeners generally show less FMB, suggesting a dip-listening deficit attributable to suprathreshold spectral or temporal distortion. However, reduced FMB might instead result from different test signal-to-noise ratios (SNRs), reduced absolute audibility of otherwise unmasked speech segments, or age differences. This study examined the FMB for nine age-matched NH-HI listener pairs, while simultaneously equalizing audibility, SNR, and percentage-correct performance in stationary noise. Nonsense syllables were masked by stationary noise, 4- or 32-Hz sinusoidally amplitude-modulated noise (SAMN), or an opposite-gender interfering talker. Stationary-noise performance was equalized by adjusting the response-set size. Audibility was equalized by removing stimulus components falling below the HI absolute threshold. HI listeners showed a clear 4.5-dB reduction in FMB for 32-Hz SAMN, a similar FMB to NH listeners for 4-Hz SAMN, and a non-significant trend toward a 2-dB reduction in FMB for an interfering talker. These results suggest that HI listeners do not exhibit a general dip-listening deficit for all fluctuating maskers, but rather a specific temporal-resolution deficit affecting performance for high-rate modulated maskers.

Effects of noise overexposure on tone detection in noise in nonhuman primates

This report explores the consequences of acoustic overexposures on hearing in noisy environments for two macaque monkeys trained to perform a reaction time detection task using a Go/No-Go lever release paradigm. Behavioral and non-invasive physiological assessments were obtained before and after narrowband noise exposure. Physiological measurements showed elevated auditory brainstem response (ABR) thresholds and absent distortion product otoacoustic emissions (DPOAEs) post-exposure relative to pre-exposure. Audiograms revealed frequency specific increases in tone detection thresholds, with the greatest increases at the exposure band frequency and higher. Masked detection was affected in a similar frequency specific manner: threshold shift rates (change of masked threshold per dB increase in noise level) were lower than pre-exposure values at frequencies higher than the exposure band. Detection thresholds in sinusoidally amplitude modulated (SAM) noise post-exposure showed no difference from those in unmodulated noise, whereas pre-exposure masked detection thresholds were lower in the presence of SAM noise compared to unmodulated noise. These frequency-dependent results were correlated with cochlear histopathological changes in monkeys that underwent similar noise exposure. These results reveal that behavioral and physiological effects of noise exposure in macaques are similar to those seen in humans and provide preliminary information on the relationship between noise exposure, cochlear pathology and perceptual changes in hearing within individual subjects.

Dual sensitivity of inferior colliculus neurons to ITD in the envelopes of high-frequency sounds: experimental and modeling study

Human listeners are sensitive to interaural time differences (ITDs) in the envelopes of sounds, which can serve as a cue for sound localization. Many high-frequency neurons in the mammalian inferior colliculus (IC) are sensitive to envelope-ITDs of sinusoidally amplitude-modulated (SAM) sounds. Typically, envelope-ITD-sensitive IC neurons exhibit either peak-type sensitivity, discharging maximally at the same delay across frequencies, or trough-type sensitivity, discharging minimally at the same delay across frequencies, consistent with responses observed at the primary site of binaural interaction in the medial and lateral superior olives (MSO and LSO), respectively. However, some high-frequency IC neurons exhibit dual types of envelope-ITD sensitivity in their responses to SAM tones, that is, they exhibit peak-type sensitivity at some modulation frequencies and trough-type sensitivity at other frequencies. Here we show that high-frequency IC neurons in the unanesthetized rabbit can also exhibit dual types of envelope-ITD sensitivity in their responses to SAM noise. Such complex responses to SAM stimuli could be achieved by convergent inputs from MSO and LSO onto single IC neurons. We test this hypothesis by implementing a physiologically explicit, computational model of the binaural pathway. Specifically, we examined envelope-ITD sensitivity of a simple model IC neuron that receives convergent inputs from MSO and LSO model neurons. We show that dual envelope-ITD sensitivity emerges in the IC when convergent MSO and LSO inputs are differentially tuned for modulation frequency.

Age-related alterations in the neural coding of envelope periodicities.

This research was guided by the working hypothesis that the aging auditory system progressively loses its ability to process rapid acoustic transients efficiently, and in elderly listeners, this results in difficulties in speech perception. Neural correlates of age-related deficits in temporal processing were investigated by recording from inferior colliculus (IC) neurons from young adult and old CBA mice. Single-unit responses were recorded to sinusoidally amplitude-modulated (SAM) noise carriers, presented at 65-80 dB SPL, having modulation frequencies (MFs) that ranged from 10 to 800 Hz. Because phasic-type temporal response patterns dominate responses to tone and noise in mammalian IC, we limited our analyses to only phasic units. Modulation transfer functions (MTF) for both rate (rMTF) and synchronization (sMTF) measures were used to derive respective best modulation frequencies (rBMF and sBMF). The main age-related finding was that there was an overall increase in response rate to SAM noise carriers and a decrease in the median upper cutoff frequency in units from old mice. At rBMF, the median spike count from units from old animals was 1.63 times greater, and at the sBMF, the median spike count was 2.29 times greater than the young adult sample. We explored whether the increase in driven activity was due to a change in the transient (first cycle response) or periodic (remaining response) component of the response to SAM noise. Median spike counts of the transient component decreased with increasing MF for both young adult and old units, with median counts consistently greater in the old sample as compared with young. Median spike counts for the periodic response remained relatively constant as a function of MF; however, there was a significantly greater (3 times) response for older units in a restricted range of MFs. The greater median spike counts found for the transient and periodic response was also evident when we analyzed the cycle-by-cycle response. The magnitude of the differences between the young adult and the old spike median responses was greatest at low MFs and then declined as MF increased. Finally, the young adult distribution of rBMFs extends to higher MFs than the old, with 36.0% of units having rBMFs >100 Hz as compared with only 12.5% of the old unit sample. We postulate that this age-related difference in rate coding of SAM noise carriers is consistent with a loss, or imbalance, of excitatory and inhibitory neural mechanisms known to shape encoding of envelope periodicities in the IC.

Second-order temporal modulation transfer functions.

Detection thresholds were measured for a sinusoidal modulation applied to the modulation depth of a sinusoidally amplitude-modulated (SAM) white noise carrier as a function of the frequency of the modulation applied to the modulation depth (referred to as f'm). The SAM noise acted therefore as a "carrier" stimulus of frequency fm, and sinusoidal modulation of the SAM-noise modulation depth generated two additional components in the modulation spectrum: fm-f'm and fm+f'm. The tracking variable was the modulation depth of the sinusoidal variation applied to the "carrier" modulation depth. The resulting "second-order" temporal modulation transfer functions (TMTFs) measured on four listeners for "carrier" modulation frequencies fm of 16, 64, and 256 Hz display a low-pass segment followed by a plateau. This indicates that sensitivity to fluctuations in the strength of amplitude modulation is best for fluctuation rates f'm below about 2-4 Hz when using broadband noise carriers. Measurements of masked modulation detection thresholds for the lower and upper modulation sideband suggest that this capacity is possibly related to the detection of a beat in the sound's temporal envelope. The results appear qualitatively consistent with the predictions of an envelope detector model consisting of a low-pass filtering stage followed by a decision stage. Unlike listeners' performance, a modulation filterbank model using Q values > or = 2 should predict that second-order modulation detection thresholds should decrease at high values of f'm due to the spectral resolution of the modulation sidebands (in the modulation domain). This suggests that, if such modulation filters do exist, their selectivity is poor. In the latter case, the Q value of modulation filters would have to be less than 2. This estimate of modulation filter selectivity is consistent with the results of a previous study using a modulation-masking paradigm [S. D. Ewert and T. Dau, J. Acoust. Soc. Am. 108, 1181-1196 (2000)].

The role of envelope modulation in spectrally unresolved iterated rippled noise

Iterated rippled noise (IRN) produces a pitch corresponding to the IRN delay. The pitch persists even when the sound is high-pass filtered at 12 times the reciprocal of the IRN delay, i.e., in the absence of resolved spectral peaks. Typically, when a sound produces a pitch in the absence of spectral cues, the pitch is explained in terms of periodic envelope modulation, for example, the pitch of a high-pass filtered cosine-phase harmonic complex, or the pitch of sinusoidally amplitude-modulated noise (SAMN). This study presents experiments designed to search for periodic modulation in IRN. The occurrence and significance of modulation is investigated in the envelope of the stimulus waveform as well as in the IRN envelope as represented after narrow-band filtering similar to that occurring in peripheral auditory filters. The results indicate that the envelope of band-pass filtered IRN reveals modulation but that the order of modulation (corresponding to the number of envelope maxima recurring every period) increases with increasing filter bandwidth. The occurrence of first-order modulation, like that of SAMN, is indirectly demonstrated for spectrally unresolved IRN in the lower unresolved frequency range between the 10th and 20th spectral peaks. The significance of recurring transients sometimes visible in the IRN waveform with respect to their contribution to the IRN pitch was assessed by replacing portions of the IRN period with random noise. The results of this experiment indicate that this ‘waveform modulation’ is not essential for the IRN pitch perception. The presence of temporal pitch in the absence of first-order modulation is demonstrated in two experiments involving the detection of phase delays and f 0 differences for spectrally separated, narrow bands of harmonic complexes and IRNs.

Fringe effects in modulation masking

Modulation detection thresholds (20 log ms) for a sinusoidally amplitude-modulated (SAM) noise were measured in the presence of a SAM noise masker with a modulation depth (mm) of 1.0 and a modulation frequency of 16 or 64 Hz. The signal and masker carriers were presented continuously, and the signal was modulated during one of the two 500-ms observation intervals. The masker was modulated during both observation intervals and, in some conditions, for a certain amount of time before and after signal modulation. The duration of this "fringe" ranged from 62.5 ms to continuous (masker modulated throughout the thresholds estimate). The first experiment showed that a 500-ms fringe could reduce masked thresholds by 4-6 dB, but only at low signal modulation frequencies (2-8 Hz). In the second and third experiments, it was found that the fringe had to have a duration of 500 ms and a depth of about 0.75 to be maximally effective. A final, supplementary experiment indicated that the fringe effect is not due solely to the fringe that occurs prior to the observation intervals. The results are discussed in terms of both peripheral and central auditory processing.

Binaural modulation masking

Modulation thresholds were measured in three subjects for a sinusoidally amplitude-modulated (SAM) wideband noise (the signal) in the presence of a second amplitude-modulated wideband noise (the masker). In monaural conditions (Mm-Sm) masker and signal were presented to only one ear; in binaural conditions (M0-S pi) the masker was presented diotically while the phase of modulation of the SAM noise signal was inverted in one ear relative to the other. In experiment 1 masker modulation frequency (fm) was fixed at 16 Hz, and signal modulation frequency (fs) was varied from 2-512 Hz. For monaural presentation, masking generally decreased as fs diverged from fm, although there was a secondary increase in masking for very low signal modulation frequencies, as reported previously [Bacon and Grantham, J. Acoust. Soc. Am. 85, 2575-2580 (1989)]. The binaural masking patterns did not show this low-frequency upturn: binaural thresholds continued to improve as fs decreased from 16 to 2 Hz. Thus, comparing masked monaural and masked binaural thresholds, there was an average binaural advantage, or masking-level difference (MLD) of 9.4 dB at fs = 2 Hz and 5.3 dB at fs = 4 Hz. In addition, there were positive MLDs for the on-frequency condition (fm = fs = 16 Hz: average MLD = 4.4 dB) and for the highest signal frequency tested (fs = 512 Hz: average MLD = 7.3 dB). In experiment 2 the signal was a SAM noise (fs = 16 Hz), and the masker was a wideband noise, amplitude-modulated by a narrow band of noise centered at fs. There was no effect on monaural or binaural thresholds as masker modulator bandwidth was varied from 4 to 20 Hz (the average MLD remained constant at 8.0 dB), which suggests that the observed "tuning" for modulation may be based on temporal pattern discrimination and not on a critical-band-like filtering mechanism. In a final condition the masker modulator was a 10-Hz-wide band of noise centered at the 64-Hz signal modulation frequency. The average MLD in this case was 7.4 dB. The results are discussed in terms of various binaural capacities that probably play a role in binaural release from modulation masking, including detection of varying interaural intensity differences (IIDs) and discrimination of interaural correlation.

Modulation Transfer Functions

Modulation transfer functions (MTFs) were measured with three different psychoacoustical paradigms in the same normal-hearing subjects. In the temporal-probe method, the threshold of a 4-ms probe tone (frequencies of 1000 and 4000 Hz) was measured at various envelope phases within a 100% sinusoidally amplitude-modulated (SAM) noise at modulation frequencies from 2 to 256 Hz. For the derived-MTF method, the threshold of a 500-ms tone at 1000 and 4000 Hz was measured in the same noise at the same modulation frequencies. For the modulation-detection paradigm, modulation thresholds were measured as a function of modulation frequency for bandpass filtered SAM noise centered at 1000 and 4000 Hz. MTFs with lowpass shapes were observed with all three methods. Differences were observed in the cutoff frequencies and/or attenuation rates when the data were fitted with lowpass filter transfer functions. Factors influencing those differences are discussed.

Temporal resolution in frog auditory-nerve fibers.

Sinusoidally amplitude-modulated (SAM) noise was monaurally presented to the neotropical frog, Eleutherodactylus coqui, while recording intracellularly from auditory-nerve fibers. Neuronal phase locking was measured to the SAM noise envelope in the form of a period histogram. The modulation depth was changed (in 10% steps) until the threshold modulation depth was determined. This was repeated for various modulation frequencies (20-1200 Hz) and different levels of SAM noise (34-64 dB/Hz). From these data, temporal modulation transfer functions (TMTFs) were produced and minimum integration time (MIT) for each auditory fiber was calculated. The median MIT was 0.42 ms (lower quartile 0.32, upper quartile 0.68 ms). A noise level-dependent effect was noted on the shape of the TMTF as well as the minimum integration time. The latter results may be explained as a loss in spectral resolution with increasing noise level, which is consistent with the correlation that was found between minimum integration time and bandwidth.

Detection of increments and decrements in modulation depth of SAM noise

Difference limens (DLs) for the modulation depth of a sinusoidally amplitude-modulated (SAM) wideband noise were measured using a two-interval forced-choice adaptive procedure. For increment detection the reference modulation depth (mr) was 0.0, 0.25, 0.50, or 0.75. For decrement detection, mr was 0.45, 0.60, 0.75, or 0.90. Modulation frequency was 2, 4, 64, or 512 Hz. The DL is defined as 10 log|mthr2 − mr2|, where mthr is the modulation depth just discriminable from mr. For mr = 0.0, the DLs (or “absolute” modulation thresholds) varied from −25 to −10 dB as modulation frequency varied from 2–512 Hz. As mr increased, the increment DL increased monotonically for each frequency from its absolute detection level to about −6 dB at mr = 0.75. These data are in good agreement with those of Wakefleld and Viemeister [J. Acoust. Soc. Am. Suppl. 1 72, S90 (1982)]. The decrement DLs overlay and extended the increment DL functions, although the DLs for the highest mr (0.90) decreased slightly (−10 dB). In a separate manipulation, no effect on the modulation onset phase on modulation depth discrimination was found. [Work supported by NIH.]

Binaural modulation detection

A sinusoidally amplitude‐modulated (SAM) noise (modulation depth: 1.0) was used to mask another SAM noise, the signal, in a two‐interval forced‐choice experiment. Signal and masker modulation frequency were both either 4 or 32 Hz, and were added in quadrature. The same wideband noise carrier was used for both masker and signal (spectrum level: 15 dB SPL). The whole stimulus was filtered after modulation (either 4‐kHz low pass or 4‐kHz high pass). Modulation depth (m) of the signal was varied adaptively to track threshold. In the case of diotic presentation (Mo‐So), threshold m was about 0.30 for both modulation frequencies and both filtering conditions. In a binaural condition Mo‐Sm (masker presented to both ears and signal to only one ear) some subjects. showed a release from masking of up to 10 dB (m = 0.10), while others. showed none. The release from masking occurred for both filtering conditions, but only for the lower (4‐Hz) modulation rate. The results will be, discussed in relation to current models of binaural processing and the sluggish nature of the binaural system. [Work supported by NIH.]

Detection of amplitude modulation by normal‐hearing and hearing‐impaired subjects

Modulation thresholds for sinusoidally amplitude‐modulated (SAM) broadband noise were obtained as a function of modulation frequency from normal‐hearing subjects and from subjects with primarily high‐frequency hearing losses. The resulting Temporal Modulation Transfer Functions (TMTFs) indicated that the impaired subjects were generally less sensitive to modulation. In addition, unlike previously published data for normal‐hearing subjects, a level effect was observed in the impaired subjects: sensitivity decreased, particularly at high modulation frequencies, as noise level decreased. TMTFs were also obtained from normal‐hearing subjects for SAM noise that was lowpass filtered at 1.6 kHz after modulation, and presented with a continuous, 1.6‐kHz highpass masker. These TMTFs were similar to the broadband TMTFs in the impaired subjects; this suggests that the impaired temporal processing in the hearing‐impaired subjects is a result of a narrower effective bandwidth. [Supported by NIH NS 12125.]

Discrimination of modulation depth of SAM noise

Difference limens (DLs) for modulation depth of sinusoidally amplitude modulated (SAM) wideband noise were measured as a function of the modulation depth of the standard, the frequency of modulation (25, 100, 400 Hz), and the spectrum level of the noise (0, 20, 40 dB SPL). Preliminary results for one subject show that psychometric functions are linear and parallel in Log d′ and 10 Log(m12 − m02), where m1 and m0 are the modulation depths of the variable and standard stimuli, respectively. In the main study, three subjects were tested using a 2IFC adaptive procedure. The DL [10 Log(m12 − m02)] is at, or slightly above, the modulation detection threshold when m0 is small. For m0 above modulation threshold, the DL is a positively accelerating function of 20 Log(m0). Weber's Law does not describe the discrimination of modulation depth over any appreciable range. Across all conditions, DLs tend to converge to values around −9 dB when the level of the standard is −5 dB (m0 = 56%) and generally cannot be measured beyond this level. The effects of noise level on the DL are, in general, small. [Supported by NIH grant NS12125 and by USPHS HD07151 traineeship from the Center for Research in Human Learning.]

Played-again SAM: Further observations on the pitch of amplitude-modulated noise

The pitchlike sensation elicited by sinusoidally amplitude-modulated (SAM) noise remains a controversial phenomenon. The controversy centers on two major points: (1) whether this sensation is ’’really’’ pitch rather than, e.g., roughness or intermittency, and (2) the possibility that any pitch sensation is mediated by short-term spectral information rather than temporal information—thus nullifying an interesting aspect of the phenomenon. Three experiments employing SAM wideband noise, SAM wideband noise bandpass-filtered after modulation, and a SAM 10 kHz pure tone were performed: (1) open-set melody identification, (2) melodic dictation, and (3) musical-interval adjustment. These experiments extend our earlier study [Burns and Viemeister, J. Acoust. Soc. Am. 60, 863 (1976)]. The results provide further evidence that SAM noise can, at suitable modulation frequencies, elicit a sensation of pitch (as defined by the ability to carry melodic information), and that this pitch represents a purely temporal phenomenon.