An analysis of quasi-frequency-modulated noise and random-sideband noise as comparisons for amplitude-modulated noise

Experiments were performed to determine under what conditions quasi-frequency-modulated (QFM) noise and random-sideband noise are suitable comparisons for AM noise in measuring a temporal modulation transfer function (TMTF). Thresholds were measured for discrimination of QFM from random-sideband noise and AM from QFM noise as a function of sideband separation. In the first experiment, the upper spectral edge of the noise stimuli was at 2400 Hz and the bandwidth was 1600 Hz. For sideband separations up to 256 Hz, at threshold sideband levels for discriminating AM from QFM noise, QFM was indiscriminable from random-sideband noise. For the largest sideband separation used (512 Hz), listeners may have used within-stimulus envelope correlation in the QFM noise to discriminate it from the random-sideband noise. Results when stimulus bandwidth was varied suggest that listeners were able to use this cue when the carrier was wider than a critical band, and the sideband separation approached the carrier bandwidth. Within-stimulus envelope correlation was also present in AM noise, and thus QFM noise was a suitable comparison because it made this cue unusable and forced listeners to use across-stimulus envelope differences. When the carrier bandwidth was less than a critical band or was wideband, QFM noise and random-sideband noise were equally suitable comparisons for AM noise. When discrimination thresholds for QFM and random-sideband noise were converted to modulation depth and modulation frequency, they were nearly identical to those for discrimination of AM from QFM noise, suggesting that listeners were using amplitude modulation cues in both cases.     

Detection of modulation of a 4-kHz carrier

To better understand the processing of complex high-frequency sounds, modulation-detection thresholds were measured for sinusoidal frequency modulation (SFM), quasi-frequency modulation (QFM), sinusoidal amplitude modulation (SAM), and random-phase FM (RPFM). At the lowest modulation frequency (5 Hz) modulation thresholds expressed as AM depth were similar for RPFM, SAM and QFM suggesting the predominance of envelope cues. At the higher modulation frequencies (20 and 40 Hz) thresholds expressed as total frequency excursions were similar for SFM and QFM suggesting a common mechanism, one perhaps based on single-channel FM-to-AM conversion or on a multi-channel place mechanism. The fact that the nominal envelopes of SFM and QFM are different (SFM has a flat envelope), seems to preclude processing based on the envelope of the external stimulus. Also, given the 4-kHz carrier and the similarity to previously published results obtained with a 1-kHz carrier, processing based on temporally-coded fine structure for all four types of modulation appears unlikely.     

On the mechanisms involved in the recovery of envelope information from temporal fine structure

Three experiments were designed to provide psychophysical evidence for the existence of envelope information in the temporal fine structure (TFS) of stimuli that were originally amplitude modulated (AM). The original stimuli typically consisted of the sum of a sinusoidally AM tone and two unmodulated tones so that the envelope and TFS could be determined a priori. Experiment 1 showed that normal-hearing listeners not only perceive AM when presented with the Hilbert fine structure alone but AM detection thresholds are lower than those observed when presenting the original stimuli. Based on our analysis, envelope recovery resulted from the failure of the decomposition process to remove the spectral components related to the original envelope from the TFS and the introduction of spectral components related to the original envelope, suggesting that frequency- to amplitude-modulation conversion is not necessary to recover envelope information from TFS. Experiment 2 suggested that these spectral components interact in such a way that envelope fluctuations are minimized in the broadband TFS. Experiment 3 demonstrated that the modulation depth at the original carrier frequency is only slightly reduced compared to the depth of the original modulator. It also indicated that envelope recovery is not specific to the Hilbert decomposition.     

The role of temporal fine structure information for the low pitch of high-frequency complex tones

The fused low pitch evoked by complex tones containing only unresolved high-frequency components demonstrates the ability of the human auditory system to extract pitch using a temporal mechanism in the absence of spectral cues. However, the temporal features used by such a mechanism have been a matter of debate. For stimuli with components lying exclusively in high-frequency spectral regions, the slowly varying temporal envelope of sounds is often assumed to be the only information contained in auditory temporal representations, and it has remained controversial to what extent the fast amplitude fluctuations, or temporal fine structure (TFS), of the conveyed signal can be processed. Using a pitch matching paradigm, the present study found that the low pitch of inharmonic transposed tones with unresolved components was consistent with the timing between the most prominent TFS maxima in their waveforms, rather than envelope maxima. Moreover, envelope cues did not take over as the absolute frequency or rank of the lowest component was raised and TFS cues thus became less effective. Instead, the low pitch became less salient. This suggests that complex pitch perception does not rely on envelope coding as such, and that TFS representation might persist at higher frequencies than previously thought.     

Spectral modulation detection as a function of modulation frequency, carrier bandwidth, and carrier frequency region

The present study investigates the nature of spectral envelope perception using a spectral modulation detection task in which sinusoidal spectral modulation is superimposed upon a noise carrier. The principal goal of this study is to characterize spectral envelope perception in terms of the influence of modulation frequency (cycles/octave), carrier bandwidth (octaves), and carrier frequency region (defined by lower and upper cutoff frequencies in Hz). Spectral modulation detection thresholds measured as a function of spectral modulation frequency result in a spectral modulation transfer function (SMTF). The general form of the SMTF is bandpass in nature, with a minimum modulation detection threshold in the region between 2 to 4 cycles/octave. SMTFs are not strongly dependent on carrier bandwidth (ranging from 1 to 6 octaves) or carrier frequency region (ranging from 200 to 12 800 Hz), with the exception of carrier bands restricted to very low audio frequencies (e.g., 200-400 Hz). Spectral modulation detection thresholds do not depend on the presence of random level variations or random modulation phase across intervals. The SMTFs reported here and associated excitation pattern computations are considered in terms of a linear systems approach to spectral envelope perception and potential underlying mechanisms for the perception of spectral features.     

Selectivity of modulation interference for consonant identification in normal-hearing listeners

The present study sought to establish whether speech recognition can be disrupted by the presence of amplitude modulation (AM) at a remote spectral region, and whether that disruption depends upon the rate of AM. The goal was to determine whether this paradigm could be used to examine which modulation frequencies in the speech envelope are most important for speech recognition. Consonant identification for a band of speech located in either the low- or high-frequency region was measured in the presence of a band of noise located in the opposite frequency region. The noise was either unmodulated or amplitude modulated by a sinusoid, a band of noise with a fixed absolute bandwidth, or a band of noise with a fixed relative bandwidth. The frequency of the modulator was 4, 16, 32, or 64 Hz. Small amounts of modulation interference were observed for all modulator types, irrespective of the location of the speech band. More important, the interference depended on modulation frequency, clearly supporting the existence of selectivity of modulation interference with speech stimuli. Overall, the results suggest a primary role of envelope fluctuations around 4 and 16 Hz without excluding the possibility of a contribution by faster rates.     

Subcomponent cues in binaural unmasking

The addition of a signal in the N0Sπ binaural configuration gives rise to fluctuations in interaural phase and amplitude. Sensitivity to these individual cues was measured by applying sinusoidal amplitude modulation (AM) or quasi-frequency modulation (QFM) to a band of noise. Discrimination between interaurally in-phase and out-of-phase modulation was measured using an adaptive task for narrow bands of noise at center frequencies from 250 to 1500 Hz, for modulation rates of 2-40 Hz, and with or without flanking bands of diotic noise. Discrimination thresholds increased steeply for QFM with increasing center frequency, but increased only modestly for AM, and mainly for modulation rates below 10 Hz. Flanking bands of noise increased thresholds for AM, but had no consistent effect for QFM. The results suggest that two underlying mechanisms may support binaural unmasking: one most sensitive to interaural amplitude modulations that is susceptible to across-frequency interference, and a second, most sensitive to interaural phase modulations that is immune to such effects.     

Mechanisms underlying the detection of frequency modulation

Frequency modulation detection limens (FMDLs) were measured for carrier frequencies (f(c)) of 1000, 4000, and 6000 Hz, using modulation frequencies (f(m)) of 2 and 10 Hz and levels of 20 and 60 dB sensation level (SL), both with and without random amplitude modulation (AM), applied in all intervals of a forced-choice trial. The AM was intended to disrupt excitation-pattern cues. At 60 dB SL, the deleterious effect of the AM was smaller for f(m) = 2 than for f(m) = 10 Hz for f(c) = 1000 and 4000 Hz, respectively, while for f(c) = 6000 Hz the deleterious effect was large and similar for the two values of f(m). This is consistent with the idea that, for f(c) below about 5000 Hz and f(m) = 2 Hz, frequency modulation can be detected via changes in phase locking over time. However, at 20 dB SL, the deleterious effect of the added AM for f(c) = 1000 and 4000 Hz was similar for the two values of f(m), while for f(c) = 6000 Hz, the deleterious effect of the AM was greater for f(m) = 10 than for f(m) = 2 Hz. It is suggested that, at low SLs, the auditory filters become relatively sharp and phase locking weakens, so that excitation-pattern cues influence FMDLs even for low f(c) and low f(m).     

Selective adaptation to frequency-modulated tones: evidence for an information-processing channel selectively sensitive to frequency changes.

Exposure to an FM tone elevates FM threshold but not AM threshold. This holds for a wide range of frequency deviations (delta F = +/- 0.4 Hz- +/- 30 Hz at least) provided that modulation frequency is low (fm = 2 Hz), but if fm is somewhat higher (e.g., 8 Hz) the finding only holds for small frequency deviations. FM threshold can rise with time up to an adapting duration of at least 1200 s, through this buildup depends on frequency deviation. Exposure to an AM tone elevates AM threshold, but not FM threshold, over a wide range of modulation depths (at least m = 5%--50%). Quasi-FM (QFM) adapting tones resemble FM adapting tones in their effects upon FM and AM sensitivities, even though QFM and AM adapting tones have identical power spectra. Exposure to a pure tone produces no difference between FM and AM threshold elevations. These data can be explained if the human auditory pathway contains separate information-processing channels for AM and FM signals whose sensitivities do not overlap even with suprathreshold stimuli. We suppose that the FM channel (but not the AM channel) is sensitive to changing differences (or ratios) between signals from different sites along the basilar membrane.     

Moderate cochlear hearing loss leads to a reduced ability to use temporal fine structure information

The ability of normally hearing and hearing-impaired subjects to use temporal fine structure information in complex tones was measured. Subjects were required to discriminate a harmonic complex tone from a tone in which all components were shifted upwards by the same amount in Hz, in a three-alternative, forced-choice task. The tones either contained five equal-amplitude components (non-shaped stimuli) or contained many components, but were passed through a fixed bandpass filter to reduce excitation pattern changes (shaped stimuli). Components were centered at nominal harmonic numbers (N) 7, 11, and 18. For the shaped stimuli, hearing-impaired subjects performed much more poorly than normally hearing subjects, with most of the former scoring no better than chance when N=11 or 18, suggesting that they could not access the temporal fine structure information. Performance for the hearing-impaired subjects was significantly improved for the non-shaped stimuli, presumably because they could benefit from spectral cues. It is proposed that normal-hearing subjects can use temporal fine structure information provided the spacing between fine structure peaks is not too small relative to the envelope period, but subjects with moderate cochlear hearing loss make little use of temporal fine structure information for unresolved components.     

Responses to amplitude-modulated tones in the auditory nerve of the cat.

Sinusoidally amplitude-modulated (AM) tones are frequently used in psychophysical and physiological studies, yet a comprehensive study on the coding of AM tones in the auditory nerve is lacking. AM responses of single auditory-nerve fibers of the cat are studied, systematically varying modulation depth, frequency, and sound level. Synchrony-level functions were nonmonotonic with maximum values that were inversely correlated with spontaneous rate (SR). In most fibers, envelope phase-locking showed a positive gain. Modulation transfer functions were uniformly low pass. Their corner frequency increased with characteristic frequency (CF), but changed little for CFs above 10 kHz. The highest modulation frequencies to which phase locking occurred were more than 0.8 oct lower than the highest frequencies to which phase locking to pure tones occurs. Cumulative, or unwrapped, phase increased linearly with modulation frequency: The slope was inversely related to CF, and slightly higher than group delays reported for pure tones. High SR, low CF fibers showed the poorest envelope phase locking. In some low CF fibers, phase locking increased at high levels, associated with "peak-splitting" phenomena. Changes in average rate due to modulation were small, and could be enhancement or suppression.     

Lateralization of sinusoidally amplitude-modulated tones: effects of spectral locus and temporal variation

It has long been recognized that listeners are sensitive to interaural temporal disparities (ITDs) of low-frequency (i.e., below 1600 Hz) stimuli. Within the last three decades, it has often been demonstrated that listeners are also sensitive to ITDs within the envelope of high-frequency, complex stimuli. Because these studies, for the most part, employed discrimination tasks, few data exist concerning the extent of laterality produced by ITDs as a function of the spectral locus of the stimulus. To this end, we employed an acoustic "pointing" task in which listeners varied the interaural intensity difference of a 500-Hz narrow-band noise (the pointer) so that it matched the intracranial position of a second, experimenter-controlled stimulus (the target). Targets were sinusoidally amplitude-modulated tones centered on 500 Hz, 1, 2, 3, or 4 kHz and modulated at rates ranging from 50 to 800 Hz. Targets were presented with either the entire waveform delayed or with only the envelope delayed. Our results suggest that: (1) for low-frequency targets, lateralization is influenced by ITDs in the envelope but is dominated by ITDs in the fine structure; (2) for high-frequency targets, envelope-based delays produce displacements of the acoustic images which are affected greatly by the rate of modulation; rather large extents of laterality could be produced with high rates of modulation; these data are consistent with those obtained previously in discrimination experiments; (3) for low rates of modulation (e.g., 100 Hz), delays of the entire waveform (both envelope and fine structure) produce much greater displacements of the acoustic image for low-frequency than for high-frequency targets (where fine-structure-based cues are not utilizable); (4) there appear to be no consistent relations among extent of laterality, rate of modulation, and the frequency of the carrier within and across listeners.     

Speech recognition with altered spectral distribution of envelope cues.

Recognition of consonants, vowels, and sentences was measured in conditions of reduced spectral resolution and distorted spectral distribution of temporal envelope cues. Speech materials were processed through four bandpass filters (analysis bands), half-wave rectified, and low-pass filtered to extract the temporal envelope from each band. The envelope from each speech band modulated a band-limited noise (carrier bands). Analysis and carrier bands were manipulated independently to alter the spectral distribution of envelope cues. Experiment I demonstrated that the location of the cutoff frequencies defining the bands was not a critical parameter for speech recognition, as long as the analysis and carrier bands were matched in frequency extent. Experiment II demonstrated a dramatic decrease in performance when the analysis and carrier bands did not match in frequency extent, which resulted in a warping of the spectral distribution of envelope cues. Experiment III demonstrated a large decrease in performance when the carrier bands were shifted in frequency, mimicking the basal position of electrodes in a cochlear implant. And experiment IV showed a relatively minor effect of the overlap in the noise carrier bands, simulating the overlap in neural populations responding to adjacent electrodes in a cochlear implant. Overall, these results show that, for four bands, the frequency alignment of the analysis bands and carrier bands is critical for good performance, while the exact frequency divisions and overlap in carrier bands are not as critical.     

Auditory stream segregation on the basis of amplitude-modulation rate

In this study, auditory stream segregation based on differences in the rate of envelope fluctuations--in the absence of spectral and temporal fine structure cues--was tested. The temporal sequences to segregate were composed of fully amplitude-modulated (AM) bursts of broadband noises A and B. All sequences were built by the reiteration of a ABA triplet where A modulation rate was fixed at 100 Hz and B modulation rate was variable. The first experiment was devoted to measuring the threshold difference in AM rate leading subjects to perceive the sequence as two streams as opposed to just one. The results of this first experiment revealed that subjects generally perceived the sequences as a single perceptual stream when the difference in AM rate between the A and B noises was smaller than 0.75 oct, and as two streams when the difference was larger than about 1.00 oct. These streaming thresholds were found to be substantially larger than, and not related to, the subjects' modulation-rate discrimination thresholds. The results of a second experiment demonstrated that AM-rate-based streaming was adversely affected by decreases in AM depth, but that segregation remained possible as long as the AM of either the A or B noises was above the subject's AM-detection threshold. The results of a third experiment indicated that AM-rate-based streaming effects were still observed when the modulations applied to the A and B noises were set individually, either at a constant level in dB above AM-detection threshold, or at levels at which they were of the same perceived strength. This finding suggests that AM-rate-based streaming is not necessarily mediated by perceived differences in AM depth. Altogether, the results of this study indicate that sequential sounds can be segregated on the sole basis of differences in the rate of their temporal fluctuations in the absence of other temporal or spectral cues.     

7 fs超快强激光驱动Ar原子产生支持单个阿秒脉冲的高次谐波连续谱

采用脉冲宽度为7 fs,脉冲能量为0.4 mJ的超快强激光脉冲与气体盒子中Ar原子作用获得了高次谐波截止区连续谱,并发现当驱动激光稳定在不同的载波包络相位时,高次谐波的谱结构、谱调制深度和连续谱的带宽都有很大区别。在某些载波包络相位时获得了平滑的连续谱,调制深度小于17%,连续带宽达10 eV,从而支持时域上获得变换极限500 as的单个阿秒脉冲。     

The ability of cochlear implant users to use temporal envelope cues recovered from speech frequency modulation

Previous studies have demonstrated that normal-hearing listeners can understand speech using the recovered "temporal envelopes," i.e., amplitude modulation (AM) cues from frequency modulation (FM). This study evaluated this mechanism in cochlear implant (CI) users for consonant identification. Stimuli containing only FM cues were created using 1, 2, 4, and 8-band FM-vocoders to determine if consonant identification performance would improve as the recovered AM cues become more available. A consistent improvement was observed as the band number decreased from 8 to 1, supporting the hypothesis that (1) the CI sound processor generates recovered AM cues from broadband FM, and (2) CI users can use the recovered AM cues to recognize speech. The correlation between the intact and the recovered AM components at the output of the sound processor was also generally higher when the band number was low, supporting the consonant identification results. Moreover, CI subjects who were better at using recovered AM cues from broadband FM cues showed better identification performance with intact (unprocessed) speech stimuli. This suggests that speech perception performance variability in CI users may be partly caused by differences in their ability to use AM cues recovered from FM speech cues.     

Effects of periodic interruptions on the intelligibility of speech based on temporal fine-structure or envelope cues

The intelligibility of speech signals processed to retain either temporal envelope (E) or fine structure (TFS) cues within 16 0.4-oct-wide frequency bands was evaluated when processed stimuli were periodically interrupted at different rates. The interrupted E- and TFS-coded stimuli were highly intelligible in all conditions. However, the different patterns of results obtained for E- and TFS-coded speech suggest that the two types of stimuli do not convey identical speech cues. When an effect of interruption rate was observed, the effect occurred at low interruption rates (<8 Hz) and was stronger for E- than TFS-coded speech, suggesting larger involvement of modulation masking with E-coded speech.     

Detection of quasitrapezoidal frequency and amplitude modulation

It has been proposed that the detection of frequency modulation (FM) of sinusoidal carriers can be mediated by two mechanisms; a place mechanism based on FM-induced amplitude modulation (AM) in the excitation pattern, and a temporal mechanism based on phase locking in the auditory nerve. The temporal mechanism appears to be "sluggish" and does not play a role for FM rates above about 10 Hz. It also does not play a role for high carrier frequencies (above about 5 kHz). This experiment provided a further test of the hypothesis that the effectiveness of the temporal mechanism depends upon the time spent close to frequency extremes during the modulation cycle. Psychometric functions for the detection of AM and FM were measured for two carrier frequencies, 1 and 6 kHz. The modulation waveform was quasitrapezoidal. Within each modulation period, P, a time Tss was spent at each extreme of frequency or amplitude. The transitions between the extremes, with duration Ttrans had the form of a half-cycle of a cosine function. The modulation rate was 2, 5, 10, or 20 Hz, giving values of P of 500, 200, 100, and 50 ms. TSS varied from 0 ms (sinusoidal modulation) up to 160, 80, 40, or 20 ms, for rates of 2, 5, 10, and 20 Hz, respectively. The detectability of AM was not greatly affected by modulation rate or by the value of TSS, except for a slight improvement with increasing TSS for the lowest modulation rates; this was true for both carrier frequencies. For FM of the 6-kHz carrier, the pattern of results was similar to that found for AM, which is consistent with an excitation-pattern model of FM detection. For FM of the 1-kHz carrier, performance improved markedly with increasing TSS, especially for the lower FM rates; there was no change in performance with TSS for the 20-Hz modulation rate. The results are consistent with the idea that detection of FM of a 1-kHz carrier is partly mediated by a sluggish temporal mechanism. That mechanism benefits from greater time spent at frequency extremes of the modulation cycle for rates up to 10 Hz.     

Behaviors of cubic distortion product otoacoustic emissions evoked by amplitude modulated tones

Distortion product otoacoustic emissions (DPOAEs) were measured using sinusoidal amplitude modulation (AM) tones. When one of the primary stimuli (f(1) or f(2), f(1)?< f(2)) was amplitude modulated, a series of changes in the cubic difference tone (CDT) were observed. In the frequency domain, multiple sidebands were present around the CDT and their sizes grew with the modulation depth of the AM stimulus. In the time domain, the CDT showed different modulation patterns between two major signal conditions: the AM tone was used as the f(1) or the f(2). The CDT amplitude followed the AM tone when the f(1) was amplitude modulated. However, when the AM tone acted as the f(2), the CDT showed a more complex modulation pattern with a notch present at the AM tone peak. The relatively linear dependence of CDT on f(1) and the nonlinear relation with f(2) can be explained with a variable gain-control model representing hair cell functions at the DPOAE generation site. It is likely that processing of AM signals at a particular cochlear location depends on whether the hair cells are tuned to the frequency of the carrier. Nonlinear modulation is related to on-frequency carriers and off-frequency carriers are processed relatively linearly.     

The ability of listeners to use recovered envelope cues from speech fine structure

Recent work has demonstrated that auditory filters recover temporal-envelope cues from speech fine structure when the former were removed by filtering or distortion. This study extended this work by assessing the contribution of recovered envelope cues to consonant perception as a function of the analysis bandwidth, when vowel-consonant-vowel (VCV) stimuli were processed in order to keep their fine structure only. The envelopes of these stimuli were extracted at the output of a bank of auditory filters and applied to pure tones whose frequency corresponded to the original filters' center frequencies. The resulting stimuli were found to be intelligible when the envelope was extracted from a single, wide analysis band. However, intelligibility decreases from one to eight bands with no further decrease beyond this value, indicating that the recovered envelope cues did not play a major role in consonant perception when the analysis bandwidth was narrower than four times the bandwidth of a normal auditory filter (i.e., number of analysis bands > or =8 for frequencies spanning 80 to 8020 Hz).