Lateralization of bands of noise and sinusoidally amplitude-modulated tones: effects of spectral locus and bandwidth

Lateralization of narrow bands of noise was investigated while varying interaural temporal disparity (ITD), center frequency, and bandwidth, utilizing an acoustic pointing task. Stimuli were narrow bands of noise centered at octave intervals between 500 Hz and 4 kHz with bandwidths ranging from 50-400 Hz. In a second experiment, lateralization for bands of noise and sinusoidally amplitude-modulated (SAM) tones, whose spectral content was constrained to be no lower than 3.8 kHz, was assessed. Overall, relatively large extents of laterality were obtained from all four listeners for ITDs of low-frequency bands of noise. Increasing the bandwidth of these noises did not yield consistent changes in the extent of laterality across ITDs and listeners. Most targets centered at high frequencies were lateralized near the midline. However, three of the four listeners did exhibit rather large displacements of the intracranial image when the bandwidth of the high-frequency noises was 400 Hz or greater. Interestingly, ITDs within high-frequency SAM tones were relatively ineffective. Thus, it appears that ITDs of relatively wide-band, high-frequency stimuli can mediate rather substantial extents of laterality. However, these effects are highly listener-dependent.     

Across-critical-band processing of amplitude-modulated tones

Two experiments using two-tone sinusoidally amplitude-modulated stimuli were conducted to assess cross-channel effects in processing low-frequency amplitude modulation. In experiment I, listeners were asked to discriminate between two sets of two-tone amplitude-modulated complexes. In one set, the modulation phase of the lower frequency carrier tone was different from that of the upper frequency carrier tone. In the other stimulus set, both amplitude-modulated carriers had the same modulator phase. The amount of phase shift required to discriminate between the two stimulus sets was determined as a function of the separation between the two carriers, modulation depth, and modulation frequency. Listeners could discriminate a 50 degrees-60 degrees phase shift between the modulated envelopes for tones separated by more than a critical band. In experiment II, the modulation depth required to detect modulation of a probe carrier was measured in the presence of an amplitude-modulated masker. The threshold for detecting probe modulation was determined as a function of the separation between the masker and probe carriers, the phase difference between the masker and probe modulators, and masker modulation depth (in all conditions, the rate of probe and masker modulation was 10 Hz). The threshold for detecting probe modulation was raised substantially when the masker tone was also modulated. The results are consistent with theories suggesting that amplitude modulation helps form auditory objects from complex sound fields.     

Listeners' sensitivity to "onset/offset" and "ongoing" interaural delays in high-frequency, sinusoidally amplitude-modulated tones

The relative potency of onset/offset and envelope-based ongoing interaural time delays (ITDs) was assessed using high-frequency stimuli. A two-cue, two-alternative, forced-choice adaptive task was employed to measure threshold ITDs with 100% sinusoidally amplitude-modulated tones centered at 4 kHz. Modulation rates of 125, 250, and 350 Hz were tested with durations of 32, 90, or 240 ms. In the first experiment, ITDs to be detected were imposed only at the onset/offset, only within the ongoing portion, or within both the onset/offset and ongoing portions of the stimuli. Results indicated that ongoing ITDs dominated onset/offset ITDs. The relative potency of ongoing ITDs was directly proportional to duration and inversely proportional to modulation rate. Quantitative analysis suggested that listeners effectively combine onset/offset and ongoing ITDs. Furthermore, the data could be largely accounted for by assuming that listeners attend to the interaural decorrelation of the stimulus resulting from onset/offset and/or ongoing ITDs. A second experiment showed that, (1) overall, an ongoing ITD of one-half period of the envelope had little impact on listeners' sensitivity to delays of the onset/offset and (2) sensitivity to delays within the onset/offset portion of the waveform was reduced by roving the delay within the ongoing portion of the waveform.     

Discrimination of interaural phase differences in the envelopes of sinusoidally amplitude-modulated 4-kHz tones as a function of modulation depth

Psychometric functions were measured for the discrimination of the interaural phase difference (IPD) of the envelope of a sinusoidally amplitude-modulated (SAM) 4-kHz pure tone for modulation frequencies of 128 and 300 Hz and modulation depths (m) of 0.2, 0.6, 0.9, and 1.0. Contrary to recent modeling assumptions, it was found that a constant change in normalized interaural envelope correlation, with or without additional model stages to simulate peripheral auditory processing, did not produce a constant level of performance. Rather, in some cases, performance could range from chance to near perfect across modulation depths for a given change in normalized interaural envelope correlation. This was also true for the maximum change in normalized interaural envelope correlation computed across the cross-correlation functions for the stimuli to be discriminated. The change in the interaural time difference (ITD) computed from the IPD accounted for discriminability across modulation depths better than the change in normalized interaural envelope correlation, although ITD could not account for all the data, particularly those for lower values of m.     

Temporal interactions between pure tones and amplitude-modulated noise

An auditory interaction between the temporal fine structure of a low-frequency tone and the envelope of a high-frequency waveform was observed at very large frequency separations. Thresholds for detection of sinusoidal amplitude modulation of a high-frequency, narrow-band noise were measured as a function of the relative phase between the modulator and a pure tone with the same frequency as the modulator. These "phase functions" were determined at various intensities of the noise and tone for three different modulation frequencies. In general, the phase functions show that low-frequency stimulation has a cyclic effect on the sensitivity to amplitude modulation; over a limited range of relative phases, the modulation threshold is lower than that measured without low-frequency stimulation whereas over a broader range of relative phases, the modulation threshold is much higher. The difference between minimum and maximum modulation thresholds was observed to be as great as 23 dB. Despite this substantial degree of temporal interaction, little, if any, masking by the low-frequency tone of the high-frequency noise was observed.     

Lateralization of low-frequency tones and narrow bands of noise

It is well known and universally accepted that people's ability to use ongoing interaural temporal disparities conveyed via pure tones is limited to frequencies below 1600 Hz. We wish to determine if this limitation is the result of the constant amplitude and periodic axis-crossings which characterize pure tones. To this end, an acoustic pointing task was employed in which listeners varied the interaural intensitive difference of a 500-Hz narrow-band noise (the pointer) so that the position of its intracranial image matched that of a second, experimenter-controlled stimulus (the target). Targets were either pure tones or narrow bands of noise (50 or 100 Hz wide). The narrow bands of noise were delayed interaurally in two distinct manners: Either the entire waveform or only the carrier was delayed. In the latter case, the envelopes and phase-functions of the bands of noise were identical interaurally. This resulted in noises which resemble the pure tone case in that the interaural delay is manifested as a constant phase-shift and resemble ordinary noises in that the envelope and phase are random functions of time. Surprisingly, it appears that all three targets were lateralized virtually identically regardless of frequency or bandwidth. Apparently, the dynamically changing envelopes and phases did not affect the listeners' use of interaural temporal disparities in any discernible fashion.     

Lateralization of low-frequency tones: relative potency of gating and ongoing interaural delays.

Several types of interaural delay can affect the lateral position of binaural signals. Delays can occur within the gating (onset and/or offset) or ongoing portions of the signal, or both. Extent of laterality produced by each of these delays was measured for low-frequency tones with an acoustic pointing task. Relative potency was assessed by presenting the delays singly or in combinations (where the types of delay were consistent or in opposition). Rise/decay time, duration, and frequency of the tonal targets were also varied. The major finding was that ongoing delays were much more potent than gating delays in determining extent of laterality. Gating delays were most effective when the interaural phase of the ongoing portion of the tones was more or less ambiguous with respect to which ear was leading. Many of our findings are qualitatively well described by considering properties of patterns of activity produced within a cross-correlation network by such interaurally delayed signals.     

Influence of spectral locus and F0 changes on the pitch and timbre of complex tones.

Harmonic complex tones comprising components in different spectral regions may differ considerably in timbre. While the pitch of "residue" tones of this type has been studied extensively, their timbral properties have received little attention. Discrimination of F0 for such tones is typically poorer than for complex tones with "corresponding" harmonics [A. Faulkner, J. Acoust. Soc. Am. 78, 1993-2004 (1985)]. The F0 DLs may be higher because timbre differences impair pitch discrimination. The present experiment explores effects of changes in spectral locus and F0 of harmonic complex tones on both pitch and timbre. Six normally hearing listeners indicated if the second tone of a two-tone sequence was: (1) same, (2) higher in pitch, (3) lower in pitch, (4) same in pitch but different in "something else," (5) higher in pitch and different in "something else," or (6) lower in pitch and different in "something else" than the first. ("Something else" is assumed to represent timbre.) The tones varied in spectral loci of four equal-amplitude harmonics m, m + 1, m + 2, and m + 3 (m = 1,2,3,4,5,6) and ranged in F0 from 200 to 200 +/- 2n Hz (n = 0,1,2,4,8,16,32). Results show that changes in F0 primarily affect pitch, and changes in spectral locus primarily affect timbre. However, a change in spectral locus can also influence pitch. The direction of locus change was reported as the direction of pitch change, despite no change in F0 or changes in F0 in the opposite direction for delta F0 < or = 0-2%. This implies that listeners may be attending to the "spectral pitch" of components, or to changes in a timbral attribute like "sharpness," which are construed as changes in overall pitch in the absence of strong F0 cues. For delta F0 > or = 2%, the direction of reported pitch change accord with the direction of F0 change, but the locus change continued to be reported as a timbre change. Rather than spectral-pitch matching of corresponding components, a context-dependent spectral evaluation process is thus implied in discernment of changes in pitch and timbre. Relative magnitudes of change in derived features of the spectrum such as harmonic number and F0, and absolute features such as spectral frequencies are compared. What is called "spectral pitch," contributes to the overall pitch, but also appears to be an important dimension of the multidimensional percept, timbre.     

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.     

A high-precision magnetoencephalographic study of human auditory steady-state responses to amplitude-modulated tones

The cerebral magnetic field of the auditory steady-state response (SSR) to sinusoidal amplitude-modulated (SAM) tones was recorded in healthy humans. The waveforms of underlying cortical source activity were calculated at multiples of the modulation frequency using the method of source space projection, which improved the signal-to-noise ratio (SNR) by a factor of 2 to 4. Since the complex amplitudes of the cortical source activity were independent of the sensor position in relation to the subject's head, a comparison of the results across experimental sessions was possible. The effect of modulation frequency on the amplitude and phase of the SSR was investigated at 30 different values between 10 and 98 Hz. At modulation frequencies between 10 and 20 Hz the SNR of harmonics near 40 Hz were predominant over the fundamental SSR. Above 30 Hz the SSR showed an almost sinusoidal waveform with an amplitude maximum at 40 Hz. The amplitude decreased with increasing modulation frequency but was significantly different from the magnetoencephalographic (MEG) background activity up to 98 Hz. Phase response at the fundamental and first harmonic decreased monotonically with increasing modulation frequency. The group delay (apparent latency) showed peaks of 72 ms at 20 Hz, 48 ms at 40 Hz, and 26 ms at 80 Hz. The effects of stimulus intensity, modulation depth, and carrier frequency on amplitude and phase of the SSR were also investigated. The SSR amplitude decreased linearly when stimulus intensity or the modulation depth were decreased in logarithmic steps. SSR amplitude decreased by a factor of 3 when carrier frequency increased from 250 to 4000 Hz. From the phase characteristics, time delays were found in the range of 0 to 6 ms for stimulus intensity, modulation depth, and carrier frequency, which were maximal at low frequencies, low intensities, or maximal modulation depth.     

Lateralization of low-frequency, complex waveforms: the use of envelope-based temporal disparities

Several recent investigations suggest that listeners either cannot or do not use envelope-based interaural temporal disparities (ITDs) to lateralize low-frequency sounds [G.B. Henning, J. Acoust. Soc. Am. 68, 446-453 (1980); G.B. Henning and J. Ashton, Hear. Res. 4, 185-194 (1981); G.B. Henning, Hear. Res. 9, 153-172 (1983)]. We believe listeners in those studies may have been unable to process envelope-based ITDs principally because of the types of stimuli utilized. In this study we employed an acoustic "pointing" task in which listeners varied the interaural intensitive 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 or 1 kHz, and modulated at 25, 50, or 100 Hz. Targets were presented with either the entire waveform delayed or with only the envelope delayed. The results suggest that delays of the envelope do affect the lateral position of low-frequency targets. However, the envelope-based cues appear to interact with those provided by the dominant fine structure.     

Tunable spectral shift of high-order harmonic generation in atoms using a sinusoidally phase-modulated pulse

High-order harmonic generation (HHG) from an atom illuminated by a sinusoidally phase-modulated pulse is investigated by solving the time-dependent Schrödinger equation. The spectral shift that occurs in atomic HHG can be achieved easily using our laser pulse. It is shown that the photon energy of the generated harmonics is controllable within the range of 1 eV. The shift of the frequency peak position is rooted in the asymmetry of the rising and falling parts of the laser pulse. We also show that by varying the phase parameters in the frequency domain of the laser one can adjust and control the shift in atomic harmonic spectra.     

Lateralization produced by interaural temporal and intensitive disparities of high-frequency, raised-sine stimuli: data and modeling

An acoustic pointing task was used to measure extents of laterality produced by combinations of ongoing envelope-based interaural temporal disparities (ITDs) and interaural intensitive disparities (IIDs) of 4-kHz-centered raised-sine stimuli [Bernstein and Trahiotis, J. Acoust. Soc. Am. 125, 3234-3242 (2009),] while varying, parametrically, their peakedness, depth of modulation, and frequency of modulation. The study was designed to assess whether IIDs act as "weights" within the putative "binaural display" at high spectral frequencies (where the envelopes convey ITD-information) as appears to be the case at low spectral frequencies (where the waveforms, i.e., fine-structure and envelopes, convey ITD-information). The data indicate that envelope-based IIDs do principally act as weights and that they appear to exert their influence on lateral position independently of the influence of ITDs. Quantitative analyses revealed that an augmented form of the cross-correlation-based "position-variable" model of Stern and Shear [J. Acoust. Soc. Am. 100, 2278-2288 (1996)] accounted for 94% of the variance in the data. This success notwithstanding, for a small subset of the data, predictions could be improved by assuming that the listeners utilized information within auditory filters having center frequencies above 4 kHz.     

Lateralization produced by envelope-based interaural temporal disparities of high-frequency, raised-sine stimuli: empirical data and modeling

An acoustic pointing task was used to measure extents of laterality produced by ongoing interaural temporal disparities (ITDs) conveyed by the envelopes of 4-kHz-centered raised-sine stimuli while varying, parametrically, their peakedness, depth of modulation, and frequency of modulation. One purpose of the study was to determine whether such manipulations would produce changes in laterality logically consistent with those found for ITD-discrimination thresholds reported by Bernstein and Trahiotis [J. Acoust. Soc. Am. 125, 3234-3242 (2009)]. The data obtained revealed that they did in that (1) increasing depth of modulation, peakedness, or frequency of modulation between 32 and 128 Hz produced smaller threshold ITDs and greater laterality and (2) increasing frequency of modulation to 256 Hz produced modest increases in threshold ITDs and modest decreases in laterality. The extents of laterality measured were successfully accounted for via an augmentation of the cross-correlation-based "position-variable" modeling approach developed by Stern and Shear [J. Acoust. Soc. Am. 100, 2278-2288 (1996)] to account for ITD-based extents of laterality obtained at low spectral frequencies.     

Asymmetry of masking between complex tones and noise: the role of temporal structure and peripheral compression

Thresholds for the detection of harmonic complex tones in noise were measured as a function of masker level. The rms level of the masker ranged from 40 to 70 dB SPL in 10-dB steps. The tones had a fundamental frequency (F0) of 62.5 or 250 Hz, and components were added in either cosine or random phase. The complex tones and the noise were bandpass filtered into the same frequency region, from the tenth harmonic up to 5 kHz. In a different condition, the roles of masker and signal were reversed, keeping all other parameters the same; subjects had to detect the noise in the presence of a harmonic tone masker. In both conditions, the masker was either gated synchronously with the 700-ms signal, or it started 400 ms before and stopped 200 ms after the signal. The results showed a large asymmetry in the effectiveness of masking between the tones and noise. Even though signal and masker had the same bandwidth, the noise was a more effective masker than the complex tone. The degree of asymmetry depended on F0, component phase, and the level of the masker. The maximum difference between masked thresholds for tone and noise was about 28 dB; this occurred when the F0 was 62.5 Hz, the components were in cosine phase, and the masker level was 70 dB SPL. In most conditions, the growth-of-masking functions had slopes close to 1 (on a dB versus dB scale). However, for the cosine-phase tone masker with an F0 of 62.5 Hz, a 10-dB increase in masker level led to an increase in masked threshold of the noise of only 3.7 dB, on average. We suggest that the results for this condition are strongly affected by the active mechanism in the cochlea.     

Acoustics of children's speech: developmental changes of temporal and spectral parameters

Changes in magnitude and variability of duration, fundamental frequency, formant frequencies, and spectral envelope of children's speech are investigated as a function of age and gender using data obtained from 436 children, ages 5 to 17 years, and 56 adults. The results confirm that the reduction in magnitude and within-subject variability of both temporal and spectral acoustic parameters with age is a major trend associated with speech development in normal children. Between ages 9 and 12, both magnitude and variability of segmental durations decrease significantly and rapidly, converging to adult levels around age 12. Within-subject fundamental frequency and formant-frequency variability, however, may reach adult range about 2 or 3 years later. Differentiation of male and female fundamental frequency and formant frequency patterns begins at around age 11, becoming fully established around age 15. During that time period, changes in vowel formant frequencies of male speakers is approximately linear with age, while such a linear trend is less obvious for female speakers. These results support the hypothesis of uniform axial growth of the vocal tract for male speakers. The study also shows evidence for an apparent overshoot in acoustic parameter values, somewhere between ages 13 and 15, before converging to the canonical levels for adults. For instance, teenagers around age 14 differ from adults in that, on average, they show shorter segmental durations and exhibit less within-subject variability in durations, fundamental frequency, and spectral envelope measures.     

Lateralization of bands of noise: effects of bandwidth and differences of interaural time and phase

The effects of stimulus bandwidth on lateralization of narrow bands of noise were investigated with an acoustic pointing task. Stimuli were narrow bands of noise (centered on 500 Hz with bandwidths ranging from 50-400 Hz) that contained interaural time delays and/or interaural phase shifts. The overall extent of lateralization and sidedness was found to vary greatly as a function of stimulus bandwidth, as insightfully discussed earlier by Jeffress [L. A. Jeffress, Foundations of Modern Auditory Theory, edited by J. V. Tobias (Academic, New York, 1972)]. The data are qualitatively consistent with a weighted-image model [Stern et al., J. Acoust. Soc. Am. 84, 156-165 (1988)] that specifies and utilizes the shapes and locations of patterns of hypothesized neural activity. These patterns are topographically organized along a two-dimensional surface, and they describe the cross-correlation function of the stimuli as a joint function of frequency and the delay parameter of the cross-correlation operation. In this fashion, lateralization depends upon individual modes of such patterns that are weighed with respect to their straightness (consistency of interaural delay over frequency) and centrality (the extent to which interaural delays are small in magnitude).     

Phase effects on the perceived elevation of complex tones

Free-field source localization experiments with 30 source locations, symmetrically distributed in azimuth, elevation, and front-back location, were performed with periodic tones having different phase relationships among their components. Although the amplitude spectra were the same for these different kinds of stimuli, the tones with certain phase relationships were successfully localized while the tones with other phases led to large elevation errors and front-back reversals, normally growing with stimulus level. The results show that it is not enough to have a smooth, broadband, long-term signal spectrum for successful sagittal-plane localization. Instead, temporal factors are important. A model calculation investigates the idea that the tonotopic details that mediate localization need to be simultaneously, or almost simultaneously, accessible in the auditory system in order to achieve normal elevation perception. A qualitative model based on lateral inhibition seems capable in principle of accounting for both the phase effects and level effects.     

Decoupling of temporal/spatial broadening effects in Doppler wind LiDAR by 2D spectral analysis

Pulse echo accumulation is commonly employed in coherent Doppler wind LiDAR(light detection and ranging)under the assumption of steady wind. Here, the measured spectral data are analyzed in the time dimension and frequency dimension to cope with the temporal wind shear and achieve the optimal accumulation time. A hardware-efficient algorithm combining the interpolation and cross-correlation is used to enhance the wind retrieval accuracy by reducing the frequency sampling interval and then reduce...