Lateralization discrimination of interaural time delays in four-pulse sequences in electric and acoustic hearing

This study examined the sensitivity of four cochlear implant (CI) listeners to interaural time difference (ITD) in different portions of four-pulse sequences in lateralization discrimination. ITD was present either in all the pulses (referred to as condition Wave), the two middle pulses (Ongoing), the first pulse (Onset), the last pulse (Offset), or both the first and last pulse (Gating). All ITD conditions were tested at different pulse rates (100, 200, 400, and 800 pulses/s pps). Also, five normal hearing (NH) subjects were tested, listening to an acoustic simulation of CI stimulation. All CI and NH listeners were sensitive in condition Gating at all pulse rates for which they showed sensitivity in condition Wave. The sensitivity in condition Onset increased with the pulse rate for three CI listeners as well as for all NH listeners. The performance in condition Ongoing varied over the subjects. One CI listener showed sensitivity up to 800 pps, two up to 400 pps, and one at 100 pps only. The group of NH listeners showed sensitivity up to 200 pps. The result that CI listeners detect ITD from the middle pulses of short trains indicates the relevance of fine timing of stimulation pulses in lateralization and therefore in CI stimulation strategies.     

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.     

The effects of intensity on the detection of interaural differences of time in high-frequency trains of clicks

Threshold values of interaural differences of time (delta IDTs ) were measured for trains of dichotic clicks whose levels were 20, 40, or 60 dB SPL. All clicks were bandpass filtered at 4 kHz, and the number of clicks in the train (n) was 1, 2, 4, 8, 16, or 32. The interclick interval (ICI) was 5, 2, or 1 ms. Performance was compared to that of an ideal integrator of information, which produces slopes of - 0.5 when log delta IDT versus log n is plotted. The results showed that increases in level had no effect on the slopes of the log-log functions regardless of the ICI but did decrease the intercepts. Shortening the ICI caused the slopes to go from nearly - 0.5 towards 0.0. The improvement with level could be explained by either a decrease in the temporal variability of neural discharges, or by an increase in the number of samples of IDT at higher intensities brought on by increased firing rates or the activation of more auditory units. A review of the physiological literature found the most parsimonious explanation to be that the decline in threshold IDT was mediated by an increase in the number of active units, each possessing the same degree of adaptation.     

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).     

Masking effects in binaural detection and interaural time discrimination

This study was designed to investigate the effects of masker level and frequency on binaural detection and interaural time discrimination. Detection and interaural time discrimination of a 700-Hz sinusoidal signal were measured as a function of the center frequency and level of a narrow-band masking noise. The masker was a continuous, diotic, 80-Hz-wide noise that varied in center frequency from 250 to 1370 Hz. In the detection experiment, the signal was presented either diotically (NoSo) or interaurally phase reversed (NoS pi). In the interaural time discrimination experiment, the signal level needed to discriminate a 30-microseconds interaural delay was measured. As would be expected, the presence of the masker has a greater effect on NoSo detection than NoS pi detection, and for masker frequencies at or near the signal frequency. In contrast, interaural time discrimination can be improved by the presence of a low-level masker. Also, performance improves more rapidly as the signal/masker frequency separation increases for NoSo detection than for interaural time discrimination and NoS pi detection. For all three tasks, significant upward spread of masking occurs only at the highest masker level; at low masker levels, there is a tendency toward downward spread of masking.     

Directional dependence of interaural envelope delays

Interaural envelope delays were measured in six human subjects as a function of the location of a movable sound source, bandpassed between 3 and 16 kHz. A total of 324 source locations were tested in horizontal and vertical increments of 10 degrees. A method is described for estimating the complex directional transfer function of the external ear, independent of the position of the recording microphone in the ear canal. To compute interaural envelope delays, directional transfer functions from the left and right ears were convolved with a critical-band filter, the envelopes were computed, and the envelopes were cross correlated. Interaural envelope delays, as well as interaural group delays, varied somewhat with the center frequency of the critical-band filter and with the vertical location of the sound source. Nevertheless, to a first approximation, envelope delays measured in the ear canals increased monotonically with increasing angle of incidence relative to the median plane, as they would for two microphones on the surface of a rigid sphere. The results are discussed in relation to the possible contribution of interaural envelope delays to sound localization behavior.     

Generation and propagation of pulse trains with ultrashort pulse separation

We investigate the nonlinear Schrödinger equation with variable coefficients by employing perturbation method. The analysis solution of the harmonic form is presented. The solution is one of forms to describe pulse trains with ultrashort pulse separation, which is about two orders of magnitude shorter than one of sech-type solitons considered before. And we could systematically adjust the perturbation parameter to obtain different pulse separation. As an example, we consider a nonlinear dispersive system with spatial parameter variations, and the results show that, the pulse train with ultrashort pulse separation presented by analysis solution may keep its shape even if the velocity is changed. The stability of the solution is discussed numerically, and the results reveal that the finite initial perturbations, such as white noise could not influence the main character of the solution. In addition, the stability of the solution is also discussed under more general conditions.     

Sensitivity to brief changes of interaural time and interaural intensity

The purpose of this study was to measure listeners' abilities to detect brief changes in interaural temporal disparities (ITDs) or interaural intensitive disparities (IIDs) conveyed by bursts of noise (probes) temporally and symmetrically flanked by segments of diotic or uncorrelated noise. Thresholds were measured using a four-interval, two-alternative, forced-choice adaptive task and the total duration of the bursts of noise was either 20, 40, or 100 ms. Probes were temporally centered within each burst and the durations of the probes ranged from 2 to 100 ms, depending upon the duration of the (longer) total burst of noise within which they were embedded. The results indicate that, for a given total duration of noise, there is a rapid decrease in threshold ITD or threshold IID as the duration of the probe is increased so that it occupies a larger portion of the total burst of noise. Mathematical analyses revealed that both threshold ITDs and threshold IIDs could be well accounted for by assuming that the listener processes both types of binaural cues via a single, symmetric, double-exponential temporal window. Interestingly, the shapes of the temporal windows that fit the data obtained from the human listeners resemble the shapes of the temporal windows derived by Wagner [H. Wagner, J. Comp. Physiol. A 169, 281-289 (1991)], who studied the barn owl. The time constants and relative weightings yielded temporal window functions that heavily emphasize information occurring within the very temporal center of the window. This temporal window function was found to be generalizable in the sense that it also accounts for classic data reported by Grantham and Wightman [D.W. Gratham and F.L. Wightman, J. Acoust. Soc. Am. 63, 511-523 (1978)] concerning sensitivity to dynamically changing interaural disparities.     

The influence of different segments of the ongoing envelope on sensitivity to interaural time delays

The auditory system is sensitive to interaural timing disparities in the fine structure and the envelope of sounds, each contributing important cues for lateralization. In this study, psychophysical measurements were conducted with customized envelope waveforms in order to investigate the isolated effect of different segments of a periodic, ongoing envelope on lateralization. One envelope cycle was composed of the four segments attack flank, hold duration, decay flank, and pause duration, which were independently varied to customize the envelope waveform. The envelope waveforms were applied to a 4-kHz sinusoidal carrier, and just noticeable envelope interaural time differences were measured in six normal hearing subjects. The results indicate that attack durations and pause durations prior to the attack are the most important stimulus characteristics for processing envelope timing disparities. The results were compared to predictions of three binaural lateralization models based on the normalized cross correlation coefficient. Two of the models included an additional stage to mimic neural adaptation prior to binaural interaction, involving either a single short time constant (5 ms) or a combination of five time constants up to 500 ms. It was shown that the model with the single short time constant accounted best for the data.     

Temporal weighting of binaural cues revealed by detection of dynamic interaural differences in high-rate Gabor click trains

Listeners detected interaural differences of time (ITDs) or level (ILDs) carried by single 4000-Hz Gabor clicks (Gaussian-windowed tone bursts) and trains of 16 such clicks repeating at an interclick interval (ICI) of 2, 5, or 10 ms. In separate conditions, target interaural differences favored the right ear by a constant amount for all clicks (condition RR), attained their peak value at onset and diminished linearly to 0 at offset (condition R0), or grew linearly from 0 at onset to a peak value at offset (condition 0R). Threshold ITDs and ILDs were determined adaptively in separate experiments for each of these conditions and for single clicks. ITD thresholds were found to be lower for 16-click trains than for single clicks at 10-ms ICI, regardless of stimulus condition. At 2-ms ICI, thresholds in RR and R0 conditions were similar to single click thresholds at 2-ms ICI; thresholds in the 0R condition were significantly worse than for single clicks at 2-ms ICI, consistent with strong rate-dependent onset dominance in listeners' temporal weighting of ITD. ILD thresholds, in contrast, were predominantly unaffected by ICI, suggesting little or no onset dominance for ILD of high-rate stimuli.     

Accurate detection of interaural time differences by a population of slowly integrating neurons

For localization of a sound source, animals and humans process the microsecond interaural time differences of arriving sound waves. How nervous systems, consisting of elements with time constants of about and more than 1 ms, can reach such high precision is still an open question. In this Letter we present a hypothesis and show theoretical and computational evidence that a rather large population of slowly integrating neurons with inhibitory and excitatory inputs (EI neurons) can detect minute temporal disparities in input signals which are significantly less than any time constant in the system.     

External shaping of laser pulse trains

Two simple passive systems are presented for modifying the gaussian pulse train envelope produce by a dye-mode-locked Nd: YAG laser oscillator. The first system truncates the gaussian to produce a flat-topped envelope; the second system sharpens the envelope to a triangular form. The second system is analyzed as an intensity-dependent filter.     

Rate discrimination of high-pass-filtered pulse trains

Difference limens for trains of 30-microseconds pulses were determined for repetition rates of 50, 100, 200, 400, and 800 pulses per second under conditions of no filtering and high-pass filtering (115 dB/oct) with corner frequencies of 2.5, 5.0, 7.5, and 10 kHz. Low-pass-filtered noise was mixed with the trains of impulses to preclude discrimination on the basis of potential low-frequency signal components. Measures were obtained from four trained listeners at a signal level of 30 dB SL relative to individually determined thresholds for each filter condition and repetition rate. The data support the hypothesis that resolution of pulse-train repetition rate involves both temporal- and frequency-based processes--the latter becoming ineffective when frequency resolution of the ear is insufficient to resolve separate harmonics of the signal. Inter- and intra-individual differences are interpreted as reflecting frequency resolution capacity.     

Physiological detection of interaural phase differences

Auditory evoked cortical responses to changes in the interaural phase difference (IPD) were recorded using magnetoencephalography (MEG). Twelve normal-hearing young adults were tested with amplitude-modulated tones with carrier frequencies of 500, 1000, 1250, and 1500 Hz. The onset of the stimuli evoked P1m-N1m-P2m cortical responses, as did the changes in the interaural phase. Significant responses to IPD changes were identified at 500 and 1000 Hz in all subjects and at 1250 Hz in nine subjects, whereas responses were absent in all subjects at 1500 Hz, indicating a group mean threshold for detecting IPDs of 1250 Hz. Behavioral thresholds were found at 1200 Hz using an adaptive two alternative forced choice procedure. Because the physiological responses require phase information, through synchronous bilateral inputs at the level of the auditory brainstem, physiological "change" detection thresholds likely reflect the upper limit of phase synchronous activity in the brainstem. The procedure has potential applications in investigating impaired binaural processing because phase statistic applied to single epoch MEG data allowed individual thresholds to be obtained.     

XFEL experiments: jitter of pump–probe time delays and pulse intensities

Jitter of XFEL signals due to fluctuations in shot‐to‐shot time delays and intensities are explored in the frame of a statistical theory of X‐ray diffraction from liquids. Deformed signals are calculated at different levels of pump–probe jitter. A new method is proposed to eliminate these distortions.