Observer weighting of interaural delays in filtered impulses

Onset dominance in sound localization was examined by estimating observer weighting of interaural delays for each click of a train of high-frequency filtered clicks. The interaural delay of each click was a normal deviate that was sampled independently on each trial of a single-interval design. In Experiment 1, observer weights were derived for trains ofn=2, 4, 8, or 16 clicks as a function of interclick interval (ICI=1.8, 3.0, or 12.0 msec). For smalln and short ICI (1.8 msec), the ratio of onset weight to remaining weights was as large as 10. As ICI increased, the relative onset weight was reduced. For largen and all ICIs, the ongoing train was weighted more heavily than the onset. This diminishing relative onset weight with increasing ICI andn is consistent with optimum distribution of weights among components. Efficiency of weight distribution is near ideal when ICI=12 msec andn=2 and very poor for shorter ICIs and larger ns. Further experiments showed that: (1) onset dominance involves both within- and between-frequency-channel mechanisms, and (2) the stimulus configuration (ICI,n, frequency content, and temporal gaps) affects weighting functions in a complex way not explained by cross-correlation analysis or contralateral inhibition (Lindemann, 1986a, 1986b).     

Imperfect pitch: Gabor’s uncertainty principle and the pitch of extremely brief sounds

How brief must a sound be before its pitch is no longer perceived? The uncertainty tradeoff between temporal and spectral resolution (Gabor’s principle) limits the minimum duration required for accurate pitch identification or discrimination. Prior studies have reported that pitch can be extracted from sinusoidal pulses as brief as half a cycle. This finding has been used in a number of classic papers to develop models of pitch encoding. We have found that phase randomization, which eliminates timbre confounds, degrades this ability to chance, raising serious concerns over the foundation on which classic pitch models have been built. The current study investigated whether subthreshold pitch cues may still exist in partial-cycle pulses revealed through statistical integration in a time series containing multiple pulses. To this end, we measured frequency-discrimination thresholds in a two-interval forced-choice task for trains of partial-cycle random-phase tone pulses. We found that residual pitch cues exist in these pulses but discriminating them requires an order of magnitude (ten times) larger frequency difference than that reported previously, necessitating a re-evaluation of pitch models built on earlier findings. We also found that as pulse duration is decreased to less than two cycles its pitch becomes biased toward higher frequencies, consistent with predictions of an auto-correlation model of pitch extraction.     

Evaluation of maximum-likelihood estimators in nonintensive auditory psychophysics

This is a brief report on the use of maximum-likelihood (ML) estimators in auditory psychophysics. Slope parameters of psychometric functions are characterized for three nonintensive auditory tasks: forced-choice discrimination of interaural time differences (ΔITD), frequency (Δf), and duration (Δt). Using these slope estimates, the ML method is implemented and threshold estimates are obtained for the three tasks and compared with previously published data. ΔITD thresholds were additionally measured for human observers by means of two other psychophysical procedures: the constant-stimuli (CS) and the 2-down 1-up methods (Wetherill & Levitt, 1965). Standard errors were smallest for the ML method. Finally, simulations showed ML estimates to be more efficient than the CS andk-down 1-up procedures fork=2 to 5. For up—down procedures, efficiency was highest fork values of 3 and 4. The entropy (Shannon, 1949) of ML estimates was the smallest of the simulated procedures, but poorer than ideal by 0.5 bits.     

Lateralization of two-transient stimuli

In this study of the precedence effect in binaural hearing, subjects adjusted the interaural delay of a wideband acoustic pointer to match the perceived intracranial position of transient test stimuli presented over headphones. The test stimuli had leading and lagging components (either brief noise bursts or clicks), each with its own interaural delay. In some test conditions, the leading and lagging stimuli were coherent copies of one another, whereas in others, they were independent samples of noise. The duration of the stimuli and the delay from the leading component to the lagging component were also varied. All the stimulus conditions showed a moderate or strong precedence effect (i.e., covariation of perceived lateral position of the composite two-transient stimulus with the interaural delay of the leading component). Predictions of the lateralization data are presented for variants of models based on temporal weighting and/or bandpass correlation. In one model variant, the binaural stimuli are temporally weighted to emphasize the onset and then subjected to bandpass correlation analysis. In another variant, it is assumed that the onset mechanism provides a rough estimate of the initial interaural delay that guides a slower and more focused bandpass correlation analysis. The accuracies of these two model's predictions were equivalent and superior to those of models that either represent leading and lagging cues equally (bandpass correlation with no onset effect) or do not represent lagging cues at all (a complete precedence effect). The results of these analyses show the need for both a strong onset effect and for bandpass correlation analysis and suggest two modeling approaches for achieving that goal.     

Fast Fourier-based DSP algorithm for auditory motion experiments

A digital signal-processing (DSP) technique for rapid generation of complex auditory motion stimuli based on dynamic linear changes in interaural delay is described. In this technique, a pair of comple-mentary discrete Fourier transforms (DFTs) for which the component spacing in one series is different than that of the other is used. The appeal of this technique is its wide applicability, since it can generate real-time motion stimuli of any velocity and starting interaural delay for complex broadband or filtered noise waveforms and nonstationary sounds such as speech, music, and other natural sounds.     

Detection of spatial cues in linear and logarithmic frequency-modulated sweeps

Frequency- and amplitude-modulated (FM and AM) sounds are the building blocks of complex sounds. In the present study, we investigated the ability of human observers to process spatial information in an important class of FM sounds: broadband directional sweeps common in natural communication signals such as speech. The stimuli consisted of linear or logarithmic unidirectional FM pulses that swept either up or down in frequency at various rates. Spatial localization thresholds monotonically improved as sweep duration decreased and as sweep rate increased, but no difference in performance was observed between logarithmic and linear or between upand down-frequency sweeps. Counterintuitive reversals in localization were observed which suggested that the localization of high-frequency sweeps may be strongly dominated by amplitude information even in situations in which one might consider timing cues to be critical. Implications of these findings for the localization of complex sounds are discussed.     

Auditory psychomotor coordination and visual search performance

In Experiments 1 and 2, the time to locate and identify a visual target (visual search performance in a two-alternative forced-choice paradigm) was measured as a function of the location of the target relative to the subject's initial line of gaze. In Experiment 1, tests were conducted within a 260 degree region on the horizontal plane at a fixed elevation (eye level). In Experiment 2, the position of the target was varied in both the horizontal (260 degrees) and the vertical (+/- 46 degrees from the initial line of gaze) planes. In both experiments, and for all locations tested, the time required to conduct a visual search was reduced substantially (175-1,200 msec) when a 10-Hz click train was presented from the same location as that occupied by the visual target. Significant differences in latencies were still evident when the visual target was located within 10 degrees of the initial line of gaze (central visual field). In Experiment 3, we examined head and eye movements that occur as subjects attempt to locate a sound source. Concurrent movements of the head and eyes are commonly encountered during auditorily directed search behavior. In over half of the trials, eyelid closures were apparent as the subjects attempted to orient themselves toward the sound source. The results from these experiments support the hypothesis that the auditory spatial channel has a significant role in regulating visual gaze.     

A detection-theoretic model of echo inhibition

A detection-theoretic analysis of the auditory localization of dual-impulse stimuli is described, and a model for the processing of spatial cues in the echo pulse is developed. Although for over 50 years "echo suppression" has been the topic of intense theoretical and empirical study within the hearing sciences, only a rudimentary understanding of its mechanisms has emerged. In this article, psychometric functions and results from matching studies are used in developing a model that specifies the perceived position of the echo pulse as a normal deviate, with an expectation that is a logistic function of the echo delay and a variance that depends on interaural time difference. Loss of information in the echo event is quantified as a decline in the efficiency with which the binaural system receives information from the lag impulse.