Thomas N. Buell

Combination of binaural information across frequency bands

Perceptual grouping of the frequency components from a source into a single auditory object is needed when localizing a complex sound in an environment where other sounds are also present. Two acoustic regularities that might allow for such grouping are a harmonic relation among the components and a commonality of their spatial positions. The utility of these cues was examined in a forced choice psychophysical task by measuring sensitivity to interaural differences of time (IDT) for low-frequency stimuli presented via earphones. In the first experiment, stimuli were composed of either one, two, or three frequencies. A signal detection analysis used to predict the effects of combining information across frequencies found summation to be optimal, regardless of the harmonicity of the complex. A second experiment presented two-frequency complexes in which one tone, the target, contained the IDT to be detected while the other, the distractor, was constant across all three intervals of the forced choice. For inharmonic complexes, performance for the target-distractor combinations was equivalent to that found for targets presented alone, suggesting segregation of the targets and distractors into separate auditory objects. However, for harmonic target-distractor combinations, performance was diminished. A signal detection analysis of these data supports the idea that for purposes of lateralization, the interaural information in the targets and distractors was combined into a variance-weighted value, even though it meant a lowering of performance. Thus it seems that for the grouping of complex acoustic stimuli in space, harmonic structure is more important than commonality of spatial position.

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.

Judgments of sound volume: Effects of signal duration, level, and interaural characteristics on the perceived extensity of broadband noise

While auditory stimuli are often described in terms of their apparent extensity, such descriptions have usually not been collected systematically. Moreover, the few deliberate attempts to evaluate image size have rarely gone beyond the classic parameters of stimulus frequency and intensity. In the present study a direct magnitude estimation procedure was employed. Seventeen subjects numerically estimated the apparent size of images produced by broadband noise stimuli. Under earphone listening conditions, signals were presented either dichotically (uncorrelated noise), diotically (correlated noise), or monaurally (noise led to a single earphone). The signals in each of these modes varied in duration (100, 300, 1000, and 3000 msec) and intensity level (60, 75, and 90 dB A weighted). Size estimates were plotted as power functions and analyzed with a repeated measures design analysis of variance. Consistent with previous research, the main effects of duration and intensity were both highly significant (p less than 0.001). In addition, a highly significant effect for mode of presentation was found (p less than 0.001). Across conditions, dichotic stimulation produced the largest images and monaural stimulation the smallest (about half the size of the diotic images). This last result is the first quantification of previous anecdotal observations. General implications of these results were discussed.

Interaural temporal discrimination using two sinusoidally amplitude-modulated, high-frequency tones : conditions of summation and interference

This paper concerns sensitivity to interaural temporal delays (ITD) in the envelopes of two, sometimes simultaneously presented, sinusoidally amplitude-modulated (SAM) tones. The SAM tones were fully modulated (typically at a rate of 250 Hz) and had carrier frequencies of either 2 or 4 kHz. Of particular interest were cases in which the delay to be detected (target ITD) occurred in only one spectral region or in both. The reference ITD, to which the target ITD was added, was either 0, 300, or 600 microseconds. There were three general outcomes: (1) When both regions contained a target ITD, there was an improvement in sensitivity that was quantitatively consistent with an optimal use of independent information. This type of summation was seen even when the target ITDs were added to a pair of SAM tones with two different reference ITDs; (2) when the target ITD was restricted to the higher spectral region, sensitivity was reduced, indicating interference. Interference occurred when the two spectral regions had the same or different reference ITDs and the same or different rates of modulation; (3) sensitivity was unaffected (i.e., no interference occurred) when the target ITD was restricted to the lower spectral region of the pair of SAM tones. These results supplement the observations of Buell and Hafter [J. Acoust. Soc. Am. 90, 1894-1900 (1991)], who used low-frequency tones.

Lateralization of bands of noise as a function of combinations of interaural intensitive differences, interaural temporal differences, and bandwidth

Listeners indicated the intracranial position of bands of noise (from 50 to 400 Hz in width) for several combinations of interaural intensive differences (IID), and interaural temporal differences (ITD), and/or interaural phase differences (IPD). All ITD and IPD combinations produced an interaural delay of 1500 microseconds at the center frequency of the noise. The interaural phase spectra were constructed to produce several patterns of putative cross-correlation functions. Potency of IIDs depended greatly on particular combinations of bandwidth, ITD and IPD. For some combinations, changing the IID by only 3 dB resulted in large shifts in laterality (sometimes moving the image from near one ear to near the other). The complex interactions observed make the results incompatible with the traditional notion that IIDs simply act as weights or scalars. Rather, IIDs act in two distinct manners: (1) as independent scalar quantities and (2) by interacting with specific combinations of bandwidth and ITD/IPD, which is believed to reflect an action within the cross correlation surface.

On the use of adaptive procedures in binaural experiments

Adaptive psychophysical procedures have been routinely used in monaural experiments for many years, but only sparsely used in binaural experiments. In this letter, (1) the increasing use of adaptive procedures in binaural experiments is documented; (2) factors that determine their appropriateness are discussed; and (3) data that attest to their usefulness are presented.

Potency of interaural differences of intensity on the lateralization of interaurally delayed bands of noise.

Listeners indicated the intracranial position of noises (bands of from 50 to 400 Hz) for several combinations of interaural differences of intensity (IDI), time (IDT), and/or phase (IDP). All IDT and IDP combinations produced an interaural delay of 1500 μs at the center frequency of the noise. The interaural phase spectra were constructed to produce several patterns of putative cross‐correlation functions. Potency of IDIs depended greatly on particular combinations of bandwidth, IDT, and IDP. For some combinations, changing the IDI by only 3 dB resulted in large shifts in laterality (sometimes moving the image from near one ear to near the other). Generally, potency of the IDIs was found to be greatest for narrow bands of noise having pure IDTs. The complex interactions observed make the results incompatible with the traditional notion that IDIs simply act as weights or scalars. Instead, the data are aptly described by cross‐correlation models that incorporate ‘‘pattern analysis.’’ [Research supported by ...

Parametric study of onset effects in lateralization

When lateralization signals made up of trains of high‐frequency clicks (n per train), optimum usage of the information carried in each click results in binaural thresholds that decrease at the rate 1/n. However, when the interclick interval (ICI) is short, the relation between threshold and n is best described by raising n to the exponent k [where 0 ⩽ k ⩽ 1 and k = f(ICI)]. This implies that shorter ICIs produce greater dependence on information contained in the signals onset, with k = 0 meaning that no information is derived beyond the first click in the train while k = 1 means that each click is equally effective. Based on discriminations of interaural differences of time (IDT), it will be shown that the relation between k and ICI is reasonably linear, ranging from a k of 0.0 for ICIs of 1–2 ms to k equal 1.0 for ICIs of about 11–13 ms. Interestingly, if the ICIs are made even longer, the k remains at 1.0 but the absolute performance grows worse. It is what one might expect if the internal jitter added...

Recovery from saturation in lateralization of high‐frequency stimuli

Lateralization of trains of high‐frequency clicks improves with both the number of dicks in a train (n) and interclick interval (ICI). Functions relating log threshold to log n are linear; however, the absolute values of the slopes decrease from the optimal—0.5 toward 0.0 with shorter ICIs (generally 10 ms or less). We have argued that shallower slopes are the result of a form of rate‐dependent saturation which reduces the amount of binaural information derived from successive clicks in the train. The decay of the proposed saturation was measured here by presenting trains with an ICI of 2.5 ms but with each train broken by either one or three pauses. Of interest was the duration of the break needed to fully recover from saturation. A pause of 7.5 ms proved to be sufficient, implying that the saturation responsible for shallow slopes in log‐threshold, log‐n plots is the result of an active process rather than of fatigue. [Supported by NIH.]

Auditory localization acuity in free field and simulated environments: Reaction time measures

A reaction time measure was used to compare localization acuity in a free field and a simulated environment (where headphone‐presented stimuli are processed so as to approximate the waveforms that would occur at each ear canal for a free‐field source). Four listeners discriminated the relative direction of a broadband noise (whether it was to the left or right of midline or whether it was in front or back of the interaural axis). The results across four listeners demonstrated that psychometric functions could be plotted for both reaction time as well as percent correct versus angle. Localization was better in the free‐field conditions (higher percent correct scores as well as faster reaction times) for front–back discrimination; however, the free‐field advantage was negligible for left–right discrimination. Under headphone presentation, there was large inter‐subject variability in reaction times for front–back discrimination, but not for left–right discrimination. Although the above findings were not espe...