Leslie R. Bernstein

Auditory Processing of Interaural Timing Information: New Insights

Differences in the time‐of‐arrival of sounds at the two ears, or interaural temporal disparities (ITDs), constitute one of the major binaural cues that underlie our ability to localize sounds in space. In addition, ITDs contribute to our ability to detect and to discriminate sounds, such as speech, in noisy environments. For low‐frequency signals, ITDs are conveyed primarily by “cycle‐by‐cycle” disparities present in the fine‐structure of the waveform. For high‐frequency signals, ITDs are conveyed by disparities within the time‐varying amplitude, or envelope, of the waveform. The results of laboratory studies conducted over the past few decades indicate that ITDs within the envelopes of high‐frequency are less potent than those within the fine‐structure of low‐frequency stimuli. This is true for both measures of sensitivity to changes in ITD and for measures of the extent of the perceived lateral displacement of sounds containing ITDs. Colburn and Esquissaud ( 1976 ) hypothesized that it is differences in the specific aspects of the waveform that are coded neurally within each monaural (single ear) channel that account for the greater potency of ITDs at low frequencies rather than any differences in the more central binaural mechanisms that serve these different frequency regions. In this review, the results of new studies are reported that employed special high‐frequency “transposed” stimuli that were designed to provide the high‐frequency channels of the binaural processor with envelope‐based information that mimics waveform‐based information normally available only in low‐frequency channels. The results demonstrate that these high‐frequency transposed stimuli (1) yield sensitivity to ITDs that approaches, or is equivalent to, that obtained with “conventional” low‐frequency stimuli and (2) yield large extents of laterality that are similar to those measured with conventional low‐frequency stimuli. These findings suggest that by providing the high‐frequency channels of the binaural processor with information that mimics that normally available only at low frequencies, the potency of ITDs in the two frequency regions can be made to be similar, if not identical. These outcomes provide strong support for Colburn and Esquissauds ( 1976 ) hypothesis. The use of high‐frequency transposed stimuli, in both behavioral and physiological investigations offers the promise of new and important insights into the nature of binaural processing. J Neurosci. Res. 66:1035–1046, 2001.

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.

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 profile‐analysis bandwidth

Detection of a change in spectral shape, or profile analysis, appears to be mediated by comparisons across widely separated frequency ‘‘channels’’ rather than by local comparisons among adjacent frequency regions [e.g., Green et al., J. Acoust. Soc. Am. 73, 639–643 (1983)]. Two experiments were conducted in order to determine the ‘‘resolution bandwidth’’ of these channels. The first involved detection of an increment to a single component of a multicomponent background as a function of the number of components in the background. Performance improved as the number of components was increased from 3 to 21. Further increases yielded poorer performance and the estimate of the ‘‘resolution bandwidth’’ from these data suggests that this poorer performance was due simply to masking. The second experiment involved discrimination of a multicomponent complex having a flat amplitude spectrum from one having a sinusoidally ‘‘rippled’’ amplitude spectrum. The latter experiment yielded somewhat larger estimates of the ...

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.

Binaural beats at high frequencies: Listeners’ use of envelope‐based interaural temporal and intensitive disparities

Two experiments were conducted in order to assess listeners’ ability to utilize interaural disparities within interaurally ‘‘mistuned’’ envelopes of two‐tone complexes centered at 3500 Hz. In the first experiment, listeners were required to distinguish between rightward or leftward directions of intracranial movement produced by the beat. The stimuli and experimental paradigm were designed so that such judgments would be made on the basis of the dynamically varying, envelope‐based interaural temporal disparities (ITDs). In the second experiment, listeners were required only to distinguish between the presence or absence of an envelope‐based binaural beat in a paradigm similar to that employed by McFadden and Pasanen [Science 190, 394–396 (1975)]. For both experiments, monaural envelope (beat) rates of between 10 and 640 Hz were employed with interaural beat rates of 0.25, 0.5, and 1.0 Hz. The data indicate that listeners are indeed capable of discriminating the direction of intracranial movement based on time‐varying ITDs conveyed by the envelopes of high‐frequency two‐tone complexes. Changes in sensitivity produced by altering the rate of change of the ITD and/or its extent appear to be accounted for by assuming that there is a relatively simple trade‐off between these two parameters. When the task is changed to one that is similar to that employed by McFadden and Pasanen [Science 190, 394–396 (1975)] such that listeners are merely required to discriminate between the presence and absence of an envelope‐based interaural beat, the data appear to be accounted for quite parsimoniously by assuming that listeners base their decisions upon the presence or absence of dynamically varying interaural intensitive disparities.

Spectral interference in a binaural detection task

Several investigations suggest that sensitivity to changes in interaural disparities within select spectral regions may be degraded by the presence of energy at other, even remote, spectral regions. This study assessed whether similar degradations would be observed in an MLD paradigm. Detection thresholds were measured for NoSo and NoS pi. The signal, an 800-Hz tone (100-ms), was presented in continuous, broadband noise. Thresholds were also measured in the presence of a 400-Hz tone (the interferer) presented with an interaural phase disparity of 180 degrees and gated simultaneously with the signal or presented continuously. NoS pi thresholds increased by about 7 dB with the gated interferer at 80 dB SPL. Smaller increases were observed with lower levels of the interferer. Presenting the interferer continuously reduced substantially its effect. NoSo thresholds were affected only slightly by the interferer. Reversing the roles of the signal and interferer (400-Hz signal, 800-Hz interferer) led to smaller, but reliable degradations in performance. Diotic interferers had, in general, smaller effects on performance. The possible relation between the mechanisms that produce interference and those that foster an ability to segregate sources of sound is discussed.

The normalized correlation: Accounting for NoSπ thresholds with Gaussian and ‘‘low‐noise’’ masking noise

Recently, both Eddins and Barber [J. Acoust. Soc. Am. 103, 2578–2589 (1998)] and Hall et al. [J. Acoust. Soc. Am. 103, 2573–2577 (1998)] demonstrated that ‘‘low‐noise’’ noise produced more masking of antipha‐ sic tones than did Gaussian noise. Eddins and Barber could not account for this finding by considering changes in the normalized interaural correlation produced by adding antiphasic signals to the diotic maskers. They calculated the normalized correlation subsequent to half‐wave, square‐law rectification and low‐pass filtering. That approach was used successfully by Bernstein and Trahiotis [J. Acoust. Soc. Am. 100, 3774–3784 (1996)] to account for binaural detection performance as a function of frequency. In this presentation, it will be shown that one can account quantitatively for thresholds obtained with ‘‘low‐noise’’ noise and with Gaussian noise by incorporating a stage of compression prior to rectification and low‐pass filtering. The compressive function employed produces effects similar to tho...

Some physical and psychological effects produced by selective delays of the envelope of narrow bands of noise

One can construct narrow bands of noise that contain delays of either the envelope, the phase, or the carrier separately or in combination. Delayed and undelayed noises will have identical spectra if, and only if, both the envelope and the phase undergo delays of the same magnitude. To study lateralization of these signals, an acoustic pointing task was employed in which listeners varied the interaural intensitive disparity of a narrow band of noise (the pointer) so that it matched the position of a second, experimenter-controlled stimulus (the target) which contained symmetric interaural delays of only the envelope. Targets were narrow bands of noise with center frequencies chosen at octave intervals between about 500 Hz and about 4000 Hz. The smallest bandwidth was 100 Hz and the largest was 800 Hz. For high-frequency stimuli, delays of only the envelope of a narrow band of noise appear to mediate lateralization which is greatest for bands centered near 2000 Hz. For low frequencies, delays larger than 800 microseconds were required to produce acoustic images appreciably away from the midline. These findings confirm the notion that listeners are sensitive to interaural temporal disparities in the envelopes of high-frequency, complex stimuli.