Interaural Amplitude Research Articles

Masker bandwidth and the MLD

Thresholds for binaurally in-phase (SO) and out-of-phase (Sπ) tones masked by diotic noise were measured as a function of noise bandwidth at 250 and 4000 Hz. Results show that the familiar dependence of the MLD for a tone in wideband noise on the frequency of the tone derives primarily from the dependence of critical bandwidth on frequency and only secondarily from a frequency dependence, per se. MLD's measured with narrowband (≈10 Hz) noise muskets are 25–30 dB at 250 Hz and about 20 dB at 4000 Hz. The difference between Sπ thresholds at 250 Hz and 4 kHz measured with narrowband maskers supports the hypothesis that the detection cue is interaural phase difference at low frequencies and interaural amplitude difference at high frequencies. In both frequency regions, “binaural sluggishness,” which has been documented by Grantham and Wightman [J. Acoust. Soc. Am. 63, 511–523 (1978)], is evident in the increase of the Sπ threshold with an increase in noise bandwidth at narrow bandwidths. [Work supported by NIH.]

The use of temporal and amplitude cues by schizophrenics, psychiatric controls, and aged normals in auditory lateralization.

The present study investigated the ability of paranoid and nonparanoid schizophrenics, psychiatric controls, and aged controls to use temporal and amplitude cues to make judgments about the direction of auditory stimuli on an auditory lateralization task. Twelve subjects in each group were tested on three lateralization conditions: temporal--fixed, temporal--shifted, and amplitude. The subjects' task was to identify the apparent direction (left or right) of a train of clicks presented binaurally through headphones. In the temporal conditions interaural click onset differences provided the lateralization cues, while in the amplitude condition interaural amplitude differences provided the cues. In the temporal--fixed condition, onset asynchrony was constant for the duration of each stimulus. In the temporal--shifted condition, onset asynchrony was introduced midway through each stimulus presentation. A forced-choice random staircase method was used to determine each subject's 70 per cent correct response threshold. Results indicated that paranoids had particular difficulty with the temporal--fixed condition relative to the temporal--shifted condition. The opposite was true for the nonparanoids. Both control groups showed no significant difference in performance on the two temporal conditions. Psychiatric controls performed significantly better than aged controls in both the fixed and shifted conditions. There were no significant differences among groups on the amplitude condition. The data were discussed in terms of schizophrenics' clinical symptoms and previous perceptual research with schizophrenics.

Interaural time and amplitude differences together

Interaural time jnds were measured for a 500-Hz and 1000-Hz tone-burst signal using a 2IFC adaptive procedure. The time delay was presented to one ear in one interval. In addition, an interaural amplitude difference was presented simultaneously. Thus, perceptual movement was created by both an interaural time difference and an interaural amplitude difference, which either reinforced or opposed each other (i.e., time delay accompanied by amplitude increase or decrease). With the interaural amplitude difference remaining constant for each jnd measurement, the interaural time jnd was determined for alpha ranging from +2 dB to −2 dB in 12 dB steps. Results indicate that combining the interaural amplitude difference and interaural time difference so that they reinforce each other decreases the interaural time jnds. That is, when each is below threshold, they combine to be detectable. Similarly, when they oppose each other the interaural time jnd increases. The trading ratio (μsec/dB) is substantially larger when time and amplitude are in opposition than when they reinforce each other.

Binaural hearing with a bilateral, high‐frequency loss

Results from four binaural experiments are presented for subjects with bilateral, high‐frequency hearing losses. The experiments are interaural time discrimination, interaural amplitude discrimination, interaural correlation discrimination, and binaural detection of tones masked by noise. The stimuli are third‐octave noises and tones at six different center frequencies. Preliminary data with noise at a center frequency in the normal range (500 Hz) and at a center frequency in the loss range (3 kHz) of the audiogram show abnormal performance for all tests, at both frequencies. Specifically, reduced MLDs (∼6 dB), increased interaural correlation jnds (∼0.25 from a reference correlation of 1 and ∼0.7 from a reference correlation of 0), increased interaural amplitude jnds (∼3 dB), and increased interaural time jnds (⩾60 μs). [Work supported by NIH.]

The effect of noise on time-intensity trading in lateralization

A two-interval forced-choice lateralization task was used to determine interaural delays and interaural amplitude ratios which, when opposed in quiet, led to chance lateralization; the opposed cues formed time-intensity trades. Three types of signal were used: (1) 20-μs clicks; (2) 250-ms bursts of a 305 Hz tone; and (3) 250-ms bursts of 3965 Hz carrier amplitude modulated to a depth of 100% at a modulation rate of 305 Hz. The addition of various levels of broad-band white noise did not influence the relative effectiveness of the two lateralization cues with pure tone signals; however, the addition of noise to clicks and to AM waveforms led to lateralization judgements in which the cue based on interaural delay appeared to be less effective than in quiet.

Binaural interaction of impaired listeners. A review of past research.

This paper reviews past studies of binaural interaction in impaired listeners. The topics covered include localization and lateralization; equal loudness, centering and lateralization adaptation; detection of signals in noise; and discrimination of interaural time delay and interaural amplitude ratio. Comments concerning experimental design and procedures, as well as interpretation of results, are often included with descriptions of the studies.

Comparison of interaural time and amplitude jnds in noise

Interaural time jnds and interaural amplitude jnds for a 500‐Hz sinusoid were measured in the presence of either perfectly correlated noise (M0), negatively correlated noise (Mn), or no additional noise using a 21FC adaptive procedure. The correlated noise was presented at a level which would produce a 30‐dB threshold shift and this level was maintained for the negatively correlated noise condition. Signal levels in the noise were 10, 20, 30, and 40 dB above detection threshold in the correlated noise (M0S0), and signal levels in quiet were 10, 20, 30, and 40 dB SL. The data indicate that at equal sensation levels, both interaural time and amplitude jnds are smaller when measured against a background of correlated noise than when measured in quiet, and that the largest jnds occur for the negatively correlated noise. Although the differences are greater at low signal levels, they continue to exist at the higher levels for both types of interaural discrimination.

Interaural time and amplitude discrimination in noise

Just‐noticeable differences (jnds) of interaural time delay (ITD) and interaural amplitude differences (IAD) were measured for 50‐dB SPL 500‐Hz binaural tones in the presence of 100–1000 Hz broadband markers. Data were collected using maskers with various combinations of ITD and IAD. The time and amplitude jnds exhibit similar dependencies on target‐to‐masker ratio, and they vary with masker type in a fashion previously described by Cohen [J. Acoust. Soc. Am. Suppl. 1 64, S35 (1978)] and Ito et al. [J. Acoust. Soc. Am. Suppl. 1 65, S121 (1979)]. Many of these data trends are consistent with the predictions of simple models based on the instantaneous interaural time and amplitude differences of the stimuli and their variability. Our model that generates lateral position estimates from operations on auditory nerve activity [Stern and Colburn, J. Acoust. Soc. Am. 64, 127–140 (1978)] does not accurately describe these results, for reasons at least partly related to inadequacies in its description of the auditory‐nerve response to these stimuli. [Supported by NIH.]

Weber's law, the "near miss," and binaural detection.

Weber functions (ΔI/I in dB) for gated 250-Hz tones were studied for monaural and several binaural stimulus configurations (homophasic, and antiphasic with varying phase angle for addition of signal to masker). The various cues for discrimination of signal plus masker from masker alone are functions of intensity increments at one or both ears, an intensity increment at one ear coupled with a decrement at the other, or the introduction of a phase difference between the ears. The decline of the Weber fraction with increasing masker level (the “near miss” to Weber’s law) was confirmed for monaural discrimination over the entire 40-dB range, and a similar rate of decline was found for various binaural stimuli over the lower half of that range. The data also confirm the individual differences found in other studies for sensitivity favoring either interaural amplitude or interaural phase shifts.

The precedence effect and its possible role in the avoidance of interaural ambiguities.

The precedence effect, the observation that sound-source localization is determined largely by the interaural cues associated with the earlier-arriving direct sound to the neglect of later-arriving reflections, was investigated in several psychophysical experiments. The first experiment employed a stimulus composed of a continuous noise and its delayed repetition to simulate a direct sound and a single reflection. Comparison of the interaural amplitude and phase differences in this stimulus with its judged lateral position showed that the interaural amplitude and phase differences in this stimulus with its judged lateral position showed that the interaural differences do not predict lateralization judgments as simply as does a knowledge of the temporal pattern of stimulation, given the precedence effect. Most of the experiments were attempts to outline the time course of the precedence effect by measuring just-noticeable differences in interaural time and intensity of brief (less than 5 ms) wide-band noise bursts. The principal finding of these experiments was that interaural sensitivity to changes in both time and intensity follows a nonmonotonic course after the abrupt onset of as ound. Sensitivity is degraded for a period from approximately 0.5 to 10 ms after onset, with the largest jnd's at delays of 2-3 ms. The precedence effect can be understood as resulting from this temporary lapse of interaural sensitivity. It was speculated that this temporal variation in interaural acuity serves to inhibit: (1) the representation of multiple and ambiguous interaural time delays between corresponding points on the waves of periodic stimuli; and (2) the extraction of interaural phase at frequencies greater than approximately 1400 Hz.

An in‐depth psychoacoustical study of a patient with vestibular schwannoma

Monaural and binaural capabilities of a thirty‐year‐old male with a unilateral vestibular schwannoma were studied. His right ear appeared to be within normal limits for all tests. The threshold in his left ear was elevated approximately 20 dB from 250 to 1400 Hz and fluctuated sharply above 1400 Hz. Stimuli were pure tones and one‐third‐octave noise bands at three frequencies: 0.5, 1, and 3.3 kHz. Alternate‐binaural‐loudness matches showed decruitment. Frequency discrimination and frequency selectivity were reduced in the impaired ear, as indicated by large difference limens and broad tips of the psychoacoustical tuning curves. Intensity discrimination in the impaired ear was impoverished at all levels and frequencies except near threshold at 1 kHz. Experiments on interaural discrimination failed to demonstrate any sensitivity. For interaural amplitude, all above‐chance performance could be attributed to monaural cues in the normal ear. For interaural time and interaural correlation, no sensitivity could be demonstrated. [Work supported by NIH.]

The effect of noise on time/intensity trading in lateralization

A standard two‐interval forced‐choice lateralization task was used to determine interaural delays and interaural amplitude ratios which, when opposed in quiet, led to chance lateralization, i.e., the opposed cues formed time/intensity trades. The signals were 20‐μs clicks or 250‐ms bursts of: (1) a 304‐Hz tone, and (2) a 3952‐Hz carrier amplitude modulated to a depth of 100% at a modulation rate of 304 Hz. The addition of various levels of broadband white noise did not influence the relative effectiveness of the two lateralization cues with pure tone signals; however, the addition of noise to clicks and to AM waveforms led to lateralization judgments in which the cue based on interaural delay was less effective than it had been in quiet. This effect can be explained by changes in the frequency region used by Observers listening to clicks but not to AM waveforms. [This research was supported by both Science and the Medical Research Council.]

Sound localization with impaired hearing

Discrimination tests were made on persons with hearing impairments and on patients with multiple sclerosis. Results include estimates of horizontal minimum audible angle (MAA) at eight azimuths, vertical MAA straight ahead, and just‐noticeable differences in interaural time delay (time JND) and interaural amplitude ratio (amplitude JND). The standard stimulus was broadband (0.25–10 kHz), pulsed (1 s) noise. Conductive cases (>35‐dB loss) gave abnormal values in almost all tests. Symmetric sensori‐neural cases with speech discrimination scores above 90% gave roughly normal values in all tests, while the corresponding group with scores below 80% gave elevated values for the vertical MAA and the horizontal MAA on the sides. Meniere's cases had normal MAAs and time JNDs and an amplitude JND that was normal only at high levels. Neurinoma cases gave abnormal values in at least one measurement each, with large intersubject variability. Persons with only one functional ear showed no ability to discriminate intera...

Theory of binaural interaction based on auditory-nerve data. III. Joint dependence on interaural time and amplitude differences in discrimination and detection.

This paper is primarily concerned with issues of symmetry-asymmetry in the joint dependence of binaural performance on interaural time delay and interaural amplitude ratio. The predictions of our model based on auditory-nerve data are shown to be inadequate for available data showing asymmetries in interaural time discrimination. Measured detection thresholds for a 500-Hz tone burst masked by wide-band noise are reported for nonunity amplitude ratios and both polarities of delays in order to test for asymmetries analogous to those observed in the time discrimination case. The detection results show only small asymmetries and are not inconsistent with the model. A general discussion of our model and the implications of available empirical results leads to a suggested modification of the model. We show by an example that modifications of the type we are suggesting are capable of describing the detection data and at least the gross trends of the discrimination data.

Adaptation effects in binaural experiments

Experiments were conducted to examine some of the effects of imbalanced interaural parameters on binaural lateralization. We examined adaptation both to stimuli with interaural amplitude differences and to stimuli with interaural time differences. To measure the adaptation over time, 30‐trial runs were made with each run lasting about 15 min. On each trial, the subject was required to adjust the interaural time delay from an initial random value to a value that resulted in a centered image. For a low‐pass filtered pulse train with an interaural amplitude difference of 10 dB, the amount of interaural time delay needed to center the image changed from 250 μs at the outset to 100 μsec at the end of the run. These results are consistent with earlier results using a paradigm nearly identical to ours [M. P. Bertrand, Proc. R. Soc. Med. 65, 3–4 (Sept. 1972)]. For the same low‐pass filtered pulse train with no interaural amplitude differences, but with the initial random delay restricted to one ear, the amount of time delay needed to center the image changed by as much as 50 μs during the course of a run, i.e., at the end of a run the subjects no longer heard a diotic presentation (zero delay) in the center of their heads. Two possible mechanisms of adaptation will be discussed. [Work supported by NIH.]

Localization accuracy in the horizontal plane

The accuracy and bias of 12 subjects in localizing sound sources in the horizontal plane are described. All tests were performed with unrestricted head motion using band limited pink noise. Unlike most previous localization work, circular statistics were employed for data analysis and the effects of front-rear confusion are examined in detail. The effect of varying stimulus frequency between 500 Hz and 4 kHz was investigated and the results are considered in relation to the tradeoff of interaural amplitude and phase cues. The dependence of both accuracy and bias on stimulus length was studied, showing little improvement for sounds longer than 0.33 s. The perceived directions were biased away from the median plane, the bias being greater for sounds behind the listener than in front. The variability of responses vs. direction follows a cardioid shape, sources behind the listener producing the highest variability. Extensive regression analysis was performed to determine the statistical significance of these dependencies. This also resulted in an empirical model which predicts the perceived direction era sound given its actual direction, frequency, and duration.

Lateral position and interaural discrimination.

Experimental results are described for the dependence on interaural stimulus parameters of subjective lateral-position matches and just-noticeable differences (jnd’s) in interaural amplitude and time. All experiments employed 500-Hz tones presented through acoustically monitored headphones. The overall amplitudes of the stimuli were iteratively adjusted to eliminate subjective loudness cues in the interaural-amplitude jnd experiments. Performance in the experiments is analyzed within the framework of a probabilistic lateralization model in which internal noise is explicitly treated. The model predicts a specific relation between position matches and the interaural amplitude and time jnd’s. The data are qualitatively consistent with the predictions, and the quantitative agreement is good for one subject, moderate for the second, and poor for the third. In particular, the model predicts the observed asymmetry in the dependence of the interaural-time jnd on interaural time delay for nonunity interaural amplitude ratios. Calculated values of the internal noise of the subjective image position are in qualitative agreement with reports by observers of image diffuseness as a function of interaural parameters of the stimulus.

Discrimination of symmetric, time-intensity traded stimuli

New experiments have been conducted to determine the extent to which listeners can discriminate between different combinations of interaural time and interaural amplitude for stimulus configurations which eliminate loudness, lateralization, and image-diffuseness cues. A 21, 2AFC paradigm was used and the task of the subject was to determine the order of two stimuli, each of which was a slowly gated 500-Hz tone with an interaural time and amplitude combination that resulted in a centered image. The two stimuli to be discriminated were symmetric in that they differed only in the polarity of the interaural differences. Also, in order to reduce artifacts introduced by variations in the coupling of the earphones to the head, acoustic monitoring was performed both before and after each experimental run. For three of the four subjects tested, discrimination performance was at the chance level. However, the fourth subject (the experimenter) was able to perform well above chance after extensive practice utilizing a special listening technique. His perceptions were not consistent with models predicting a “time” image and a “time-intensity traded” image. [Work supported by NIH.]

Lateralization by frequency in dichotic tonal sequences as a function of interaural amplitude and time differences

When listeners are presented with a dichotic sequence of tones which alternate in pitch between 400 and 800 Hz, such that when one ear receives 400 Hz the other receives 800 Hz (and vice versa) many hear a single high tone in one ear alternating with a single low tone in the other. This effect is based in part on a lateralization of each fused tonal percept towards the ear receiving the 800-Hz signal. It was found that in four out of five subjects, lateralization towards the 800-Hz signal occurred even when the 400-Hz signal was substantially higher in amplitude, and clearly higher in apparent loudness. However, although this effect was prominent with sequences consisting of 20 dichotic tone pairs, it was weak with sequences consisting of two dichotic tone pairs. With long sequences, lateralization to the 800-Hz signal was also prominent even when it lagged or led the 400-Hz signal by several milliseconds.

Lateral position and interaural discrimination: Data and blackbox model

Experimental results are described for the dependence on interaural stimulus parameters of subjective lateral position and just-noticeable differences in interaural amplitude and time. The experiments employed 500-Hz tones presented through acoustically monitored headphones [R. H. Domnitz, J. Acoust. Soc. Am. 58, 510–511 (1975)] at 55 dB SPL. The overall amplitudes of the stimuli in the interaural amplitude discrimination experiment were iteratively adjusted to eliminate subjective loudness cues. Performance in the experiments is analyzed within the framework of a probabilistic subjective lateralization model in which internal noise is explicitly treated. One prediction of the model relates the ratio of the interaural time and amplitude jnd's to subjective position. Although the data are qualitatively consistent with this prediction, there is close quantitative agreement for only one of three subjects. The model also predicts values of the internal noise of the subjective image position and these are in qualitative agreement with reports of image diffuseness as a function of interaural parameters. Nontradability between large increments of interaural amplitude and time is implied by the experimental results, and can be interpreted in the model as discrimination between tones with equal position means but different position variances. [Work supported by NIH.]