Ervin R. Hafter

Discrimination of interaural delays in complex waveforms: Spectral effects

Sensitivity to interaural delays in high‐frequency waveforms was examined using amplitude‐modulated signals. Carrier frequencies ranged from 2 to 6 kHz, and modulation was varied from 50 to 550 Hz. The signals’ components were not harmonically related. Findings include: (a) As carriers exceed 3 kHz, a broad region appears (from 150 to 350 Hz of modulation) over which there is little change in sensitivity to interaural time differences in the modulation envelope. (b) Signals with interaurally‐discrepant carriers are more difficult to lateralize, but for carriers at and above 4 kHz, a region of constant performance can also be found as the interaural frequency difference is increased from zero. (c) Variations in modulation depth are useful in approximating the sideband attenuation that may occur as wideband signals are filtered by the auditory system. Such a technique allows an assessment of the importance of critical bands in these phenomena.

Lateralization of complex waveforms: effects of fine structure, amplitude, and duration.

In recent years there has been an increased interest in the use of interaural time as a cue for lateralization at high frequencies. Our study examines this question using AM complexes with carriers of 1800, 2400, 3000, 3600, 3900, 4200, and 4500 Hz, modulated at 300 Hz and interaurally delayed. The findings indicate that (a) the ability to lateralize these stimuli does not depend on low‐frequency distortion, since this ability remains despite the presence of intense low‐frequency masking; (b) the task is easier when the entire waveform, rather than just the modulation envelope, is delayed, though this difference seems to decline with practice; (c) the effects of stimulus amplitude and duration on the detection of interaural time differences are reminiscent of those observed with pure tones, although ongoing time differences may be more fully utilized with AM signals; and (d) lateralization declines with interaural carrier frequency discrepancy. We were unable to obtain lateralization of AM complexes at on...

Listening bandwidths and frequency uncertainty in pure‐tone signal detection

The effect of frequency uncertainty on the detection of tonal signals in noise was studied using a modified probe-signal method. Widths of the listening bands used during detection were measured directly, allowing for an analysis that separates the effects of having to monitor multiple independent bands from those due to limited frequency resolution. Uncertainty was varied by beginning each trial with a cue consisting of one, two, or four randomly chosen, simultaneously presented tones. An expected signal, whose frequency matched one of the components in a cue, was presented on a majority of trials. However, on remaining trials, the signal was a probe, which meant that its frequency differed from one of the components in the cue by a constant ratio. Performance as measured in percent correct declined for probes at increasingly distant ratios from the expected values. The results were converted to dB using individual psychometric functions for expected signals and listening bands were fitted using the rounded exponential filter of Patterson et al. [J. Acoust. Soc. Am. 72, 1788-1803 (1982)]. The obtained bandwidths are comparable to those reported using notched-noise maskers, but there is a small but consistent increase in bandwidth with increased numbers of components in the cues. The primary results is that the effects due to uncertainty are well described by a 1-of-M orthogonal band model, which takes into consideration limitations of the detector, including the widths of the listening bands.

Two‐Image Lateralization of Tones and Clicks

When interaural differences of time and level are set into opposition, subjects may report a single image that is determined in its lateralization largely by the interaural level difference. Such images show a fairly large “trading‐ratio.” They may report hearing an image that is little affected by the difference of level, but is dependent largely upon the interaural difference of time. Such images show a very small trading‐ratio. With practice, subjects can learn to hear and respond to both images, and center either at will. The present study is concerned with both images as they arise from tonal stimuli of various durations and rise fall times, and from high‐pass clicks.

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.

Binaural Interaction in Low‐Frequency Stimuli: The Inability to Trade Time and Intensity Completely

Subjects were presented with low‐frequency tonal signals, leading in time to one ear but more intense to the other. For differing values of interaural time (Δt) and intensity (ΔI), the listeners task was to detect the difference between the dichotic signals and diotic tones of the same frequency. The shapes of the psychometric functions demonstrate that time and intensity do trade but that the trade is incomplete—a result shown to be in accord with other experiments where listeners presented with such signals report the presence of two distinct “images.” A model of time‐intensity trading is presented in which it is assumed that differences of intensity are converted to time by two different trading mechanisms and that the two sums of Δt and converted ΔI act as different loci on a single interaural dimension. It is further suggested that the inner and outer hair cells may represent the site of the two trades.

Masking-level differences obtained with a pulsed tonal masker.

Masking‐Level Differences (MLDs) were obtained for the binaural condition N0Sπ with pulsed tonal signals masked by pulsed tones of the same frequency and duration. MLDs were measured relative to the condition N0S0. Frequencies tested were 250, 500, and 1000 Hz; the duration was 125 msec. Detection was measured with four values of the signal‐to‐masker phase, which for N0Sπ produced: (a) an interaural difference of time (Δt); (b) an interaural difference of intensity (ΔI); (c) Δt and ΔI both favoring the same ear (consonance); or (d) Δt and ΔI favoring opposite ears (dissonance). The magnitudes of the MLDs were largest for conditions of Δt alone, next largest for consonance, and least for dissonance and ΔI alone. The MLDs were positive in sign, in contrast to the negative MLDs that have previously been reported for similar stimuli. Data were shown to fit a lateralization model in which differential time‐intensity trading occurs for consonance and dissonance.

Cuing mechanisms in auditory signal detection

Detection of auditory signals under frequency uncertainty can be improved by presenting cues to the listeners. Since various cues have been found to differ in effectiveness, three conceivable mechanisms were considered which might account for these differences. Cuing might reduce the number and/or width of the employed auditory filters or listening bands. Also, cues could modulate the precision of frequency tuning of the filters. Psychometric functions were collected in a detection experiment with frequency uncertainty employing three kinds of cues: pure tones whose frequency was identical to that of the signal (iconic cues), complex tones with a missing fundamental equal to the signal (complex cues), and pure tones with a certain frequency relation to the signal (relative cues). Compared with a no-cue condition, all cue types improved detection performance. Fitting models to the data suggests that in the no-cue condition as well as the complex-cue condition, multiple bands were utilized, and that the iconic and relative cues induced single-band listening. There is no indication that accuracy of frequency tuning was responsible for cue-efficiency differences.

Responses of inferior colliculus neurons to free field auditory stimuli

Abstract The cues for auditory localization depend, in the natural state, on the size and shape of an animals head and pinnae and on the frequency of the acoustic stimulus. This study examined the responses of neurons in the inferior colliculus to sounds presented from various locations in a free field. A group of loudspeakers in an anechoic chamber was arranged in a semicircle around the cats head with a minimum speaker resolution of 20°. The frequency characteristics of the speakers were compensated for by a computer controlled attnuator. The relation of firing rate to auditory direction was quite varied among inferior colliculus neurons. In many cells the form of functions relating number of stimulus elicited spikes to position in the horizontal plane varied with frequency. These observations suggested that neurons of inferior colliculus did not have the property of acoustic space maps. Many cells could define a median directiton as suggested by marked slope changes. The ease with which the median plane is represented has suggested that the control of various acoustically driven movements of the eyes, pinnae and head may be a primary functional output of neurons at this level. From this perspective, the neurons of the inferior colliculus may drive a center-null device which restores the organism to a region of maximum sensitivity, i.e., the median plane.

Quantitative Evaluation of a Lateralization Model of Masking‐Level Differences

A lateralization model is proposed in which binaural detection in those cases that produce a masking‐level difference is of a shift in the spatial locus of the auditory image. The shift is presumed to be a function of interaural time and intensity differences in the signal‐plus‐masker; it is quantified as a weighted sum of these two differences, averaged across the observation interval. The model is applied to a broad range of experimental data in which detection is given as a function of interaural phase and amplitude differences in the signal. It is shown here that, when applied to noise‐masked low‐frequency detection, the lateralization model is identical in its predictions to the time‐difference model of Jeffress (1965) and the equalization‐cancellation model of Durlach (1963). However, when applied to noise‐masked high‐frequency detection and to detection of signals masked by pure tones, the lateralization model better describes the data.