Raymond H. Dye

Discrimination of interaural differences of level as a function of frequency

Discrimination of interaural differences of level (IDLs) was measured for pure tones as a function of frequency and as a function of the interaural difference of phase or level of a standard. Varying the interaural difference of the standard was assumed to change the lateral position of its intracranial image. Threshold IDLs were approximately constant over a frequency range from 200-5000 Hz, except in a region near 1000 Hz where they were slightly elevated. Thresholds increased as the value of the standard interaural differences of phase or level increased, implying that interaural resolution declines as the lateral image moves away from midline. The results are generally consistent with the predictions of current models of lateralization, but additions to these models are required in order for them to account for the slight frequency dependence of threshold IDLs.

THE COMBINATION OF INTERAURAL INFORMATION ACROSS FREQUENCIES : LATERALIZATION ON THE BASIS OF INTERAURAL DELAY

Three experiments were carried out that employed low-frequency tone complexes with interaural delays that varied across the frequency domain. In the first experiment, threshold interaural delays were measured for three-tone complexes for which one, two, or all three components were delayed. The center frequency was 750 Hz and the frequency spacing (delta f) between components was 20, 50, 100, 250, or 450 Hz. For all delta fs, the presence of two diotic components elevated the threshold interaural delays obtained for the third component relative to that obtained for a pure tone of the same frequency. In the second experiment, observers made left-right judgments regarding the direction of movement of signals for which two components were delayed by 25 microseconds to the left ear during one interval and to the right ear during the other interval, while a third component of a variable time difference was delayed to the opposite side as the tone pair. Subjects reported single intracranial images during each interval, and the data showed that interaural delays of one component to one ear could be offset by interaural delays of the other two components to the other ear. In the final experiment, threshold interaural delays were measured for five-tone complexes in which one, two, three, four, or five components were delayed. The center frequency was 750 Hz and delta f was fixed at 100 Hz. Thresholds decreased in a linear fashion as the number of delayed components increased, falling by about a factor of 5 as the number of delayed components went from one to five. These results are consistent with spectrally synthetic binaural processing, with the lateral position of intracranial images determined by a combination of interaural information across the spectrum. These effects could be brought about by a linear combination of the outputs of frequency-specific cross-correlation networks or by a wideband cross correlation of the signals at the two ears.

Statistical and receiver operating characteristic analysis of empirical spike‐count distributions: Quantifying the ability of cochlear nucleus units to signal intensity changes

Analytical methods from signal detection theory were applied in an effort to quantify the ability of cochlear nucleus (CN) units to signal changes in intensity. Of particular interest was the relation between this ability and the different patterns of discharge that characterize auditory neurons. Single-unit responses to best-frequency (BF) tone bursts were recorded from neurons in the gerbil cochlear nucleus, and empirical spike-count distributions were generated. The mean-to-variance ratios for regular units were generally larger than those of irregular units. Receiver operating characteristic (ROC) curves were generated from empirical spike-count distributions. The area under the ROC curve [P(A)] was computed and used to define the performance of an observer detecting whether or not a change in firing rate has occurred, thus signaling a change in intensity. For a given change in mean spike count, units characterized by regular interspike-interval (ISI) histograms typically gave larger P(A) values than did units characterized by irregular ISI histograms. In addition, onset units gave larger values of P(A) than did irregular units for a given change in mean spike count. These results suggest that regular and onset units are better able to signal intensity changes than are irregular units.

Observer weighting of interaural delays in source and echo clicks

A correlational analysis was used to assess the relative weight given to the interaural differences of time (IDTs) of source and echo clicks for echo delays ranging from 1-256 ms. In three different experimental conditions, listeners were instructed to discriminate the IDT of the source, the IDT of the echo, or the difference between the IDTs of the source and echo. The IDT of the target click was chosen randomly and independently from trial-to-trial from a Gaussian distribution (mu = 0 microsecond, sigma = 100 microseconds). The IDT of the nontarget click was either fixed at 0 microsecond or varied in the same manner as the IDT of the target. The data show that for echo delays of 8 ms or less, greater weight was given to the IDT of the source than to that of the echo in all experimental conditions. For echo delays from 16-64 ms, the IDT of the echo was weighted slightly more than that of the source and the weights accounted for a greater proportion of the responses when the echo was the target, indicating that the binaural information in the echo was dominant over the binaural information in the source. The data suggested the possibility that for echo delays from 8-32 ms, listeners were unable to resolve the temporal order of the source and echo IDTs. Listeners were able to weight the binaural information in the source and echo appropriately for a given task only when the echo delay was 128 ms or greater.

The combination of interaural time and intensity in the lateralization of high‐frequency complex signals

In an effort to examine the rules by which information arising from interaural differences of time (IDT) and interaural differences of intensity (IDI) is combined, ds were measured for trains of high-frequency clicks (4000 Hz, bandpass) possessing various combinations of IDT and IDI. The number of clicks was either 1 or 8, with the interclick interval either 2 or 10 ms. A 2-IFC task was employed in which the paired values of IDT and IDI favored one side during one interval and the other side during the other interval. Data obtained with the combined cues are compared to those obtained with IDTs or IDIs alone in order to determine the degree to which processing of the two cues is done independently. Results suggest that lateralization with such stimuli is based on the sum of the temporal and intensive differences and not on independent evaluations of their separate presences.

The effects of intensity on the detection of interaural differences of time in high‐frequency trains of clicks

Threshold values of interaural differences of time (delta IDTs ) were measured for trains of dichotic clicks whose levels were 20, 40, or 60 dB SPL. All clicks were bandpass filtered at 4 kHz, and the number of clicks in the train (n) was 1, 2, 4, 8, 16, or 32. The interclick interval (ICI) was 5, 2, or 1 ms. Performance was compared to that of an ideal integrator of information, which produces slopes of - 0.5 when log delta IDT versus log n is plotted. The results showed that increases in level had no effect on the slopes of the log-log functions regardless of the ICI but did decrease the intercepts. Shortening the ICI caused the slopes to go from nearly - 0.5 towards 0.0. The improvement with level could be explained by either a decrease in the temporal variability of neural discharges, or by an increase in the number of samples of IDT at higher intensities brought on by increased firing rates or the activation of more auditory units. A review of the physiological literature found the most parsimonious explanation to be that the decline in threshold IDT was mediated by an increase in the number of active units, each possessing the same degree of adaptation.

The effect of distractor frequency on judgments of target laterality based on interaural delays

A two-dimensional stimulus-classification paradigm was used to examine the ability of listeners to judge the laterality of an interaurally delayed low-frequency target component presented concurrently with a distractor component. Of primary interest was the effect on performance of the frequency difference (Delta f) between the target and distractor. In one set of conditions, the target was fixed at 753 Hz and the distractor was 353, 553, 653, 703, 803, 853, 953, or 1153 Hz (fixed within a block of trails). In a second set of conditions, the distractor was fixed at 753 Hz and the target frequency was 353, 553, 653, 703, 803, 953, or 1153 Hz. The listeners were presented with a target component with an interaural delay that varied from trial to trial, taking on one of ten values, five leading to the left ear and five leading to the right. A distractor component was simultaneously presented with an interaural delay that also took on one of the same ten values. Delays ranged from -90 to (+)90 microseconds in 20-microsecond steps. during a block of 100 trials, each of the possible combinations of target and distractor delay was presented once and only once in a random order. Listeners were instructed to make left-right judgments based on the target delay. Each condition was repeated ten times, and the slopes of the best linear boundaries between left and right responses were used to derive the relative weights given to the target and distractor. The duration of the signals was 200 microseconds. Two of the eight listeners weighted the target heavily when the target and distractor were spectrally remote but gave the two components equal weight when the different in frequency was small. These two listeners yielded similar target weights regardless of which component was designated as the target. One listener gave nearly equal weight to the target and the distractor regardless of Delta f. Five of the listeners gave greater weight to the higher of the two frequencies regardless of which was assigned as the target. This high-frequency dominance is explained in terms of cross-correlation functions based on the composite two-tone waveforms.

Discrimination of interaural envelope delays: The effect of randomizing component starting phase

The goal of this study was to examine the nature of envelope extraction in the discrimination of high-frequency waveforms on the basis of envelope delay. Threshold interaural envelope delays were measured for complexes consisting of three or five components for which the starting phases of all sinusoids were either sine phase or randomized between intervals of a two-alternative forced-choice (2-AFC) task. The center frequency was 4 kHz and the frequency separation was varied from 25 to 500 Hz. The results showed that thresholds were greater for the phase-randomized conditions than the sine-phase conditions. The phase effect tended to diminish with increasing frequency separation for three-component complexes but not for the five-component complexes. Sensitivity to envelope delay was better for five-component complexes than for three-component complexes at most frequency separations. In general, the results showed superior lateralization performance for conditions in which the envelope fluctuations were greater, a finding that is consistent with models of high-frequency binaural processing that include envelope extraction prior to binaural comparison.

The influence of later arriving sounds on the ability of listeners to judge the lateral position of a source

This study examined the deleterious effects of a later-arriving sound on the processing of interaural differences of time (IDTs) from a preceding sound. A correlational analysis assessed the relative weight given to IDTs of source and echo clicks for echo delays of 1-64 ms when the echo click was attenuated relative to the source click (0-36 dB). Also measured were proportion correct and the proportion of responses predicted from the weights. The IDTs of source and echo clicks were selected independently from Gaussian distributions (mu=0 s, sigma = 100/s). Listeners were instructed to indicate the laterality of the source click. Equal weight was given to the source and echo clicks for echo delays of 64 ms with no echo attenuation. For echo delays of 16-64 ms, attenuating the echo had no substantial effect on source weight or proportion correct until the echo was attenuated by 18-30 dB. At echo delays < or =4 ms, source weights and proportions correct remained high regardless of echo attenuation. The proportions of responses predicted from the weights were lower at echo delays > or =16 ms. Results were discussed in terms of backward recognition masking and binaural sluggishness and compared to measurements of echo disturbance.

Observer weighting of binaural information in source and echo clicks

Relative weights given to interaural differences of time (IDTs) of source and echo clicks were computed for a range of echo delays (1–256 ms) for stimuli presented over headphones. On each trial, source and echo IDTs were selected randomly and independently (μ=0 μs, σ=100 μs). Listeners were instructed to indicate the direction of the IDT, left or right, of the source or echo (in separate conditions). Weights were obtained by computing the correlation between the source or echo IDT and the listener’s binary responses. In all conditions, little weight was given to the echo at short echo delays ( 32 ms, listeners gave greater weight to the source or echo, as appropriate. For all echo delays, when listeners were instructed to indicate the direction of the source IDT, the percent correct was lower when the echo DT varied across trials than when it was fixed at 0 μs, indicati...