Observer weighting of monaural level information in a pair of tone pulses.

A correlational analysis was used to assess the relative weight given to the levels of two monaurally presented tone pulses for interpulse intervals (IPIs) ranging from 2-256 ms. In three different experimental conditions, listeners were instructed to discriminate the level of the first pulse, the level of the second pulse, or the difference between the levels of the two pulses. The level of the target pulse was chosen randomly and independently from trial to trial from a Gaussian distribution. The level of the nontarget pulse was either fixed at 75 dB SPL or varied in the same manner as the level of the target. In the tasks in which one pulse was to be ignored, listeners gave increasing weight to the nontarget component as IPI decreased. Listeners weighted the level information in the pulses appropriately only when the IPI approached 256 ms. When the listeners were instructed to compare the pulse levels to one another, two of three listeners weighted the levels optimally at all IPIs, while the third listener did so only at the longest IPI. For the two listeners who weighted the pulses optimally, a minimum in performance was achieved at IPIs around 16-32 ms. Intensity discrimination thresholds were also measured for one pulse in the presence of a second fixed pulse for IPIs of 2-256 ms. Thresholds were higher in all the two-pulse conditions relative to a one-pulse condition, and were dependent on the level of the nontarget pulse but not on IPI. The results indicate that level information is integrated to some extent over fairly long durations, but not in a manner that is consistent with simple temporal integration.

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.

Spectral weights in level discrimination by preschool children: synthetic listening conditions.

On most auditory discrimination and detection tasks young children perform more poorly than adults. The current experiment applies a technique which potentially can reveal the extent to which the adult-child performance difference results from suboptimal attentional strategies or simply greater internal noise in the children. In this experiment preschool children and adults were asked to discriminate between complex tones comprised of three random-amplitude sinusoidal components. A trial-by-trial correlational analysis [R. A. Lutfi, J. Acoust. Soc. Am. 97, 1333-1334 (1995)] provided an estimate of the weight listeners placed on the level information from individual spectral components in making the discrimination. The patterns of weights were interpreted as measures of "attentional strategy." Both children and adults produced reliable patterns of weights. This is an especially important result since measuring a single weighting pattern requires large numbers of trials and hence multiple sessions with the children. While individual weighting patterns were reliable, weighting patterns differed both within and across groups. Moreover, neither the children nor the adults produced weighting patterns that would maximize percent correct in the task. A substantial proportion of the responses from both children and adults could be predicted from their weighting patterns even when performance was near chance. However, differences in overall performance between children and adults could not be accounted for by differences in their weighting functions.

Spectral weights in level discrimination by preschool children: analytic listening conditions.

In this series of experiments, adult and child listeners were required to attend to a target tone in the presence of two distracters and to indicate in which of two intervals the target tone had the higher level. The attentional weight listeners placed on each component was estimated by computing the correlation between the level change of each component across intervals and the listener's response. In the first experiment, weights were obtained as a function of the mean level of the distracters (250 and 4000 Hz) for a 1000-Hz target. No consistent differences between the weighting functions of children and adults were observed. In a second experiment, weights were obtained as a function of the harmonic relationship between the distracters (250 and 4000 Hz, or 270 and 4320 Hz) and the 1000-Hz target. No difference was observed between the weighting functions computed with harmonic and inharmonic complexes. In the final experiment, each component of the complex (250, 1000, and 4000 Hz) was identified as the target in separate blocks of trials. In general, adults were able to weight the target component appropriately regardless of its frequency, while children tended to weight all components equally. The results suggest that preschool listeners may exhibit poorer attentional selectivity than adults.

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.

Observer weighting of concurrent binaural information.

Two experiments examined how listeners weight binaural information of individual components in a multicomponent complex when they are instructed to use this information in various ways. On each experimental trial of the first experiment, a two-component complex consisting of a 553- and 753-Hz pure tone was presented to listeners over headphones. Each component had an interaural difference of time (IDT) that was randomly chosen from a single distribution. Listeners were to indicate the apparent position of the intracranial image relative to the midline on the basis of either the IDT of the 753-Hz component, the sum of the IDTs of the two components, or the difference between the two IDTs. Observer weights for the component IDTs were derived by computing the point-biserial correlations between the IDTs of the components and the listeners' left-right judgments. Three of the four listeners were found to adjust their weighting of the binaural information appropriately for each listening task, while the fourth listener consistently gave almost no weight to the 553-Hz component regardless of the listening condition. In a second experiment, listeners were instructed to attend to the randomly selected IDT of one of three components (553, 753, or 953 Hz) and to indicate whether the intracranial image produced by that component was to the left or right of midline. Only one of six listeners gave greatest weight to the target component regardless of its frequency. The other five listeners gave significant weight to all three components in all listening conditions. In all cases, individual differences in percent correct performance could be attributed largely to individual differences in weights with little variation in internal noise.

Stimulus classification procedure for assessing the extent to which binaural processing is spectrally analytic or synthetic.

A two-dimensional stimulus classification paradigm was used to assess the extent to which listeners' processing of interaural delays at low frequencies is spectrally analytic or synthetic. Listeners were presented with a 753-Hz target 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 553-Hz distractor component was simultaneously presented, with its interaural delay also presented at one of ten different values. 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 in judgments of laterality. Six of the nine listeners gave increasing weight to the target as the duration of the signals was increased from 25 or 50 to 400 ms. Three listeners showed little change with duration; one consistently gave equal weight to the target and distractor, two consistently gave greater weight to the target than to the distractor. The utility of classification paradigms in the study of multidimensional acoustic signals is discussed.

The reduction of binaural interference by the temporal nonoverlap of components.

Threshold interaural delays were measured for a single interaurally delayed 753-Hz target component presented against a background of six diotic distractor components (453, 553, 653, 853, 953, and 1053 Hz). In one set of conditions, all components were gated on simultaneously, but 200-ms after their onset the distractors were turned off for 10, 25, 50, 100, or 200 ms, then turned on again for the remainder of the 500-ms listening interval. In a second set of conditions, the target component was present only during the temporal notch in the distractors for notch durations of 50, 100, and 200 ms. Threshold interaural delays were also measured for the target component in isolation at durations of 50, 100, 200, and 500 ms. In both sets of conditions in which the distractors were present, it was observed that a brief isolated presentation of the target reduced threshold interaural differences of time to nearly the level of thresholds for the target presented in total isolation, while thresholds were many times higher when the distractors had no temporal notch. Possible sources of binaural interference are discussed and the evidence supporting each is evaluated.

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 combination of interaural information across frequencies: the effects of number and spacing of components, onset asynchrony, and harmonicity.

Threshold interaural delays were measured for a single interaurally delayed low-frequency target component presented against a background of two, four, six, or eight diotic "distractor" components. In the first experiment, a 753-Hz target and the flanking distractor components were gated on and off simultaneously. In subsequent experiments, the distractors were gated on 25-200 ms prior to the target. In addition, the target and distractor components were given various harmonic configurations. In general, threshold interaural delays were higher in all conditions in which distractors were present relative to thresholds obtained for the target component in isolation. Subjects reported that the pitch of the target component was more salient when an onset asynchrony between the target and distractors was present, but the components were perceived as occupying a single intracranial position in spite of the various interaural delays across the frequency domain. These results suggest that binaural processing of stimuli consisting of a small number of low-frequency temporally overlapping components occurs in a spectrally synthetic manner in which interaural information is combined across the spectrum, even in situations in which the segregation of pitch information occurs.