The threshold of a short interaurally phase-inverted probe tone (20 ms, 500 Hz, S pi) was obtained in the presence of a 750-ms noise masker that was switched after 375 ms from interaurally phase-inverted (N pi) to interaurally in-phase (No). As the delay between probe-tone offset and noise phase transition is increased, the threshold decays from the N pi S pi threshold (masking level difference = 0 dB) to the No S pi threshold (masking level difference = 15 dB). The decay in this "binaural" situation is substantially slower than in a comparable "monaural" situation, where the interaural phase of the masker is held constant (N pi), but the level of the masker is reduced by 15 dB. The prolonged decay provides evidence for additional binaural sluggishness associated with "binaural forward masking." In a second experiment, "binaural backward masking" is studied by time reversing the maskers described above. Again, the situation where the phase is switched from No to N pi exhibits a slower transition than the situation with constant interaural phase (N pi) and a 15-dB increase in the level of the masker. The data for the binaural situations are compatible with the results of a related experiment, previously reported by Grantham and Wightman [J. Acoust. Soc. Am. 65, 1509-1517 (1979)] and are well fit by a model that incorporates a double-sided exponential temporal integration window.

The sound localization ability of human observers has been frequently examined in quiet environments, but there have been relatively few studies that have considered the effect of noise on sound localization. In this study, three subjects judged the perceived direction of broadband click-train signal in the quiet and in the presence of a broadband noise at nine signal-to-noise ratios, which varied over a 23 dB range. The signal could originate from any of 239 spatial locations that completely surrounded the subjects in azimuth 360 degrees) and ranged from -45 degrees to (+)90 degrees in elevation; the masker (when present) was always located directly in front of the subjects at 0 degrees azimuth and 0 degree elevation. The subjects indicated the perceived direction of the signal by pointing at a 20-cm-diam spherical model of auditory space. As the signal-to-noise ratio was lowered, the accuracy of localization judgments decreased nearly monotonically. However, the accuracy of judgments relative to the median plane (i.e., the left/right dimension) was less strongly influenced by the presence of noise than was the accuracy of judgments relative to the horizontal plane (i.e., the up/down dimension). The accuracy of judgments relative to the frontal plane (i.e., the front/back dimension) was most strongly influenced by noise.

Data from a simple tone-in-noise simultaneous masking task were used to evaluate each of two common adaptive staircase rules (a "1 up 2 down" rule and a "1 up 3 down" rule) and the parameter estimation by sequential testing (PEST) technique in combination with each of two psychophysical procedures [a two-alternative forced-choice (2AFC) and a three-alternative forced-choice (3AFC) procedure]. These human data were compared to predictions generated by a mathematical model based on Markov theory. The model predicts that threshold estimates obtained with the adaptive techniques should be equal to those derived with equivalent "fixed signal level" techniques. However, the human data indicate that the adaptive techniques tend to yield lower thresholds. The model predicts that the standard error of a threshold estimate obtained from an adaptive technique will decrease and approach zero as the number of trials used to compute the estimate increases. The human data show greater variability than predicted and approach a nonzero value as the number of trials increases. The predictions of the model suggest that the commonly used combination of the 2AFC procedure and the 1 up 2 down rule is the least efficient method of estimating a threshold and that the 3AFC procedure in combination with the 1 up 3 down rule is the most efficient method. The human data are less consistent, but generally show the combination of the 2AFC procedure and the 1 up 2 down rule to be one of the least efficient methods. Possible explanations for the differences between the model's predictions and the human data, as well as suggestions for laboratory practice, are discussed.

This study investigated binaural detection of tonal targets (500 Hz) using sets of individual masker waveforms with two different bandwidths. Previous studies of binaural detection with wideband noise maskers show that responses to individual noise waveforms are correlated between diotic (N0S0) and dichotic (N0S(pi)) conditions [Gilkey et al., J. Acoust. Soc. Am. 78, 1207-1219 (1985)]; however, results for narrowband maskers are not correlated across interaural configurations [Isabelle and Colburn, J. Acoust. Soc. Am. 89, 352-359 (1991)]. This study was designed to allow direct comparison, in detail, of responses across bandwidths and interaural configurations. Subjects were tested on a binaural detection task using both narrowband (100-Hz bandwidth) and wideband (100 Hz to 3 kHz) noise maskers that had identical spectral components in the 100-Hz frequency band surrounding the tone frequency. The results of this study were consistent with the previous studies: N0S0 and N0S(pi) responses were more strongly correlated for wideband maskers than for narrowband maskers. Differences in the results for these two bandwidths suggest that binaural detection is not determined solely by the masker spectrum within the critical band centered on the target frequency, but rather that remote frequencies must be included in the analysis and modeling of binaural detection with wideband maskers. Results across the set of individual noises obtained with the fixed-level testing were comparable to those obtained with a tracking procedure which was similar to the procedure used in a companion study of rabbit subjects [Zheng et al., J. Acoust. Soc. Am. 111, 346-356 (2002)].