Huanping Dai

Effective attenuation of signals in noise under focused attention

When attending to a tone at a given frequency, listeners are most sensitive to that tone and others within a restricted band of frequencies surrounding it. This region of enhanced sensitivity defines the attention band that was measured in two experiments using a modified version of the probe-signal method of Greenberg and Larkin [J. Acoust. Soc. Am. 44, 1513-1523 (1968)]. Experiment 1 showed that at five center frequencies, from 0.25 to 4.0 kHz, the shape of the attention band resembles that of the auditory filter as inferred from notched-noise masking experiments by other investigators. The width of the attention band is close to the critical band at higher frequencies, but only half as wide at 0.25 and 0.5 kHz. Experiment 2 produced psychometric functions for unattended probe tones at least 0.23 kHz away from a fully attended, 1-kHz target tone. From these functions, the effective attenuation, measured as the threshold difference between the 1-kHz target and the probes, was estimated to be 7 dB; the amount of attenuation appeared to be about the same regardless of how far the probe frequency was from the attended band. One interpretation of these results is that bands centered on the unattended tones contribute to the decision process with some small but measurable weight and are not entirely ignored.

On measuring psychometric functions: A comparison of the constant‐stimulus and adaptive up–down methods

Psychometric functions were obtained using the conventional constant-stimulus method and an adaptive up-down method, for both computer-simulated and human observers. Except when the stimuli are closely placed, psychometric functions obtained with the adaptive method are as accurate as those obtained with the constant-stimulus method. Empirically, the adaptive method has some potential advantages owing to its ability to automatically concentrate the trials within the dynamic range of the psychometric function. It needs no pilot measurements for setting the signals as required by the constant-stimulus method. Furthermore, following a marked change in the underlying psychometric function, the distribution of the trials is automatically readjusted. Thus, on the basis of empirical considerations, the adaptive method is a better choice than the constant-stimulus method for measuring psychometric functions.

Detection of unexpected tones with short and long durations

The detectability of short and long unexpected tones masked by a continuous wideband noise was assessed using a probe-signal method. This method leads the listener to expect a target frequency by presenting the signal most often at that frequency, and only occasionally at other unexpected probe frequencies. The probe-signal contour (percent correct as a function of probe frequency) was considerably broader with 5-ms than with 295-ms signals. However, auditory filter shapes measured using the notched-noise technique were very similar for those two signal durations, indicating that the results obtained in the probe-signal conditions do not simply reflect peripheral frequency selectivity. Further supporting this interpretation, probe tones having the same frequency but a different duration from the target were poorly detected. It is proposed that the subject listens through a time-frequency window whose location and shape in the time-frequency plane is determined by the duration and frequency of the target.

The optimum decision rules in the same-different paradigm

In this paper we derive the optimum (likelihood-ratio) decision statistic for asame-different paradigm. The likelihood ratio is dependent on the degree of correlation between the two observations on each trial. For the two extreme cases in which the observations are either independent or highly correlated, the optimum decision rule is identical to each of two previously suggested decision rules. For these two cases, the receiver-operating characteristic (ROC) curves are calculated. Finally, an experimental procedure is suggested for assessing the decision rule actually used by the observer in asame-different task.

Auditory intensity perception: Successive versus simultaneous, across‐channel discriminations

This study measures the ability of observers to compare the intensities of two stimuli occupying different frequency regions. It includes three experiments, each experiment having two conditions. In one condition, the two stimuli to be compared were presented simultaneously within each interval; this condition has been called profile analysis. In the other condition, the two stimuli were presented successively within each interval. Because the overall level of the stimuli was randomized between intervals, the observers were encouraged to compare the intensities of the two stimuli within each observation interval rather than between intervals. The stimuli were two simple tones in experiment 1 and two tonal complexes in both experiments 2 and 3. The stimuli used in experiments 2 and 3 differed in frequency. The results show that simultaneous comparisons are superior to successive comparisons. For simple tones, the difference in threshold is about 8 dB; for complexes with 10 to 11 components, the difference in threshold is about 15 dB. These differences can be explained by assuming that internal noises in different channels were partially correlated when stimuli in those channels were presented simultaneously and were independent when the stimuli were presented successively. Cancellation of the correlated noise is therefore possible with simultaneous comparisons, making such discrimination better than that achievable with successive comparisons.

The optimum decision rules for the oddity task

This paper presents the optimum decision rule for an m-interval oddity task in whichm-1 intervals contain the same signal and one is different or odd. The optimum decision rule depends on the degree of correlation among observations. The present approach unifies the different strategies that occur with “roved” or “fixed” experiments (Macmillan & Creelman, 1991, p. 147). It is shown that the commonly used decision rule for anm-interval oddity task corresponds to the special case of highly correlated observations. However, as is also true for thesame-different paradigm, there exists a different optimum decision rule when the observations are independent. The relation between the probability of a correct response andd’ is derived for the three-interval oddity task. Tables are presented of this relation for the three-, four-, and five-interval oddity task. Finally, an experimental method is proposed that allows one to determine the decision rule used by the observer in an oddity experiment.

DECISION RULES OF LISTENERS IN SPECTRAL-SHAPE DISCRIMINATION WITH OR WITHOUT SIGNAL-FREQUENCY UNCERTAINTY

A correlation technique was used to assess the decision rules of three listeners in two cases of spectral-shape discrimination tasks. In one case the signal frequency was fixed, and in the other it was randomly varied within each block of trials. In order to estimate the decision rule of the listeners, the experimenter superimposed random level perturbations on each frequency component upon each stimulus presentation. Over many trials, correlation coefficients were computed between the random perturbations and the binary responses of the listeners, and were expressed as a function of signal level. For both the fixed- and random-signal cases, the measured correlation functions were in reasonably good agreement with those predicted based on the likelihood-ratio decision rules. Thus the listeners appeared to use information nearly optimally in discriminating spectral shapes. This investigation demonstrated that the correlation technique can be used to reveal the decision rules for cases where the decision statistics are nonlinear functions of the observations.

Detection of sinusoidal amplitude modulation at unexpected rates

The detectability of sinusoidal amplitude modulation at unexpected modulation rates was assessed using a probe-signal method. With this method, three listeners were led to expect a target modulation rate (4, 32, or 256 Hz) by presenting the signal most often at that rate, and sensitivity to modulation at six other unexpected rates between 4 and 256 Hz was measured via occasionally presented probe modulation rates. The modulation phase was random on each two-interval forced-choice trial and the overall level of the 500-ms broadband carrier was randomly varied between 55 and 75 dB SPL across intervals. The modulation depth at each rate was set so that the modulation was detected on about 90% of the trials when only that rate was presented. Performance at the unexpected rates depended upon the target rate. For the 4-Hz target, modulation at all rates was detected on about 80% of the trials. For the 32- and 256-Hz targets, unexpected modulation rates of 16 Hz and above were detected on 80%-90% of the trials, but modulation rates below 16 Hz were detected nearly at chance. The influence of expectation of modulation rate on the detection of sinusoidal amplitude modulation is not readily predicted by current models of modulation detection.

A two-filter model for frequency discrimination

In this paper, we explore a two-filter model, the simplest version of multi-channel models for frequency discrimination of simple tones. According to this model, frequency discrimination is based on a change in the relative output levels of two auditory filters, one centered below and the other above the frequency of the tone. This idea can explain the experimental results that frequency discrimination is relatively unaffected by randomization of stimulus level. Moreover, it suggests a close relationship between the ability of listeners to perform frequency discrimination of simple tones and spectral-shape discrimination of two-tone complexes. The ability of three listeners to perform these two tasks was measured at six frequencies (from 0.25 to 8 kHz). The results from the spectral-shape-discrimination task were used to predict frequency-difference limens. There was a high correlation between obtained and predicted values.

Effect of gating the masker on frequency-selective listening

When presented with variable-frequency tones in a continuous random-noise masker, observers detect the most frequently presented tone (the target tone) better than other infrequently presented tones (probe tones). The present paper shows that the detection of the probe tones improves, and detection of the target tone worsens, when the masker is gated a few hundred milliseconds before the signal onset. The results suggest that the observers are led to listen for several bands when the masker is gated.