Binaural detection at high frequencies with time-delayed waveforms.

Recent research has demonstrated that the binaural system can utilize ongoing interaural time differences for lateralization at high frequencies as well as at low frequencies. The requirement is that the signal be complex so that the time difference appears as a delay in the envelope of the waveform at one ear. Reported here are several masking experiments that examine detection performance with time-delayed signals or maskers. In the first experiment, the signal was a 50-Hz band of noise centered at 4000 Hz that was time delayed by different amounts on different blocks of trials; the masker was similar band of noise, presented diotically. Large masking-level differences (MLDs) were obtained for some values of time delay, but the MLDs did not increase monotonically within time delay as they should were envelope time delay the basis for detection performance. Subsequent experiments in which the masker was time delayed and the signal was a diotic, high-frequency tone, revealed that detectability follows the autocorrelation function, and that MLDs as large as 24 dB can be obtained at 4000 Hz at time delays corresponding to negative values in the autocorrelation function. Examination of the signal-plus masker waveforms in these conditions reveals that ongoing interaural differences in level and cycle-by-cycle time exist in those conditions that yield MLDs. Since the time differences are small by usual standards, the basis for detection performance in these conditions appears to be the ongoing interaural level differences. In a final experiment, lateralization performance was measured for a time-delayed, complex waveform in the presence of maskers of various intensities. The results show that subjects are able to extract information about the time delay in the envelope even when the signal is added to a masker of equal intensity or greater. Thus, at the small signal-to-noise ratios used in our detection experiments, extraction of envelope time information was impossible, but also unnecessary, for detection was accomplished on the basis of another cue--most likely the ongoing interaural level differences.