Sequential streaming due to manipulation of interaural time differences.

The effect of apparent spatial location on sequential streaming was investigated by manipulating interaural time differences (ITDs). The degree of obligatory stream segregation was inferred indirectly from the threshold for detecting a rhythmic irregularity in an otherwise isochronous sequence of interleaved "A" and "B" tones. Stimuli were bandpass-filtered harmonic complexes with a 100-Hz fundamental. The A and B tones had equal but opposite ITDs of 0, 0.25, 0.5, 1, or 2 ms and had the same or different passbands. The passband ranges were 1250-2500 Hz and 1768-3536 Hz in experiment 1, and 353-707 Hz and 500-1000 Hz in experiment 2. In both experiments, increases in ITD led to increases in threshold, mainly when the passbands of A and B were the same. The effects were largest for ITDs above 0.5 ms, for which rhythmic irregularities in the timing of the A or B tones alone may have disrupted performance. It is concluded that the differences in apparent spatial location produced by ITD have only weak effects on obligatory streaming.

Temporal masking curves for hearing-impaired listeners.

The decay of forward masking was investigated for three subjects with moderate sensorineural hearing loss. For such subjects, compression on the basilar membrane (BM) is thought to be largely absent, enabling one to determine the decay of masking without the influence of compression. Temporal masking curves (TMCs), plots of the masker level at threshold against delay between masker offset and signal onset, were measured for delays of 0, 15, 30, 45, 60, and 75 ms, for signal frequencies, fs, of 500, 1000, 2000, 4000, and 6000 Hz. Masker frequencies were 0.5, 0.8, 1.0, 1.15, and 1.3 times fs. Most of the TMCs were well fitted with single-segment straight lines, which, except for high masker levels, were roughly parallel for each fs, supporting the belief that BM compression was largely absent in these subjects. However, the slopes of the TMCs were greater for fs = 500 and 1000 Hz than for higher frequencies, which may indicate that the decay of forward masking is not the same for all signal frequencies. The results suggest that it may not be valid to infer BM compression at low signal frequencies by using a reference TMC for a high fs.

Sequential streaming and effective level differences due to phase-spectrum manipulations.

Roberts et al. [J. Acoust. Soc. Am. 112, 2074-2085 (2002)] demonstrated that sequential stream segregation occurs with stimuli that differ only in phase spectrum. We investigated if this was partly due to differences in effective excitation level. Stimuli were harmonic complexes with a 100 Hz fundamental, 1250-2500 Hz passband, and cosine, alternating, or random component phase. In experiment 1, the complex tones were used as forward maskers of 20-ms probe tones at 1000, 1250, 1650, 2050, 2500, and 3000 Hz. While there was no significant difference in the masking produced by the cosine- and alternating-phase stimuli, the random-phase stimulus produced significantly greater masking, equivalent to a difference in overall effective excitation level of 12.6 dB. Experiments 2 and 3 used the asynchrony detection and subjective streaming tasks of Roberts et al. Successive stimuli had identical phase, but differed in level by 0, 1, 3, 5, 10, or 15 dB. Stream segregation increased once the level difference reached 5 dB. While some of the stream segregation observed by Roberts et al. may have been due to a difference in effective excitation level, this does not account for the stream segregation between cosine- and alternating-phase stimuli.