Perceptual grouping of alternating word pairs: Effect of pitch difference and presentation rate

When listeners hear sequences of tones that slowly alternate between a low frequency and a slightly higher frequency, they tend to report hearing a single stream of alternating tones. However, when the alternation rate and/or the frequency difference increases, they often report hearing two distinct streams: a slowly pulsing high and low frequency stream. This experiment used repeating sequences of spondees to investigate whether a similar streaming phenomenon might occur for speech stimuli. The F0 difference between every other word was varied from 0 - 18 semitones. Each word was either 100 or 125 ms in duration. The inter-onset intervals (IOIs) of the individual words were varied from 100 - 300 ms. The spondees were selected in such a way that listeners who perceived a single stream of sequential words would report hearing a different set of spondees than ones who perceived two distinct streams grouped by frequency. As expected, F0 differences was a strong cue for sequential segregation. Moreover, the number of 'two' stream judgments were greater at smaller IOIs, suggesting that factors that influence the obligatory streaming of tonal signals are also important in the segregation of speech signals.