Effect of spectral frequency range and separation on the perception of asynchronous speech.

The use of across-frequency timing cues and the effect of disrupting these cues were examined across the frequency spectrum by introducing between-band asynchronies to pairs of narrowband temporal speech patterns. Sentence intelligibility by normal-hearing listeners fell when as little as 12.5 ms of asynchrony was introduced and was reduced to floor values by 100 ms. Disruptions to across-frequency timing had similar effects in the low-, mid-, and high-frequency regions, but band pairs having wider frequency separation were less disrupted by small amounts of asynchrony. In experiment 2, it was found that the disruptive influence of asynchrony on adjacent band pairs did not result from disruptions to the complex patterns present in overlapping excitation. The results of experiment 3 suggest that the processing of speech patterns may take place using mechanisms having different sensitivities to exact timing, similar to the dual mechanisms proposed for within- and across-channel gap detection. Preservation of relative timing can be critical to intelligibility. While the use of across-frequency timing cues appears similar across the spectrum, it may differ based on frequency separation. This difference appears to involve a greater reliance on exact timing during the processing of speech energy at proximate frequencies.