The present investigation focused on how temporal degradation affected intelligibility in two types of languages, i.e., a tonal language (Mandarin Chinese) and a non-tonal language (Japanese). The temporal resolution of common daily-life sentences spoken by native speakers was systematically degraded with mosaicking (mosaicising), in which the power of original speech in each of regularly spaced time-frequency unit was averaged and temporal fine structure was removed. The results showed very similar patterns of variations in intelligibility for these two languages over a wide range of temporal resolution, implying that temporal degradation crucially affected speech cues other than tonal cues in degraded speech without temporal fine structure. Specifically, the intelligibility of both languages maintained a ceiling up to about the 40-ms segment duration, then the performance gradually declined with increasing segment duration, and reached a floor at about the 150-ms segment duration or longer. The same limitations for the ceiling performance up to 40 ms appeared for the other method of degradation, i.e., local time-reversal, implying that a common temporal processing mechanism was related to the limitations. The general tendency fitted to a dual time-window model of speech processing, in which a short (~ 20–30 ms) and a long (~ 200 ms) time-window run in parallel.
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