Perceptual restoration of degraded speech: The effects of linguistic structure.

Listeners can understand speech even when its temporal structure is acoustically distorted. Ishida et al. (Frontiers in Psychology, 9, 1749, 2018) reported that native English speakers could comprehend English sentences using two types of temporal distortions: (1) speech signals divided into equally timed segments, with each segment reversed in time (locally time-reversed speech), and (2) speech signals with reduced modulation-frequency components shaping the amplitude envelope (modulation-filtered speech). While the results showed a similar pattern of intelligibility decline across these two conditions in English (a syllable-oriented language with consonant clusters) when degradation increased in six steps, it remained unclear whether this pattern holds in a linguistically distinct language like Japanese (a mora-oriented language with CV and V as basic linguistic units). The current study investigates how native Japanese speakers comprehend Japanese sentences under the same temporal distortions. In Experiment 1, participants listened to locally time-reversed Japanese sentences with segment intervals reversed at 10 ms, 30 ms, 50 ms, 70 ms, 90 ms, and 110 ms. In Experiment 2, the same participants listened to modulation-filtered Japanese sentences, where the modulation frequency components were low-pass filtered at cut-off frequencies of 32 Hz, 16 Hz, 8 Hz, 4 Hz, 2 Hz, and 1 Hz. Results showed that the intelligibility of locally time-reversed and modulation-filtered Japanese sentences decreased as distortion increased, with longer reversed segment lengths and lower cut-off frequencies. However, the patterns of intelligibility degradation in Japanese differed significantly from those in English. Thus, perceptual restoration may function differently depending on the basic linguistic units (mora vs. syllable).