Perceptual restoration of locally time-reversed speech: Japanese words are very tolerant of severe temporal distortion.

People can understand speech even when the speech signal is divided into equally long segments and each segment is reversed in time (locally time-reversed speech). In addition, Ishida (Attention, Perception & Psychophysics, 83(6), 2675-2693, 2021) reported that Japanese words - composed of consonant-vowel (CV) units - were significantly more intelligible than English words when locally time-reversed. The current study investigates how tolerant and robust Japanese words are under more severe temporal distortions. In Experiment 1, native Japanese speakers listened to Japanese words and pseudowords that were locally time-reversed at intervals of 100, 120, 140, 160, 180, and 200 ms, which had not been previously examined. These lexical items contained either many fricatives or stops. Results showed that Japanese words were highly tolerant of local time reversal, even at these extreme durations. Perceptual restoration was sustained by dominant phoneme type (fricative-dominant > stop-dominant) and lexicality (words > pseudowords). In Experiment 2, participants listened to stop-dominant Japanese words and pseudowords, which were more susceptible to temporal distortion in Experiment 1. Temporal distortion was further increased by introducing extreme speech rates (fast vs. slow) while reversing the signal at 10, 30, 50, 70, 90, and 110 ms, commonly used intervals with normal speech rates. Results showed that stop-dominant Japanese words remained intelligible with increasing distortions, while pseudowords remained intelligible only up to 50 ms in the slow condition and became unintelligible in the fast condition. Overall, recognition of Japanese CV-based words was highly tolerant of severe temporal distortion, with perceptual restoration supported by dominant phoneme type, slower speech rate, and lexicality.