Mental Representation of Japanese Mora; Focusing on its Intrinsic Duration

Japanese is one of the typical languages in which vowel quantity plays a key role. In Japanese, a phonological structure called “mora” is a fundamental rhythmic unit, and theoretically, each mora is supposed to have a similar duration (isochronicity). The rhythm of a native language has great importance on spoken language processing, including second language speaking; therefore, in order to get a clear picture of bottom-up speech processing, it is crucial to discern how morae are mentally represented. Various studies have been conducted to understand the nature of speech processing as a cognitive construct; however, most of this research was conducted with the target stimuli embedded in words or carrier sentences to clarify on specifically the relative duration of morae. In this study, two reaction-time experiments were conducted to investigate whether morae are mentally represented and how long the duration is. The isolated vowels /i/, /e/, /a/, /o/, /u/, and syllable /tan/ were chosen as target stimuli, and the first morae were digitally manipulated into 15 durations with 20 ms variations in length, from 150 ms to 330 ms. The results revealed the existence of a durational threshold between one and two morae, ranging around 250 ms.