Doctoral Dissertation Research: Predictability and Recoverability of High Vowel Reduction in Speech

New York University, New York NY

Investigators

Abstract

Human cognition relies on direct and indirect information to draw conclusions. Processes that rely on direct information are called bottom-up processes, where a conclusion is drawn based solely on the provided information; processes that rely on indirect information are called top-down processes, where elements such as context, pattern-recognition, and probability are used to form an expectation for a particular outcome. Studies have shown that a strong top-down process often blocks bottom-up processing of certain information available in the stimulus, but how the two types of processes interact is poorly understood. As an integral part of human cognition, language also shows interactions of top-down and bottom-up processes, where differences that are meaningful in one language may be readily ignored as uninformative in another. Language, therefore, is an excellent tool for shedding light on how the processing of direct and indirect information interacts in the mind. This research focuses on Japanese as a case study. The syllable structure of Japanese overwhelmingly follows a simple consonant-vowel structure. However, the process of high vowel reduction in Japanese, where unaccented vowels /i, u/ are weakened between two voiceless consonants (e.g., /suki/ > [ski] 'like'), often results in the complete deletion of a vowel during speech production. However Japanese speakers often report perceiving a vowel between two consecutive voiceless consonants, even in the complete absence of a vowel (e.g., [ski] perceived as /suki/). It is unclear at this point what drives this misperception of apparently non-existent vowels, but there are two likely sources. The first is an indirect source. Given the simple syllable structure of Japanese, Japanese speakers are conditioned to expect a vowel after a consonant, in which case the misperception of a vowel would be the result of a strong top-down process. The alternative is a direct source. Because high vowel reduction is common in Japanese, Japanese speakers may be hypersensitive even to subtle acoustic information that indicates the presence of a vowel, in which case the perception of a vowel would be the result of a bottom-up process. A production experiment and a corpus study will first test what acoustic information for a weakened vowel exists in naturally produced speech. A perception experiment will then manipulate the direct and indirect information to test which of these Japanese listeners rely on more when perceiving a weakened vowel. The experimental results will inform a computational model, which investigates how a reliance on the direct and indirect cues might be learned. The experiments, together with the model, will lead to a deeper understanding of the extent in which bottom-up and top-down processes are shaped by experience both in language and more broadly in human cognition as a whole.