Neural and Behavioral Mechanisms of Clear Speech

Clear speech is a speaking style that has been shown to improve intelligibility in adverse listening conditions, for various listener and talker populations. Clear-speech phonetic enhancements include a slowed speech rate, expanded vowel space, and expanded pitch range. Although clear-speech phonetic enhancements have been demonstrated across a variety of talkers, only a subset of these changes may be required for listeners to benefit perceptually from clear speech. Furthermore, while current literature has provided some understanding of the phonetic enhancements that are typical of clear speech and the improvements in intelligibility resulting from its use, less is understood regarding how listeners benefit from clear speech. Understanding the mechanisms underlying the effectiveness of clear speech will provide insight into speech processing more generally. To that end, two studies were conducted to investigate the possible mechanisms underlying clear-speech benefits. The first study tests the hypothesis that clear speech reduces the amount of information needed for syllable identification. The amount of information presented to listeners was controlled using a silent-center syllable paradigm, in which various amounts of the center or edge of the syllables were removed. The second study tests the hypothesis that phonetic processing of clear speech requires fewer neuro-cognitive resources than typical, or conversational, speech. An Event Related Potential (ERP) paradigm, focused on the Phonological Mismatch Negativity (PMN) component, was used to compare participants’ neurophysiological responses to conversational- and clear-speech stimuli. Results from the first experiment supported the hypothesis of a clear-speech benefit in partial syllables, although the effect was stronger for some vowels than for others. The second experiment demonstrated differences in the way the brain responds to clear versus conversational speech, for syllables in which only the nucleus varied across stimuli. Match-mismatch differences were found in the MMN and PMN components, while the N600 component was found to respond to style differences