Understanding speech requires that listeners be able to adjust to a wide variety of talkers and listening conditions. As such, there are multiple influences of the speech a listener has already heard on how they process incoming speech signals. One such effect of heard speech on speech processing involves the timing of speech information; the rate of context speech influences whether listeners report hearing acoustically blended words (e.g., “or” in “leisure or time”). Competing hypotheses posited that 1) listeners formulate predictions about the timing of upcoming speech signals based on the rate of the preceding context and 2) speech rate information can influence lexical competition in a way that favors words that are consistent with context speech rate. To differentiate between these accounts, we conducted event-related potential (ERP) research that measured the electrical activity of the brain that is time locked to specific speech events. The neurophysiological response to blended function words in acoustically identical portions of speech streams differed based on context speech rate by 100 ms after onset. Specifically, the same speech sounds that failed to elicit ERPs associated with auditory onsets and function words when presented with fast, casual contexts did elicit these brain responses when presented with slowed contexts. The ERP results were consistent with transcriptions of the phrases. The acoustically blended function words were typically not reported in the faster context condition and typically were reported in the slowed context condition. The timing of the differences in the brain response indicates that distal speech rate affects early perceptual processing of speech sounds. This result is consistent with listeners formulating predictions about the timing of upcoming speech signals, and those predictions influencing perception of incoming speech signals.

This research significantly expands our shared knowledge of the mechanism by which context influences speech processing. The finding that context speech rate affects early perceptual processing of speech sounds has important implications for models of speech processing. The results suggest that listeners automatically extract rate information from speech signals under typical listening conditions. Further, they demonstrate that predictions about the timing of speech sounds are typically employed when listening to speech for comprehension. Finally, the results show that factors which influence predictive processes can be functionally equivalent to acoustic modulations.

This research involved the contributions of seven undergraduate students and two post-baccalaureate assistants, five of whom are under-represented minorities. The research experience on this project compelled six of those students (three under-represented minorities) to apply to graduate programs; four have already begun their graduate work. One of the students who has started her PhD program is an author on a manuscript describing this research that has been submitted for publication.

Last Modified: 10/30/2017
Modified by: Lisa D Sanders