TY - JOUR T1 - Cortical Measures of Phoneme-Level Speech Encoding Correlate with the Perceived Clarity of Natural Speech JF - eneuro JO - eNeuro DO - 10.1523/ENEURO.0084-18.2018 SP - ENEURO.0084-18.2018 AU - Giovanni M. Di Liberto AU - Michael J. Crosse AU - Edmund C. Lalor Y1 - 2018/03/29 UR - http://www.eneuro.org/content/early/2018/03/29/ENEURO.0084-18.2018.abstract N2 - In real-world environments, humans comprehend speech by actively integrating prior knowledge and expectations with sensory input. Recent studies have revealed effects of prior information in temporal and frontal cortical areas, and have suggested that these effects are underpinned by enhanced encoding of speech-specific features, rather than a broad enhancement or suppression of cortical activity. However, in terms of the specific hierarchical stages of processing involved in speech comprehension, the effects of integrating bottom-up sensory responses and top-down predictions are still unclear. In addition, it is unclear whether the predictability that comes with prior information may differentially affect speech encoding relative to the perceptual enhancement that comes with that prediction. One way to investigate these issues is through examining the impact of prior knowledge on indices of cortical tracking of continuous speech features. Here, we did this by presenting participants with degraded speech sentences that either were or were not preceded by a clear recording of the same sentences while recording non-invasive electroencephalography. We assessed the impact of prior information on an isolated index of cortical tracking that reflected phoneme-level processing. Our findings suggests the possibility that prior information affects the early encoding of natural speech in a dual manner. Firstly, the availability of prior information, as hypothesized, enhanced the perceived clarity of degraded speech, which was positively correlated with changes in phoneme-level encoding across subjects. In addition, prior knowledge induced an overall reduction of this cortical measure, which we interpret as resulting from the increase in predictability.Significance Statement The human ability to comprehend speech despite challenges such as loud noise and competing speech derives in large part from the use of prior knowledge of the upcoming speech. Here, we examine the cortical underpinnings of this process by using prior knowledge to modulate the perceived intelligibility of degraded stimuli. We find two distinct effects of prior knowledge: A positive correlation between perceptual enhancement and phoneme-level encoding and an overall suppression of this cortical encoding. ER -