When listening to speech under adverse conditions, expectancies resulting from semantic context can have a strong impact on comprehension. Here we ask how minimal variations in semantic context (cloze probability) affect the unfolding comprehension of acoustically degraded speech. Three main results are observed in the brain electric response. First, auditory evoked responses to a degraded sentence's onset (N100) correlate with participants' comprehension scores, but are generally more vigorous for more degraded sentences. Second, a pronounced N400 in response to low-cloze sentence-final words, reflecting the integration effort of words into context, increases linearly with improving speech intelligibility. Conversely, transient enhancement in Gamma band power (γ, ~40-70 Hz) during high-cloze sentence-final words (~600 ms) reflects top-down-facilitated integration. This γ-band effect also varies parametrically with signal quality. Third, a negative correlation of N100 amplitude at sentence onset and the later γ-band response is found in moderately degraded speech. This reflects two partly distinct neural strategies when dealing with moderately degraded speech; a more "bottom-up," resource-allocating, and effortful versus a more "top-down," associative and facilitatory strategy. Results also emphasize the non-redundant contributions of phase-locked (evoked) and non-phase-locked (induced) oscillatory brain dynamics in auditory EEG.
Copyright © 2010 Elsevier Inc. All rights reserved.