More than Words: Neurophysiological Correlates of Semantic Dissimilarity Depend on Comprehension of the Speech Narrative

Speech comprehension relies on the ability to understand the meaning of words within a coherent context. Recent studies have attempted to obtain electrophysiological indices of this process by modelling how brain activity is affected by a word9s semantic dissimilarity to preceding words. While the resulting indices appear robust and are strongly modulated by attention, it remains possible that, rather than capturing the contextual understanding of words, they may actually reflect word-to-word changes in semantic content without the need for a narrative-level understanding on the part of the listener. To test this possibility, we recorded EEG from subjects who listened to speech presented in either its original, narrative form, or after scrambling the word order by varying amounts. This manipulation affected the ability of subjects to comprehend the narrative content of the speech, but not the ability to recognize the individual words. Neural indices of semantic understanding and low-level acoustic processing were derived for each scrambling condition using the temporal response function (TRF) approach. Signatures of semantic processing were observed for conditions where speech was unscrambled or minimally scrambled and subjects were able to understand the speech. The same markers were absent for higher levels of scrambling when speech comprehension dropped below chance. In contrast, word recognition remained high and neural measures related to envelope tracking did not vary significantly across the different scrambling conditions. This supports the previous claim that electrophysiological indices based on the semantic dissimilarity of words to their context reflect a listener9s understanding of those words relative to that context. It also highlights the relative insensitivity of neural measures of low-level speech processing to speech comprehension.