Heard or understood? Neural tracking of language features in a comprehensible story, an incomprehensible story and a word list

Speech comprehension is a complex neural process on which relies on activation and integration of multiple brain regions. In the current study, we evaluated whether speech comprehension can be investigated by neural tracking. Neural tracking is the phenomenon in which the brain responds in a time-locked fashion to specific features of the speech. These features can be acoustic, i.e., acoustic tracking, or derived from the content of the speech using language properties, i.e., language tracking. We evaluated whether neural tracking of speech differs between a comprehensible story, an incomprehensible story, and a word list. We evaluated the neural responses to speech of 19 participants (6 men). No significant difference regarding acoustic tracking was found. However, significant language tracking was only found for the comprehensible story. The most prominent effect was visible to word surprisal, a language feature at the word level. The neural response to word surprisal showed a prominent negativity between 300 and 400 ms, similar to the N400 in evoked response paradigms. This N400 was significantly more negative when the story was be comprehended, i.e., when words could be integrated in the context of previous words. These results show that language tracking can capture the effect of speech comprehension. Significance statement Most neural tracking studies focus on how the brain tracks acoustic speech features. However, whether acoustic tracking reflects speech comprehension is unclear. Therefore, in the pursuit of finding a neural marker for speech comprehension, language tracking might be a more suitable candidate. The results of this study showed that, indeed, language tracking can capture the effect of speech comprehension. This indicates that language tracking is a potential neural marker of speech comprehension. Such a neural marker would allow testing speech comprehension in populations that are currently difficult to test with behavioral tests, such as young children and persons with cognitive impairment.