Dissociating speech perception and comprehension at reduced levels of awareness

Abstract

We used functional MRI and the anesthetic agent propofol to assess the relationship among neural responses to speech, successful comprehension, and conscious awareness. Volunteers were scanned while listening to sentences containing ambiguous words, matched sentences without ambiguous words, and signalcorrelated noise (SCN). During three scanning sessions, participants were nonsedated (awake), lightly sedated (a slowed response to conversation), and deeply sedated (no conversational response, rousable by loud command). Bilateral temporal-lobe responses for sentences compared with signal-correlated noise were observed at all three levels of sedation, although prefrontal and premotor responses to speech were absent at the deepest level of sedation. Additional inferior frontal and posterior temporal responses to ambiguous sentences provide a neural correlate of semantic processes critical for comprehending sentences containing ambiguous words. However, this additional response was absent during light sedation, suggesting a marked impairment of sentence comprehension. A significant decline in postscan recognition memory for sentences also suggests that sedation impaired encoding of sentences into memory, with left inferior frontal and temporal lobe responses during light sedation predicting subsequent recognition memory. These findings suggest a graded degradation of cognitive function in response to sedation such that ''higher-level'' semantic and mnemonic processes can be impaired at relatively low levels of sedation, whereas perceptual processing of speech remains resilient even during deep sedation. These results have important implications for understanding the relationship between speech comprehension and awareness in the healthy brain in patients receiving sedation and in patients with disorders of consciousness.anesthesia ͉ functional MRI ͉ language ͉ memory ͉ sedation

A Temporal Bottleneck in the Language Comprehension Network

“…However, perisylvian language areas in the left STS and inferior frontal gyrus demonstrated a temporally invariant response profile up to ϳ40% compression, followed by a sudden collapse. These results confirm an association of language intelligibility with the left perisylvian inferior frontal gyrus and the superior temporal gyrus (Davis and Johnsrude, 2003;Davis et al, 2007), including for compressed speech (Poldrack et al, 2001;Peelle et al, 2004Peelle et al, , 2010Adank and Devlin, 2010). Adank et al (2010) found that, as subjects adapt to compressed speech, activation increases in bilateral superior temporal and midline premotor cortices.…”

Brain–computer interfacing in disorders of consciousness

“…Firstly, fMRI evidence suggests that some patients might retain near-normal levels of language comprehension [36][37][38]. In addition, they appear to be able to selectively attend to and process their own names as compared to unfamiliar names [39][40][41] This finding has been confirmed with EEG data: ERPs evoked by increased mismatch negativity (MMN) have been observed when some patients with DoC heard their own names presented infrequently amongst tones and other names [42].…”

Detecting consciousness in a total locked-in syndrome: An active event-related paradigm

“…However, a previous study of Perrin et al, 2006) showed a larger amplitude in response to the patients' own name as compared to unfamiliar names in all controls but also in 3 out 5 patients diagnosed as being in a vegetative state. Furthermore, it is known that speech processing can be observed in unconscious state such as anesthesia (Davis et al, 2007) or sleep (Perrin et al, 1999). Therefore, responses observed during passive listening conditions probably reflect automatic speech processing and are not sufficient to suggest voluntary and therefore conscious brain activity.…”