Investigating EEG Markers of Responsiveness to Surgical Noxious Stimuli under Propofol Anaesthesia

Background : While general anaesthesia typically induces unconsciousness, some patients retain the capacity to respond behaviourally to noxious stimulation. Reduction in frontal alpha power has been proposed as a marker of arousal, but its clinical reliability is inconsistent. This exploratory study aimed to identify multichannel EEG spectral and connectivity markers associated with intraoperative behavioural responsiveness around noxious stimulation. Methods : Sixty-four-channel EEG was recorded intraoperatively from seven patients undergoing microlaryngoscopy under propofol anaesthesia accompanied by analgesia. Responsiveness was determined by reactions to verbal commands. Alpha-band spectral features of responders and non-responders were compared pre- and post-noxious stimulation using descriptive statistics. The relationship between spectral power of individual channels and the factors of noxious stimulation and responsiveness was investigated using a linear mixed model (LMM). Event-related synchronization/desynchronization (ERS/ERD) and weighted Symbolic Mutual Information (wSMI) were computed across frequency bands and response categories, with correction for multiple comparisons. Results : Alpha power and global coherence showed minimal changes after stimulation in both responders (n = 3) and non-responders (n = 4). In contrast, volitional (or cognitive) responses to noxious stimulation were reliably associated with increased high-beta and gamma power in sensory-motor and auditory cortices. These responses showed event-related synchronisation in left-central channels, whereas reflexive movements were marked by desynchronisation in same areas. Theta-band activity also differentiated response types: cognitive responses showed suppression, while reflexive responses showed enhancement, particularly over the same regions. Connectivity analysis further revealed that cognitive responses were associated with increased global whole-brain integration, especially in the theta-band linking motor, auditory, and premotor cortices. Reflexive responses, by contrast, were associated with reductions in global brain connectivity. Conclusions : Frontal alpha EEG markers did not reliably index intraoperative responsiveness. Instead, localized high-frequency power increases and enhanced theta-band connectivity more robustly reflected ‘connected consciousness’, a state in which patients remain capable of perceiving and processing sensory inputs despite anaesthesia. Despite the limitations behind small sample size and variability in anaesthetic and patient factors, the surgical setting of the study strengthens the clinical relevance of these multichannel EEG features for guiding intraoperative monitoring and analgesic management before noxious stimulation.