Data Sheet 1_GRASP: a response-locked paradigm that reveals low-beta desynchronisation is a participant-invariant signature of semantic decoding in EEG.docx

Participants were instructed to press a spacebar when they felt reasonably confident in their answer, focusing the EEG analysis on the pre-response retrieval interval. Data were captured from 13 neurologically healthy British English speakers. Preprocessed 64-channel EEG epochs were analysed using Morlet time-frequency decomposition over 2–35 Hz.

Concept-minus-dummy contrast maps were computed per participant for each category and entered into a whole-scalp three-dimensional Threshold-Free Cluster Enhancement (TFCE)-based permutation analysis across channel × frequency × time space. All three categories produced significant TFCE-corrected effects in the pre-response interval, consistently located in the low-beta range and peaking at approximately 15 Hz. Meaningful concept trials showed lower low-beta power than dummy trials, with complete directional consistency across all 13 participants in all three contrasts.

Body and music trial effects were strongest over centro-parietal and posterior scalp regions, whereas food showed a broader frontal-central-posterior distribution. These findings identify a robust, participant-invariant low-beta desynchronisation signature distinguishing meaningful semantic retrieval from a controlled pseudo-word baseline. The consistency of this effect across categories and participants suggests that the high-semantic vs. low-semantic boundary may provide a more cross-participant-stable electrophysiological target than fine-grained concept vs. concept discrimination, providing a principled signal-characterisation framework for future sBCI development.</p>