Ongoing theta oscillations predict encoding of subjective memory type

- Prestimulus theta oscillations separate episodic from nonepisodic encoding.
- Pre- and post-stimulus theta correlation is early for nonepisodic encoding.
- Pre- and post-stimulus theta correlation is late for episodic encoding.
- First evidence of alpha in the episodic versus non-episodic encoding distinction.

Recent human electro-encephalography (EEG) studies show that ongoing brain states support successful encoding of human memory, including recognition. However it is not known whether ongoing cortical activity qualitatively determines different memory types at encoding. In this study, using a remember/know procedure, we measured the EEG oscillations that emerge before and during the encoding of abstract visual stimuli in episodic and non-episodic memory, focusing on the theta (2-8 Hz) and alpha (9-12 Hz) oscillation range. We found that enhanced prestimulus theta oscillations precede episodic memory encoding, compared to non-episodic encoding. The prestimulus difference appeared at frontal and temporal sites. Furthermore, the theta enhancement reappeared after stimulus onset. Enhanced upper alpha oscillations suggested increased working memory processing in the case of episodic memory. Finally, the pre- and post-stimulus theta and alpha amplitudes showed different correlation patterns for episodic and non-episodic encoding. Our results are the first to suggest that encoding of episodic memory depends on preparatory processing in the form of frontal and temporal theta oscillations.