Theta and Gamma Oscillations Predict Encoding and Retrieval of Declarative Memory

TLDR

Brain oscillations are known to support declarative memory, yet the temporal dynamics and source of gamma and theta activity in healthy humans remain poorly understood, with gamma possibly reflecting top‑down reinforcement. The study proposes that theta activity is directly engaged in mnemonic operations. Gamma‑band synchronization in occipital areas may drive activity to downstream regions, facilitating encoding and retrieval. Gamma (60–90 Hz) and theta (4.5–8.5 Hz) oscillations were stronger for remembered and recognized items, with theta localized to right parietotemporal cortex and gamma to occipital Brodmann 18/19, supporting their role in encoding and retrieval.

Abstract

Although studies in animals and patients have demonstrated that brain oscillations play a role in declarative memory encoding and retrieval, little has been done to investigate the temporal dynamics and sources of brain activity in healthy human subjects performing such tasks. In a magnetoencephalography study using pictorial stimuli, we have now identified oscillatory activity in the gamma (60–90 Hz) and theta (4.5–8.5 Hz) band during declarative memory operations in healthy participants. Both theta and gamma activity was stronger for the later remembered compared with the later forgotten items (the “subsequent memory effect”). In the retrieval session, theta and gamma activity was stronger for recognized items compared with correctly rejected new items (the “old/new effect”). The gamma activity was also stronger for recognized compared with forgotten old items (the “recognition effect”). The effects in the theta band were observed over right parietotemporal areas, whereas the sources of the effects in the gamma band were identified in Brodmann area 18/19. We propose that the theta activity is directly engaged in mnemonic operations. The increase in neuronal synchronization in the gamma band in occipital areas may result in a stronger drive to subsequent areas, thus facilitating both memory encoding and retrieval. Alternatively, the gamma synchronization might reflect representations being reinforced by top-down activity from higher-level memory areas. Our results provide additional insight on human declarative memory operations and oscillatory brain activity that complements previous electrophysiological and brain imaging studies.