Abstract
The hippocampus supports episodic memory via interaction with a distributed brain network. Previous experiments using network-targeted noninvasive brain stimulation have identified episodic memory enhancements and modulation of activity within the hippocampal network. However, mechanistic insights were limited because these effects were measured long after stimulation and therefore could have reflected various neuroplastic aftereffects with extended timecourses. In this experiment with human subjects of both sexes, we tested for immediate stimulation impact on memory-related activity of the hippocampus and surrounding cortex of the medial-temporal lobe (MTL) by delivering theta-burst transcranial magnetic stimulation (TBS) concurrent with fMRI, as an immediate impact of stimulation would suggest an influence on neural activity. We reasoned that TBS would be particularly effective for influencing the MTL because rhythmic neural activity in the theta band is associated with MTL memory processing. First, we demonstrated that it is possible to obtain robust fMRI signals of MTL activity during concurrent TBS. We then identified immediate effects of TBS on memory encoding of visual scenes. Brief volleys of TBS targeting the hippocampal network increased MTL activity during scene encoding and strengthened subsequent recollection. Stimulation did not influence MTL activity during an interleaved numerical task with no memory demand. Control conditions using beta-band stimulation and out-of-network stimulation also did not influence MTL activity or memory. These findings indicate that TBS targeting the hippocampal network immediately impacts MTL memory processing. This suggests direct, beneficial influence of stimulation on MTL neural activity related to memory and supports the role of theta-band activity in human episodic memory.
Significance Statement Theta-burst noninvasive stimulation targeting the human hippocampal network immediately impacted memory processing measured during concurrent fMRI, suggesting that this rhythm is relatively privileged in its ability to influence neural activity related to memory.
Footnotes
Conflict of interest: The authors declare no competing financial interests.