Evidence for Immediate Enhancement of Hippocampal Memory Encoding by Network-Targeted Theta-Burst Stimulation during Concurrent fMRI

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 time courses. In this experiment with human subjects of both sexes, we tested for immediate stimulation impact on encoding-related activity of the hippocampus and immediately adjacent medial-temporal cortex 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 hippocampus because rhythmic neural activity in the theta band is associated with hippocampal memory processing. First, we demonstrated that it is possible to obtain robust fMRI correlates of task-related activity during concurrent TBS. We then identified immediate effects of TBS on encoding of visual scenes. Brief volleys of TBS targeting the hippocampal network increased activity of the targeted (left) hippocampus during scene encoding and increased subsequent recollection. Stimulation did not influence activity during an intermixed numerical task with no memory demand. Control conditions using beta band and out-of-network stimulation also did not influence hippocampal activity or recollection. TBS targeting the hippocampal network therefore immediately impacted hippocampal memory processing. This suggests direct, beneficial influence of stimulation on hippocampal neural activity related to memory and supports the role of theta-band activity in human episodic memory.SIGNIFICANCE STATEMENT Can noninvasive stimulation directly impact function of indirect, deep-brain targets, such as the hippocampus? We tested this by targeting an accessible region of the hippocampal network via transcranial magnetic stimulation during concurrent fMRI. We reasoned that theta-burst stimulation would be particularly effective for impacting hippocampal function, as this stimulation rhythm should resonate with the endogenous theta-nested-gamma activity prominent in hippocampus. Indeed, theta-burst stimulation targeting the hippocampal network immediately impacted hippocampal activity during encoding, improving memory formation as indicated by enhanced later recollection. Rhythm- and location-control stimulation conditions had no such effects. These findings suggest a direct influence of noninvasive stimulation on hippocampal neural activity and highlight that the theta-burst rhythm is relatively privileged in its ability to influence hippocampal memory function.

Keywords: hippocampus; memory; noninvasive stimulation; recollection; simultaneous TMS/fMRI; theta-burst.

Copyright © 2020 the authors.

Figures

Figure 1.

Trial-specific stimulation during episodic memory formation. A, Scene-encoding and numeric-judgment trials were randomly intermixed during each study phase, with ∼2 s of stimulation delivered immediately before stimulus onset for a subset of trials (ON) and no preceding stimulation for remaining trials (OFF). Study phases were completed during fMRI scanning with memory test phases after scanning. There were four stimulation conditions for ON and OFF trials. B, Stimulation was delivered as either a theta-burst pattern (TBS: 50 Hz triplet pulses delivered at 5 Hz) or at beta (single pulses delivered at 12.5 Hz). These conditions had the same overall number of pulses during each stimulation period delivered at the same intensity. C, Stimulation was delivered to the HNT parietal location (based on its fMRI connectivity with left hippocampus, as indicated by the blue arrow), or a control out-of-network SMA location. Achieved stimulation locations confirmed via MRI for each condition and subject are indicated by colorized spheres on a template brain. Bar plots indicate mean ± SEM baseline resting-state fMRI connectivity of the subject-specific stimulation locations with the hippocampal network, confirming relatively higher connectivity for the HNT than SMA location. *p < 0.05, main effect of stimulation location by one-way repeated-measures ANOVA. D, HNT or SMA locations were targeted for one of the two study phases in each experimental session. After both study phases were complete, subjects exited the scanner for a ∼15 min break before taking the memory test. A different stimulation pattern (TBS or beta) was used for each experimental session. Black arrows indicate stimulation conditions with order counterbalanced across subjects.

Figure 2.

Interleaved TMS/fMRI scan sequences. Depiction of one imaging volume in the (A) TBS and (B) beta scan sequences. Each colored cube represents one MR EPI slice acquisition (purple represents TBS scan; green represents beta scan). Each grey line indicates a TMS pulse. The extent of imaging coverage (22 EPI slices for TBS; 30 EPI slices for beta) is shown on a template brain, with EPI slices colorized by condition (purple represents TBS scan; green represents beta scan).

Figure 3.

Expected fMRI signals of scene processing and stimulation sensations confirm fMRI data quality during concurrent TMS. Voxelwise contrasts confirm that the fMRI signal could distinguish the task stimuli (scenes vs numbers) and the presence of stimulation (ON vs OFF). A, Group-level contrast of scenes versus numbers, regardless of stimulation location or presence, identified significantly greater activation by scenes in areas that are typically scene-sensitive for both TBS and beta stimulation, including bilateral hippocampus and occipital, fusiform, and posterior parahippocampal cortex. B, Group-level contrast of TMS ON versus OFF, regardless of stimulation location or the stimuli type (scenes and numbers) identified significantly greater activation for TMS ON in the bilateral auditory cortex for both TBS and beta stimulation. Direct contrasts of TBS versus beta did not identify significant differences for either comparison. Plots represent suprathreshold voxels on a template brain. Color bars represent t statistic calculated by 3dMEMA.

Figure 4.

Theta-patterned stimulation of the hippocampal network during encoding selectively increased subsequent recollection. A, Subjects discriminated studied scenes (old; depicted here with a yellow border) from randomly intermixed novel lures (new) using “Remember” responses to indicate the experience of hippocampal-dependent recollection, “Familiar” responses to indicate the experience of familiarity, and “New” responses to indicate lures. Example correct responses are underlined. B, To identify stimulation effects on memory while holding sensory entrainment qualities (e.g., somatosensory, auditory) relatively constant, we compared the difference in the proportion of recollected hits for stimulation targeting HNT versus SMA for each of the stimulation patterns (TBS, beta, and off). *p = 0.02, interaction between TMS presence and rhythm reported in the text. C, The proportion of recollected hits for HNT TBS was greater than for the aggregate of all control conditions. **PB = 0.008 (reported in the text). None of the control conditions increased the proportion of recollected hits relative to the aggregate of all other control conditions (nonsignificant values reported in the text). White bars next to each condition represent the average of the control conditions that the corresponding condition of interest (colorized bars) was tested against. Bars and error bars indicate mean ± SEM.

Figure 5.

HNT TBS selectively increased left hippocampal activity during recollection memory formation. A, Stimulation-evoked activity was measured within hippocampal/MTL units centered along the hippocampal longitudinal axis in each hemisphere, shown as colorized voxels in coronal slices of a template brain. The left hippocampus/MTL was targeted indirectly via its network functional connectivity (see Materials and Methods) and the right hippocampus/MTL served as a nontargeted control. B, fMRI activity evoked by later-recollected scenes for each stimulation condition (colorized bars) and the aggregate of all other control conditions (white bards adjacent to each colorized bars), plotted separately for left and right hippocampus. Bars and error bars indicate mean ± SEM estimated fMRI activity (arbitrary units). Activity evoked by later-recollected scenes for HNT TBS was greater than for the aggregate of all control conditions. *PB = 0.03 (reported in the text). None of the control conditions increased active relative to the aggregate of all other control conditions (nonsignificant values reported in the text). C, Activity evoked by numbers displayed as in B. Significant difference of the effect of HNT TBS relative to the average of all controls for scene-evoked recollection-related activity versus activity evoked by numeric judgments is indicated (*p = 0.01 as reported in the text).

Figure 6.

Exploratory analysis of differential stimulation effects on activity along the hippocampal longitudinal axis. The same fMRI analyses as described in Figure 5 are presented but segregated for longitudinal segments of the hippocampus in each hemisphere. A, The five segments along the longitudinal axis in each hemisphere are shown as colorized spheres on a template brain. B, fMRI activity evoked by scenes later-recollected was extracted from each of the spherical ROIs in the left and right hemisphere for each stimulation condition. Line plots indicate mean estimated fMRI activity (arbitrary units) ± SEM (shaded area), colorized for each of the five stimulation conditions. C, fMRI activity evoked by numbers displayed as in B. *p < 0.05 main effect of stimulation condition by five-way repeated-measures ANOVA reported in the text are outlined. Follow-up pairwise comparisons among stimulation conditions are described in the text.