How Can Transcranial Magnetic Stimulation Be Used to Modulate Episodic Memory?: A Systematic Review and Meta-Analysis

Abstract

A systematic review and meta-analysis were conducted to synthesize the existing literature on how transcranial magnetic stimulation (TMS) has been used to modulate episodic memory. Given the numerous parameters of TMS protocols and experimental design characteristics that can be manipulated, a mechanistic understanding of how changes in the combination of parameters (e.g., frequency, timing, intensity, targeted brain region, memory task) modulate episodic memory is needed. To address this, we reviewed 59 studies and conducted a meta-analysis on 245 effect sizes from 37 articles on healthy younger adults ( N = 1,061). Analyses revealed generally more beneficial effects of 1-Hz rTMS vs. other frequencies on episodic memory. Moderation analyses revealed complex interactions as online 20-Hz rTMS protocols led to negative effects, while offline 20-Hz rTMS led to enhancing effects. There was also an interaction between stimulation intensity and frequency as 20-Hz rTMS had more negative effects when applied below- vs. at-motor threshold. Conversely, 1-Hz rTMS had more beneficial effects than other frequencies when applied below- vs. at- or above-motor threshold. No reliable aggregate or hypothesized interactions were found when assessing stimulation site (frontal vs. parietal cortex, left vs. right hemisphere), stimulated memory process (during encoding vs. retrieval), the type of retrieval (associative/recollection vs. item/familiarity), or the type of control comparison (active vs. sham or no TMS) on episodic memory. However, there is insufficient data to make strong inference based on the lack of aggregate or two-way interactions between these factors, or to assess more complex (e.g., 3-way) interactions. We reviewed the effects on other populations (healthy older adults and clinical populations), but systematic comparison of parameters was also prevented due to insufficient data. A database of parameters and effects sizes is available as an open source repository so that data from studies can be continuously accumulated in order to facilitate future meta-analysis. In conclusion, modulating episodic memory relies on complex interactions among the numerous moderator variables that can be manipulated. Therefore, rigorous, systematic comparisons need to be further investigated as the body of literature grows in order to fully understand the combination of parameters that lead to enhancing, detrimental or null effects on episodic memory.

Repetitive Transcranial Magnetic Stimulation Improved Source Memory and Modulated Recollection-Based Retrieval in Healthy Older Adults

“…In addition, the present results showed that 10 Hz rTMS stimulus is an effective parameter to enhance episodic memory. A meta-analysis indicated that off-line 20, 10, and 5 Hz rTMS were all effective to enhance episodic memory (Yeh and Rose, 2019). The present study further supported the high-frequency rTMS could promote source memory.…”

“…The reason for the contradictory results might be that some studies have used single-session stimulation while others have used multi-session stimulation. Yeh and Rose (2019) reviewed all the rTMS studies on episodic memory, and they proposed that the implementation of high-frequency multi-session off-line rTMS could promote episodic memory in MCI and AD patients. Only a few studies have implemented multi-session off-line stimulation in healthy older adults.…”

Transcranial magnetic stimulation of right dorsolateral prefrontal cortex does not affect associative retrieval in healthy young or older adults

“…Moreover, it has been reported that the relationship between the intensity of cTBS stimulation and degree cortical suppression is complex and possibly non-linear, at least in some participants, casting doubt on the assumption that the behavioral effects of such stimulation should correlate positively with stimulation intensity (Sasaki et al, 2018). Further work is needed to establish the stimulation parameters that have the greatest likelihood of impacting episodic memory processes (Yeh & Rose, 2019;Widhalm & Rose, 2019).…”

Medial prefrontal cortex has a causal role in selectively enhanced consolidation of emotional memories after a 24-hour delay: A TBS study

“…It is unclear if our results would differ had we used a different TMS protocol, such as 1Hz rTMS. Of relevance to this point, a recent meta-analysis of TMS effects on episodic memory found that offline 1 Hz rTMS led to larger enhancing effects on episodic memory compared to other stimulation protocols, including iTBS (Yeh & Rose, 2019). Second, we only examined the effects of mPFC stimulation during memory encoding.…”

“…For example, as described above, Bovy and colleagues found a causal role of the mPFC in gist-based false memories implementing cTBS while our iTBS protocol revealed a causal role of the mPFC in emotional memory (collapsed across specific and gist-based memories). Of relevance to this point, a recent meta-analysis of TMS effects on episodic memory found that offline 1 Hz rTMS led to larger enhancing effects on episodic memory compared to other stimulation protocols, including iTBS (Yeh & Rose, 2019). Second, we only examined the effects of mPFC stimulation during memory encoding.…”