Cognitive functioning before and after repetitive transcranial magnetic stimulation (rTMS): A quantitative meta-analysis in healthy adults

Repetitive transcranial magnetic stimulation (rTMS) is thought to modulate brain function through methods of electromagnetic induction. Over the last few decades, a large body of studies have investigated the clinical applications of rTMS in a variety of patient populations for a diverse range of symptoms from depressive symptomology to post-stroke motor functioning. There is still no clear consensus, however, on how rTMS influences cognitive functioning in the healthy brain. We conducted a quantitative meta-analysis in order to evaluate whether offline rTMS (the delivery of rTMS when not actively engaged in a cognitive task) influences cognition in healthy adults. More specifically, we examined studies that applied rTMS to the dorsal lateral prefrontal cortex (DLFPC) and that tracked cognitive outcomes both before and after a prescribed period of rTMS. Fifteen studies met our inclusion criteria. Cognitive performance was pooled and examined across studies for four cognitive domains (working memory, executive functioning, episodic memory, and visual perception) and under two types of stimulation conditions (excitatory and inhibitory rTMS). Whereas excitatory rTMS was associated with statistically reliable effects for improving executive functioning, inhibitory rTMS was associated with statistically reliable effects for improving episodic memory and visual perception. However, the magnitude of these effects was small and no other significant effects were observed. Though future studies are still needed, our findings suggest that offline forms of rTMS may have limited utility in affecting cognitive functioning when applied to the DLPFC in healthy adults, irrespective of cognitive domain or stimulation type.