Why does invasive brain stimulation sometimes improve memory and sometimes impair it?

Abstract

Episodic memory is a critical aspect of everyday life that allows us to recall events, make decisions, and navigate our environments. It is estimated that 1 in 25 Americans suffer from episodic memory loss from Alzheimer 's disease, dementia, or following a traumatic brain injury. Clinicians and researchers have recently tried to help supplement or repair memory in individuals with episodic memory disorders using electrical brain stimulation. However, although there is evidence of invasive brain stimulation improving memory, studies also show that it can have variable effects, and sometimes cause memory impairment. In this Essay, we investigate these differences and try to explain this variability. We propose that key aspects of the memory network are heterogeneous across individuals, requiring customized stimulation protocols to create successful brain stimulation systems for memory enhancement. We provide a plan for applying brain stimulation while overcoming this variability by using closed-loop stimulation protocols that are customized according to a person's own anatomical structure, functional connectivity, and memory-related electrophysiological patterns.