Mapping long-term memory modulation with transcranial alternating current stimulation

BackgroundLong-term memory (LTM), which is the ability to retain information over a long period of time, is governed by neural oscillations in various parts of the cerebral cortex. To investigate the relationship between oscillations and memory performance, transcranial alternating current stimulation (tACS) has been used to modulate LTM. However, due to the variability in stimulation parameters and locations, no clear picture has arisen on where tACS needs to be applied to improve LTM. MethodsWe collected behavioral data from 20 studies (58 effect sizes) and computationally modeled the corresponding brain electric fields based on each studys unique tACS parameters. As a result, we generated a map of performance-electric field index (PEI) values using linear and quadratic fits. ResultsTheta-tACS showed no regions with a positive linear association with LTM. However, left lateral prefrontal and temporal areas exhibited negative associations, meaning that theta stimulation of that region led to decreased LTM performance. A quadratic analysis revealed a U-shaped relation in the medial temporal and ventromedial prefrontal cortex, suggesting improved memory at low and high, but not moderate, electric field strength values. Gamma-tACS showed positive linear associations in parieto-occipital cortex and negative ones in frontal regions. For gamma, we found no regions that expressed a quadratic relationship. ConclusionThese findings indicate that LTM-relevant tACS effects are frequency- and site-specific and potentially nonlinear with respect to intensity. Integrating meta-analytic effect sizes with individualized field estimates provides a brain-wide framework for identifying targets of memory modulation and for guiding future fundamental and translational research.