Noninvasive Stimulation of Prefrontal Cortex Strengthens Existing Episodic Memories and Reduces Forgetting in the Elderly

Abstract

Memory consolidation is a dynamic process. Reactivation of consolidated memories by a reminder triggers reconsolidation, a time-limited period during which existing memories can be modified (i.e., weakened or strengthened). Episodic memory refers to our ability to recall specific past events about what happened, including where and when. Difficulties in this form of long-term memory commonly occur in healthy aging. Because episodic memory is critical for daily life functioning, the development of effective interventions to reduce memory loss in elderly individuals is of great importance. Previous studies in young adults showed that the dorsolateral prefrontal cortex (DLPFC) plays a causal role in strengthening of verbal episodic memories through reconsolidation. The aim of the present study was to explore the extent to which facilitatory transcranial direct current stimulation (anodal tDCS) over the left DLPFC would strengthen existing episodic memories through reconsolidation in elderly individuals. On Day 1, older adults learned a list of 20 words. On Day 2 (24 h later), they received a reminder or not, and after 10 min tDCS was applied over the left DLPFC. Memory recall was tested on Day 3 (48 h later) and Day 30 (1 month later). Surprisingly, anodal tDCS over the left DLPFC (i.e., with or without the reminder) strengthened existing verbal episodic memories and reduced forgetting compared to sham stimulation. These results provide a framework for testing the hypothesis that facilitatory tDCS of left DLPFC might strengthen existing episodic memories and reduce memory loss in older adults with amnestic mild cognitive impairment.

The optimal timing of stimulation to induce long-lasting positive effects on episodic memory in physiological aging

“…A noninvasive brain stimulation technique that can be used to modulate memory in physiological aging is transcranial Direct Current Stimulation (tDCS) [5,6]. TDCS has been used for two main purposes in memory research: (1) to test the causal relationship between activity of a cortical region and a memory function; and (2) to investigate whether tDCS might modulate memory formation and learning, an issue of relevance for research and neurorehabilitation [5,[7][8][9].…”

“…tDCS studies have shown that the left lateral prefrontal cortex (PFC) contributes to verbal episodic memories along the life span [8][9][10][11][12][13] and enhancement of these abilities has been reported in healthy older adults after the application of tDCS over this brain area [8,9,13]. Specifically, Sandrini and coworkers [8,9] showed that anodal tDCS strengthened memories when applied during the encoding phase or after the consolidation process in elderly individuals. However, an aspect that has not been directly investigated in previous studies is the optimal timing of stimulation to induce long-lasting positive effects on episodic memory function.…”

“…In the present study, we re-analyzed data acquired from two previous studies [8,9] in which we showed episodic memory enhancement in older adults when anodal tDCS was applied to the left PFC during the encoding phase [9] or after the memory consolidation process [24h after the encoding session, 8] using the same word-learning paradigm.…”

“…after consolidation) they received tDCS over the left PFC with or without a contextual reminder (respectively, same or different experimental room as done in previous studies [18,19]). On Day 3 and Day 30 (respectively 48h and 30 days from the learning phase) they were asked to recall the words [8]. In the successive study, elderly participants learned the list of words while receiving tDCS over the left PFC during encoding.…”

“…tDCS was applied over the left PFC (intensity: 1.5 mA, duration: 15 minutes, current density 0.043 mA/cm2, size of electrodes: 5x7 cm2) [8,9,13,19]: the anode was placed over F3 according to the 10-20 EEG international system for electrode placement, and the cathode was placed over the right supraorbital area (see Figure 1). The placement of the reference electrode over the supraorbital region was motivated by the assumption that, since this area is not specifically involved in memory processing, this electrode would not actively contribute to modulation.…”