Preprint · Jan 2026
The effect of non-invasive brain stimulation on associative episodic memory
By Ngubo C, Sandrini M, Brandt KR
Among the different types of long-term memory, episodic memory (EM) is the one that is most important for preserving one's sense of self (Yeh & Rose, 2019). EM is a form of declarative memory that reflects contextual acquisition, such as the time and location of an event, as well as knowledge of specific events that one has encountered. It can be tested using associate memory (AM) tests such as face-name tasks. Studies have shown that EM declines more markedly with healthy ageing than other forms of memory, with deficits becoming particularly apparent from the age of 60 (Kinugawa et al., 2013). A key feature of this decline is AM impairment, where healthy older adults (OA) around age 60 show greater difficulty remembering links between items (e.g., a face and a name) compared to single-item memory (Leach et al., 2017; Leach et al., 2018). While cognitive training, lifestyle-based treatments, and pharmacological interventions have been utilised to compensate for AM decrease with healthy ageing (Bjekić et al., 2023), studies show that memory consolidation can be more enhanced with non-invasive brain stimulation (NIBS) to increase activity in associated cortical brain regions. Although the hippocampus would be the most effective area to activate to potentially improve AM, this is not feasible because of this area’s depth inside the cortex. To indirectly impact the hippocampus, the alternative is to stimulate other areas with functional connection to it, such as the right dorsolateral prefrontal cortex (rDLPFC) which has been associated with EM retrieval, and the posterior parietal cortex (PC) which is associated with memory development and consolidation, essential for EM (Zanatta et al., 1997; Sandrini et al., 2003). Manenti et al. (2013) found that transcranial direct current stimulation (tDCS), a NIBS technique using low direct current, over the DLPFC and PC improved EM retrieval. However, high-definition tDCS (HD-tDCS) is favoured over conventional tDCS due to its enhanced accuracy in delivering current to specific brain regions through smaller electrodes, which minimises stimulation effects outside of the target area (Rezakhani et al., 2024). This method could further enhance EM in both young and older adults but there is a gap in research exploring this. This study aims to fill that gap by examining the effects of safe HD-tDCS (Reckow et al., 2018) over the rDLPFC and the left posterior PC (lPPC) to enhance EM in adults. This study also aims to investigate if HD-tDCS over rDLPFC and lPPC can enhance EM consolidation and retrieval in healthy adults (aged 18-80 years).