A Systematic Review of Non-invasive Brain Stimulation Applications to Memory in Healthy Aging

Abstract

It has long been acknowledged that memory changes over the course of one's life, irrespective of diseases like dementia. Approaches to mitigate these changes have however yielded mixed results. Brain stimulation has been identified as one novel approach of augmenting older adult's memory. Thus far, such approaches have however been nuanced, targeting different memory domains with different methodologies. This has produced an amalgam of research with an unclear image overall. This systematic review therefore aims to clarify this landscape, evaluating, and interpreting available research findings in a coherent manner. A systematic search of relevant literature was conducted across Medline, PsycInfo, Psycarticles and the Psychology and Behavioral Sciences Collection, which uncovered 44 studies employing non-invasive electrical brain stimulation in healthy older adults. All studies were of generally good quality spanning numerous memory domains. Within these, evidence was found for non-invasive brain stimulation augmenting working, episodic, associative, semantic, and procedural memory, with the first three domains having the greatest evidence base. Key sites for stimulation included the left dorsolateral prefrontal cortex (DLPFC), temporoparietal region, and primary motor cortex, with transcranial direct current stimulation (tDCS) holding the greatest literature base. Inconsistencies within the literature are highlighted and interpreted, however this discussion was constrained by potential confounding variables within the literature, a risk of bias, and challenges defining research aims and results. Non-invasive brain stimulation often did however have a positive and predictable impact on older adult's memory, and thus warrants further research to better understand these effects.

Randomized trial of cognitive training and brain stimulation in non‐demented older adults

“… 14 , 16 Recent research suggests that other cognitive tasks or domains, such as episodic memory, may be more amenable to atDCS‐induced improvements during or immediately after intervention. 6 , 11 , 28 With regard to the stimulation protocol, there is no consent yet about the efficacy of specific parameters over others, leaving the question of optimal stimulation intensity and electrode configuration unresolved. Previous studies have applied the same (1 mA) or higher intensities (1.5 or 2 mA) with the same (left prefrontal) or other electrode configurations (bilateral frontal, right prefrontal) with mixed evidence overall.…”

“… 11 , 15 With regard to electrode configuration, a recent review suggests that simultaneous targeting of multiple nodes of the frontoparietal network with tDCS during cognitive training may be needed to produce benefits on the behavioral level in older adults. 28 …”

Modulation of network centrality and gray matter microstructure using multi‐session brain stimulation and memory training

“…Combined interventions of tDCS and cognitive training present a promising means to counteract cognitive impairments in healthy as well as in pathological aging (Berryhill & Martin, 2018 ; de Sousa et al, 2020 ; Goldthorpe et al, 2020 ). Improved understanding of targeted neuronal network effects of combined interventions in older adults will therefore inform the development of specific therapeutic interventions against age‐associated cognitive decline.…”

“…Transcranial direct current stimulation (tDCS), especially when applied over multiple sessions and combined with training interventions, has emerged as a promising means to modulate cognitive functions in younger adults, older adults, and clinical populations (Berryhill & Martin, 2018 ; Goldthorpe et al, 2020 ; Polania et al, 2018 ). However, neural mechanisms underlying such improvements are poorly understood (Berryhill & Martin, 2018 ; Horne et al, 2020 ).…”

Working Memory and Transcranial-Alternating Current Stimulation—State of the Art: Findings, Missing, and Challenges

“…Interestingly, all NIBS studies that targeted two regions in healthy older adults significantly improved WM outcomes ( Park et al, 2014 ; Arciniega et al, 2018 ; Borghini et al, 2018 ; Reinhart and Nguyen, 2019 ), while those that targeted one region showed mixed results ( Goldthorpe et al, 2020 ). Therefore, it is reasonable to call for studies targeting two regions, both in healthy participants and in patients with schizophrenia, depression, dementia, MCI, etc.…”