Increased variability in response to transcranial direct current stimulation in healthy older compared to young...

Healthy aging is associated with a decline in cognitive and motor functions, affecting daily activities and quality of life. Combining transcranial direct current stimulation (tDCS) with behavioral training may be a promising intervention against this decline. However, individual response variability may obscure group-level effects and mislead conclusions about tDCS efficacy. Quantifying this variability is crucial for accurately assessing stimulation effects and understanding individual response factors, like age. Yet, no study has quantitatively compared tDCS variability across age groups. This systematic review and meta-analysis examine age-related variability in cognitive and motor responses to tDCS.

Following PRISMA guidelines, we searched PubMed and Cochrane for studies directly comparing young and healthy older adults under similar experimental conditions. Across 19 studies comprising 390 older adults (mean ± SD age: 67 ± 5 years) and 384 young adults (mean ± SD age: 24 ± 3 years) receiving transcranial direct current (tDCS), we quantified behavioral variability using the log-transformed coefficient of variation ratio (lnCVR).

Results revealed substantially higher response variability in healthy older compared to young adults during active tES (21 %, lnCVR_active = -0.24 [-0.43, -0.04], p = 0.02), but not during sham conditions (lnCVR_sham = -0.18 [-0.42, 0.05], p = 0.13).

These findings provide the first quantitative evidence that advanced age increases behavioral tDCS response variability, highlighting the need to develop personalized tDCS approaches to optimize their efficacy in older populations.