Localized Analysis of Normalized Distance from Scalp to Cortex and Personalized Evaluation (LANDSCAPE): Focusing on Age- and Dementia-Specific Changes.

Background: Scalp to cortex distance (SCD), as a key technological parameter, has been highlighted in the guidelines of non-invasive brain stimulation. However, in the context of age-related brain changes, the region-specific SCD and its impact on stimulation-induced electric field remain unclear.
Objective: This study aimed to investigate the region-specific SCD and its relationship with morphometric features and cognitive function in age- and disease-specific populations.
Methods: We analyzed the SCD and cortical thickness (CT) of left primary motor cortex (M1) and dorsolateral prefrontal cortex (DLPFC) in 214 cognitively normal adults and 43 dementia patients. CT-adjusted SCD was used to control the influence of CT on SCD. Head model was developed to simulate the impact of SCD on the electric field induced by transcranial electrical stimulation.
Results: We found age-related increased SCD in the left DLPFC (p < 0.001), but not M1 (p = 0.134), and dementia-related increased SCD in both left DLPFC (p < 0.001) and M1 (p < 0.001). CT-adjusted SCD showed greater region-specific impact on left DLPFC rather than M1. The electric field induced by stimulation was consequently decreased with the increased SCD across normal aging and dementia groups.
Conclusions: Age and dementia have differential impacts on the SCDs of left DLPFC and M1. The findings suggest that it is important to be aware of region-specific distance measures when conducting neuromodulation in individuals with old age and dementia.