Scalp‐to‐cortex distance of left primary motor cortex and its computational head model: Implications for personalized neuromodulation

BACKGROUND: Non‐invasive brain stimulation (NIBS) is increasingly used as a probe of function and therapeutics in experimental neuroscience and neurorehabilitation. Scalp‐to‐cortex distance (SCD), as a key parameter, has been shown to potentially impact on the electric field. This study aimed to examine the region‐specific SCD and its relationship with cognitive function in the context of age‐related brain atrophy. METHODS: We analyzed the SCD and cortical thickness (CT) of left primary motor cortex (M1) in 164 cognitively normal (CN) adults and 43 dementia patients drawn from the Open Access Series of Imaging Studies (OASIS). The degree of brain atrophy was measured by the volume of ventricular system. Computational head model was developed to simulate the impact of SCD on the electric field. RESULTS: Increased SCD of left M1 was only found in dementia patients (P < .001). When considering CT, the ratio of SCD to CT (F = 27.41, P < .001) showed better differential value than SCD. The SCD of left M1 was associated with worse global cognition (r = −.207, P = .011) and enlarged third ventricle (r = .241, P < .001). The electric field was consequently reduced with the increased SCD across cognitively normal elderly and dementia groups. CONCLUSIONS: Scalable distance measures, including SCD and CT, are markedly correlated with reduced electric field in dementia patients. The findings suggest that it is important to be aware of region‐specific distance measures when conducting NIBS‐based rehabilitation in individuals with brain atrophy.