Abstract

Tuning brain activity with non-invasive brain stimulation critically depends on optimized targeting of the brain structures of interest, especially in populations with accelerated interindividual morphological variations as they are typical in aging. Here, we explored the electric field distribution when targeting the right dorsolateral prefrontal cortex (DLPFC) using High-Definition Transcranial Alternating Current Stimulation (HD-tACS) in both young (YA) and older adults (OA). We modeled the electric field of ten datasets (OA: n=5, 63-76 y/o, 68.4±6.5 y/o; YA: n=5, 18-39 y/o, 27.4±7.5 y/o) comprising T1-and T2-weighted images using SimNIBS in combination with FreeSurfer. We simulated two montages (2x1 and 4x1) to deliver a net current of 2 mA. The simulations revealed that the peak electric field and mean electric field at the DLPFC were approximately three times stronger when using the 2x1 montage compared to the 4x1 montage. However, this increase in field strength came at the expense of focality, resulting in the spread of the electric field into the ocular area, where peak values were observed in some participants. In these cases, a viable approach to enhance the efficacy of the 2x1 montage was to individualize the current. Our simulations showed that it is possible to generate a similar electric field among participants while mitigating the spread of the electric field into the eye, thereby reducing the likelihood of phosphenes.