Personalizing the electrode to neuromodulate an extended cortical region

Background:Among transcranial electric stimulation (tES) parameters, personalizing the electrodegeometry might help overcome the individual variability of the induced effects.Objective/hypothesis:To test the need for electrode personalization, instead of a universal electrode foreveryone, to induce neuromodulation effects on the bilateral primary motor cortex (M1) devoted toupper and lower limb representation.Methods:By an ad-hoc neuronavigation procedure, we shaped the personalized electrode and positionedit matching the projection on the scalp of the individual central sulcus by a 2 cm strip, with total area of35 cm2. The non-personalized electrode, i.e., equal for all subjects, was a 2 cm wide strip size-matchedwith the personalized electrode but shaped on a standard modelfitting the curve passing through C3-CZ-C4 sites of the electroencephalographic (EEG) 10-20 International System. To test neuromodulationelectrode-dependent efficacy, we induced a 20 Hz sinusoidal modulated current (transcranial alternatingcurrent stimulation, tACS) because it produces online effects. We simultaneously collected left and righthand and leg motor potentials (MEP) that were evoked by a rounded transcranial magnetic stimulation(TMS) coil. Through each electrode we delivered both real and sham stimulations.Results:While cortical excitability during tACS increased during both the non-personalized and thepersonalized electrodes for the leg, the hand representation excitability enhancement was inducedselectively when using the personalized electrode. The results were consistent bilaterally.Conclusions:We documented that by using a personalized electrode it is possible to induce the neuro-modulation of a predetermined extended cortical target, which did not occur with a non-personalizedelectrode. Ourfindings can help in building neuromodulation methods that might compensate forindividual alterations across specific brain networks.