Transcranial Direct Current Stimulation (tDCS) is an upcoming therapeutic tool to improve post-stroke upper extremity motor function by modulating cortical excitability. However, research shows mixed findings on tDCS in stroke patients. The functional and structural brain reorganization may explain the mixed findings: (i) The individual motor function, i.e. the target for tDCS, may be relocated to a non-affected brain region, and (ii) the structural characteristics, i.e. conductivity, of the lesion influences the current propagation in the brain.
This study evaluated the effect of individual optimized tDCS configurations for 21 chronic stroke patients and ten healthy controls using model simulations. We optimized tDCS configurations for two individual targets: (i) a functional target based on EEG during a functional hand task and (ii) an anatomical target based on MRI. The individual optimized configurations are compared with conventional configurations targeting the contralateral and ipsilateral primary motor cortex. The configurations are compared based on the normal component of the simulated electric field at both the functional and anatomical targets. Here we consider a negative field strength inhibitory, therefore undesirable.
The functional target was found ipsilateral in eight of the twenty-one stroke subjects, which indicates the relevance of individual functional targets. The conventional configurations simulated for all healthy subjects a positive field strength in the anatomical target. However, our simulations showed that conventional tDCS can result in negative stimulation for seven stroke subjects in the functional target and two stroke subjects in the anatomical target. The simulations may explain the lack of consistent therapeutic effects of conventional tDCS in stroke patients, and highlight the need for individualized tDCS configurations.