Evaluating the Effects of Model-Based Optimal Bipolar tDCS Configurations on Cortical Excitability

Many positive effects of tDCS have been demonstrated, but these effects can be small or short-lived. Volume conduction modeling results have suggested that the effects of tDCS may improve by using different electrode placements. Some novel configurations have been put forward, but experimental validation is scarce. In a recent modeling study, optimal configurations were found for maximizing either the electric field strength or the electric field strength in the presumably most effective direction in a cortical target area. Configurations with two large electrodes were optimized as this is still the most widely available setup and this approach allows for a comparison with the conventional setup solely based on placement. In the current study, these optimal configurations were compared with the conventional configuration for motor cortex stimulation for their effects on neuronal excitability. Three tDCS configurations were compared in a randomized crossover design withtwenty healthy subjects. Corticospinal excitability was evaluated from electromyography of the right first dorsal interosseus muscle using single-pulse transcranial magnetic stimulation. Excitability was measured before and until 25 minutes after 15 minutes of 2 mA tDCS. The configuration that was optimized for field strength in the most effective direction produced significantly larger effects on a group level than the configuration optimized for absolute field strength, and slightly larger effects than the conventional configuration. However, individual differences were large and should be taken into account.