a-tDCS differential modulation of corticospinal excitability: the effects of electrode size

Novel noninvasive brain stimulation techniques such as transcranial direct current stimulation (tDCS) have been developed in recent years. tDCS-induced corticospinal excitability changes depend on two important factors: current density and electrodes size. Despite clinical success with existing tDCS parameters; optimal protocols are still not entirely set.

The current study aimed to investigate the effects of anodal tDCS (a-tDCS) with three electrode sizes on corticospinal excitability.

a-tDCS was applied with three active electrode sizes of 12, 24 and 35 cm(2) with a constant current density of 0.029 mA/cm(2) on twelve right handed healthy individuals (mean age: 34.5 ± 10.32 years) in different sessions at least 48 h apart. a-tDCS was applied continuously for 10 min, with a constant reference electrode size of 35 cm(2). The corticospinal excitability of extensor carpi radialis muscle (ECR) was measured before and immediately after the intervention and at 10, 20 and 30 min thereafter.

We found that smaller electrode may produce more focal current density and could lead to more effective and localized neural modulation than the larger ones. Post hoc comparisons showed that active electrode of 12 cm(2) size induces the biggest increase in the corticospinal excitability compared to bigger electrode sizes, 24 cm(2) (P = 0.002) and 35 cm(2) (P = 0.000). There was no significant difference between two larger electrode sizes (24 cm(2) and 35 cm(2)) (P = 0.177). a-tDCS resulted in significant excitability enhancement lasting for 30 min after the end of stimulation in the 12 and 24 cm(2) electrode size conditions (P < 0.005). However, in 35 cm(2) electrode size condition, the MEP amplitudes of the ECR did not differ significantly from baseline value in 20 and 30 min post stimulation (P > 0.005).

Reducing stimulation electrode size to one third of the conventional one results in spatially more focused stimulation and increases the efficacy of a-tDCS for induction of larger corticospinal excitability. This may be due to the fact that larger electrodes stimulate nearby cortical functional areas which can have inhibitory effects on primary motor cortex.