Differential modulation of corticospinal excitability by different current densities of anodal transcranial direct current stimulation

Abstract

Background: Novel non-invasive 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 intensity and stimulation duration. Despite clinical success with existing tDCS parameters, optimal protocols are still not entirely set.

Objective/hypothesis: The current study aimed to investigate the effects of four different anodal tDCS (a-tDCS) current densities on corticospinal excitability.

Methods: Four current intensities of 0.3, 0.7, 1.4 and 2 mA resulting in current densities (CDs) of 0.013, 0.029, 0.058 and 0.083 mA/cm(2) were applied on twelve right-handed (mean age 34.5±10.32 yrs) healthy individuals in different sessions at least 48 hours apart. a-tDCS was applied continuously for 10 minute, with constant active and reference electrode sizes of 24 and 35 cm(2) respectively. The corticospinal excitability of the extensor carpi radialis muscle (ECR) was measured before and immediately after the intervention and at 10, 20 and 30 minutes thereafter.

Results: Post hoc comparisons showed significant differences in corticospinal excitability changes for CDs of 0.013 mA/cm(2) and 0.029 mA/cm(2) (P = 0.003). There were no significant differences between excitability changes for the 0.013 mA/cm(2) and 0.058 mA/cm(2) (P = 0.080) or 0.013 mA/cm(2) and 0.083 mA/cm(2) (P = 0.484) conditions.

Conclusion: This study found that a-tDCS with a current density of 0.013 mA/cm(2) induces significantly larger corticospinal excitability changes than CDs of 0.029 mA/cm(2). The implication is that might help to avoid applying unwanted amount of current to the cortical areas.

Figure 1. Experimental design.

Comparison of the effects of different CDs (D1–D4) on corticospinal excitability.

Figure 2. The effects of different CDs on the MEPs size over the 30 minutes.

Filled symbols indicate significant deviation of the post-a-tDCS MEP amplitudes compared to baseline (A, B). The asterisks mark significant differences between time points during the 30 minutes after cessation of a-tDCS (B). The only significant differences were seen within D2 condition. Error bars represent SEM.

Figure 3. Percentage increase in corticospinal excitability after the intervention.

The asterisks mark significant differences between ECR muscle MEP amplitudes after the end of a-tDCS in all time points of T0, T10, T20 and T30. Error bars represent SEM.