Abstract
Transcranial direct current stimulation (tDCS) has been used as a useful interventional brain stimulation technique to improve unilateral upper-limb motor function in healthy humans, as well as in stroke patients. Although tDCS applications are supposed to modify the interhemispheric balance between the motor cortices, the tDCS after-effects on interhemispheric interactions are still poorly understood. To address this issue, we investigated the tDCS after-effects on interhemispheric inhibition (IHI) between the primary motor cortices (M1) in healthy humans. Three types of tDCS electrode montage were tested on separate days; anodal tDCS over the right M1, cathodal tDCS over the left M1, bilateral tDCS with anode over the right M1 and cathode over the left M1. Single-pulse and paired-pulse transcranial magnetic stimulations were given to the left M1 and right M1 before and after tDCS to assess the bilateral corticospinal excitabilities and mutual direction of IHI. Regardless of the electrode montages, corticospinal excitability was increased on the same side of anodal stimulation and decreased on the same side of cathodal stimulation. However, neither unilateral tDCS changed the corticospinal excitability at the unstimulated side. Unilateral anodal tDCS increased IHI from the facilitated side M1 to the unchanged side M1, but it did not change IHI in the other direction. Unilateral cathodal tDCS suppressed IHI both from the inhibited side M1 to the unchanged side M1 and from the unchanged side M1 to the inhibited side M1. Bilateral tDCS increased IHI from the facilitated side M1 to the inhibited side M1 and attenuated IHI in the opposite direction. Sham-tDCS affected neither corticospinal excitability nor IHI. These findings indicate that tDCS produced polarity-specific after-effects on the interhemispheric interactions between M1 and that those after-effects on interhemispheric interactions were mainly dependent on whether tDCS resulted in the facilitation or inhibition of the M1 sending interhemispheric volleys.
Highlights
Transcranial direct current stimulation is a widely used interventional brain stimulation technique that improves unilateral upper-limb motor function in healthy humans [1,2,3,4,5,6] and hemiparetic stroke patients [7,8,9,10,11]
Post-hoc analysis revealed that after real-anodal Transcranial direct current stimulation (tDCS) over the right M1, the motor evoked potential (MEP) elicited from the right M1 was increased (232.0¡144.7%, p,0.001) and the MEP elicited from the left M1 was unchanged (111.1¡41.7%, p50.54) compared with the baseline (Figure 1A)
After real-cathodal tDCS over the left M1, the MEP elicited from the left M1 was decreased (76.2¡27.6%, p50.01) and the MEP elicited from the right M1 was unchanged (109.0¡36.5%, p50.45, Figure 1C)
Summary
Transcranial direct current stimulation (tDCS) is a widely used interventional brain stimulation technique that improves unilateral upper-limb motor function in healthy humans [1,2,3,4,5,6] and hemiparetic stroke patients [7,8,9,10,11]. Gilio et al [27] demonstrated that 1 Hz repetitive TMS (rTMS) given to the left M1 suppressed IHI from the left M1 to right M1 with minor effects on iSP Given these physiological backgrounds, we hypothesized that tDCS given at rest would induce polarity-specific after-effects on IHI from the stimulated M1 in which the corticospinal excitability was changed. We hypothesized that tDCS given at rest would induce polarity-specific after-effects on IHI from the stimulated M1 in which the corticospinal excitability was changed To examine this hypothesis, we investigated the after-effects of tDCS applied at rest with three different electrode montages (i.e., unilateral anodal, unilateral cathodal, and bilateral). Before and after each tDCS, single-pulse TMS and paired-pulse TMS were given to the left M1 and right M1 in order to assess the corticospinal excitability and mutual direction of IHI
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