Abstract
Individual responses to transcranial direct current stimulation (tDCS) are varied and therefore potentially limit its application. There is evidence that this variability is related to the contributions of Indirect waves (I-waves) recruited in the cortex. The latency of motor-evoked potentials (MEPs) can be measured through transcranial magnetic stimulation (TMS), allowing an individual’s responsiveness to tDCS to be determined. However, this single-pulse method requires several different orientations of the TMS coil, potentially affecting its reliability. Instead, we propose a paired-pulse TMS paradigm targeting I-waves as an alternative method. This method uses one orientation that reduces inter- and intra-trial variability. It was hypothesized that the paired-pulse method would correlate more highly to tDCS responses than the single-pulse method. In a randomized, double blinded, cross-over design, 30 healthy participants completed two sessions, receiving 20 min of either anodal (2 mA) or sham tDCS. TMS was used to quantify Short interval intracortical facilitation (SICF) at Inter stimulus intervals (ISIs) of 1.5, 3.5 and 4.5 ms. Latency was determined in the posterior-anterior (PA), anterior-posterior (AP) and latero-medial (LM) coil orientations. The relationship between latency, SICF measures and the change in suprathreshold MEP amplitude size following tDCS were determined with Pearson’s correlations. TMS measures, SICI and SICF were also used to determine responses to Anodal-tDCS (a-tDCS). Neither of the latency differences nor the SICF measures correlated to the change in MEP amplitude from pre-post tDCS (all P > 0.05). Overall, there was no significant response to tDCS in this cohort. This study highlights the need for testing the effects of various tDCS protocols on the different I-waves. Further research into SICF and whether it is a viable measure of I-wave facilitation is warranted.
Highlights
Transcranial direct current stimulation is a non-invasive brain stimulation technique that can modulate corticospinal excitability in a polarity-dependent manner
We aimed to determine the individual variability of responses to anodal Transcranial direct current stimulation (tDCS) (a-tDCS) using the difference in motor-evoked potential (MEP) latencies, and compared it to pairedpulse transcranial magnetic stimulation (TMS) at indirect waves (I-waves) periodicities
None of the MEP latency measures, nor any of the short-interval intracortical facilitation (SICF) measures correlated to the change in excitability following a-tDCS in resting or active conditions
Summary
Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that can modulate corticospinal excitability in a polarity-dependent manner. Later studies have further demonstrated the relationship between early I-wave recruitment and a-tDCS response (McCambridge et al, 2015; Davidson et al, 2016), with McCambridge et al (2015) finding the relationship only existing in the distal muscles of the upper limb (i.e., extensor carpi radialis (ECR)), not those of the proximal upper limb (i.e., biceps brachii). Studies of patients implanted with high cervical epidural electrodes for pain suggested a-tDCS preferentially modulates cortical circuits generating D- and early I-wave activity (Lang et al, 2011; Di Lazzaro et al, 2013) These seminal studies suggest a seemingly close interaction between early I-wave recruitment and a-tDCS responses
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