Abstract
BackgroundMeasurements and models of current flow in the brain during transcranial Direct Current Stimulation (tDCS) indicate stimulation of regions in-between electrodes. Moreover, the folded cortex results in local fluctuations in current flow intensity and direction, and animal studies suggest current flow direction relative to cortical columns determines response to tDCS. MethodsHere we test this idea by using Transcranial Magnetic Stimulation Motor Evoked Potentials (TMS-MEP) to measure changes in corticospinal excitability following tDCS applied with electrodes aligned orthogonal (across) or parallel to M1 in the central sulcus. ResultsCurrent flow models predicted that the orthogonal electrode montage produces consistently oriented current across the hand region of M1 that flows along cortical columns, while the parallel electrode montage produces non-uniform current directions across the M1 cortical surface. We find that orthogonal, but not parallel, orientated tDCS modulates TMS-MEPs. We also show modulation is sensitive to the orientation of the TMS coil (PA or AP), which is thought to select different afferent pathways to M1. ConclusionsOur results are consistent with tDCS producing directionally specific neuromodulation in brain regions in-between electrodes, but shows nuanced changes in excitability that are presumably current direction relative to column and axon pathway specific. We suggest that the direction of current flow through cortical target regions should be considered for targeting and dose-control of tDCS.
Highlights
To date, the majority of studies in humans using transcranial direct current stimulation to modulate cortical functionAbbreviations: PA, postero-anterior; AP, antero-posterior; ML, medio-lateral; tDCS, transcranial direct current stimulation; Motor-evoked potentials (MEPs), motor evoked potential; M1, primary motor cortex; TMS, transcranial magnetic stimulation; AP-Transcranial Magnetic Stimulation Motor Evoked Potentials (TMS-MEP), motor evoked potentials elicited with anterior-posterior directed TMS; PA-TMSMEPs, motor evoked potentials elicited with posterior-anterior directed TMS.employ a bipolar electrode montage: one electrode is usually placed over the target site and the other at a distance
Experiment 1 In experiment 1, we investigated whether corticospinal excitability could be modulated with an electrode montage for which the region of interest (M1 hand region) was positioned between our stimulating electrodes
There is a notable and clear difference between the two electrode montages: whereas ML-tDCS does not produce any uniformly directed electrical fields through the main surface of motor cortex located in the anterior bank of the central sulcus, PA-tDCS leads to relatively uniform inward and outward electrical fields, which are perpendicular relative to the cortical surface of M1
Summary
The majority of studies in humans using transcranial direct current stimulation (tDCS) to modulate cortical functionemploy a bipolar electrode montage: one electrode is usually placed over the target site and the other at a distance. For the hand area of motor cortex (M1), a large anode is conventionally centred over the anatomical location of the “hand knob” of the precentral gyrus, with a cathode over the contralateral orbit [1] This montage, based on canonical studies by Nitsche, Paulus and colleagues on how the position of large electrodes influences population-averaged modulation of TMS-MEPs [2e4], is widely applied for targeting diverse cortical target regions [5,6] though rarely with consideration for nuanced dose response [7e11]. Conclusions: Our results are consistent with tDCS producing directionally specific neuromodulation in brain regions in-between electrodes, but shows nuanced changes in excitability that are presumably current direction relative to column and axon pathway specific.
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