Focal and bi-directional modulation of lower limb motor cortex using anodal transcranial direct current stimulation

Abstract

Background: Because we are interested in non-invasive transcranial brain stimulation as an adjuvant to post-stroke walking therapy, we applied direct current stimulation (tDCS) preferentially to either the left or right lower limb motor cortex (M1) in two separate sessions and assessed the resulting modulation in both cortices.

Objective/hypothesis: We hypothesized that tDCS applied preferentially to one lower limb M1 of healthy subjects would induce between-hemisphere opposite sign modulation.

Methods: Transcranial magnetic stimulation (TMS) with the coil offset 2 cm either side of vertex was used to assess the percent change in rectified motor evoked potential (MEP) area recorded bilaterally from vastus lateralis (VL) and tibialis anterior (TA) of 10 subjects during weak tonic contraction.

Results: ANOVA revealed an up-regulation of the target cortex and a down-regulation of the non-target cortex (p = 0.001) and no effects of hemisphere (left, right) or muscle (TA, VL). Significant modulation was evident in 78% of VL and TA muscles (all p < 0.05). Excitability increased in 60%, but decreased in 18%. For 43% when excitability increased, a simultaneous decrease in excitability was evident in homologous muscle responses providing support for our hypothesis.

Conclusions: The results indicate a modest effectiveness and focality of anodal tDCS when applied to lower limb M1, suggesting in a human model that the strength and depth of polarizing cortical currents induced by tDCS likely depend on inter-individual differences in the electrical properties of superficial brain structures.

Keywords: Cortical excitability; Lower limb; Transcranial direct current stimulation; Transcranial magnetic stimulation.

Figures

Figure 1

Schematic representation of tDCS electrode positions and expected outcome. The anode (dark grey) was placed lateral to the vertex with the medial edge of the electrode on the mid sagittal line. The cathode (light grey) was placed over the orbit of the contralateral eye. EMG traces represent pre-tDCS (dotted) and post-tDCS (solid) motor evoked responses (MEPs) obtained from the contralateral hemisphere. Anodal tDCS was expected to cause facilitation of contralateral MEPs evoked from the target cortex and inhibition of MEPs from the non-target cortex.

Figure 2

Representative examples of contralateral motor evoked responses from: A. vastus lateralis (VL) and B. tibialis anterior (TA) muscles showing background and MEP area windows used for analysis. Traces are rectified EMG of a single trial from a single subject. In this example, MEP window area (including the added 10 ms for ipsilateral response) for the VL and TA was 52 ms and 40 ms respectively. A window of identical width was set prior to the stimulus artifact to measure background activity. Note the difference in the Y axis scales for the two muscles.

Figure 3

Average changes in normalized MEP areas for the VL (black bars) and TA (white bars) following anodal tDCS. Responses obtained from the target and non-target cortices are shown. ANOVA showed a significant effect of cortex, ***, p

Figure 4

Plot A represents the distribution of muscles that showed no modulation (I), inhibition of target cortex (II), facilitation of target and non-target cortex (III) and opposite sign modulation (IV). Plots B, C and D represent average changes in MEP window area following anodal tDCS. Each point represents individual subject data showing modulation of contralateral responses evoked from the target and non-target motor cortex. B, subjects who showed inhibition of responses from the target cortex (II). C, subjects who showed facilitation of responses from the target and non-target cortex (III). D, subjects who showed opposite sign modulation (facilitation of responses from the target cortex and inhibition of responses from the non-target cortex) (IV). Vertical bars represent averages within each group.

Figure 5

Examples of rectified averaged EMG responses from the VL are shown for two representative subjects in whom ipsilateral connectivity was indicated (left panel) and ipsilateral connectivity was not indicated (right panel). Each panel has three sets of traces; the top set (A) shows contralateral responses with the coil preferentially positioned over the up-regulated hemisphere; the middle set (B) shows contralateral responses with the coil preferentially positioned over the down-regulated hemisphere; and the bottom set (C) shows ipsilateral responses with the coil preferentially positioned over the up-regulated hemisphere. Each EMG trace is an average of 15 MEPs showing pre-aTDCS (grey lines) and post-aTDCS (black lines) responses. Note the facilitation of ipsilateral responses in Example 1 and inhibition of ipsilateral responses in Example 2. Also note the low amplitude of background EMG due to the phase canceling that is inherent in trace averaging.