Effects of Cathode Location and the Size of Anode on Anodal Transcranial Direct Current Stimulation Over the Leg Motor Area in Healthy Humans

Abstract

Objective: Non-invasive brain stimulation such as transcranial direct current stimulation (tDCS) involves passing low currents through the brain and is a promising tool for the modulation of cortical excitability. In this study, we investigated the effects of cathode location and the size of anode for anodal tDCS of the right-leg area of the motor cortex, which is challenging due to its depth and orientation in the inter-hemispheric fissure.Methods: We first computationally investigated the effects of cathode location and the size of the anode to find the best montage for specificity of stimulation effects for the targeted leg motor area using finite element analysis (FEA). We then compared the best electrode montage found from FEA with the conventional montage (contralateral supraorbital cathode) via neurophysiological testing of both, the targeted as well as the contralateral leg motor area.Results: The conventional anodal tDCS electrode montage for leg motor cortex stimulation using a large-anode (5 cm × 7 cm, current strength 2 mA) affected the contralateral side more strongly in both the FEA and the neurophysiological testing when compared to other electrode montages. A small-anode (3.5 cm × 1 cm at 0.2 mA) with the same current density at the electrode surface and identical contralateral supraorbital cathode placement improved specificity. The best cathode location for the small-anode in terms of specificity for anodal tDCS of the right-leg motor area was T7 (10–10 EEG system).Conclusion: A small-anode (3.5 cm × 1 cm) with the same current density at the electrode surface as a large-anode (5 cm × 7 cm) resulted in similar cortical excitability alterations of the targeted leg motor cortex respresentation. In relation to the other stimulation conditions, the small-anode montage with the cathode positioned at T7 resulted in the best specificity.

Transcranial Direct Current Stimulation at 4 mA Induces Greater Leg Muscle Fatigability in Women Compared to Men

“…Similar to previous tDCS studies targeting unilateral M1 leg areas [33,34], the anode bordered or covered the center of the skull (Cz). Therefore, the anode also covered the leg area of the dominant M1, which is located in the longitudinal fissure [34]. During active tDCS conditions, the stimulation ramped-up over a 30 s period and then remained at the target intensity for 20 min before ramping back down to 0 mA over 30 s. During sham tDCS, the stimulator performed a 30 s ramp-up to 2 mA followed by an immediate 30 s ramp-down to 0 mA in order simulate stimulation-related sensations and maintain condition blinding [35].…”

“…This montage was chosen to maximize motor performance by targeting the dominant M1 [32]. Similar to previous tDCS studies targeting unilateral M1 leg areas [33,34], the anode bordered or covered the center of the skull (Cz). Therefore, the anode also covered the leg area of the dominant M1, which is located in the longitudinal fissure [34].…”

Walking in multiple sclerosis improves with tDCS: a randomized, double‐blind, sham‐controlled study

“…Even if some studies conceptualized experimental alternative motor electrode montages varying the position of the cathode, evidence is still mixed concerning whether these variations can enhance the neuromodulatory effects. 49,50 In addition to the electrode montage, dosing parameters for tDCS, such as optimal intensity, timing, and duration of stimulation, still remain largely undefined. As a result, there is a general lack of standardization of the stimulation methodologies and dosing features, making comparison across studies difficult.…”

Deep Cerebellar Transcranial Direct Current Stimulation of the Dentate Nucleus to Facilitate Standing Balance in Chronic Stroke Survivors—A Pilot Study

“…These different brain tissues for the volume mesh were modeled as different volume conductors for finite element analysis (FEA) in the ROAST. Here, isotropic conductivity based on prior works was used for different brain tissues [30] which were (in S/m): Scalp = 0.465; Skull = 0.01; CSF = 1.654; GM = 0.276; WM = 0.126 [7,30,41,42]. A subject-specific cap fitted to the individual head model was created using the high-density 10-05 EEG locations [43], EGI net-based system (https://www.egi.com), and extra electrodes from ROAST [27] along with nine custom locations that were defined on the neck and the lower head.…”