Monday, 16 July 2012

TDCs electrode positioning

Anode Electrode PositioningCathode Electrode PositioningObservationsCaveats
Primary Motor cortex (M1)Supra-OrbitalThis is the most used montage. It has been proven that the cortical excitability can be changed up to 40%6 (Figure 6). Anodal stimulation results in neuronal depolarisation and increasing neuronal excitability while cathodal stimulation has opposite results6.Only one motor cortex is stimulated – might be a problem for bilateral pain syndromes. Also the confounding effect of the supra-orbital electrode needs to be considered.
Primary Motor cortex (M1)Primary Motor cortex- Interesting approach when there is a bi-hemispheric imbalance between motor cortices (such as in stroke)
- Can be used with two anodal stimulation electrodes (see sixth row), where cathodal electrode is placed in the supraorbital area for instance.
Electrodes might be too close to each other- issue of shunting.
A decrease of the area of the electrodes will increase the degree of shunting along the skin 19
Therefore shunting might be related not only to electrode positioning but also to electrode size.
The relative resistance of the tissues is dependent upon the electrode position and size- the overall resistance on which the current flows is dependent upon the electrode properties19.
Dorsolateral Prefrontal Cortex (DLPFC)Supra-OrbitalMost used for DLPFC stimulation – positive results for treatment of depression20 and also chronic pain3.Only unilateral DLPFC stimulation situation is possible with this montage.
Dorsolateral Prefrontal CortexDorsolateral Prefrontal Cortex- Interesting approach when there is a bi-hemispheric imbalance.
- Can be used for a two anodal stimulation situation (see sixth row), where cathodal electrode is placed in the supraorbital area for instance.
Electrodes might be too close to each other- issue of shunting 19. (Please see second row, fourth column).
OccipitalVertexInteresting active control for chronic pain trials or for modulation of visual cortex.When used as active control, reference electrodes are placed in different locations- problem of comparability between intra- and inter- experimental approaches.
Two anodal electrodes, e.g. both Motor corticesSupra-OrbitalSimultaneous change in cortical excitabilityTranscallosal inhibition might add a confounding factor21
One electrode over a cortical target, e.g. Primary Motor cortex (M1)Extra-CranialAvoid the confounding effect of two electrodes with opposite polarities in the brain7.Depending on intended target, current distribution might not be optimal and therefore induce ineffective stimulation22

Table 2. Electrode Positioning

Images Of Several Protocol Placements:

Demonstration of 4 typical electrode locations on the skull surface when using tDCS. The four figures illustrate the typical placement of anode and cathode during stimulation of the primary motor cortex (A), somatosensory cortex (B), primary visual cortex (C), anterior language cortex (D). Note that in (C) one electrode is placed at the back of the head (see small image of the head), while the other electrode is placed at the right supra-orbital area. One electrode is placed on the area of the skull covering the target structure and the other electrode is typically placed either over the supraorbital area of the other hemisphere or over the corresponding area of the contralateral hemisphere. Note, that other stimulation positions have been used as well .
Figure d is that used for "Working Memory" enhancement. The Anode is placed on the Dorsolateral prefrontal cortex of the left hisphere, the cathode on the supra orbital above the right eye.

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