The spatial resolution of tDCS tends to be limited by electrode size (usually 5cm x 7cm) and the necessity of including at least one return (or ‘cathode’) electrode. Consequently, tDCS tends to affect a very large amount of brain tissue, limiting the spatial resolution of the technique. These limitations, combined with the shunting of current along the skull tend to result in tES affect undesired brain regions. For example, under certain conditions the position of return electrode can impact on performance during a task, in addition to an ‘anodal’ effect (). As a result, a means of focally stimulated a desired brain region whilst avoiding stimulation of other regions is required.
‘High definition’-tDCS (HD-tDCS) is a means of focally stimulating ‘one’ cortical region without undesired stimulation effects associated with the position of the return electrode. HD-tDCS relies on placing electrodes in a 4 x 1 ring arrangement (Datta et al., 2009). This can involve surrounding one anode with four cathodes or vice versa. An M1 anodal HD-tDCS montage successfully produces a peak in electric field in M1 but does not lead to electric field elsewhere (Datta et al., 2009), suggesting that this technique can focally stimulate a desired region and avoid stimulation elsewhere. The use of these montages has successfully produced effects on the lateralisation of visual attention (Spooner et al., 2019). The application of HD-tDCS to either the left or right dorsolateral prefrontal cortex (DLPFC). The use of electroencephalography (EEG) measurements after a 20 minute stimulation period revealed that left DLPFC HD-tDCS modulate functional connectivity within the theta range between frontal cortex and visual cortex (Spooner et al., 2009), compared to right DLPFC HD-tDCS.
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