Counteractive effect of transcranial direct current stimulation on spatial attention biases

Transcranial direct current stimulation (tDCS) has the potential to alter neuronal activity and modify behaviour. The enhancement of activity in one hemisphere relative to the other is assumed to benefit to the rehabilitation of hemineglect, a disorder of spatial attention consecutive to a unilateral lesion [1, 2]. Previous studies on neglect patients showed that anodal tDCS over the left or right posterior parietal cortex (PPC) reorient attention toward the left hemifield [3, 4, 5]. However, this mechanism of action has been mainly studied in visuospatial tasks, while visuomotor and representational aspects have received little consideration so far. In the present study, we investigated how tDCS modulates spatial attention during visuomotor behaviour and visual imagery in healthy humans. The visuomotor task required participants to cross out a maximum of targets randomly distributed over a digital tablet within thirty seconds. The digital tablet allowed us to track the participant's performance over time and compute the centre of gravidity of the crossed targets. The visual imagery task required participants to mentally compare the similarity of two previously seen geometrical shapes that could differ either on their left or right side. The anode was positioned over the left or right parietal cortex and the cathode was positioned over the controlateral frontal cortex. In the sham condition, the centre of gravity of the crossed targets revealed a leftward bias in half of the participants and a rightward bias in the other half. In the active condition, the attention of participants was shifted in the direction opposite to their natural bias in the sham condition, irrespective to the stimulated hemisphere. No effect was observed in the visual imagery task, possibly because this task relied on allocentric rather than egocentric spatial coordinates. The present findings demonstrate that activating the attention network through tDCS counteracts natural biases in visual search. Because the left-to-right or right-to-left current direction did not matter, we suggest that the net effect of tDCS is to re-balance activity within the whole network, which makes it an optimal technique to restore the full range of attention after a unilateral lesion.

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Geers, L., Dormal, V., Vandermeeren, Y., Masson, N., & Andres, M. (2018). Counteractive effect of transcranial direct current stimulation on spatial attention biases. Frontiers in Human Neuroscience. Published. Belgian Brain Congress 2018 — Belgian Brain Council, Liège, Belgium. https://doi.org/10.3389/conf.fnins.2018.95.00062 (Original work published 2018)