Enhancing multisensory spatial orienting by brain polarization of the parietal cortex

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that induces polarity-specific excitability changes in the human brain, therefore altering physiological, perceptual and higher-order cognitive processes. Here we investigated the possibility of enhancing attentional orienting within and across different sensory modalities, namely visual and auditory, by polarization of the posterior parietal cortex (PPC), given the putative involvement of this area in both unisensory and multisensory spatial processing. In different experiments, we applied anodal or sham tDCS to the right PPC and, for control, anodal stimulation of the right occipital cortex. Using a redundant signal effect (RSE) task, we found that anodal tDCS over the right PPC significantly speeded up responses to contralateral targets, regardless of the stimulus modality. Furthermore, the effect was dependant on the nature of the audiovisual enhancement, being stronger when subserved by a probabilistic mechanism induced by blue visual stimuli, which probably involves processing in the PPC. Hence, up-regulating the level of excitability in the PPC by tDCS appears a successful approach for enhancing spatial orienting to unisensory and crossmodal stimuli. Moreover, audiovisual interactions mostly occurring at a cortical level can be selectively enhanced by anodal PPC tDCS, whereas multisensory integration of stimuli, which is also largely mediated at a subcortical level, appears less susceptible to polarization of the cortex.