P 141. Global/local attention: Effects of tDCS on brain dynamics

Abstract

Introduction Posterior parietal cortex (PPC) has been identified as a critical region for the control of spatial and non-spatial attention. Hierarchical letter stimuli (large letters composed of smaller letters) have been used to investigate the contribution of PPC to global/local processing of features in the visual field, and also to the switching of attention between the global and local level. Objectives We utilized transcranial direct current stimulation (tDCS) and high-density EEG to characterize brain dynamics during global/local attentional switching. Materials and methods 128-channel EEG data was recorded from 14 healthy, right-handed adults who made motor responses to hierarchical letter stimuli. A cue before each block of 4–8 hierarchical stimuli directed attention to either global or local features. tDCS was applied over left posterior parietal cortex (electrode P3, forearm reference; 2mA for 20min) during separate anodal, cathodal and sham stimulation sessions. The EEG data were analyzed using Brainstorm (Tadel et al., 2011) to extract brain-based activation waveforms for regions defined in the BrainVisa Tzourio-Mazoyer Atlas. Time–frequency plots were computed for left and right superior and inferior parietal and mid occipital corteces. Results Behavioral responses differed for the first stimuli following switching of attention between global/local features, but not for visual processing per se . Attentional shifts from local-to-global features were degraded for at least 20min following anodal stimulation (Stone and Tesche, 2009). Practice effects were observed in the theta-band between baseline data and data recorded immediately after sham stimulation for both G/L cues to switch attention and for the first compound letter stimuli after the G/L cues. In left PPC, anodal and cathodal stimulation increased theta for global-to-local switches for the first hierarchical letters after a cue to switch, whereas anodal increased and cathodal decreased theta for local-to-global switches. In right PPC, anodal tDCS increased and cathodal tDCS decreased theta for both global-to-local and local-to-global switches. Differential time–frequency activations were observed in left (but not right) mid occipital cortex following tDCS; anodal stimulation enhanced theta/alpha activity for local-to-global, but not global-to-local, switches. Conclusion The data suggest that tDCS of left PPC induces changes in theta-band activity in a distributed network which includes left and right PPC and occipital cortex, and that theta-band activity may be salient for PPC control of early(100–300ms) global/local processing.