Two spatially distinct posterior alpha sources fulfill different functional roles in attention
Directing attention helps to extract relevant information and suppress distracters. Alpha brain oscillations (8-12Hz) play a fundamental role in this process, with a power decrease facilitating processing of important information and power increase inhibiting brain regions processing irrelevant information. Evidence for this phenomenon arises from visual attention studies (Worden et al., 2000), however, the effect also exists in other modalities, including the somatosensory system (Haegens et al., 2011) and inter-sensory attention tasks (Foxe and Snyder, 2011). We investigated what happens when attention is divided between two modalities using both a multi- and unimodal attention paradigm while recording EEG over 128 scalp electrodes in two separate experiments. In Experiment 1 participants divided their attention between the visual and somatosensory modality to determine the temporal or spatial frequency of a target stimulus (vibrotactile stimulus or Gabor grating). In Experiment 2, participants divided attention between two visual hemifields to identify the orientation of a target Gabor grating. In both experiments, pre-stimulus alpha power in visual areas decreased linearly with increasing attention to visual stimuli. In contrast, alpha power in parietal areas showed lower pre-stimulus alpha power when attention was divided between modalities, compared to unimodal attention. These results suggest that there are two different alpha sources, where one reflects the visual spotlight of attention and the other reflects attentional effort. To our knowledge, this is the first study to show that attention recruits two spatially distinct alpha sources in occipital and parietal brain regions, which act simultaneously but serve different functions in attention.\n\nSIGNIFICANCE STATEMENTAttention to one spatial location/sensory modality leads to power changes of alpha oscillations (~10Hz) with decreased power over regions processing relevant information and power increases to actively inhibit areas processing to-be-ignored information. Here, we used detailed source modelling to investigate EEG data recorded during separate uni-modal (visual) and multi- (visual and somatosensory) attention tasks. Participants either focused their attention on one modality/spatial location or directed it to both. We show for the first time two distinct alpha sources are active simultaneously but play different roles. A sensory (visual) alpha source was linearly modulated by attention representing the visual spotlight of attention. In contrast, a parietal alpha source was modulated by attentional effort, showing lowest alpha power when attention was divided.