From local inhibition to long-range integration: a functional dissociation of alpha-band synchronization across cortical scales in visuospatial attention.

When attention is allocated to one visual hemifield, increased alpha power is observed in ipsilateral visual cortex. This has been attributed to synchronization of alpha-band oscillations within cortical regions which reflects inhibitory processing. Recent results, however, indicate that synchronization of alpha oscillations between cortical regions is relevant for transient functional coupling. Such coupling is thought to be involved in orienting attention to a specific region of the visual field. We thus hypothesized that alpha-band synchronization between low-level visual cortex and higher-level visual brain regions would be increased in the hemisphere contralateral to an attended location. To test this hypothesis we calculated phase synchronization between attention-related EEG source activations occurring between predictive directional cues and expected visual targets. Alpha amplitude (understood as an index of local synchronization) within low-level visual cortex was increased ipsilateral to attended locations and decreased contralateral to attended locations, consistent with alpha-band scalp topography and previous research relating local alpha power to active inhibition. Increased long-range alpha-band synchronization between low-level visual cortex and parietal cortex, however, was observed contralateral to the attended visual hemifield, whereas decreased synchronization (phase scattering) was observed in the ipsilateral hemisphere. These results identify a potential mechanism for the enhanced processing of stimuli appearing at attended locations, as long-range synchronization is thought to increase the fidelity and effectiveness of communication between brain areas. Our observation of inhibitory amplitude changes, interpreted as increased local-area synchronization, and facilitatory long-range synchronization demonstrates a functional dissociation for alpha-band synchronization across cortical scales.