Distinct top-down and bottom-up attention networks revealed through high-resolution electrocorticography
Brain-wide networks operating at a millisecond timescale are thought to underlie our cognitive functions, but have never been observed directly. Neuroimaging studies based on hemodynamic signals visualized the precise topographies of brain-wide functional networks, but at low temporal resolution. Neurons and areas within these networks likely cooperate through rhythmic synchronization in multiple frequency bands. However, limitations of current recording methods have restricted our ability to detect and investigate these putative brain-wide synchronization networks. Only if extended corticocortical synchronization networks are observed directly and in behaving subjects, will we simultaneously reveal their topographies, frequencies, directions of information flow and cognitive functions, and thereby the relations among those properties. I will present data from large-scale, high-density electrocorticography grids, combining millisecond temporal and millimeter spatial resolution with coverage of large parts of one hemisphere. I will show that a given brain area may simultaneously participate in different networks that synchronize in distinct frequencies and mediate influences in counter-streams. A gamma-band (50 90 Hz) network synchronizes visual-occipital areas and parts of parietal cortex, and gamma-mediated inter-areal influences are bottom-up. A beta-band (peaking at 14 18 Hz) network synchronizes parietal and frontal areas and parts of visual cortex, and beta-mediated inter-areal influences are mostly top-down. Both networks subserve the cognitive function of attention: gamma- and beta-mediated inter-areal influences are enhanced when they mediate behaviorally relevant signals. The direct topographical demonstration of rhythmic synchronization-defined networks constitutes a new quality of brain network investigation and opens an important window onto their function.