Dynamic cortical and tractography atlases of proactive and reactive alpha and high-gamma activities
Alpha waves - posterior-dominant rhythms at 8-12 Hz reactive to eye opening and closure - are among the most fundamental EEG findings in clinical practice and research since Hans Berger first documented them in the early 20th century. Yet, the exact network dynamics of alpha waves in regard to eye movements remains unknown. High-gamma activity at 70-110 Hz is also reactive to eye movements and a summary measure of local cortical activation supporting sensorimotor or cognitive function. We aimed to build the first-ever brain atlases directly visualizing the network dynamics of eye movement-related alpha and high-gamma modulations, at cortical and white matter levels. We studied 28 patients (age: 5-20 years) who underwent intracranial EEG and electrooculography recordings. We measured alpha and high-gamma modulations at 2,170 electrode sites outside the seizure onset zone, interictal spike-generating areas, and MRI-visible structural lesions. Dynamic tractography animated white matter streamlines modulated significantly and simultaneously beyond chance, on a millisecond scale. Before eye closure onset, significant alpha augmentation occurred at the occipital and frontal cortices. After eye closure onset, alpha-based functional connectivity was strengthened, while high gamma-based connectivity was weakened extensively in both intrahemispheric and interhemispheric pathways involving the central visual areas. The inferior fronto-occipital fasciculus supported the strengthened alpha coaugmentation-based functional connectivity between occipital and frontal lobe regions, whereas the posterior corpus callosum supported the interhemispheric functional connectivity between the occipital lobes. After eye opening offset, significant high gamma augmentation and alpha attenuation occurred at occipital, fusiform, and inferior parietal cortices. High gamma coaugmentation-based functional connectivity was strengthened, whereas alpha-based connectivity was weakened in the posterior interhemispheric and intrahemispheric white matter pathways involving central and peripheral visual areas. Proactive and reactive alpha waves involve extensive, distinct white matter networks that include the frontal lobe cortices, along with low- and high-order visual areas. High-gamma co-attenuation coupled to alpha co-augmentation in shared brain circuitry after eye closure supports the notion of an idling role for alpha waves during eye closure. These dynamic tractography atlases may improve understanding of the significance of EEG alpha waves in assessing the functional integrity of brain networks in clinical practice; they also may help elucidate the effects of eye movements on task-related brain network measures observed in cognitive neuroscience research.