Localization of alpha activity in humans using EEG and functional MRI

The objective of this dissertation is to image neuronal generators of spontaneous alpha rhythm in human brain. We have developed two different approaches: (i) using individual EEG (electroencephalography) measurements, and (ii) using concurrent measurements of EEG and fMRI (functional magnetic resonance imaging).; For our EEG study, we have developed a distributed-source imaging approach in frequency-domain. Generators of spontaneous activity are characterized by a specific frequency range, and therefore, a frequency-domain approach may be more accurate than a time-domain model. We carried out a frequency-domain analysis of alpha activity (8-12 Hz) under the assumption of synchronous sources. Since the alpha rhythm is probably produced by several independent generators, a distributed-source imaging approach was considered more appropriate than a multiple equivalent dipole model. This imaging approach was based on the generalized maximum entropy principle. The reconstructed sources were superposed on corresponding anatomical images obtained using MRI.; The central premise of the second technique to localize alpha activity using fMRI derives from the reported reductions in local glucose metabolism and local blood flow in regions associated with alpha activity. If the decrease in oxygen consumption is not commensurate with the decrease in blood flow (a situation akin to regular fMRI but in the opposite direction) then the measured signal will reduce in accordance with the BOLD (blood oxygenation level dependent) effect. Also, the reduction in blood flow could produce a negative in-flow signal. Thus, the combined negative BOLD/in-flow effect would reduce the fMR signal during alpha activity. To monitor alpha activity during fMRI study, we performed fMRI and EEG experiments simultaneously, where we recorded the EEG activity at two electrode locations near the occipital region. The alpha spectral power was then used to design a proper reference function and to construct a functional image revealing negative BOLD/in-flow signal. This functional image was superposed on corresponding anatomy to depict functional/structural relationships.; Both of our approaches found the highest concentration of alpha generators in occipital and parietal lobe areas at or near the parieto-occipital sulcus, middle occipital gyrus, cuneus, cingulate gyrus and precuneus (brain areas 18 and 19).