Combining Electromagnetic and Hemodynamic Signals to Derive Spatiotemporal Brain Activation Patterns: Theory and Results

The construction of accurate spatio-temporal maps of human brain activity during higher cognitive tasks poses substantial technical and theoretical difficulties. Since no single technique has the ability to provide accurate and certain resolution in both the temporal and spatial domains, it is necessary to combine techniques with complementary sensitivities. Functional magnetic resonance imaging (fMRI) has emerged äs the best technique for localizing neural activation due to cognition. It provides high, certain, and fairly uniform spatial accuracy (to about 3 mm), but low temporal resolution (>1 s), due to its indirect relationship to neural activity. In contrast, electromagnetic measures, the electroencephalogram and magnetoencephalgram (EEG and MEG) have temporal accuracy that is limited only by the sampling rate (typically a few ms), but have limited, uncertain and biased spatial accuracy [1]. In this paper, we discuss the physiological justification for combining these measures, and some initial results of their combination in studying the neural basis of cognition. The limited temporal resolution of fMRI is due to its origin in the Blood Oxygenation Level Dependent (BOLD) response [2]. Although it is not wellestablished, it is thought that the BOLD response is produced by an increase in blood flow that in turn is caused by byproducts of increased energy utilization during neural activity [3]. Each of these linkages, from neural activity to metabolic demand to changes in microcirculation, may act äs low-pass filier, with the relatively sluggish dynamic response of the capillaries and arterioles forming a biophysical limit on the ability of the BOLD response to follow rapid fluctuations of neural activity [4]. In contrast to the fMRI, the EEG and MEG result directly from current flow across post-synaptic membranes. Aligned intracellular currents generate the MEG, whereas aligned extracellular currents are measured äs EEG [5, 6J. Consequently, the EEG and MEG have identical generators but propagate differently. Thus, they provide complementary localizing Information [7], and are considered äs a unit in the current chapter. The uncertain spatial resolution of EEG/MEG was articulated in the 19* Century by Helmholtz [8]: for any given extracranial distribution of electromagnetic activity, there are an infinite number of possible generator configurations in the brain. Even given a simple dipolar pattern äs measured outside the head, it is easy to imagine complex arrangements of dipoles that cancel each other except for a residual that can appear äs a small dipole at a distance [9]. Furthermore, a focal source may produce an external pattern that is indistinguishable from a more superficial field of activation distributed over a broad area [10]. For these reasons, uncertainty in the location of generating sources is worse for their depth than their other dimensions, i.e. there is a nonuniformity in the spatial localizing ability of EEG/MEG [7]. Despite these uncertainties, it must be emphasized that there is still a great deal of spatial Information in the EEG/MEG.