Electromagnetic brain imaging

Abstract

There is a long history to detection of neural current flow via specific changes in the electromagnetic field that is measured outside the brain. Electroencephalography (EEG) has been extensively investigated and utilized for the last 80 years [Berger, 1929] and electrocorticography (ECoG) for 60 years [Jasper and Penfield, 1949]. Magnetoencephalography (MEG) was introduced 40 years ago [Cohen, 1968] and it has since experienced intense technical development and growing interest in various fields of neuroscience. Clinicians and basic scientists who have chosen to utilize these time-sensitive techniques have always appreciated their great potential and pertinence to questions in both the clinical domain and integrative neuroscience, while understanding their respective limitations. These electromagnetic techniques have recently fostered increasing interest from investigators who originally entered the field of human brain mapping via other modalities, particulary functional magnetic resonance imaging (fMRI). An aspiration to reach beyond the hemodynamic response and its limited time resolution is likely to be at the origin of such interest which has also manifested as notable investments in cutting-edge MEG and fMRI-compatible EEG systems. For example, the organization and internal mechanisms of brain-wide functional networks are considered by many investigators a tough problem of great interest that can ultimately not be addressed with the time scale of several hundreds of milliseconds that is accessible with blood oxygen level dependent (BOLD) effects. Consequently, the functional neuroimaging community now seems to increasingly encompass the relevance of timing—in a very broad sense—as a natural complement to spatial mapping, when seeking to characterize and understand human brain function and its disorders as well as the relationship between neural processes and behavior. Each new development in electromagnetic methods and the more recent interplay between electromagnetic and hemodynamic techniques has brought along new views, challenges and disputes that have, eventually, moved the field forward. EEG was initially collected with a relatively small number of electrodes and, for several decades, the focus was set primarily on the identification and chronometry of typical scalp waveforms or components, with only indirect concern of the areas in the brain the signals would originate from—although there has been an unwarranted tendency to associate changes in EEG signals (or event-related potentials, ERPs) at specific electrodes with activation of the brain areas directly underneath. Emergence of the MEG technique, particularly devices covering large areas of the scalp (cortex), resulted in a palpable tension between EEG and MEG users and developers, akin to the brawls witnessed in the hemodynamic imaging community between users of positron emission tomography (PET) and the proponents of the rapidly emerging fMRI technique. Early MEG investigators—mostly from physics laboratories sought to localize—from the outset, the neural sources of the surface signals and determine the temporal variation of their activation. The prominent spatiotemporal (or source localization) emphasis in MEG had strong foundations: the volume currents that are generated by the intracellular (primary) currents and circulate within the head tissues have markedly less influence on the surface magnetic fields (MEG) than surface electric potentials (EEG), as *Correspondence to: Riitta Salmelin, Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, FIN02015 TKK, Finland. E-mail: riitta@neuro.hut.fi