Modern High Resolution Electroencephalography

High temporal resolution is necessary to resolve the rapidly changing patterns of brain activity underlying mental function. While electroencephalography (EEG) provides temporal resolution in the millisecond-range, which would seem to make it an ideal complement to other imaging modalities, traditional EEG technology and practice provides insufficient spatial detail to identifu relationships between brain electrical events and structures and functions visualized by magnetic resonance imaging (MRI) or positron emission tomography (PET). Recent advances overcome this problem by recording EEGs from more electrodes, by registering EEG data with anatomical information from each subject's MRI, and by correcting the distortion caused by volume conduction of EEG signals through the scull and scalp. Example applications are presented from studies of language, attention and working memory. Along with its ability to record how brains think when performing everyday activities in the real world, these advances make modern EEG an invaluable complement to other functional neuroimaging modalities. The purpose of functional brain mapping is to measure patterns of neuronal activity associated with sensory, motor, and cognitive functions, or with disease processes. To be truly effective in this regard, an imaging modality needs both near millimeter precision in localizing regions of activated tissue and subsecond temporal precision for characterizing changes in patterns of activation over time. Increasingly fine anatomical resolution is available from positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) technologies, but the length of the temporal sample they require is still an order of magnitude or more too long to resolve the rapidly shifting patterns of activity that are characteristic of actual neurophysiological processes. The electroencephalogram (EEG), in contrast, has a temporal resolution as fine as the analog-to-digital sampling rate used to record it (typically in the one to five millesecond range), and its exquisite sensitivity to changes in mental activity has been recognized since Hans Berger reported a decrease in the amplitude of the dominant (alpha) rhythm of the EEG during mental arithmetic. In addition to the type of tonic alterations in brain electrical activity reported by Berger, EEG measurements of phasic, stimulus-related changes in brain activity (such as Evoked Potentials of Eps) are wellsuited for measuring subsecond component processes of sensory, motor and cognitive processes ll,2l. For simplicity we use the term EEG in a general sense to refer 2.0.01