Cortical Lead Fields of Electroencephalographic and Magnetoencephalographic Sensors
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چکیده
The spatial relationship of electroencephalography (EEG) and magnetoencephalography (MEG) to brain activation is quantified as their cortical lead fields: the sensitivity of each sensor to activation of each cortical location. Here we map the lead fields of EEG and MEG across the cortical surface using a realistic boundary element model with four compartments. These calculations demonstrate that inclusion of CSF is essential for accurate calculation of EEG lead fields. The typical referential EEG derivation is affected by activity in all lobes of both hemispheres, while the typical MEG planar gradiometer is sensitive to part of one lobe. The proportion of cortical dipoles that have an effect on the average sensor that is ≥25% of the most effective dipole, is ~40% for referential EEG and <2% for MEG gradiometers. Gradiometers are substantially more focal than magnetometers and both are typically more focal than EEG. These findings indicate that gradiometer signals usually arise from the underlying cortex and that their coherence can often be inferred to reflect neural interaction rather than overlapping lead fields; such inferences are unwarranted for referential EEG. Quantitative differences between EEG and MEG lead fields may give rise to qualitative differences in the neural systems that they record.
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تاریخ انتشار 2011