![]() ![]() The coherence measure is generally distinct from synchrony ( Singer, 1999), which in the neuroscience literature normally refers to signals oscillating at the same frequency with identical phases. EEG coherence may yield information about network formation and functional integration across brain regions. EEG coherence provides one important estimate of functional interactions between neural systems operating in each frequency band. Any pair of EEG signals may be coherent in some frequency bands and incoherent in others the dependence of detailed extracranial coherence patterns on frequency and brain state is now well established ( Nunez, et al. One of the most promising measures of such dynamics is coherence, a correlation coefficient (squared) that estimates the consistency of relative amplitude and phase between any pair of signals in each frequency band ( Bendat and Piersol, 2000). The central goal of EEG studies is to relate various measures of neural dynamics to functional brain state, determined by behavior, cognition, or neuropathology. EEG, Laplacian, and MEG coherence emphasize different spatial scales and orientations of sources. However, MEG coherence estimates reflect fewer sources at a smaller scale than EEG coherence and may only partially overlap EEG coherence. In comparison to long-range (> 20 cm) volume conduction effects in EEG, widely spaced MEG sensors show smaller field spread effects, which is a potentially significant advantage. This effect is most apparent at sensors separated by less than 15 cm in tangential directions along a surface passing through the sensors. MEG coherence estimates are inflated at all frequencies by the field spread across the large distance between sources and sensors. Surface Laplacian EEG methods minimize the effect of volume conduction on coherence estimates by emphasizing sources at smaller spatial scales than unprocessed potentials (EEG). In contrast, while lower frequency EEG coherence appears to result from a mixture of volume conduction effects and genuine source coherence. Cortical sources generating spontaneous EEG in this band are apparently uncorrelated. This volume conduction effect was readily observed in experimental EEG at high frequencies (40-50 Hz). We found that volume conduction can elevate EEG coherence at all frequencies for moderately separated ( 20 cm) electrodes. We estimated this effect using simulated brain sources and a model of head tissues (CSF, skull, and scalp) derived from MRI. However, moderate to large EEG coherence can also arise simply by the volume conduction of current through the tissues of the head. EEG coherence is often used to assess functional connectivity in human cortex. We contrasted coherence estimates obtained with EEG, Laplacian, and MEG measures of synaptic activity using simulations with head models and simultaneous recordings of EEG and MEG. ![]()
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