In fact, the primary sources of EEG/MEG are the synaptic currents flowing through the apical dendrites of large pyramidal neurons within the cortical gray matter [8, 13, 25, 26]. The apical dendric trees extend toward the pial surface and are organized into columns – a unique structure facilitating the formation of regionally synchronized synaptic currents [12]. It has been approximated that the simultaneous activation of as few as 0.1% synapses within a cortical area of around 40 mm2 would suffice to produce a detectable EEG/MEG signal [9]. Moreover, the view that the primary EEG/MEG sources are synaptic currents is also supported by a general similarity between the frequency spectra of scalp potentials and post-synapc potentials [27]. This is regardless of the fact that extracranial potentials measured with EEG have much smaller amplitudes and lower frequency than intracranial potentials, such as local field potentials (LFP) and electrocorticograms (ECoG), owing to the different spatial scale to which each recording technique is applied. In general, low-frequency signals tend to be synchronized in a larger spatial scale, whereas synchronous high-frequency activity is often confined to a smaller scale. The same relationship also holds true for MEG.
