Specifically, we propose that two mechanisms contribute to the observed discrepancies between scalp EEG and intracranial EEG. First, studies in non-human primates have simultaneously recorded scalp EEG and LFPs and show that spectral power in scalp EEG represents a linear combination of LFP power and inter-electrode synchrony. In fact, increases in synchrony can produce positively modulated spectral power at the scalp even in the presence of a negative LFP modulation [70–72]. As schematized in Figure 5A, intracranial electrodes record relative decreases in theta power (blue shading), but the oscillation itself is highly correlated across electrodes (red lines). Accordingly, scalp electrodes detect this synchronization as a relative increase in theta power (red shading). In view of these findings, the predominantly positive effects in theta phase locking associated with successful memory that we have described above might be observed as positive power modulations in scalp EEG, even in situations where local activity measured intracranially shows decreases in power. It is less clear how this analysis would generalize to MEG. Studies that simultaneously measured MEG and scalp EEG have shown that MEG may
