Separation of low-voltage EEG-activity during mental activation from that during transition to drowsiness

When dealing with electroencephalograms (EEGs) recorded under resting conditions, periods of low-voltage activity might indicate drowsiness, but mental activation as well. Thus, low-voltage EEG retrieves a notorious source of confusion. The simultaneous occurrence of drowsiness related slow horizontal eye movements (SEM) allow to assign low-voltage EEG-activity to a brain state of reduced vigilance instead of mental activation. The aim of this study was to investigate, whether under resting conditions with eyes closed low-voltage EEG with SEM (B1+) and without SEM (B1-) differ in spectral and spatial distribution of EEG-activity. EEGs of 35 healthy subjects where analyzed, each containing at least 10 s of low-voltage EEG recorded during a calculation task (calc, as control condition), as well as 10 s of each B1- and B1+, recorded during following about 20 min of rest. Using standardized, low resolution brain electromagnetic tomography, cortical current density was computed in four individually adapted frequency bands (delta, theta, alpha, beta) for calc, B1- and B1+. Paired test comparison of cortical current densities revealed significant differences for B1- compared to B1+. In detail, B1- exhibited lower delta- and theta band activity, especially in the cingulate- and adjacent medial portions of the frontal, parietal and occipital cortices, as well as higher beta band activity in temporal cortices. Similar results where found in calc versus B1+. These findings support the association of B1- to a higher level of vigilance compared to B1+, thus justifying the separation of low-voltage EEG-activity by means of SEM.