Slow wave oscillations in awake healthy subjects - methodological and physiological considerations

INTRODUCTION:: Detection of pathological slow wave oscillations (0.5–7 Hz) in awake subjects have gained increasing interest in clinical diagnostics. Their significance, however, is hampered by the occasional presence of slow waves in healthy subjects, as well as the abundance of artefactual signals at low measurement frequencies. Our aim was to assess the occurrence of slow wave oscillations in healthy subjects, and to sharpen the management of possible measurement artifacts, in order to create a normative database for neurological patients. METHODS:: We analyzed magnetoencephalography (MEG) recordings of spontaneous brain oscillations in 139 awake healthy adults. Sources of artifacts were first identified and suppressed by temporal extension of signal space separation method (tSSS), and the remaining artifact components were projected out using signal space projection (SSP). Individual amplitude spectra were compared with the channel-level average spectra over all subjects. RESULTS:: Slow wave oscillations deviating ± 2 standard deviations (SD) from the average spectrum were detected in 12 subjects (∼9%). In ten subjects, the oscillations were considered as normal physiological phenomena. Only two subjects showed activity that could have been interpreted as pathological: one subject with widespread parietal bilateral polyrhythmic slow wave activity, and one with focal rolandic 2.7-Hz slow wave activity. CONCLUSIONS:: The prevalence of slow oscillations in a healthy adult population is low. Knowledge about their occurrence, however, is essential for interpreting their significance in brain diseases. Artifacts and benign oscillatory variants at slow frequencies have to be recognized.