Monochromatic ultra-slow oscillations in the human electroencephalogram
Introduction: Ultra-slow electroencephalographic oscillations (< 0.1 Hz) have been extensively investigated in humans and animals and were found to occur in a variety of conditions. Despite the ample evidence for their presence in the human brain, less is known about their biological mechanisms. Here, we report on the existence of monochromatic ultra-slow oscillations (MUSO) in the human scalp electroencephalogram (EEG).
Methods: In order to address the biological MUSO origin we performed simultaneous recordings with multi-channel EEG, near infrared spectroscopy (NIRS) and non-invasive continuous measurement of arterial blood pressure in 10 healthy subjects. To test the relevance of hemodynamic contribution to MUSO variability, we performed recordings before and after a 60 degrees bed tilting manoeuvre.
Results: MUSO were detected in 8 out of 10 subjects during restful wakefulness. The striking signature of these oscillations was their almost monochromatic spectral profile restricted to the frequency range 0.08 – 0.15 Hz. The spatial topography of MUSO was complex, showing multiple foci variable across subjects. Application of spatio-spectral decomposition revealed multiple MUSO components in a given subject. These oscillations were most pronounced after the application of spatial derivatives, such as bipolar or Laplacian montages. While the peak frequency of NIRS and blood pressure was often similar across subjects, the peak frequencies of MUSO differed considerably from both NIRS and blood pressure oscillations. In 8/10 subjects we observed a coherence between MUSO and Oxy-Hb (NIRS)/arterial blood pressure. After a 60 degrees tilt we observed a significative correlation at the MUSO spectral power peak between EEG and blood pressure.
Discussion: We hypothesize that MUSOs can occur due to the modulation of the blood-brain barrier DC potential by mechanical fluctuations related to hemodynamics or due to skin vasomotion. The discovery of such possibly extra-neuronal pronounced oscillations opens an avenue for studying hemodynamic responses with EEG technology and in addition poses a number of questions concerning the interpretation of previously recorded low-frequency neuronal oscillations in humans.