Abstract
Physiological pulsations have been shown to affect the global blood oxygen level dependent (BOLD) signal in human brain. While these pulsations have previously been regarded as noise, recent studies show their potential as biomarkers of brain pathology. We used the extended 5 Hz spectral range of magnetic resonance encephalography (MREG) data to investigate spatial and frequency distributions of physiological BOLD signal sources. Amplitude spectra of the global image signals revealed cardiorespiratory envelope modulation (CREM) peaks, in addition to the previously known very low frequency (VLF) and cardiorespiratory pulsations. We then proceeded to extend the amplitude of low frequency fluctuations (ALFF) method to each of these pulsations. The respiratory pulsations were spatially dominating over most brain structures. The VLF pulsations overcame the respiratory pulsations in frontal and parietal gray matter, whereas cardiac and CREM pulsations had this effect in central cerebrospinal fluid (CSF) spaces and major blood vessels. A quasi‐periodic pattern (QPP) analysis showed that the CREM pulsations propagated as waves, with a spatiotemporal pattern differing from that of respiratory pulsations, indicating them to be distinct intracranial physiological phenomenon. In conclusion, the respiration has a dominant effect on the global BOLD signal and directly modulates cardiovascular brain pulsations.
Highlights
Roy and Sherrington noted in their pioneering 1890 study on animal cerebral hemodynamics that, in addition to hemodynamic coupling to electrical stimuli, “the brain expands with each rise of the blood pressure and contracts with each successive fall” during the frequently detected spontaneous Mayer blood-pressure waves (Roy & Sherrington, 1890)
It has been difficult to ascribe exact physiological source of these signals due to the problem of signal aliasing and further spatiotemporal mixing from interleaved data sampling the blood oxygen level dependent (BOLD) signal, which is usually sampled at a low frequency
As this study focuses on the sources of physiological BOLD signals, we wanted as much as possible to retain the physiological pulsations in the data
Summary
Roy and Sherrington noted in their pioneering 1890 study on animal cerebral hemodynamics that, in addition to hemodynamic coupling to electrical stimuli, “the brain expands with each rise of the blood pressure and contracts with each successive fall” during the frequently detected spontaneous Mayer blood-pressure waves (Roy & Sherrington, 1890). A decade later, Hans Berger showed that there were three sources of intracranial brain pressure pulsations, namely the “ pulsatory, respiratory and vasomotor waves” (Berger, 1901). To this day, an understanding of the physiological significance of these pulsations remains elusive. Conventional fMRI uses blood oxygen level dependent (BOLD) signals to measure hemodynamic changes following neuronal brain activity that can be either cued or spontaneous in nature It has been difficult to ascribe exact physiological source of these signals due to the problem of signal aliasing and further spatiotemporal mixing from interleaved data sampling the BOLD signal, which is usually sampled at a low frequency
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