Spontaneous low frequency oscillations of cerebral hemodynamics and metabolism in human adults

We investigated slow spontaneous oscillations in cerebral oxygenation in the human adult's visual cortex. The rationale was (1) to demonstrate their detectability by near infrared spectroscopy (NIRS); (2) to analyze the spectral power of as well as the phase relationship between the different MRS parameters (oxygenated and deoxygenated hemoglobin and cytochrome-oxidase; oxy-Hb/deoxy-Hb/Cyt-ox). Also (3) influences of functional stimulation and hypercapnia on power and phase shifts were investigated. The results show that-in line with the literature-low frequency oscillations (LFO) centred around 0.1 s(-1) and even slower oscillations at; about 0.04 s(-1) (very low frequency, VLFO) can be distinguished. Their respective power differs between oxy-Hb, deoxy-Hb, and Cyt ox. Either frequency (LFO and VLFO) is altered in magnitude by functional stimulation of the cortical area examined. Also we find a change of the phase shift between the vascular parameters (oxy-Hb, tot-Hb) and the metabolic parameter (Cyt-ox) evoked by the stimulation. It is shown that hypercapnia attenuates the LFO in oxy-Hb and deoxy-Hb. Conclusions: (1) spontaneous vascular and metabolic LFO and VLFO calm be reproducibly detected by MRS in the human adult. (2) Their spectral characteristics and their response to hypercapnia are in line with those described in exposed cortex (for review see (Hudetz ef al., 1998)) and correspond to findings with transcranial doppler sonography (TCD) (Diehl ct al., 1995) and fMRI (Biswal et al., 1997). (3) The magnitude of and phase relation between MRS-parameters at the LFO may allow for a local noninvasive assessment of autoregulatory mechanisms in the adult brain. (C) 2000 Academic Press.