Low-Frequency Spontaneous Oscillations of Cerebral Hemodynamics Investigated With Near-Infrared Spectroscopy: A Review

Hemodynamic low-frequency oscillations (LFOs), in the range 0.01-0.1 Hz, are intriguing, yet elusive phenomena, which occur spontaneously in the vascular system. More than 150 years have passed since their discovery, but the cellular mechanisms at their origin and their physiological implications have yet to be unraveled. The study of LF hemodynamic oscillations is considered to be relevant in areas of brain research such as cerebral autoregulation, functional and effective connectivity, and their related pathologies. These research areas have been traditionally investigated by transcranial Doppler (autoregulation) and functional MRI [functional connectivity (FC)]. The purpose of this paper is to review the work that has been done in this area using near-infrared spectroscopy (NIRS), which is a noninvasive technique used for monitoring and imaging tissue hemodynamics. NIRS has the advantage of being sensitive to both oxy- and deoxyhemoglobin concentration changes. Therefore, not only can it be used in autoregulation and FC studies (for broader populations of subjects), it can also help understand the local interplay between vascular and metabolic parameters. Finally, we present a novel approach to the study of LFOs and to the physiological interpretation of the amplitude and relative phase of oscillatory components of oxy- and deoxyhemoglobin concentrations.