Identification of the regional variability of the brain hemodynamic response to spontaneous and step-induced CO <inf>2</inf> changes using function expansions

The cerebrovascular bed is very sensitive to CO 2 changes, particularly in respiratory-related areas, such as the brainstem. Therefore, the hemodynamic response to such changes is of interest as it quantifies this sensitivity. Here, we examine in detail the regional characteristics of the hemodynamic response to spontaneous and larger, externally induced step CO 2 changes CO 2 (end-tidal forcing) by utilizing BOLD functional magnetic resonance imaging (fMRI) measurements from healthy humans. We first obtain estimates of the impulse response between CO 2 and BOLD signal in several anatomically and functionally defined regions of interest, using function expansions with different basis sets. These include the Laguerre basis, which has been widely used in linear and nonlinear systems identification particularly for biological/physiological systems, as well as different variants of gamma functions, which have been widely used in functional neuroimaging due to physiological considerations with regards to the characteristics of the BOLD response to external (sensory or other) stimuli. Based on the aforementioned comparisons, we perform the same analysis in smaller anatomical areas, considering voxel neighborhoods that span the entire image, in order to map key features of the hemodynamic response function such as peak value, time-to-peak and area, in finer spatial resolution. © 2012 IFAC.