Biophysical Aspects of the BOLD Effect in Human Brain Mapping
The aim of the research described in this thesis was to study basic mechanisms of functional magnetic resonance imaging (fMRI) and to develop an improved understanding of the Blood Oxygen Level Dependent (BOLD) effect. In addition to the assessment of the nonlinear nature of brain activity during dynamic visuo-spatial imagery using event-related functional magnetic resonance imaging (er-fMRI) and in vivo optical imaging and electrophysiology, several methodological issues have been investigated. Moreover, the interpretation of various BOLD responses has been aided by developing an improved model of the neurovascular coupling during brain activation. The theoretical part of this thesis consists of a selective overview of fMRI and Near-Infrared Spectroscopy (NIRS) and Event-related Potentials (ERPs), and of the advantages of their combination for functional neuroimaging. The methodological and experimental chapters describe projects performed to investigate different BOLD responses to transient activation and deactivation and to investigate the feasibility of a method for detecting neuronal current induced magnetic fields by MRI. An improved Balloon Model is described which explicitly includes nonlinear physiological variables to predict the BOLD signal in various circumstances. This model better explains the BOLD response for transient stimuli.