HANDBOOK OF FUNCTIONAL NEUROIMAGING OG COGNITION
Since 1990 cognitive neuroscience has emerged as a very important growth area in neuroscience. Cognitive neuroscience combines the experimental strategies of cognitive psychology with various techniques to examine how brain function supports mental activities. Leading this research in normal humans are the new techniques of functional brain imaging: positron emission tomography (PET) and magnetic resonance imaging (MRI), along with event-related potentials (ERPs) obtained from electroencephalography (EEG) or magnetoencephalography (MEG). The signal used by PET is based on the fact that changes in the cellular activity of the brain of normal, awake humans and unanesthetized laboratory animals are invariably accompanied by changes in local blood flow (for a review see Raichle, 1987). This robust, empirical relationship has fascinated scientists for well over a century, but its cellular basis remains largely unexplained despite considerable research. More recently it has been appreciated that these changes in blood flow are accompanied by much smaller changes in oxygen consumption (Fox & Raichle, 1986; Fox et al., 1988). This leads to changes in the actual amount of oxygen remaining in blood vessels at the site of brain activation (i.e., the supply of oxygen is not matched precisely with the demand). Because MRI signal intensity is sensitive to the amount of oxygen carried by hemoglobin (Ogawa et al., 1990), this change in blood oxygen con-tent at the site of brain activation can be detected with MRI (Ogawa et al., 1992; Kwong et al., 1992; Bandettini et al., 1992; Frahm et al., 1992).