For centuries, philosophers and biologists alike dreamed of watching the brain operate to see its active response to sensations, actions, or even thoughts. In some ways, functional magnetic resonance imaging (fMRI) provides such a view. This technology essentially measures blood flow to areas of the brain, and presumably neural activity, in real time. The technique could help researchers specifically map the brain. The visual cortex, for example, appears to light up when a subject sees something. Still, scientists remain unsure about exactly what the signal, the flashing spots on an fMRI screen, actually mean.
Indeed, neuroscientists debate what fMRI measures and how it can best be used clinically. Paul Matthews of the University of Oxford calls it "perhaps the single most important general technique in cognitive neuroscience." But, he cautions, "It is being widely used with relatively little understanding of its fundamental basis."