How does the coordinated activity of neurons translate into a sensation or a thought? Experimental methods to address this central question of neuroscience range from characterization of the molecular and cellular basis of neuronal activity to noninvasive studies of the living animal. Recent techniques that can image the functioning brain offer a promising bridge between the cells and molecules of neuroscience and the complexities of the mind.

Through positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), it is now possible to see “the brain at work,” to visualize which brain areas are activated (and in certain cases inhibited) by specific tasks. The spatial and temporal resolution of PET and fMRI provide an intermediate step in linking neuronal activity to behavior: The signals detected reflect the activity of neuronal ensembles in∼ 1 mm 3 with acquisition times as short as seconds. Until recently, however, it has not been clear exactly what neuronal activity is measured in PET and fMRI experiments.