Imaging the mind.

At the forefront of cognitive neuroscience research in normal humans are the new techniques of functional brain imaging: positron emission tomography (PET) and, more recently, magnetic resonance imaging (MRI). The signal used by PET is based on the fact that changes in the cellular activity of the brain of normal, awake humans and laboratory animals are accompanied almost invariably by changes in local blood flow. This robust, empirical relationship has fascinated scientists for well over a hundred years. PET provided a level of precision in the measurement of blood flow that opened up the modern era of functional human brain mapping. Further, the discovery with PET that these changes in blood flow are unaccompanied by quantitatively similar changes in oxygen consumption has paved the way for the explosive rise in the use of MRI in functional brain imaging. The remarkable success of this enterprise is a fitting tribute to men like Michel Ter-Pogossian. He pioneered the use of positron emitting radionuclides in biology and medicine when most had abandoned them in favor of more conventional nuclear medicine radionuclides. Importantly, also, he welcomed into his laboratory young scientists with a broad range of talents, many of whom subsequently became leaders in imaging the mind in research centers throughout the world.