This theory has reinforced the idea that the connectivity provided by white matter occupies a central place in the elaboration of human behavior. After centuries of being recognized but not well understood, white matter gained growing importance as neuroscientists turned to the study of brain-behavior relationships.Īt the beginning of the 21st century, scientists generally accept the theory that distributed neural networks-widely scattered neurons or clusters of neurons that tend to fire in synchrony-underlie our conscious experiences. Without functioning white matter, the brain could be like a group of people in proximity to each other but unable to communicate with each other. This suggests that the cortical regions act in concert to perform mental operations and no cortical area acts in isolation. All of the well-known cortical areas such as Broca’s area, Wernicke’s area, the prefrontal cortex, and the hippocampus are connected by white matter tracts to other regions of the brain. White matter is a vast, intertwining system of neural connections that join all four lobes of the brain (frontal, temporal, parietal, and occipital), and the brain’s emotion center in the limbic system, into the complex brain maps being worked out by neuroscientists. Central to his concept was the idea that an intricate web of white matter pathways course within and between the brain’s hemispheres. 2 Geschwind’s seminal article advanced the view that the dense connectivity of the brain underlay its mental operations. In 1965, however, Norman Geschwind, M.D., proposed that one mechanism underlying dysfunctional brain-behavior relationships might be cerebral disconnection, a view that pointed directly at the need to study white matter lesions.
In the 19th century, the Parisian physician Jean Martin Charcot greatly advanced understanding of white matter’s role with his detailed studies of multiple sclerosis (MS), a disease of young adults characterized by primary damage to the white matter of the central nervous system.Īt the turn of the 20th century, the ascendancy of Sigmund Freud turned biomedical thinking toward psychoanalytic explanations of behavior, and for more than 50 years all of the brain-white and gray matter alike-was relatively neglected. 1 Only with time, though, did the role of white matter in providing structural and functional connections between gray matter areas within the brain become apparent. In 1543, in the seventh book of his monumental work, De Humani Corporis Fabrica, the Renaissance anatomist Andreas Vesalius was the first to distinguish clearly between white matter and the gray matter that overlay regions of the cerebral cortex. The Discovery (and “Rediscovery”) of White Matter So diverse and important are these contributions that it is time to consider the need for a new field: the behavioral neurology of white matter. The scope and variety of syndromes that result from disruption of white matter suggest that white matter makes a pivotal contribution to all realms of human behavior, a contribution we are just beginning to fathom. We know this because, when this connectivity is disrupted by disease or other damage to white matter, the result is often a dramatic disturbance of normal mental function. In the normal brain, white matter appears to provide the essential connectivity, uniting different regions into networks that perform various mental operations. Only quite recently have neuroscientists in the laboratory and clinic begun to understand the importance of this long-neglected part of the brain. If gray matter supposedly “is” the brain, then what is all this white matter doing in our heads? Almost one half of the brain’s volume is not gray but white matter, the densely packed collection of myelinated (insulated) projections of neurons that course between widely dispersed gray matter areas. Yet, a glance at the anatomy of the human brain reveals that cortical gray matter comprises only the brain’s outermost one to four millimeters, a layer about the thickness of heavy cloth, over a brain that in the average human adult weighs three pounds. In common parlance, gray matter has become virtually synonymous with the brain, as when Agatha Christie’s famous detective Hercule Poirot boasts about his “little gray cells.” Much of neuroscience, as presented today, tends to reinforce this view that the gray matter of the brain’s cerebral cortex makes possible our distinctive mental capacities, such as memory, language, thought, and emotion. Given the crossword puzzle clue “brain stuff,” what would come to your mind? Quite a few readers might answer “gray matter” and with reason.