Communication du Professeur FILLEY au XXVIIIème Congrès du GRAL
Behavioral neurology has traditionally focused of gray matter – primarily of the cerebral cortex – in the study of brain-behavior relationships. Yet white matter makes up about half the brain, and connects all gray matter regions into distributed neural networks subserving cognition and emotion. Classic clinical-pathologic correlations first demonstrated the role of white matter damage in disorders of higher function, and a major advance occurred in the 1980s as magnetic resonance imaging (MRI) enabled elegant in vivo assessment of white matter and its disorders. From these studies emerged a behavioral neurology of white matter. This discipline examines the role of myelinated systems in human behavior, and a wealth of new information is appearing to further the understanding of brain-behavior relationships.
The most familiar neurobehavioral disorders associated with white matter are the disconnection syndromes. Many of these syndromes were described by 19th century European neurologists, and in 1965 Norman GESCHWIND renewed interest in this area with the publication of his seminal paper on disconnection. Conduction aphasia, pure alexia, and callosal disconnection are well known neurobehavioral syndromes related to focal white matter damage. Neuropsychiatric syndromes constitute another category of illness in which white matter involvement is relevant. Many psychiatric syndromes can develop in patients with known white matter disorders, and white matter dysfunction has been postulated in a wide range of psychiatric disorders, including schizophrenia, depression, and autism. Cognitive dysfunction can also occur in the white matter disorders, often sufficiently severe to meet criteria for dementia. In 1988, my colleagues and I introduced the term « white matter dementia » to call attention to this underappreciated consequence of white matter dysfunction or damage. Disorders within ten neuropathological categories – genetic, demyelinative, infectious, inflammatory, toxic, metabolic, vascular, traumatic, neoplastic, and hydrocephalic – came to be recognized as being capable of producing white matter dementia, with a consistent cognitive profile featuring impaired processing speed and executive dysfunction with relative sparing of language. As the future unfolds, the study of white matter will extend our knowledge of brain-behavior relationships and offer insights into many disorders that remain poorly understood. Recent findings, for example, have suggested that white matter pathology may be critical in the etiopathogenesis of ALZHEIMER’s disease, and in the newly described entity of chronic traumatic encephalopathy. The behavioral neurology of white matter is a promising area of investigation that usefully complements the traditional corticocentric perspective of cognitive neuroscience.