Ann Martin Graybiel (born 1942) is an Institute Professor and a faculty member in the Department of Brain and Cognitive Sciences at the Massachusetts Institute of Technology. She is also an investigator at the McGovern Institute for Brain Research. She is an expert on the basal ganglia and the neurophysiology of habit formation, and her work is relevant to Parkinson's disease, Huntington's disease, obsessive–compulsive disorder, substance abuse and other disorders that affect the basal ganglia.
For much of her career, Graybiel has focused on the physiology of the striatum, a basal ganglia structure implicated in the control of movement, cognition, habit formation, and decision-making. In the late 1970s, Graybiel discovered that while striatal neurons appeared to be an amorphous mass, they were in fact organized into chemical compartments, which she termed striosomes. Later research revealed links between striosomal abnormalities and neurological disorders, such as mood dysfunction in Huntington's disease and depletion of dopamine in Parkinson's disease.
Graybiel's subsequent research demonstrated how modular organization of the striatum relates to cognition, learning, and habit formation. She found that neurons project from areas in the sensory and motor cortices governing the same body part and cluster together in the striatum, forming matrisomes. Graybiel went on to show that matrisomes exist for each body part and were organized into loops connecting the neocortex, a region responsible for cognition, perception and motor control, to the brain stem, a region coordinating movement. Studies of rodents and primates revealed that matrisomes were crucial to habit formation.
In later work, Graybiel demonstrated, first in the striatum and later in the infralimbic cortex, that a task-bracket or “chunking” pattern of neuronal activity emerges when a habit is formed, wherein neurons activate when a habitual task is initiated, show little activity during the task, and reactivate when the task is completed.
In more recent work, Graybiel has focused on identifying specific pathways underlying aspects of behavior such as habit formation, learning and cognition, and decision-making, including being the first to analyze the effect of dopamine depletion on the activity of neurons affected by Parkinson's disease during behavioral tasks.
Graybiel majored in biology and chemistry at Harvard University, receiving her bachelor's degree in 1964. After receiving an MA in biology from Tufts University in 1966, she began doctoral study in psychology and brain science at MIT under the direction of Hans-Lukas Teuber and Walle Nauta. She received her PhD in 1971 and joined the MIT faculty in 1973.
In 1994, she was named the Walter A. Rosenblith Professor Neuroscience in the Department of Brain and Cognitive Science and was named an Investigator at the MIT McGovern Institute for Brain research in 2001. She was named Institute Professor in 2008.
In 2001, Graybiel was awarded the President's National Medal of Science for "her pioneering contributions to the understanding of the anatomy and physiology of the brain, including the structure, chemistry, and function of the pathways subserving thought and movement." In 2012, she was awarded the Kavli Prize in Neuroscience, along with Cornelia Bargmann and Winfried Denk, "for elucidating basic neuronal mechanisms underlying perception and decision." 
She is a member of the US National Academy of Sciences, the American Academy of Arts and Sciences, the American Philosophical Society, and the National Academy of Medicine (formerly Institute of Medicine).
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