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The characteristic movement of a musician's hand is captured in a painting by Giacomo Balla from 1912, "The Hand of the Violinist". Balla studied violin as a boy and, like his contemporaries in the Futurist movement, was interested in depicting motion and speed. The rhythmic brushstrokes evoke the energy of the performer and the vibrations of the music. (Reproduced, with permission, from the Copyright 2011 Artists Rights Society (ARS), New York/SIAE, Rome; and the Bridgeman Art Library International, NY.)
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The capacity for movement, as many dictionaries remind us, is a defining feature of animal life. As Sherrington, who pioneered the study of the motor system pointed out, "to move things is all that mankind can do, for such the sole executant is muscle, whether in whispering a syllable or in felling a forest."*
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The immense repertoire of motions that humans are capable of stems from the activity of some 640 skeletal muscles—all under the control of the central nervous system. After processing sensory information about the body and its surroundings, the motor centers of the brain and spinal cord issue neural commands that effect coordinated, purposeful movements.
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The task of the motor systems is the reverse of the task of the sensory systems. Sensory processing generates an internal representation in the brain of the outside world or of the state of the body. Motor processing begins with an internal representation: the desired purpose of movement. Critically, however, this internal representation needs to be continuously updated by internal (efference copy) and external sensory information to maintain accuracy as the movement unfolds.
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Just as psychophysical analysis of sensory processing tells us about the capabilities and limitations of the sensory systems, psychophysical analyses of motor performance reveal regularities and invariances in the control rules used by the motor system.
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Because many of the motor acts of daily life are unconscious, we are often unaware of their complexity. Simply standing upright, for example, requires continual adjustments of numerous postural muscles in response to the vestibular signals evoked by miniscule swaying. Walking, running, and other forms of locomotion involve the combined action of central pattern generators, gated sensory information, and descending commands, which together generate the complex patterns of alternating excitation and inhibition to the appropriate sets of muscles. Many actions, such as serving a tennis ball or executing an arpeggio on a piano, occur far too quickly to be shaped by sensory feedback. Instead, centers, such as the cerebellum, make use of predictive models that simulate the consequences of the outgoing commands and allow very short latency corrections. Motor learning provides one of the most fruitful subjects for studies of neural plasticity.
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Motor systems are organized in a functional hierarchy, with each level concerned with a different decision. The highest and most abstract level, likely requiring the prefrontal cortex, deals with the purpose ...