We used electrical microstimulation to study the organization of motor cortex in awake monkeys. Stimulation on a behaviourally relevant time scale (0.5-1 s) evoked coordinated, complex postures that involved many joints. For example, stimulation of one site caused the mouth to open and also caused the hand to shape into a grip posture and move to the mouth. Stimulation of this site always drove the joints towards this final posture, regardless of the initial posture. Stimulation of other cortical sites evoked different postures. Postures that involved the arm were arranged across cortex to form a map of hand positions around the body. This map encompassed both primary motor and lateral premotor cortex. Primary motor cortex appears to represent the central part of the workspace, where monkeys most often manipulate objects with their fingers. These findings suggest that primary motor and lateral premotor cortex might not be arranged in a hierarchy, but instead might operate in parallel, serving different parts of the workspace.
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