information:classification, information:highlighting
Segregated map of the body

One of the most common misconceptions about the primary motor cortex is that the map of the body is cleanly segregated. Yet it is not a map of individuated muscles or even individuated body parts. The map contains considerable overlap. This overlap increases in more anterior regions of the primary motor cortex. One of the main goals in the history of work on the motor cortex was to determine just how much the different body parts are overlapped or segregated in the motor cortex. Researchers who addressed this issue found that the map of the hand, arm, and shoulder contained extensive overlap. Studies that map the precise functional connectivity from cortical neurons to muscles show that even a single neuron in the primary motor cortex can influence the activity of many muscles related to many joints. In experiments on cats and monkeys, as animals learn complex, coordinated movements, the map in the primary motor cortex becomes more overlapping, evidently learning to integrate the control of many muscles. In monkeys, when electrical stimulation is applied to the motor cortex on a behavioral timescale, it evokes complex, highly integrated movements such as reaching with the hand shaped to grasp, or bringing the hand to the mouth and opening the mouth. This type of evidence suggests that the primary motor cortex, while containing a rough map of the body, may participate in integrating muscles in meaningful ways rather than in segregating the control of individual muscle groups. It has been suggested that a deeper principle of organization may be a map of the statistical correlations in the behavioral repertoire, rather than a map of body parts. To the extent that the movement repertoire breaks down partly into the actions of separate body parts, the map contains a rough and overlapping body arrangement.