<P> A second modification of the classical somatotopic ordering of body parts is a double representation of the digits and wrist studied mainly in the human motor cortex . One representation lies in a posterior region called area 4p, and the other lies in an anterior region called area 4a . The posterior area can be activated by attention without any sensory feedback and has been suggested to be important for initiation of movements, while the anterior area is dependent on sensory feedback . It can also be activated by imaginary finger movements and listening to speech while making no actual movements . This anterior representation area has been suggested to be important in executing movements involving complex sensoriomotor interactions . It is possible that area 4a in humans corresponds to some parts of the caudal premotor cortex as described in the monkey cortex . </P> <P> In 2009, it was reported, that there are two evolutionary distinct regions, an older one on the outer surface, and a new one found in the cleft . The older one connects to the spinal motorneurons through interneurons in the spinal cord . The newer one, found only in monkeys and apes, connects directly to the spinal motorneurons . The direct connections form after birth, are dominant over the indirect connections, and are more flexible in the circuits they can develop which allows the post-natal learning of complex fine motor skills . "The emergence of the' new' M1 region during evolution of the primate lineage is therefore likely to have been important for the enhanced manual dexterity of the human hand ." </P> <P> Certain misconceptions about the primary motor cortex are common in secondary reviews, textbooks, and popular material . Three of the more common misconceptions are listed here . </P> <P> 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 . </P>

Damage to the cortex of the left cerebral hemisphere