Anatomical and physiological definition of the motor cortex of the marmoset monkey

We used a combination of anatomical and physiological techniques to define the primary motor cortex (M1) of the marmoset monkey and its relationship to adjacent cortical fields. Area M1, defined as a region containing a representation of the entire body and showing the highest excitability to intracortical microstimulation, is architecturally heterogeneous: it encompasses both the caudal part of the densely myelinated "gigantopyramidal" cortex (field 4) and a lateral region, corresponding to the face representation, which is less myelinated and has smaller layer 5 pyramidal cells (field 4c). Rostral to M1 is a field that is strongly reminiscent of field 4 in terms of cyto- and myeloarchitecture but that in the marmoset is poorly responsive to microstimulation. Anatomical tracing experiments revealed that this rostral field is interconnected with visual areas of the posterior parietal cortex, whereas M1 itself has no such connections. For these reasons, we considered this field to be best described as part of the dorsal premotor cortex and adopted the designation 6Dc. Histological criteria were used to define other fields adjacent to M1, including medial and ventral subdivisions of the premotor cortex (fields 6M and 6V) and the rostral somatosensory field (area 3a), as well as a rostral subdivision of the dorsal premotor area (field 6Dr). These results suggest a basic plan underlying the histological organization of the caudal frontal cortex in different simian species, which has been elaborated during the evolution of larger species of primate by creation of further morphological and functional subdivisions.