Orderly Somatotopy in Primary Motor Cortex: Does It Exist?

In the current issue of NeuroImage and an upcoming issue of Cerebral Cortex appear data relevant to a fundamental question about the functional organization of the primary motor cortex (M1) of primates (Beisteiner et al., 2001; Hlustik et al., 2001; Indovina and Sanes, 2001), that is, does there exist an orderly somatotopy in M1 and, by extension, in other major motor areas of the brain. A fundamental finding of these papers provides support for separation between representations for finger and hand movement that adheres to a somatotopic organization. The new findings extend previous reports of somatotopically ordered representations for voluntary movements of various joints of the human upper extremity, fingers, wrist, elbow, and shoulder (Grafton et al., 1993; Kleinchmidt et al., 1997; Lotze et al., 2000), but conflict with thers (Rao et al., 1995; Sanes et al., 1995). However, as others and we have noted, the degree of somatotopic representation within the upper extremity representation appears rather limited (Schieber, 1999; Sanes and Donoghue, 2000). Clearly the major body parts—lower limb (hindlimb), upper limb (forelimb), and head—have functional and largely independent subdivisions to represent the muscles and movements controlled by the respective parts of M1. These functional subdivisions of M1 are commonly laid out along the cortical surface of primates with the lower (hind) limb most medial, the head most lateral, and the upper (fore) limb in between; they have acquired the designation of “areas,” such as the “M1 arm area,” though the term “representation” might provide a more suitable functional name. No serious challenge has emerged for this basic large-scale organization pattern in M1, but