Pli de passage fronto-pariétal moyen of broca separates the motor homunculus

The complete form of the pli de passage fronto-pariétal moyen of Broca, a gyrus connecting the pre- and postcentral gyri at the level of the presumable primary motor (M1) hand area, represents a rare anatomic variation. By using functional MR imaging in a healthy subject incidentally found to harbor this configuration, we attempted to determine whether such an accessory gyrus would be functionally active and what effect it has on the complex somatotopic within-arm organization of M1. We found a specific and consistent activation pattern along the lateral and medial cortical boundaries of the pli de passage fronto-pariétal moyen. The gyrus completely segregated the M1 finger from the M1 elbow representation, one being laterally and the other medially located. Furthermore the M1 wrist representation was consistently split by the pli de passage fronto-pariétal moyen into a medial and lateral activation cluster. These findings demonstrate that this accessory gyrus not only contains functionally active neurons, but also leads to a functional separation of the motor homunculus at the level of the M1 wrist representation. This is a remarkable finding, because the region of within-arm representations in M1 was previously thought to be necessarily organized in a complex and intermingled fashion, without a topographic segregation between single body parts.