The role of the medial wall and its anatomical variations for bimanual antiphase and in-phase movements

The medial wall of the frontal cortex is thought to play an important role for bimanual coordination. However, there is uncertainty regarding the exact neuroanatomical regions involved. We compared the activation patterns related to bimanual movements using functional magnetic resonance imaging in 12 healthy right-handed subjects, paying special attention to the anatomical variability of the frontal medial wall. The subjects performed unimanual right and left and bimanual antiphase and in-phase flexion and extension movements of the index finger. Activation of the right supplementary motor area (SMA) proper, right and left caudal cingulate motor area (CMA), and right and left premotor cortices was significantly stronger during bimanual antiphase than bimanual in-phase movements, indicating an important function of these areas with bimanual coordination. A frequent anatomical variation is the presence of the paracingulate sulcus (PCS), which might be an anatomical landmark to determine the location of activated areas. Seven subjects had a bilateral, three a unilateral right, and two a unilateral left PCS. Because the area around the PCS is functionally closer coupled to the CMA than to the SMA, activation found in the area around the PCS should be attributed to the CMA. With anatomical variations such as the presence of a PCS or a vertical branch of the cingulate sulcus, normalization and determination of the activation with the help of stereotaxic coordinates can cause an incorrect shift of CMA activation to the SMA. This might explain some of the discrepancies found in previous studies.