Somatotopy in human primary motor and somatosensory hand representations revisited

High-resolution functional magnetic resonance imaging of healthy volunteers was used to study the functional anatomy of the human primary motor (M1) a nd somatosensory (S1) cortical hand representations during simple movements of thumb, little finger and wrist and a sequential movement of the middle three fingers. Rest served as a control state. The results demonstrated an orderly somatotopy in both M1 and S1, even though the cortical areas active with individual movements significantly overlapped. Moreover, the activati on patterns in M1 and S1 differed in three aspects: (i) S1 activation was d istributed into significantly more clusters than M1 and the primary cluster was smaller; (ii) the overlaps of areas active with different movements we re significantly larger in M1 than in S1; (iii) the difference between the three-finger sequential movement and the single-finger movements was more p ronounced in S1 than in M1. The sequence-activated SI cortex was distribute d into significantly more clusters. There was also a trend for a bigger vol ume difference between sequence and the single finger movements in S1 than M1. These data suggest that while the distributed character dominates in M1 and S1, a somatotopic arrangement exists for both M1 and S1 hand represent ations, with the S1 somatotopy being more discrete and segregated, in contr ast to the more integrated and overlapping somatotopy in M1.