Specialized neural systems underlying representations of sequential movements

The ease by which movements are combined into skilled actions depends on ma ny factors, including the complexity of movement sequences. Complexity can be defined by the surface structure of a sequence, including motoric proper ties such as the types of effecters, and by the abstract or sequence-specif ic structure, which is apparent in the relations amongst movements, such as repetitions. It is not known whether different neural systems support the cognitive and the sensorimotor processes underlying different structural pr operties of sequential actions. We investigated this question using whole-b rain functional magnetic resonance imaging (FMRI) in healthy adults as they performed sequences of five key presses involving up to three fingers. The structure of sequences was defined by two factors that independently lengt hen the time to plan sequences before movement: the number of different fin gers (1-3; surface structure) and the number of finger transitions (0-4; se quence-specific structure). The results showed that systems involved in vis ual processing (extrastriate cortex) and the preparation of sensor aspects of movement (rostral inferior parietal and, ventral premotor cortex (PMv)) correlated with both properties of sequence structure. The number of differ ent fingers positively correlated with activation intensity in the cerebell um and superior parietal cortex (anterior), systems associated with sensori motor, and kinematic representations of movement, respectively. The number of finger transitions correlated with activation in systems previously asso ciated with sequence-specific processing, including the inferior parietal a nd the dorsal premotor cortex (PMd), and in interconnecting superior tempor al-middle frontal gyrus networks. Different patterns of activation in the l eft and right inferior parietal cortex were associated with different seque nces, consistent with the speculation that sequences are encoded using diff erent mnemonics, depending on the sequence-specific structure. In contrast, PMd activation correlated positively with increases in the number of trans itions, consistent with the role of this area, in the retrieval or preparat ion of abstract action plans. These findings suggest that the surface and t he sequence-specific structure of sequential movements can be distinguished by distinct distributed systems that support their underlying mental opera tions.