Studies in brain damaged patients conclude that the left hemisphere is dominant for controlling heterogeneous sequences performed by either hand, presumably due to the cognitive resources involved in planning complex sequential movements. To determine if this lateralized effect is due to asymmetries in primary sensorimotor or association cortex, whole-brain functional magnetic resonance imaging was used to measure differences in volume of activation while healthy right-handed subjects performed repetitive (simple) or heterogeneous (complex) finger sequences using the right or left hand. Advanced planning, as evidenced by reaction time to the first key press, was greater for the complex than simple sequences and for the left than right hand. In addition to the expected greater contralateral activation in the sensorimotor cortex (SMC), greater left hemisphere activation was observed for left, relative to right, hand movements in the ipsilateral left superior parietal area and for complex, relative to simple, sequences in the left premotor and parietal cortex, left thalamus, and bilateral cerebellum. No such volumetric asymmetries were observed in the SMC. Whereas the overall MR signal intensity was greater in the left than right SMC, the extent of this asymmetry did not vary with hand or complexity level. In contrast, signal intensity in the parietal and premotor cortex was greater in the left than right hemisphere and for the complex than simple sequences. Signal intensity in the caudal anterior cerebellum was greater bilaterally for the complex than simple sequences. These findings suggest that activity in the SMC is associated with execution requirements shared by the simple and complex sequences independent of their differential cognitive requirements. In contrast, consistent with data in brain damaged patients, the left dorsal premotor and parietal areas are engaged when advanced planning is required to perform complex motor sequences that require selection of different effectors and abstract organization of the sequence, regardless of the performing hand.

The ease by which movements are combined into skilled actions depends on many factors, including the complexity of movement sequences. Complexity can be defined by the surface structure of a sequence, including motoric properties such as the types of effectors, and by the abstract or sequence-specific 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 properties of sequential actions. We investigated this question using whole-brain 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 lengthen the time to plan sequences before movement: the number of different fingers (1-3; surface structure) and the number of finger transitions (0-4; sequence-specific structure). The results showed that systems involved in visual processing (extrastriate cortex) and the preparation of sensory aspects of movement (rostral inferior parietal and ventral premotor cortex (PMv)) correlated with both properties of sequence structure. The number of different fingers positively correlated with activation intensity in the cerebellum and superior parietal cortex (anterior), systems associated with sensorimotor, and kinematic representations of movement, respectively. The number of finger transitions correlated with activation in systems previously associated with sequence-specific processing, including the inferior parietal and the dorsal premotor cortex (PMd), and in interconnecting superior temporal-middle frontal gyrus networks. Different patterns of activation in the left and right inferior parietal cortex were associated with different sequences, consistent with the speculation that sequences are encoded using different mnemonics, depending on the sequence-specific structure. In contrast, PMd activation correlated positively with increases in the number of transitions, consistent with the role of this area in the retrieval or preparation of abstract action plans. These findings suggest that the surface and the sequence-specific structure of sequential movements can be distinguished by distinct distributed systems that support their underlying mental operations.