Serial organization of human behavior in the inferior parietal cortex.

The parietal cortex is involved in a wide range of cognitive functions in humans including associative functions between multiple sensorimotor spaces, attentional control, and working memory. Little is known, however, about the role and the functional organization of the parietal cortex in action planning and sequential cognition. Moreover, the respective contributions of parietal and frontal regions to action planning remains poorly understood. To address this issue, we designed a functional magnetic resonance imaging protocol requiring subjects to perform overlearned sequences of motor acts and sequences of cognitive tasks. The results reveal only a single bilateral region in the cerebral cortex located in the intraparietal sulcus (IPS; Brodmann's area 40) exhibiting sustained activations during the execution of both motor and task sequences. Additional analyses of phasic activations during sequence execution further suggest a functional dissociation between the left IPS, involved in representing and processing the abstract serial structure of ongoing behavioral sequences regardless of their hierarchical structure, and the right IPS, involved in preparing successive sensorimotor sets that compose such behavioral sequences. We show that this parietal system functionally differs from the frontal system that was previously identified as controlling action selection with respect to the hierarchical rather than serial structure of behavioral plans. Thus, our results reveal the central role of the bilateral intraparietal sulcus in high-order sequential cognition and suggest a major functional segregation within the frontoparietal network mediating action planning, with the frontal and parietal sector involved in processing the hierarchical and serial structure of action plans, respectively.