Functional Rather than Effector-Specific Organization of Human Posterior Parietal Cortex

Neurophysiological and neuroimaging studies have shown that the posterior parietal cortex (PPC) distinguishes between the planning of eye and hand movements. This distinction has usually been interpreted as evidence for a modular, effector-specific organization of this cerebral region. However, the eyes differ markedly from other body parts both in terms of their functional purpose and with regard to the spatial transformations required to plan goal-directed movements. PPC may therefore provide specialized subregions for eye movements, but distinguish less for other effectors. Using functional magnetic resonance imaging, we compared activity during memory-guided eye, hand, and foot movements in human participants. The results did not reveal any significant activation differences during the planning of hand and foot movements, except in the most anterior part of PPC [Brodmann's area (BA) 5], marginally extending into anterior BA 7/40. This region showed a lateral-to-medial gradient for hand versus foot movement planning. The limb-unspecific PPC regions were functionally connected with hand and foot motor regions. In contrast, a gradient-like organization was found for all of PPC for the planning of eye versus hand and foot movements. Although planning-related activity across the three effectors considerably overlapped, saccade planning activated occipitoparietal regions more than limb movements, whereas limb movements activated anterior regions of the superior parietal lobule more than saccades. We infer that PPC does not follow a strict effector-specific organization. Rather, the large-scale organization of this region might reflect the different computational constraints that need to be satisfied when planning eye and limb movements.