Abstract
It has been proposed that posterior parietal cortex (PPC) is characterized by an effector-specific organization. However, strikingly similar fMRI activation patterns have been found in PPC for hand and foot movements. Because the fMRI signal is related to average neuronal activity, similar activation levels may result either from effector-unspecific neurons, or from intermingled subsets of effector-specific neurons within a voxel. We distinguished between these possibilities using fMRI repetition suppression (RS). Participants made delayed, goal-directed eye, hand, and foot movements to visual targets. In each trial, the instructed effector was identical or different to that of the previous trial. RS effects indicated an attenuation of the fMRI signal in repeat trials. Caudal PPC was active during the delay, but did not show RS, suggesting that its planning activity was effector-independent. Hand and foot-specific RS effects were evident in anterior superior parietal lobule (SPL), extending to premotor cortex, with limb overlap in anterior SPL. Connectivity analysis suggested information flow between caudal PPC to limb-specific anterior SPL regions, and between limb-unspecific anterior SPL toward limb-specific motor regions. These results underline that both function and effector-specificity should be integrated into a concept of PPC action representation not only on a regional, but also on a fine-grained, sub-voxel level.
