Understanding effector selectivity in human posterior parietal cortex by combining information patterns and activation measures.

The posterior parietal cortex (PPC) has traditionally been viewed as containing separate regions for the planning of eye and limb movements, but recent neurophysiological and neuroimaging observations show that the degree of effector specificity is limited. This has led to the hypothesis that effector specificity in PPC is part of a more efficient than strictly modular organization, characterized by both distinct and common activations for different effectors. It is unclear, however, what differentiates the distinctions and commonalities in effector representations. Here, we used fMRI in humans to study the cortical representations involved in the planning of eye, hand, and foot movements. We used a novel combination of fMRI measures to assess the effector-related representational content of the PPC: a multivariate information measure, reflecting whether representations were distinct or common across effectors and a univariate activation measure, indicating which representations were actively involved in movement preparation. Active distinct representations were evident in areas previously reported to be effector specific: eye specificity in the posterior intraparietal sulcus (IPS), hand tuning in anterior IPS, and a foot bias in the anterior precuneus. Crucially, PPC regions responding to a particular effector also contained an active representation common across the other two effectors. We infer that rostral PPC areas do not code single effectors, but rather dichotomies of effectors. Such combinations of representations could be well suited for active effector selection, efficiently coding both a selected effector and its alternatives.