Allocentric representations for target memory and reaching in human cortex.

The use of allocentric cues for movement guidance is complex because it involves the integration of visual targets and independent landmarks and the conversion of this information into egocentric commands for action. Here, we focus on the mechanisms for encoding reach targets relative to visual landmarks in humans. First, we consider the behavioral results suggesting that both of these cues influence target memory, but are then transformed-at the first opportunity-into egocentric commands for action. We then consider the cortical mechanisms for these behaviors. We discuss different allocentric versus egocentric mechanisms for coding of target directional selectivity in memory (inferior temporal gyrus versus superior occipital gyrus) and distinguish these mechanisms from parieto-frontal activation for planning egocentric direction of actual reach movements. Then, we consider where and how the former allocentric representations of remembered reach targets are converted into the latter egocentric plans. In particular, our recent neuroimaging study suggests that four areas in the parietal and frontal cortex (right precuneus, bilateral dorsal premotor cortex, and right presupplementary area) participate in this allo-to-ego conversion. Finally, we provide a functional overview describing how and why egocentric and landmark-centered representations are segregated early in the visual system, but then reintegrated in the parieto-frontal cortex for action.