Time-invariant reference frames for parietal reach activity.

Neurophysiological studies suggest that the transformation of visual signals into arm movement commands does not involve a sequential recruitment of the various reach-related regions of the cerebral cortex but a largely simultaneous activation of these areas, which form a distributed and recurrent visuomotor network. However, little is known about how the reference frames used to encode reach-related variables in a given "node" of this network vary with the time taken to generate a behavioral response. Here we show that in an instructed delay reaching task, the reference frames used to encode target location in the parietal reach region (PRR) and area 5 of the posterior parietal cortex (PPC) do not evolve dynamically in time; rather the same spatial representation exists within each area from the time target-related information is first instantiated in the network until the moment of movement execution. As previously reported, target location was encoded predominantly in eye coordinates in PRR and in both eye and hand coordinates in area 5. Thus, the different computational stages of the visuomotor transformation for reaching appear to coexist simultaneously in the parietal cortex, which may facilitate the rapid adjustment of trajectories that are a hallmark of skilled reaching behavior.