A causal role for the posterior corpus callosum in bimanual coordination.

Interareal communication is crucial for coordinated brain function. Given the largely contralateral organization of the brain, bimanual coordination likely depends on interactions across the two cerebral hemispheres for motor planning and execution. The parietal reach region (PRR) is an early node in the sensorimotor transformation stream. Here, we examined the contributions of direct callosal connections between left and right PRR to bimanual coordination. Using manganese-enhanced MRI, we traced callosal pathways crossing the midline and found that PRR-PRR connections are restricted to the splenium. We then temporarily blocked these fibers with lidocaine while measuring behavioral performance and interhemispheric coherence. Blockade selectively reduced PRR-PRR coherence during bimanual movements to a common target, but not during movements to separate targets. Behaviorally, blockade sped movement initiation across tasks, consistent with a functionally inhibitory influence of interhemispheric communication, reduced the temporal synchrony of bimanual movements to a common target, and reduced errors for bimanual movements to separate targets. These findings provide causal evidence that posterior callosal communication supports the temporal precision of spatially coordinated bimanual actions, while potentially constraining independent limb control.