Symmetry in frontal but not motor and somatosensory corticocortical and corticostriatal circuitry

Neocortex and striatum are topographically organized by cortical areas representing sensory and motor functions, where primary cortical areas are generally used as models for other cortical regions. But different cortical areas are specialized for distinct purposes, with sensory and motor areas lateralized for touch and motor control, respectively. Frontal areas are involved in decision making, where lateralization of function may be less important. This study contrasted the topographic precision of ipsilateral and contralateral projections from cortex based on the injection site location. While sensory cortical areas had strongly topographic outputs to ipsilateral cortex and striatum, they were weaker and not as topographically strong to contralateral targets. Motor cortex had somewhat stronger projections, but still relatively weak contralateral topography. In contrast, frontal cortical areas had high degrees of topographic similarity for both ipsilateral and contralateral projections to cortex and striatum. This contralateral connectivity reflects on the pathways in which corticostriatal computations might integrate input outside closed basal ganglia loops, enabling the two hemispheres to act as a single unit and converge on one result in motor planning and decision making. Significance Statement The mammalian brain is divided into two cerebral hemispheres, each responsible for sensation and movement on the opposite side of the body. The two sides communicate via an enormous bundle of midline-crossing fibers, the corpus callosum. The callosal projections mainly target neocortex and striatum. While callosal projections originate from most areas of the neocortex, how the anatomy and function of these projections might vary across motor, sensory, and frontal regions is unknown. In particular, callosal projections are proposed here to play a large role in frontal areas, where maintaining unity across hemispheres in value assessment and decision-making for the whole individual is key, but a smaller role for sensory representations where perceptions from the contralateral body are less informative. ### Competing Interest Statement The authors have declared no competing interest.