Single cortical neurones have axon collaterals to ipsilateral and contralateral cortex in fetal and adult primates

In the adult brain, cortical neurones that project to other cortical areas have traditionally been classified as either ‘associational’ (having connections within the ipsilateral hemisphere) or ‘callosal’ (projecting to areas in the contralateral hemisphere). Using double-labelling with fluorescent dyes1,2, we have now identified a novel category of mature cortical neurone that has both an ‘associational’ and a ‘callosal’ axon. Such neurones can direct their callosal axons either to a cytoarchitectonic area homotopic to the one in which their cell bodies reside or to an entirely different heterotopic region in the contralateral hemisphere. These cortical neurones with divergent axon collaterals in the adult neocortex differ from recently described neurones that have two axons only transiently during development3–5.