PRINCIPAL SULCUS CONTRA IPSI CONTRA IPS b WM CORPUS CALLOSUM▲ CORTICO-THALAMIC of the a ps in 、of mod in th mitive white erPRINCIPAL SULCUS CONTRA IP S1 CONTRA IP S1 CORTICAL-STRIATAL CQRTICQ-THALAMIC FIG. 3.—Simplified diagram of the modular architecture of the principal sulcus with particular refer￾ence to the location of callosal (black triangles) and associational (white) neurons that give rise to axons destined for cortical areas in die opposite (contralateral) and same (ipsilateral) hemisphere, respectively; stippling represents the tenninals of callosal (darker) and associational (lighter) reciprocal prqjectiGns to tiie principal sulcus. The major points illustrated are: (a) both types of corticocortical neurons reside in the same layers of cortex (III and V); (b) columns of same-hemisphere inputs altemate with columns of opposite-hemisphere inputs; and (c) columns of high-density callosal neurons receive callosal inputs; columns with hi^-density associational neurons receive associational inputs. Thus we may view the prefrontal cortex as made up of modules that are devoted primarily to eidier intrahemispheric or in￾terhemisiAeric processing. Integration of the information processed in the two types of modules is inte￾grated at another level and undoubtedly underlies the integration of the two hemispheres. The precise organization of die cortico-cortical connections in die principal sulcus encourages belief that cognitive fimction can ultimately be understood at a synaptic and molecular level. Roman numerals indicate layers; WM, white matter