NEURONAL CONNECTIONS OF ORBITAL CORTEX IN RATS - TOPOGRAPHY OF CORTICAL AND THALAMIC AFFERENTS

The cortical acid thalamic afferent connections of rat orbital cortex were investigated using fluorescent retrograde axonal tracers, Each of the four orbital areas has a distinct pattern of connections. Cortico cortical connections involving the ventral and ventrolateral orbital a reas are more extensive than those of the medial and lateral orbital a reas. The medial orbital area has cortical connections with the cingul ate, medial agranular (Fr2) and posterior parietal (PPC) cortices. The ventral orbital area has connections with the cingulate area, area Fr 2, secondary somatic sensory area Par2. PPC, and visual areas Oc2M and Oc2L. The ventrolateral orbital area (VLO) receives cortical input fr om insular cortex. area Fr2, somatic sensory areas Par1 and Par2, PPC and Oc2L. The lateral orbital area has cortical connections limited to the agranular and granular insular areas, and Par2. Thalamic afferent s to the four orbital fields are also topographically organized, and a re focused in the submedial and mediodorsal nuclei, The ventrolateral orbital area receives input from the entirety of the submedial nucleus , whereas the other orbital areas receive input from its periphery onl y. Each orbital area is connected with a particular segment of the med iodorsal nucleus. The medial orbital area receives its principal thala mic afferents from the parataenial nucleus, the dorsocentral portion o f the mediodorsal nucleus, and the ventromedial portion of the submedi al nucleus. The ventral orbital area receives input from the lateral s egment of the mediodorsal nucleus, the rostromedial portion of the sub medial nucleus, and the central lateral nucleus, Thalamic afferents to the ventrolateral orbital area arise from the entirety of the submedi al nucleus and from the lateral segment of the mediodorsal nucleus. Th e lateral orbital area receives thalamic afferents from the central se gment of the mediodorsal nucleus, the ventral portion of the submedial nucleus, and the ventromedial nucleus. The paraventricular, ventromed ial. rhomboid and reuniens nuclei also provide additional input to the four orbital areas. The connections of the ventrolateral orbital area are interpreted in the context of its role in directed attention and allocentric spatial localization. The present findings provide anatomi cal support for the view that areas Fr2, PPC and VLO comprise a cortic al network mediating such functions.