Connectional networks within the orbital and medial prefrontal cortex of macaque monkeys.

The intrinsic cortico-cortical connections within the orbital and medial prefrontal cortex (OMPFC) were demonstrated with retrograde and anterograde tracers injected into each of the architectonic areas that constitute this region. Although many of the connections linked neighboring areas, others selectively connected relatively distant areas. Most, but not all, of the connections were reciprocal. Altogether, the connections formed at least two distinct networks within the OMPFC. The "orbital" prefrontal network linked most of the areas within the orbital cortex, with very few connections to medial prefrontal areas. Areas Iam, Iapm, Ial, 12l, 12m, and 12r in the caudal and lateral parts of the orbital cortex (which received inputs from several sensory modalities) had convergent connections with areas 13l, 13m, and 13b in the central orbital cortex, with further connections to the rostral orbital area 11l. For the connections between areas Iapm, Iam, Ial, 13m, 13l, and 11l, rostrally directed fibers arose mainly in layer V, whereas caudally directed fibers originated mainly in layer III. The "medial" prefrontal network selectively involved medial areas 14r, 14c, 24, 25, 32, and 10m, rostral orbital areas 10o and 11m, and agranular insular area Iai in the posterior orbital cortex. Two orbital areas, 13a and 12o, had substantial connections to both networks and may serve as points of interaction between them; otherwise there were relatively few interconnections. The two networks also had distinct connections with other cortical regions, with limbic structures, and with the mediodorsal thalamic nucleus. Their role in guidance of affective behaviour is discussed.