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Rodent fear-learning models posit that amygdala–infralimbic connections facilitate extinction while amygdala–prelimbic prefrontal connections mediate fear expression. Analogous amygdala–prefrontal circuitry between rodents and primates is not established. Using paired small volumes of neural tracers injected into the perigenual anterior cingulate cortex (pgACC; areas 24b and 32; a potential homologue to rodent prelimbic cortex) and subgenual anterior cingulate cortex (sgACC, areas 25 and 14c; a potential homologue to rodent infralimbic cortex) in a single hemisphere, we mapped amygdala projections to the pgACC and sgACC within single subjects. All injections resulted in dense retrograde labeling specifically within the intermediate division of the basal nucleus (Bi) and the magnocellular division of the accessory basal nucleus (ABmc). Areal analysis revealed a bias for connectivity with the sgACC, with the ABmc showing a greater bias than the Bi. Double fluorescence analysis revealed that sgACC and pgACC projections were intermingled within the Bi and ABmc, where a proportion were double labeled. We conclude that amygdala inputs to the ACC largely originate from the Bi and ABmc, preferentially connect to the sgACC, and that a subset collaterally project to both sgACC and pgACC. These findings advance our understanding of fear extinction and fear expression circuitry across species.

A substantial amount of research has focused on the midbrain dopamine system and its role in emotional and motivational behaviors. In diseases in which dopamine function is compromised, patients exhibit a constellation of symptoms, suggesting that the dopamine system plays an important role in the integration of several functions. Subgroups of dopamine neurons receive information from limbic and association areas and project widely throughout cortex and striatum, including motor areas. A dorsal tier of dopamine neurons receive input from the ventral (limbic-related) striatum and from the amygdala and project widely throughout cortex. A more ventrally located group of dopamine cells receives input from both the limbic and association areas of striatum and projects widely throughout the striatum, including the sensorimotor regions. Through these projections, the limbic system has an enormous influence on dopamine output and can therefore affect the emotional and motivational "coloring" of a wide range of behaviors.