Connectivity of the Superior Temporal Polysensory Area With the Presubiculum, Parasubiculum, Entorhinal Cortex, and Claustrum–Endopiriform Complex in the Common Marmoset ( <i>Callithrix jacchus</i> )

The rodent hippocampal-presubicular-entorhinal circuit is thought to be a key part of the circuitry involved in memory formation. In primates, additional connections are believed to enable more complex and higher-order memory information processing. We investigated whether such additional connections exist in the cortical regions adjacent to the hippocampal formation using a tracer injection method. We discovered that the marmoset parasubiculum (ParS), presubiculum (PreS) (distal portion), and entorhinal cortex (EC) (proximal or medial portion) have strong reciprocal connections with the superior temporal polysensory area (STP), which is known as a well-developed brain region in primates and where projections from diverse sensory areas converge. When the cholera toxin B subunit (CTB), a bidirectional tracer, was injected into various portions of ParS, PreS, and EC in one hemisphere, anterograde and retrograde labeling were observed in STP in both hemispheres. When CTB was injected into STP on one side, many retrogradely labeled cells were observed in layers III-V of ParS on the same side as the injection, and anterogradely labeled axons and terminal boutons were observed in layers I-III and V of ParS on both sides. In the distal portion of PreS and proximal portion of EC that were adjacent to ParS, retrogradely labeled cells were mostly observed in layer V, and substantial anterograde labeling was observed in all layers. Both in cases of STP injection and PreS (distal)-ParS-EC (proximal) injection, substantial anterograde and retrograde labeling were found in the claustrum-endopiriform complex (Cl-En). These results suggest the existence of a neural circuit that reciprocally connects STP, Cl-En, and PreS-ParS-EC, and may support functions such as higher-order multisensory memory consolidation.