Cortical efferents of the entorhinal cortex and the adjacent parahippocampal region in the monkey (Macaca fascicularis)

Abstract

Entorhinal cortex (EC) relays information from the hippocampus to the cerebral cortex. The origin of this entorhino-cortical pathway was studied semiquantitatively and topographically with the use of 23 retrograde tracer injections in cortical areas of the frontal, temporal, and parietal lobes of the monkey. To assess possible alternative, parallel pathways, the parahippocampal region, comprised of temporal pole (TP), perirhinal (PRC), and posterior parahippocampal cortices (PPH), was included in the study. The majority of the cortical areas receive convergent projections from EC and the parahippocampal region. Strong EC layer V output is directed to temporal pole, medial frontal and orbitofrontal cortices, and the rostral part of the polysensory area of the superior temporal sulcus (sts). Moderate EC output is directed to the caudal superior temporal gyrus, area TE, and parietal cortex, and little to none to the lateral frontal cortex. With the exception of the projection to the medial frontal cortex, output from TP, PRC, and PPH surpassed that from EC, although with regional differences. TP layers II-III, V-VI project strongly to all areas injected except parietal cortex and caudal superior temporal gyrus, while PRC layers III/V-VI send strong projections to rostral parts of area TE and sts. PPH layers III/V-VI project heavily to parietal cortex and caudal superior temporal gyrus. These results suggest that the medial temporal output is primarily organized hierarchically, but at the same time, it has multiple exits of information. These parallel, alternative routes may influence local circuitry in the cerebral cortex and participate in the consolidation of declarative memory.

Shift from Hippocampal to Neocortical Centered Retrieval Network with Consolidation

“…Given the anatomical connectivity of the hippocampus to a wide range of areas in the brain (Suzuki and Eichenbaum, 2000;Muñoz and Insausti, 2005), it can be regarded as being in a highly favorable position to bind distributed neocortical nodes together during encoding and to index these nodes during retrieval. Moreover, due to high neuronal plasticity in the hippocampus, and as a consequence, even a single exposure to a stimulus changes its subsequent processing and enables later memory retrieval (Grunwald et al, 1998).…”

Working Memory for Social Cues Recruits Orbitofrontal Cortex and Amygdala: A Functional Magnetic Resonance Imaging Study of Delayed Matching to Sample for Emotional Expressions

“…Previous fMRI data has demonstrated a role for MTL structures, including the hippocampus and surrounding parahippocampal cortex, in the short term maintenance of novel information (Ranganath and D'Esposito, 2001;Stern et al, 2001;Ranganath et al, 2003Ranganath et al, , 2005Schon et al, 2004;Schon et al, 2005). Based on the rich connections between the hippocampus, amygdala, and OFC (Barbas, 2000;Insausti and Munoz, 2001;Munoz and Insausti, 2005), we suggest that together these areas form a circuit that binds together and actively maintains emotion and identity information in WM.…”

Does lateral parietal cortex support episodic memory?

“…Lateral posterior parietal cortex has reciprocal connections with entorhinal, parahippocampal, and hippocampal regions of the medial temporal lobe (Blatt et al, 2003;Clower et al, 2001;Insausti & Munoz, 2001;Laveneux et al, 2002;Munoz & Insausti, 2005;Rockland & Van Hoesen, 1999;Suzuki & Amaral, 1994), as well as with the medial parietal region (Kobayashi & Amaral, 2003;Morris et al, 1999b). It is also connected to anterior cingulate and dorsolateral prefrontal cortex, in particular BAs 6, 8, and 46 (Cavada & Goldman-Rakic, 1989;Lewis & Van Essen, 2000;Petrides & Pandya, 1984;Petrides & Pandya, 1999).…”