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From www.temporal-lobe.com

Introduction

A connectome is a comprehensive description of the network elements and connections that form the brain (Sporns et al., 2005). Such clear and comprehensive knowledge of anatomical connections lies at the basis of understanding network functions (Crick and Koch, 2003). For example, the existence of a connection between two brain regions ascertains that information transfer can occur. Likewise, when inputs from different brain regions converge onto another region, this can be interpreted as an anatomical substrate for information integration (Sporns and Tononi, 2007).

In the connectome featured on this website, the three-dimensional organization of the projection patterns between and within the hippocampal formation, the parahippocampal region and the retrosplenial cortex (see the anatomy primer) are visualized in an interactive connectome. All literature concerning projections of this system for the rat (see References) was collected and reported projections were stored in a database. Over the years, the nomenclature has changed and when necessary reported results were translated into a common frame of reference. Subsequently, the projections were drawn between all subdivisions of the hippocampal formation, the parahippocampal region and the retrosplenial cortex with as much topographical detail as current knowledge permits.

Although many additions are conceivable and in preparation See: Project status, the second version of our interactive connectome will soon become freely available for Download. It displays almost 2600 connections in the rat hippocampal – parahippocampal - retrosplenial region, which can all be interactively switched on and off. In addition, two papers are available. In the first paper, we highlighted the underexposed aspects of the HF-PHR anatomy, together with some of the functional implications of these connections (van Strien, Cappaert, Witter, 2009). In the second paper, we added the retrosplenial cortex to the connectome and discuss the challenges and advantages of connectomes [(Sugar et al, 2011)].

Below, an image of the second version of the diagram is displayed.

Hippocampus Atlas

Note that the colors of the brain areas in this connectome match with those in Oslo University's online Hippocampus atlas. Clicking the image below will navigate you away from this website, to the Hippocampus atlas.

Aims

Converging evidence suggests that encoding, consolidation and retrieval of declarative memories rely on interactions between the hippocampal formation and the parahippocampal region. The retrosplenial cortex is activated in (autobiographical) memory tasks, navigation and prospective thinking (Vann et al., 2009). The cognitive functions in which the retrosplenial cortex is engaged show a striking similarity with the hippocampal formation and the parahippocampal region. The functional relevance of the retrosplenial cortex and the striking overlap with the functional connotations attributed to the hippocampal formation and the parahippocampal region strongly suggests a functional relationship between these areas.

Anatomical studies established that these areas consist of fourteen sub-regions, which form multiple, intricately connected networks. Current functional thinking does not fully take the improved understanding of the circuitry into consideration. Here, we integrate existing knowledge into an interactive connectome in which any connection can be turned on or off at the level of cortical layers, which helps neuroscientists to conceive this complex network.

To provide an overview of all known anatomical projections of the rat hippocampal - parahippocampal – retrosplenial cortex network.
To provide a graphical interface with which users can turn on or off any connections at regional level down to the level of cortical layers.
To make it easy to find references for a particular projection.

Status

We - Natalie Cappaert and Niels van Strien - started our connectome project in an attempt to increase our own understanding of the hippocampal - parahippocampal network. Soon we discovered that creating a connectome of these connections with all its topological detail is a complex task. We started on this project in 2005 and worked together with an expert in the field, prof.dr. Menno P. Witter. In 2009 we published our first version of the connectome in Nature Reviews Neuroscience. In 2011 we moved on and added the connections within the retrosplenial cortex and between the retrosplenial cortex and the hippocampal - parahippocampal network.

The second and current version of the interactive connectome is available as a supplement to our paper in the special topic “Mapping the connectome: Multi-level analysis of brain connectivity” of Frontiers in NeuroInformatics. In this version we added the connections within the retrosplenial cortex and between the retrosplenial cortex and the hippocampal - parahippocampal network. We added a toolbar including a search button to the connectome, which simplifies the exploration of the connectome.

The first version of the connectome is available as a supplement to our review of the parahippocampal - hippocampal anatomy in the April 2009 issue of Nature Reviews Neuroscience [1].

Currently, we are:

Adding anatomical connections of the amygdala with hippocampal and parahippocampal areas.
Restructuring the database of the connections.
Constucting connectomes with the anatomical connections of hippocampal - parahippocampal region of the mouse, monkey and human.