Introduction

Why this project was started

Many researchers that study the hippocampus have made their own attempt at drawing a diagram of the connections in the hippocampus. When attempting this, one soon discovers that it is challenging to create a diagram that accurately resembles the literature. We - Natalie Cappaert and Niels van Strien - too started this project as an attempt to increase our own understanding of the network. Soon we discovered that creating a diagram of the hippocampal - parahippocampal connections with all its topological detail is indeed a complex task. We started in 2005 and worked together with an expert in the field, prof.dr. Menno P. Witter. Although many additions are conceivable, the first version of our interactive diagram is now finalized and available for download. It displays almost 1600 connections in the rat hippocampal - parahippocampal region, which can all be interactively switched on and off. In addition, a Nature Reviews Neuroscience paper is available in which the anatomy is reviewed with an emphasis on underexposed connections in the network, and some of the functional implications of these connections.

In order to comprehend the unique functional contribution of each parahippocampal - hippocampal sub-region, detailed anatomical knowledge is of pivotal importance. In the diagram featured on this website, we attempted to visualize the three-dimensional organization of the projection patterns between and within the parahippocampal region and hippocampal formation. 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 using this table). Subsequently, almost 1600 projections were drawn between all subdivisions of the HF and PHR with as much topographical detail as current knowledge permits.

Below, an image of the diagram is displayed. Clicking on the image will show a non-interactive enlargement of the image.

Project Aims

Converging evidence suggests that encoding, consolidation and retrieval of declarative memories rely on interactions between the hippocampal formation and the parahippocampal region. Anatomical studies established that these areas consist of eight sub-regions, which form multiple, intricately connected networks. Current functional thinking does not fully take the improved under-standing of the circuitry into consideration. Here, we integrate existing knowledge into an interactive diagram 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 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.

Project Status

• The diagram is now available as a supplement to our review of the parahippocampal - hippocampal anatomy in the April 2009 issue of Nature Reviews Neuroscience.
• Updated versions of the diagram will be offered for download through this website.

Future directions

• Add anatomical connections of the amygdala with hippocampal and parahippocampal areas.
• Add anatomical connections of hippocampal - parahippocampal region and amaygdala of the mouse, cat, ferret and monkey.
• Add physiologically proven connections.
• Add contralateral projections.