Functional model:Searching for the place of memory storage

RUB neuroscientists present new model for memory formation in the hippocampus

he hippocampus plays a very special role in the process of memory formation. For decades researchers have been trying to find out which regions of the hippocampus participate in this process and what their functions are. Torsten Neher, Prof. Dr. Sen Cheng and Prof. Dr. Laurenz Wiskott, neuroscientists at the interdisciplinary Collaborative Research Centre 874, have retraced this process with the help of a computer simulation. Their findings challenge the standard model of memory formation in the hippocampus. Their work has now been published in the current issue of “PLOS Computational Biology”.

Remarkable anatomy of the hippocampus

The hippocampus’ vital role in the formation of new memories has been studied intensely. If our hippocampi are damaged, no input can be archived in our long-term memory and we can access no memories older than a few minutes. The unique anatomy of the hippocampus, which has contributed to its name (hippocampus=seahorse), supports its function. Based on physiological properties the anatomical structure can be divided into several subregions. The dentate gyrus is the gateway to this part of the brain. From here nerve cells connect to the Cornu Ammonis regions CA3 and CA1. CA3 consists of a large number of pyramidal cells with many reconnections. This is presumed to be the place where our memories are stored.

A functional model retraces our memories

From this anatomical structure a functional model of the hippocampus has been established over the past years and tested in computer simulations. In this model, input from the entorhinal cortex is reduced in the dentate gyrus in such a way that distinct memory pieces are obtained – this is called “pattern separation”. The CA3 region, it has been argued, works as an auto-associative memory. It is able to deduce the complete information from only a tiny sample (“pattern completion”). CA1 then relays the complete information back to the entorhinal cortex, where it can be retrieved as a memory.

Regions of the hippocampus work differently together than previously thought

Neuroscientist Torsten Neher and his colleagues have now calculated, with the help of several computer simulations, that the original model must be fundamentally reassessed. Using an artificial neural network based on a rat’s hippocampus, they have tested how the network processes and stores information, and how this information is then recalled as memory. Their results indicate that CA1 is more involved in the completion of memory cues than it was previously assumed. Even without CA3’s recurrent connections memories can be decoded. Furthermore, there are certain types of memories, such as those that allow us to navigate in a familiar environment, which do not need to be reduced. In this case mental maps are created in CA3, which convey distance and relations. In fact, an auto-associative function, aiming to make input less similar, would be counter-productive for the memory formation process. “Seeing that the framework works so much more efficiently without pattern completion in CA3 was a big surprise for us. After all, it puts into question a framework that has been well-established for 20 years.” says Torsten Neher. The scientists’ computer model indicates that the largest part of storage and decoding happens between entorhinal cortex and CA1. This leaves CA3 free to perform other tasks, which must now be investigated. It is presumed, for example, that this area of the hippocampus could be involved in the representation of sequences of episodic or autobiographical memory.