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Modeling the hippocampus, part I: Why the hippcampus?

A note on terminology: The terms theoretical and computational neuroscience, as well as (computational) modelling are often used interchangeably. They do, however, refer to slightly different ideas: When performing theoretical neuroscience we would like to understand the processes of the brain on a fairly abstract level andask about generic principles of organization and computation. An example would be the question of how items stored in long-term memory inform upcoming behavioral choices. In contrast, computational neuroscience primarily refers to the method we use in order to answer our research question, i.e., computational modelling. When performing computational neuroscience we are primarily thinking of different architectures (neural and otherwise) that we might use to model the behavior of the brain at our chosen level of abstraction (e.g. intra-cellular, inter-cellular, cortex-wide, etc.). In short: Theoretical neuroscience refers to the kind of question we ask, while computational neuroscience refers to the sort of tool we use.

The Hippocampus

One of the primary functions that our brains provide us with is the ability to store new memories and access them later on. The central structure responsible for this ability is called the hippocampus. It looks a bit like a mix between H.R. Giger's alien and a seahorse and is found in the center of the limbic system, which is part of the medial temporal lobe (in other words, right now it sits about in the center of your skull). Besides being heavily involved in memory the hippocampus also plays a crucial role in the processing of emotions, spatial navigation, and integration of different sensory information (Bear et al,. 2007, p.744ff).

The hippocampus performs its computations within several different loops. The most studied of these is the so called trisnynaptic circuit. Neural signals from the entorhinal cortex - the input structure for the hippocampus - enter the hippocampal formation at the dentate gyrus from where they are passed on through areas CA3, CA1, the subiculum, and finally back into the entorhinal cortex (Andersen et al, 2006, p.42).

The information being passed on to the hippocampus and, subsequently, through its circuitry is highly processed: By the time it gets to the hippocampus it has been filtered through multiple stages of the relevant cortex areas (e.g. the visual cortex for visual information) and encodes high level information (e.g. a completely processed face instead of raw visual receptor information).

The hippocampus also communicates directly with the prefrontal cortex using connections found in the the parahippocampal cortex (Preston and Eichenbaum, 2013). This is highly relevant as the prefrontal cortex can be thought of as the very tip of the pyramid of cortical processing; it is essential for executive function (i.e., decision making) and tightly linked with conscious processing (such as when you repeat a phone number in your head to not forget it until you get a chance to write it down - that's your prefrontal cortex at work).

At this point we can easily see what makes the hippocampus such an interesting structure to study: It sits at the very heart of processing the kind of data we consciously work with, and links highl level sensory information with executive function. However, at this point we're still missing a central part of the hippocampal puzzle, which is:

Different mammals share the same hippocampal structure. This structure has been preserved over the deep time of evolution.

We haven't checked all mammals but for those where we have taken a closer look - including humans, primates, rats, mice, cats, and bats - the hippocampus, in terms of its internal structure, looks virtually identical across the different species. Consider for a moment what this implies: Over the course of the millenia, when these species evolved into very different organisms, the internal makeup of the hippocampus has proven not only so useful that it has survived to this day, but it has done so without differentiating itself to adapt to the individual needs of different species.

In other words:

  • The hippocampus of a given mammalian species did not, over time, adapt in order to suit the individual needs of that particular species.
  • Consequently, the computation performed by the hippocampus can be assumed to be highly generic and flexible enough to provide a useful function based on the sensory perception from a variety of different species such as bats, seals, primates - each of which experiences a vastly different habitat and displays radically different behavior.

That's amazing.

Continue reading in part II of this series, where we talk about the function the hippocampus seems most involved in: episodic memory and spatial processing.



Disclaimer: This series of articles is heavily based on the introduction chapter of my PhD thesis.

The Institut für Neuroinformatik (INI) is a central research unit of the Ruhr-Universität Bochum. We aim to understand the fundamental principles through which organisms generate behavior and cognition while linked to their environments through sensory systems and while acting in those environments through effector systems. Inspired by our insights into such natural cognitive systems, we seek new solutions to problems of information processing in artificial cognitive systems. We draw from a variety of disciplines that include experimental approaches from psychology and neurophysiology as well as theoretical approaches from physics, mathematics, electrical engineering and applied computer science, in particular machine learning, artificial intelligence, and computer vision.

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