A General Hippocampal Computational Model Combining Episodic and Spatial Memory in a Spiking Model
Aguiar, Paulo de Castro
The hippocampus, in humans and rats, plays crucial roles in spatial tasks and nonspatial tasks involving episodic-type memory. This thesis presents a novel computational model of the hippocampus (CA1, CA3 and dentate gyrus) which creates a framework where spatial memory and episodic memory are explained together. This general model follows the approach where the memory function of the rodent hippocampus is seen as a “memory space” instead of a “spatial memory”. The innovations of this novel model are centred around the fact that it follows detailed hippocampal architecture constraints and uses spiking networks to represent all hippocampal subfields. This hippocampal model does not require stable attractor states to produce a robust memory system capable of pattern separation and pattern completion. In this hippocampal theory, information is represented and processed in the form of activity patterns. That is, instead of assuming firing-rate coding, this model assumes that information is coded in the activation of specific constellations of neurons. This coding mechanism, associated with the use of spiking neurons, raises many problems on how information is transferred, processed and stored in the different hippocampal subfields. This thesis explores which mechanisms are available in the hippocampus to achieve such control, and produces a detailed model which is biologically realistic and capable of explaining how several computational components can work together to produce the emergent functional properties of the hippocampus. In this hippocampal theory, precise explanations are given to why mossy fibres are important for storage but not recall, what is the functional role of the mossy cells (excitatory interneurons) in the dentate gyrus, why firing fields can be asymmetric with the firing peak closer to the end of the field, which features are used to produce “place fields”, among others. An important property of this hippocampal model is that the memory system provided by the CA3 is a palimpsest memory: after saturation, the number of patterns that can be recalled is independent of the number of patterns engraved in the recurrent network. In parallel with the development of the hippocampal computational model, a simulation environment was created. This simulation environment was tailored for the needs and assumptions of the hippocampal model and represents an important component of this thesis.