Readers of this blog will remember a study that had shown that there were three dissociable systems in the human hippocampal regions as relevant to declarative memory. These were the anterior hippocampus (dentate gyrus) for detecting novelty; the Posterior hippocampus (CA3 )for recollecting (or using contextual cues for recall) and the posterior hippocamal gyrus for familiarity detection. Extending these to spatial memory , one can conjecture that dentate gyrus would be involved in detecting a novel cognitive map or spatial arrangement from the older stored cognitive maps; the CA3 region will actually store these cognitive maps that provide the context using which the mice (or men ) can orient oneself; while the posterior hippocampal gyrus might be involved in detecting familiarity or whether the spatial place has been visited earlier and is familiar.
Research has indicated that indeed the CA3 region contains ‘ place cells ‘ or cells that fire when a mice is near a spatial location. Multiple such cognitive maps of the environment that the mice encounters can be stored in the hippocampus.
However, as Madam Fathom has excellently elaborated, there persisted a mystery as to how widely similar, but subtly distinct cognitive maps , were distinguished within the hippocampus. As per the above model, dentate gyrus should have a prominent role to play here detecting if a new spatial location is a novel spatial location, despite it being similar in many ways to an earlier encountered spatial location.
This is exactly what has been experimentally observed. When mice which had NMDA receptors knocked off in the dentate gyrus were put in a novel environment or context, they were unable to distinguish it from the previously learned context. Thus, these mice though capable of learning could not distinguish between contexts, as presumably their ability to detect a novel context were hampered.
To me this bodes as further evidence for the cognitive map theory and I would stick my neck and say that the mechanisms and circuits involved in spatial navigation, episodic and declarative memory are same and serve a similar function. Thus, the dentate gyrus not only detects novel words in a word list (declarative memory) , but also detects novel spatial locations (cognitive maps) and novel autobiographical events (episodic memory). Similarly the CA3 region of hippocampus codes for distinct spatial maps and distinct words an facts and also distinct autobiographical memories. Similarly posterior hippocampul gyrus may detect familiarity for both facts, episodic memories (and trouble with this may lead to Deja Vu like feelings) and spatial locations.
These multiplexed use of the same brain regions, for different types of memories, may also explain why mnemonic methods like the method of loci work excellently- as the brain regions for declarative memory are the same as for discerning one’s spatial location in an environment- hence it might be computationally easy to remember lists if a associated with spatial locations or a prominent cognitive map.