We have another paper from scientists using epilepsy patients with electrodes implanted in their brains during treatment. This method gives very clear results because it measures very small groups of neurons with very accurate time measurements while the patients are able to behave in normal ways. In this case the researchers looked at place cells while the patients played a video game. The places were stores in a virtual city, the patients made deliveries to these stores, each delivery was unique and finally the patients tried to recall the delivered items. The pattern of place cells was recorded for each store just before the delivery was made, then during the delivery when the item was revealed and finally when each item was recalled. When patients recalled a delivery to that store by recalling the item (an event), their place cells showed the store’s signature (the place tag). So something that has been thought to be likely has been demonstrated.
The hippocampus appears to have (at least) two roles: it tracks location for a spatial memory; and it records events for episodic memory. These are interwined. “Our finding that spontaneous recall of a memory activates its neural geotag suggests that spatial and episodic memory functions of the hippocampus are intimately related and may reflect a common functional architecture,” said Kahana, head of one of the groups involved.
The connection between a place and what has happened in that place would be very useful to survival. Deciding how to act in that place would that take into account what was known of the place.
It has seemed to me that memories may also be time-tagged in the sense that episodic events have a serial order for some time. This does not have as strong an involvement in recall but has some. When we remember one event we have a tendency to remember what happened before and after that.
Here is the abstract (Miller etal. Neural Activity in Human Hippocampal Formation Reveals the Spatial Context of Retrieved Memories. Science, 2013):
In many species, spatial navigation is supported by a network of place cells that exhibit increased firing whenever an animal is in a certain region of an environment. Does this neural representation of location form part of the spatiotemporal context into which episodic memories are encoded? We recorded medial temporal lobe neuronal activity as epilepsy patients performed a hybrid spatial and episodic memory task. We identified place-responsive cells active during virtual navigation and then asked whether the same cells activated during the subsequent recall of navigation-related memories without actual navigation. Place-responsive cell activity was reinstated during episodic memory retrieval. Neuronal firing during the retrieval of each memory was similar to the activity that represented the locations in the environment where the memory was initially encoded.