New research shows that the brain's "place cells" fire when we recall an event.

With the 50th anniversary of Kennedy's assassination last month, you might have found yourself asking people where they were when they heard the President had been shot. Most probably remembered. Memory and place often come as a package — whether for major life events or far less significant encounters that embed themselves in the brain.

Research published last week in the journal Science offers new insight into this place-memory partnership. Cognitive scientists have known for a while about the existence of so-called "place cells," which become active during spatial navigation. The new work shows that these place cells also become active when a person remembers a non-spatial episode that occurred there.

As the researchers put it, "recalling an episodic memory involves recovery of its spatial context." In simpler terms, many of our memories are geo-tagged.

For the study, test participants traveled to the stores of a virtual town (e.g. a bakery) delivering various items (e.g. a zucchini). After the participants made their thirteenth and final delivery, the researchers asked them to remember as many of the items as they could.

Electrodes captured participant brain activity at every step of the process, from spatial navigation to item recollection. That enabled the researchers to compare how place cells fired during the navigation phase and recollection phase of the test.

During the navigation phase, as participants reached a delivery location like the bakery, their place cells fired at an increased rate. When the participants moved away from that location, this cell activity decreased a bit. These firing patterns enabled the researchers to create a sort of mental map based on place cell activity alone.

As test participants recalled an item that was delivered — say, the zucchini — the place cells also fired at an increased rate. When the researchers went back to the map, they realized that the cell activity that occurred during zucchini recollection closely resembled the cell activity that occurred when the participant had traveled to the bakery. 

Here's a visual hypothetical example that might make the process even clearer. The first figure shows what place cell activity might occur during delivery of "ice" to a store at top-left corner of the map. As distance to the store increases, the cell's activity (or firing rate) changes:

The next figure shows the activity pattern that would occur during recollection of "ice" as a delivery item. This pattern closely matches the one that occurred nearest to the store during the navigation phase above:

These matches in cell activity during navigation and recollection reflect the close neural bond between place and event. Keep in mind that during the recollection phase, the test participants only reported the item they'd delivered — not where they'd delivered it. Yet the place cells fired just as if they were in the same spot again. The researchers conclude:

This reactivation implies that each experienced item is bound to its spatial context, which in turn may be reinstated when the item comes to mind during recall.

Figures extracted from the supplementary material of J.F. Miller et al. Neural Activity in Human Hippocampal Formation Reveals the Spatial Context of Retrieved Memories, Science, 29 November 2013: 342 (6162), 1111-1114. DOI:10.1126/science.1244056.

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