Researchers have lifted the veil on the brain cells that could explain why so many people remember where they were when John F. Kennedy was assassinated in Dallas 50 years ago.
Cells in the brain “geotag” experience, helping us recall events through a spatial context, according to a study published Friday in the journal Science.
The findings offer neurological confirmation of how humans retrieve a past experience -- by restoring a context indexed on a cellular level. Populations of specialized neurons that fired during the initial experience of a place fired in the same way when a subject consciously tried to recall events that occurred there, the study found.
“Part of that remembering of the event is that those neurons that told you where you are when you learned the event reactivate,” said University of Pennsylvania computational psychologist Michael J. Kahana, one of the authors of the study.
Researchers used a rare opportunity provided by epileptic patients who had electrodes inserted into the medial temporal lobe region of their brains as part of a diagnostic procedure. The researchers were particularly interested in how cells fired in the hippocampus.
“There are a variety of cues that the memory system will use to search for the set of things that happened to you. That’s episodic memory,” Kahana said. “That type of memory depends on an intact hippocampal system. If the hippocampus is damaged, you can’t do that very well.”
Subjects played a navigational video game involving making deliveries in a virtual town. Brain activity was recorded while they familiarized themselves with the layout -- building a “mental map” -- and while the program randomly directed them to locations to deliver a package. For instance, they were asked to find the bakery, then told they had just delivered a zucchini.
Players either saw an image of what they had delivered or were just told what the item was. After 12 such scenarios, subjects were asked to recall, in any order, what they had delivered, while their brain activity was recorded.
That recall touched off activity of “place” neurons that had fired anytime the subjects had navigated near sites associated with the object, the study found.
Place cells in the human and animal hippocampus have been known and studied for some time. Experiments have shown that rats reactivate temporally sequenced events during sleep, suggesting they are consolidating memory. They also appear to rehearse such experience while awake – in reverse sequence.
Cognitive scientists have long presumed that to remember episodes, we first must restore context. That process, Kahana suggests, also may dredge up associative memories indexed by where they occurred.
“When you remember something, what you’re doing is reinstating the context of that item’s occurrence in your life,” Kahana said. “That reminds you of things that share similar context with that item.”
That could explain why memories of the JFK assassination have such a strong spatial context for many people, and why last week's remembrances may have dredged up dusty memories.
Brain cells made a traumatic event inseparable from a place.
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