Pretty much everyone has experienced some sort of memory triggering, but there hasn’t been any satisfactory explanation for this common phenomenon. But now, researchers from University of Pennsylvania have provided the first neurobiological proof that memories which are formed in the same context can become linked, the foundation of the theory of episodic memory.

This study was headed by professor Michael Kahana of the Department of Psychology in the School of Arts and Sciences and graduate student Jeremy R. Manning, of the Neuroscience Graduate Group in Penn’s Perelman School of Medicine. They also worked with Gordon Baltuch and Brian Litt of the departments of Neurology and Psychology at the medical school and Sean M. Polyn of Vanderbilt University, and their research were published in the Proceedings of the National Academy of Sciences.

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“Theories of episodic memory suggest that when I remember an event, I retrieve its earlier context and make it part of my present context,” Kahana said. “When I remember my grandmother, for example, I pull back all sorts of associations of a different time and place in my life; I’m also remembering living in Detroit and her Hungarian cooking. It’s like mental time travel. I jump back in time to the past, but I’m still grounded in the present.”

In order to verify the evidence for this theory the team analyzed a centuries-old psychological research technique (making subjects memorize and recall a list of totally unrelated words) combined with extremely precise brain information that can be acquired only through neurosurgery. The people involved in this study were all epilepsy patients, who had 50-150 electrodes connected to their brains. Due to the fact that doctors had to wait for an epilepsy crisis to occur naturally in order to study them, the study required a significant amount of time to be completed.

“We can do direct brain recordings in monkeys or rats, but with humans one can only obtain these recordings when neurosurgical patients, who require implanted electrodes for seizure mapping, volunteer to participate in memory experiments,” Kahana said. “With these recordings, we can relate what happens in the memory experiment on a millisecond-by-millisecond basis to what’s changing in the brain.”

The patients were asked to memorize a list of 15 unrelated words. After seeing the words, they were distracted by simple math problems, and were then asked to recall as many words as possible in any order. The electrodes would gather information from their brain during every step of the way.

“By examining the patterns of brain activity recorded from the implanted electrodes,” Manning said, “we can measure when the brain’s activity is similar to a previously recorded pattern. When a patient recalls a word, their brain activity is similar to when they studied the same word. In addition, the patterns at recall contained traces of other words that were studied prior to the recalled word.”

“What seems to be happening is that when patients recall a word, they bring back not only the thoughts associated with the word itself but also remnants of thoughts associated with other words they studied nearby in time,” he said.

The finding also produced an explanation for this extremely interesting yet common phenomena.

“This is why two friends you met at different points in your life can become linked in your memory,” Kahana said. “Along your autobiographical timeline, contextual associations will exist at every time scale, from experiences that take place over the course of years to experiences that take place over the course