Almost everyone has experienced one memory triggering another, but explanations for that phenomenon have proved elusive. Now, University of Pennsylvania researchers have provided the first neurobiological evidence that memories formed in the same context become linked, the foundation of the theory of episodic memory.
“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.”
To investigate the neurobiological evidence for this theory, the Penn team combined a centuries-old psychological research technique — having subjects memorize and recall a list of unrelated words — with precise brain activity data that can only be acquired via neurosurgery.
“By examining the patterns of brain activity recorded from 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 findings provide a brain-based explanation of a memory phenomenon that people experience every day.
“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 of minutes, like studying words on a list.”