Yesterday, today and tomorrow: Mount Sinai researchers are making sense of episodic memory

Many of our actions are guided by past experiences combined with insight into the future. A major mystery of biology involves understanding how brain cells can create a representation that extends backward and forward through time. A new study conducted by researchers at Mount Sinai School of Medicine published in the December 18th issue of Neuron begins to unravel the brain activity that underlies concurrent processing of the recent past, the present and the imminent future.

Memories that are organized by time and context are known as episodic memory. Dr. Matthew L. Shapiro, Associate Professor of Neurobiology at Mount Sinai School of Medicine and leader of the study offers the following example. “Imagine driving to work, parking your car, and taking an elevator to your office. During the day you may take the elevator several times without thinking of your car. Only when the end of the day arrives and you descend in the elevator to go home do you remember where your car is parked. In the present moment in the elevator, the past guides your future action.” To examine the brain processes involved in such episodic memories, Drs. Shapiro and Ferbinteanu examined cellular activity within the brain while rats searched for food in a maze where the starting and ending point was varied.

The researchers examined activity in the hippocampus, a brain region that is key for memory. The hippocampus contains cells, called place cells, which become more active in response to a particular spatial location. “We found that the activity of the place cells showed something very interesting while the rats performed the task. Some cells signaled location alone but others were additionally sensitive to recent or impending events,” explains Dr. Ferbinteanu. “These cells maintained spatial selectivity, but this activity depended upon where the animal had just been or where it intended to go.” Therefore, the hippocampus can support episodic memory by creating patterns of cellular activity for events within a temporal context.

“The pattern of cell firing suggests a model of how the hippocampus helps form episodic memories,” suggests Dr. Shapiro. “When you park your car, hippocampal neurons fire in a pattern that includes location. When the time has come to go home, the goal in the elevator is to find the car. This goal activates cells in the hippocampus that did not fire during prior trips in the elevator that day, but form a new pattern that perhaps includes the visual and verbal images that guide recollection and future action.” Further studies are needed to determine how the brain activates and decodes the signals that simultaneously integrate the past, present, and future.