Making memories

To remember an experience, the brain must be able to represent both the time and the events that occurred and put them in the right order. This sequence of events in our heads – with one coming after the other – may have something to do with so-called time cells. A study, recently published in the Journal of Neuroscience, provides evidence for how our brain knows the beginning and end of memories, despite time gaps in the middle. This discovery could lead to future strategies to restore or improve memory.

What, where, when

In this study, the researchers focused on “episodic memory.” This is the brain’s ability to remember the “what, where and when” of an experience. “When we recall a memory, we are not only able to remember what happened to us, but also where we were and when it happened to us,” Matthew Self researcher at the Netherlands Institute for Neuroscience tells us. “We think that time cells may be the underlying basis for encoding when something happened.”

Time Cells

Time cells, the nerve cells that contain time-related information, have been demonstrated before in rodents, but until now it was unclear to what extent similar cells are also present in the human brain.

To demonstrate this, a team of scientists examined the activity of the hippocampus, an important center in the brain for memory processing, in patients with epilepsy. In these patients, electrodes had been implanted in the brain for medical reasons. The subjects agreed to participate in two different experiments after their surgery.

In the first experiment, patients were repeatedly shown a sequence of five to seven pictures, always in the same order, and had to learn the order of the sequence. In a second experiment, the same series of pictures was presented, but now at random moments the patients saw a blank grey screen and had to wait 10-seconds for the series to resume.

The researchers first found that many cells in the hippocampus became active at particular times during the sequence. Some cells at the start of the sequence, other in the middle and some near the end. This time-related activity was not dependent on the pictures on the screen as even when the patients were viewing the grey screen in the second experiment cells became active at specific time points during the 10 second pause.

The results show that there are cells in the hippocampus that carry time-based information. “Different time cells become active at different moments in an experience and may provide a time-stamp for that experience” Self says. “Their activity may help us to recall when something happened and place our memories in the correct order”. In addition, the findings may also explain why some people with damage to their hippocampus can remember events but have trouble sequencing them, something often seen in patients with Alzheimer’s disease and other neurodegenerative disorders. This research contributes to the understanding of how our memory works and may help to provide a possible treatment in the future for memory problems.