I got up, showered, dressed - how do I remember this?

When we tell someone about our events on a certain morning, we are able to recall the events - detail after detail - thanks to the ability called "episodic memory" (memory of a sequence of events). This, despite the fact that in our brains there is not a single video camera that carefully records all events.

In fact, the opposite is true: the look of the clothes we wore, the smell of the coffee we made, and the writing of the note we left on the table - each of these is processed in a different part of our brain. How does the brain put all this sensory information back together and create a complete "story", in which we remember that first we got dressed, then we drank coffee, and finally we wrote the note?

Dr. Roni Paz and research student Hagar Galbard-Shagiv, from the Department of Neurobiology at the Weizmann Institute of Science, sought to understand exactly how episodic memory is formed.

Many studies have pointed to an area of ​​the brain, called the hippocampus, as being responsible for creating this type of memories. The hippocampus - which includes two seahorse-like structures, located on both sides of the brain - receives sensory information from different brain areas, and ties it together when creating memories, but the neural mechanism that enables this ability was not clear.

Together with Prof. Rafi Malach from the Department of Neurobiology at the institute, and Prof. Yitzhak Fried, a neurosurgeon from the University of California, Los Angeles and Tel Aviv University, they recruited a unique group of volunteers to help them: epilepsy patients undergoing a procedure in which electrodes were temporarily implanted in various areas of the brain, including The hippocampus, for medical treatment.

These volunteers provided scientists with a rare opportunity to closely examine the activity of individual neurons in the brains of awake and active humans.

During the experiment, the volunteers watched for about an hour about 50 videos, each of which lasts about 10 seconds. Each video was repeated several times, in more or less random order. A typical clip, for example, started with a few seconds of James Bond running towards a target, and continued with a video of another few seconds in which a video of the White House was shown. "The idea," says Dr. Paz, "was to present characters, objects or landscapes that are familiar to the viewer, but to present them in a way that puts them in new contexts - and therefore causes the volunteers to create new memories."

Comparing the patterns of neuronal activity during the first viewings of the videos versus later viewings provided scientists with a rare window through which they could "see" how memories are formed and consolidated. When the volunteers watched the videos for the second, third and fourth time, the neurons in the hippocampus showed more and more consistent patterns, so that their activity in a certain period of time made it possible to predict - more and more accurately - their activity in the period that followed. This phenomenon appeared in the hippocampus, but not in other areas of the brain - which proved to the scientists that indeed the hippocampus is responsible for linking the separate events, thus creating the internal timeline of the brain.

To test whether the neural activity they measured actually reflects a process of creating a memory of a sequence of events (and not, for example, a neural response to the character of James Bond or the White House), the scientists asked the volunteers, at the end of watching the videos, to recall and describe the videos freely, without Help or hints.
They discovered that following the index they developed, which examines the development of consistent patterns over time, allows them to predict with great accuracy which of the volunteers will perform the task better, and describe the content of more stories in the correct order.

This neural index, which the scientists discovered, not only appeared in individual neurons, but also in groups of neurons.

This discovery strengthens the current models describing how our memories are created and stored in the brain: scientists believe that memories are stored in networks of nerve cells and synapses - the junctions between nerve cells where information is transmitted from cell to cell. "While creating new memories, the networks of nerve cells dissolve one form and take on a new form," says Dr. Paz.

"In this study, the reorganization of the network that encodes the sequence of events took place gradually, in the repeated observations in the videos. In the end, one character - for example, that of James Bond running - automatically brought up the next character - the White House - even while the volunteers did not Watch the video."