"Running waves" carry your memories

You have just arrived in a new country and now you are in front of a magnificent monument. An image that will inevitably stick in your memory. Your brain “encodes” this memory: it imprints it and stores it somewhere in your neurons. But how will he do this?

Years later, the sights, sounds and smells came flooding back, suddenly recreating the feeling of being in front of the monument in question. Then you remember that memory. What’s happened ?

Well, the memories were traveling in your brain too. During encoding, it was carried by an electrical wave generated by your brain, called a “traveling wave.” It moved from the memory entry point at the temple (in an area called the hippocampus) to the front of the brain, to the frontal lobe. Then, when you retrieve that memory, the wave moves in the opposite direction to bring it back to you. And it’s right in front of your eyes.

How was this phenomenon discovered? At New York University, a team of four scientists conducted experiments on electrocorticography, a technique that involves implanting tiny electrodes on the surface of volunteers’ brains and recording the electrical discharges produced by those nerve cells as they undergo various tests. Of course, we don’t do this kind of surgery just to find out how memories move: these people need brain surgery to cure their seizures anyway, and teams of researchers are simply taking advantage of this, with their consent, to do more examinations. So the latter placed recording electrodes on the surface of the cortex of 93 patients and asked them to learn lists of words and then, after a waiting period, to recall as many of them as possible from memory. In this way, they could study the encoding and retrieval stage of memories.

The coming and going of waves of memory

The researchers then noticed that during encoding, neurons located at the temple emitted waves at a frequency of 10 hertz, or ten vibrations per second. However, these waves were not limited to neurons in the temporal cortex: they moved to the front of the brain, to the frontal lobe, at a speed of about one meter per second (3.6 km/h, about walking speed). Then, during the memory retrieval phase, when participants tried to remember the words they saw on the list, the same wave went in the opposite direction, returning from the frontal lobe to the temporal lobe to “bring back the memory.”

What do these observations tell us about the functioning of memory? It is as if the memories were sent to a central office located in the frontal lobe, ready to be retrieved at any time. When we want to remember information, this “will” is usually carried out by the frontal lobe. But observing this wave provides another piece of information: it travels relatively slowly compared to other neural phenomena, which can reach speeds of 100 meters per second (or 360 km/h). These modes of neuronal information transmission are likely to be mediated by superficial pathways along the horizontal branches of neurons called dendrites, whose conduction is slow, rather than by deep pathways that mobilize fast-conducting axons. A bit like waves on the surface of a lake. It remains to understand why…