Extraterrestrial scientists landing in a football stadium would be struck by the sight of the crowd suddenly standing up and shouting in unison. In a similar manner, since the nineties, researchers have observed a special pattern of neuronal activity in rodents: tens of thousands of nerve cells firing in unison in a part of the brain called the hippocampus. But, like an alien scientist, the researchers have not been able to understand the “language” of the rodents’ minds when these mysterious synchronous bursts occurred.

Recently Weizmann Institute scientists succeeded in recording these rapid bursts of activity – called “hippocampal ripples” – in the human brain, and they were able to demonstrate their importance as a neuronal mechanism underlying the engraving of new memories and their subsequent recall. These findings appear today in Science.

“The ripple is an amazing event in its intensity and timing. It is an orchestrated burst of synchronous activation by about 15% of hippocampal neurons – all firing together within about a tenth of a second. It’s a nerve-cell fireworks display,” explains Prof. Rafi Malach of the Institute’s Neurobiology Department.

It was first revealed that they emerge during mental states of sleep and rest, and that they play an important role in rodents’ spatial navigational memory. Only recently it was found that such synchronous electrical activity in large groups of neurons also occurs in the primate hippocampus during the awake state. However, until now, scientists have been kept in the dark as to the roles the ripples play in human cognition and mental activity.

Humans can, of course, communicate their thoughts, but most research methods do not give scientists a detailed view of what happens at the same time within the brain. Yitzhak Norman, a PhD student in Malach’s lab, who led the current research in collaboration with the group of Prof. Ashesh Mehta from the Feinstein Institute for Medical Research in the US, recruiting patients who undergo invasive recordings in the course of their medical diagnosis.

In this clinical procedure, patients suffering from intractable epilepsy get electrodes implanted in multiple brain regions to locate the epileptic focus and surgically remove it. These patients freely volunteered to participate in the memory experiments while they waited in the hospital between seizures.

The findings substantially expand our understanding of the function of the hippocampus. They emphasize the importance of synchronized neuronal group activity. The hippocampal burst, it should be remembered, involves the synchronous activation of hundreds of thousands of nerve cells.

Release/Source: Weizmann Institut