Newborn Brain Cells Encode Memory

By Margarita Nahapetyan

We all suffer lapses in memory from time to time. Do you remember how did it happen? Can you remember when did it take place? With these most common questions starts a wistful journey through our memory lane. You may remember reading an article for a few minutes, for a day or two, but not in a few days. That's because our brains handle a long-term memory, which gives us a possibility to remember events from the distant past, and a short-term memory, also known as working memory, which is responsible for the most temporary, rapid memories.

The three latest studies focused on the way the brain forms, stores and recalls memories. Scientists from the Salk Institute for Biological Studies in La Jolla, California, and the University of Queensland in Australia, found that newly born brain cells, which are generated each day by thousands, carry a time-related code that helps to "time stamp" memories. These cells do not record a precise, absolute date, instead they encode memories that are formed around the same time similarly. This allows the mind to understand whether a memory happened before, after or simultaneously with something else.

The purpose of the study was not to explain how the brain stores temporary information. The team of researchers developed a computer program designed specially to simulate the neuronal circuits in the denture gyrus, part of the brain which spawns new brain cells in adult brains. "Most modelers test a specific hypothesis and build a model around it," said Brad Aimone, a co-author of the study and graduate student in the Computational Neuroscience Program at the University of California, San Diego. "We tried not to make any big assumptions about the function of new neurons. Instead we asked, 'What is the biology, and what does the math suggest?'"

The dentate gyrus is the entryway to the hippocampus, a small seahorse-shaped area of the brain that distributes memory to appropriate storage sections in the brain after preparing the information for efficient recall. The researchers quickly realized that overly excited newborn cells respond indiscriminately and disorderly to incoming information. "The circuit in the dentate gyrus is designed to separate incoming memories into distinct events, a process called pattern separation, but immature cells get into the way by blurring the lines," says Aimone. "And if they keep muddling the picture, there's almost no point."

Each of these newborn neurons undergoes a long maturation process, during which it changes from hyper-excitable to composed and reaches out to mature brain cells. As the cells become fully mature, they take their place in the existing circuitry while the next generation of newborn neurons takes their place. People who learn and exercise assist to proliferation and survival of new neurons, while people under constant depression and stress significantly decrease their numbers. Though it is already known how new neurons become part of the existing dentate gyrus network, it is still unclear what their exact role and function is.

Yet, if the same neurons are active during two events, a memory connecting the two of them may be formed. It is possible to explain now, for example, why talking about a certain event, like going to a restaurant a while ago, might also bring memories of the play visited afterward and whose name has been erased from a memory. Therefore, different events that had nothing in common but the fact that they happened around the same time will now be connected forever in mind. "Current thinking holds that when we bring up a certain memory, it passes back to the dentate gyrus, which pulls all related bits of information from their off-site storage. Our hypothesis suggests that cells that were easily excitable bystanders when the memory was formed are engaged as well, providing a hyperlink between all events that happened during their hyperactive youth," said H. Gage, Ph.D., a professor in the Laboratory for Genetics, another co-author of the study.

In fact, the same young neurons respond to everything that happens for a while, said two of the study's co-authors. While peoples associations are formed based on sight, smell, and other senses, they may remember visited a couple of years ago football game by tasting a hamburger today.

"Even though these young cells are only a small percentage of the overall circuit, we believe that their effect may be enough to give people the sense of "this happened around the same time as" something else, Gage and Aimone explained. The computer simulation showing the time-stamp effect in the study is a novelty and it also explains why, for example, people recognize their car both when it is very dirty and very clean, and why they can remember where they parked the car today even though they had a different spot the other day.

The research was published in the January edition of the journal Neuron.