How does the brain recall memories? D Indumathi, Chennai Have you ever struggled to find a word or a formula, just when you thought you knew it very well? Especially just at the beginning of an exam? And, then when you have cooled off a little, and de-stressed, the word pops back in your memory and you heave a sigh of relief. Or perhaps you remember it only after the exam is over! Why does this happen? How do we remember? First of all, how do we store memories, and secondly, how do we recall them? The human brain is a complex collection of cells called neurons which transmit and store information. You may have heard that there are two types of memory: short-term and long-term. Short term memory is usually stored in the frontal lobe of the brain, especially the pre-frontal cortex. On the other hand, long-term memories are made by more stable and permanent changes in neural connections that are widely spread throughout the entire brain. Hippocampus One of the important parts of the brain for making and storing memories is the hippocampus. It is a seahorse-shaped region of the brain. It is thought to be the center of emotion, memory and the involuntary nervous system. It is essential for learning new information as well as in converting short-term to long-term memory although it does not seem to store any memory itself. There is still a lot of research going into how we actually store and recall memories and the subject is fascinating. In recent research, scientists have made progress in locating the centre in the brain that helps in memory recall. Putting electrodes in the brain Studying brain cells in action inside the human head is not easy. Putting electrodes in someone’s brain requires surgery. It’s not something researchers do without a good reason. The new study involved people who already had electrodes put in their brains for some medical purpose. These participants all had epilepsy. This brain disorder causes storm-like surges of electrical activity in the brain, known as seizures. Doctors put the electrodes in the patients' brains so they could pinpoint those surges. The photo shows the cross section of the brain with bead-like electrodes inserted at the bottom right of the brain. In the new study, Yitzhak Norman of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues, studied signals from cells in the brain’s hippocampus. That’s a key memory center. Triggering memory waves First, patients were shown images of people and places they already knew. For example, they might see a picture of former President Barack Obama. Or they might view an image of the Eiffel Tower in Paris, France. As each picture appeared, electrodes picked up bursts of activity in the brain. These so-called sharp-wave ripples happen when many nerve cells fire together. Earlier studies suggested that these ripples in the hippocampus were important for forming memories. But it wasn’t clear if the ripples also had a role in recalling memories. To test that, Norman’s group blindfolded the patients. Then they asked each to recall the pictures they’d seen. A second or two before the participants began describing each picture, researchers noticed an increase in sharp-wave ripples in the hippocampus. Those ripples were very similar to the ones seen earlier. Norman’s team shared its new discovery August 16 in the journal Science. The authors say their data suggest those ripples help people make and recall memories. This new finding sheds light on how the brain stores and recalls information. Adapted from Science News for Students, by Laura Sanders