“Memory” is the term we use to describe the phenomenon by which information is encoded, stored and retrieved; it is the basis for how we learn.

It is also a little mysterious: exactly how is it that vivid memories — of the cake at your seventh birthday party, the smell of your grandparents' home, the words to a song from 1992 — come to be recalled weeks, months or years later?

While scientists have long suspected that increasing the connections between neurons must underlie the formation of memories, proof had remained elusive until now.

Researchers in a variety of fields have been working on the secrets of memory for decades. Thanks to some new, cutting-edge experimental technologies, a study from the University of California San Diego (UCSD) brings us a step closer to understanding what happens at the cellular level that makes your memory — whether it's of a person, fact, event or whatever — so clear many years later.

“Beyond potential applications in disorders of memory deficiency, such as dementia, this improved understanding of how memory works may hold clues to taking control of runaway emotional memories in mental illnesses, such as post-traumatic stress disorder,” National Institute of Mental Health director, Thomas R. Insel, M.D., said in a statement on the NIMH-funded study.

The findings provide the first cause-and-effect evidence that memories are formed by strengthening connections between cells in the brain.

These cells, neurons, transmit chemical and electrical signals that form the core components of the nervous system, including the brain, spinal cord and central nervous system (CNS). While scientists have long suspected that increasing the connections between neurons must underlie the formation of memories, proof had remained elusive until now.

“Our results add to mounting evidence that the brain represents a memory by forming assemblies of neurons with strengthened connections,” researcher Roberto Malinow, professor of neurosciences at UCSD School of Medicine, explained.

Malinow's team's innovative approach relied on being able to precisely manipulate the strength of connections between neurons in genetically engineered rats. They accomplished this by using one of neuroscience's most powerful new tools — optogenetics.

For many years, researchers have been able to create fear memories in lab animals. In classic fear conditioning experiments, rats hear a sound that is paired with an electric shock and form a memory associating the two, thereby developing a fear of that particular sound.

In a major advance, scientists paired the electric shock with optogenetic stimulation which uses light to control neurons that have been genetically sensitized to light. Researchers were able to strengthen, weaken, and then strengthen again the connections between neurons using light. This made it possible to readily form a fear memory, remove it, and then bring it back.

The findings represents a major step in understanding memory formation and many hope will also lead to clinical advancements. “This work highlights the staggering potential of precision targeting and circuit manipulation for alleviating maladaptive memories,” said project officer Chiiko Asanuma, NIMH Division of Neuroscience and Basic Behavioral Science.

For people suffering from post-traumatic stress, the possibility of extinguishing the intrusive, unwanted memories that haunt them would be welcome indeed. The research could also help strengthen memory.

“We have shown that the damaging products that build up in the brains of Alzheimer's disease patients can weaken synapses in the same way that we weakened synapses to remove a memory,” said Malinow. “So this line of research could suggest ways to intervene in the process.”

The study is published in Nature.