Of Mice and Memory: What Recent Research with Mice is Teaching Scientists About Human Memory

Scientists continue to learn about how the brain works, but human memory still holds many mysteries. How exactly are memories stored and retrieved? How does the brain recognize and treat different types of memories? Can memories be changed or manipulated? How can memory be damaged or improved? Recent research has made significant strides in answering some of those questions.

Scientists Discover a Way to Identify New vs. Old Memories

Last year, researchers at the RIKEN Center for Brain Science in Japan discovered patterns in mouse brain activity that allowed them to tell the difference between old and new memories. Using a machine-learning algorithm, the team analyzed recordings from mouse brains and were able to accurately classify memories as recent or old.

The team also studied the interactions between different memory-related brain regions—including the anterior cingulate cortex (ACC) and the hippocampus (specifically area CA1 of the hippocampus)—to pinpoint where memories are formed and stored.

In memory tests with mice, the researchers determined that the neural interactions between the ACC and CA1 regions changed over time, becoming more in-sync as the memory aged. That understanding enabled the team to distinguish newer memories from older memories by looking at the correlation between ACC and CA1 interactions in brain waves.

Creating Artificial Memories in Mice

A team of researchers has successfully created false memories in mice that are indistinguishable from natural memories.

To create the false memories, researchers mapped the brain circuits used in memory formation, then stimulated the brain cells in the pattern of the natural memory to create an artificial memory that was retained and recalled as if it had been generated by an actual experience.

Using this method, researchers were able to create a fully artificial memory in mice, which could be retrieved by exposing the mice to a sensory cue (in the case of this experiment, an odor). The experiment may have implications for humans suffering from memory impairments or loss.

Improving Memory with Exercise

Researchers at the University of California, San Francisco have identified an enzyme that could help explain how exercise can slow or even reverse some signs of aging in the brain.

The researchers started out by taking blood plasma from active mice and infusing it into older mice with a sedentary lifestyle. After getting the new blood form the exercising mice, the older mice performed better at mental tasks requiring memory.

The scientists then began to look for the specific proteins in the blood that might be accounting for the improved mental activity. They found one that seemed to be the key: an enzyme called GPLD1. When researchers artificially increased production of this enzyme in older mice, they saw the same improvements in memory and cognition.

Researchers were then able to duplicate their findings in humans. By measuring the activity of a group of older people at the UCSF Memory and Aging Center, the researchers were able to determine that the people who exercised more produced more of the GPLD1 enzyme.

While the team does not expect an “exercise pill” to be possible anytime soon, the discovery does hold some promise for older adults who have memory issues but who are not able to get the benefits of increased production of the GPDL1 enzyme by exercising on their own.

The Role of Memory in Dreams

Why do we forget our dreams so quickly? A team of researchers in the United States and Japan have discovered that when certain neurons fire during REM sleep, which is when most dreams occur, they control whether the brain remembers new information.

Scientists have long pondered the role of sleep in learning and memory, including the evidence that sleep disruptions in animal models of Alzheimer’s disease can accelerate the accumulation of beta-amyloid in the brain, and that better sleep can slow that process.

In a new study, researchers identified a connection between a specific type of dream-related neuron—melanin-concentrating hormone (MCH)-producing neurons—and memory performance, but it wasn’t the connection they expected.

MCH neurons, which are active during REM sleep, seem to be connected to forgetting rather than remembering. When scientists deactivated MCH neurons in mice, the mice actually performed better in learning and memory tests. When MCH neurons were activated, the mice showed impaired memory.

The authors of the study speculated that the process may be a way for the brain to prioritize information storage. Since the content of dreams usually isn’t relevant to waking life, the brain has developed a way to help us forget it more quickly to make room for more important information.

As scientists continue to explore the mysteries of memory, their discoveries could lead to important breakthroughs in memory-related conditions including dementia, Alzheimer’s, brain injury, and even emotional disorders.