Filed in the human card catalog of the brain is an index of experience, reinforced by emotion and cross-referenced with sensory cues: chords that recall a graduation ceremony, a scent of honeysuckle that brings back a humid summer stroll, the taste of a pastry dipped in tea that restores a lost childhood.
Yet the memories that slip away are equally the stuff of life: the familiar face without a name, the missed appointment, the anniversary overlooked.
By necessity, the mirror of memory reflects an imperfect patchwork of the past onto the present. To remember too much is to drown in experience; to remember too little is to lose ourselves.
Anxious to combat the memory problems of Alzheimer's disease, amnesia and other such impairments, brain experts are working to understand how some experiences can be recalled with clarity half a century later, while other equally important memories are lost almost instantly.
For the first time, researchers now have recorded a memory in the making, identifying brain regions that influence what we can remember and what we may forget.
'A Significant Step Forward' in Science
The findings, reported recently in Science, "mark a significant step forward" in understanding why memories endure, said British memory expert Michael D. Rugg of the University of St. Andrews.
In recent years, researchers have determined that there is no single way the human brain handles memory. There appear to be interlacing systems of storage and retrieval among the brain's billions of neurons and synapses.
Learned skills, like riding a bicycle or juggling, are stored one way, knowledge of a native language another. Some information is stored only momentarily in what researchers call working memory. Unless an individual pays close attention to the new information for at least eight seconds, however, it will be lost. Short-term memory also can quickly overflow if more than seven or so items accumulate.
Through a process called encoding, some sensations, experiences, ideas and facts do find their way from the temporary working memory into a more lasting place in the brain's neural networks.
Using a noninvasive brain scanner, independent teams of neuroscientists at Stanford, Harvard and Washington University in St. Louis captured images of mental activity in the instant the brain encodes an experience into long-term memory.
Different Regions for Words and Pictures
They studied parts of the brain involved in forming memories of both words and pictures.
They discovered that the survival of a memory is determined in the moment that the brain attempts to translate an experience into a pattern of neural connections. The longer that two brain regions--the prefrontal lobes and the parahippocampal cortex--are active, the more likely a word or a picture will be remembered, the researchers found.
The brain images record how levels of blood flow change every few thousandths of a second in response to neural activity. They also showed that the formation of memories involving words was concentrated on the left side of the brain. Visual memories stimulated greater activity on the right side, the researchers said.
"We are looking at the brain activity as we are giving birth to a new memory," said Harvard psychologist Daniel L. Schacter, author of "Searching for Memory: The Brain, the Mind and the Past." He helped investigate verbal memories.
"What happens in a second or two--in the [neural] encoding operations we carry out--can determine the fate of that experience for a lifetime," Schacter said.
The two research teams took different approaches to investigating what happens when the brain turns experience into memory.
The Stanford team, led by neuroscientist James B. Brewer, focused on the formation of visual memories.
In the Stanford experiments, six volunteers were shown 24 color landscape photographs and interior scenes while the researchers recorded their brain responses with a functional magnetic resonance imaging (fMRI) machine. During several sessions, the volunteers were asked to judge whether each picture was an indoor or outdoor scene.
Thirty minutes later, they were shown the photographs again as part of a larger collection of images and asked whether they recalled any and how well they remembered them.
"As they were seeing the pictures, we were recording the brain responses picture by picture. We looked at the relationship of the brain activation to the subsequent memory . . . to predict which pictures would be well-remembered," said Stanford psychologist John D.E. Gabrieli.
"We found a number of places that had this property where the degree of activity would predict how well people would remember."
Cause of Varied Neural Responses Still Unknown