“Memory really is the key to consciousness,” said Richard Thompson, USC College neuroscientist and the Keck Professor of Psychology and Biological Sciences. “What are we without all of our memories, our learned experiences and the knowledge we’ve accumulated throughout our lifetimes?”

Authors Thompson and Stephen Madigan describe the development, experience, mechanisms and structures of memory as well as ordinary forgetting and the unreliability of memory in “Memory: The Key to Consciousness” (Joseph Henry Press, 2005). They engage readers in the scientific process – including descriptions of how scientists have figured out what they know – while avoiding overly technical language.

While scientists still do not have all the answers, “the understanding of memory has advanced exponentially in the last few decades,” Thompson said.

A member of the National Academy of Sciences, Thompson has pioneered the study of learning and memory for nearly 50 years. Using a wide variety of approaches, from psychological to genetic, he has tracked the minute physical changes that occur in the brain as learning takes place and memories are coded, stored and retrieved.

Thompson and others have shown that the brain encodes a memory by changing the physical structure of connections between neurons called synapses. When something new is learned, neurons sprout new synapses and strengthen existing connections.

Learning, he noted, is the active creation of memory. Memory is what is stored as knowledge.

“We think that most learning involves the brain associating one type of event with another, and this association also relies on memory. When something terrible happens, most people form a basic association between the environment they were in with the actual painful event,” Thompson said. “People learn to associate a specific environment or other cues with memories of pain and fear.”

Thompson recruited Madigan, his longtime colleague, to help write the book. An associate professor of psychology in the brain and cognitive sciences group, Madigan investigates facets of human memory, including mental imagery, short-term memory, forgetting and age-related changes in visual memory.

Last year, working with colleague Zhong-Lin Lu and others, Madigan published a paper exploring how tests of iconic memory might be used to detect early signs of Alzheimer’s disease.

To most, the word memory denotes the conscious recollection of events from one’s past. But Thompson and Madigan write about these so-called explicit memories and many other types of memories. Scientists have looked at differences in how people recognize visual objects and recall word meanings, remember how to drive a car or swing a bat, or recognize a face as opposed to a voice.

People use short-term memory to remember a phone number just for the few seconds necessary to dial it. But it’s in their long-term memory where they store information about how to use the phone, the person they are calling and the matter they mean to discuss.

In addition to elucidating different kinds of memories, scientists have shown that different kinds of memory and learning take place in different areas of the brain. Learning one’s native language, for example, creates memories in an area of the brain that seems specialized for spoken language. A part of the brain called the hippocampus plays a key role in creating new long-term memories of personal experiences, which are then stored in the cerebral cortex.

In terms of creating long-term memories, scientists have shown that cramming the night before an exam may result in a passing grade, but it’s not ideal for learning.

Thompson and Madigan describe a study showing that students who crammed for an exam scored only slightly worse than students who had studied regularly over a longer period of time. However, when students were retested on the same material a week later, crammers recalled only 30 percent of the original material, while those who took a slow and steady approach remembered 80 percent.

Learning a little every day with breaks between study sessions appears to be the most efficient way to learn – whether it’s a new song on the piano or all of the state capitals. “Spaced practice allows for the creation of a memory, followed by a second process called consolidation to occur,” Thompson said. Consolidation appears critical to turning a short-term memory into a permanent one.

Some people think memory consolidation occurs during sleep. One study showed that people remember more when learning is punctuated by a night’s sleep than when tested on material they learned the same day.

Those who consider themselves to have a “good” memory may be surprised to learn there is little difference between individuals in how quickly a memory disappears. Instead, the key difference seems to lie in how well the memory initially was created, or learned.

Learning, we read in “Memory,” occurs faster for some people, but forgetting affects fast and slow learners at the same rate.

The authors explore key questions about memory’s flip side: Do our memories fade or are they written over like on a computer drive? Do the memories remain stored away, but become inaccessible over time? Is forgetting active or passive?

“We just don’t know,” Thompson said. “First, remember that memory is very selective.” Memory is not a video camera, attending to every detail with equal weight, he explained. Most memories leave out the majority of sensory information from an experience.

New learning may cause some forgetting when it overwrites an earlier, similar memory through a process called interference. The book discusses a study in which researchers read children a single story. The children remembered much of the tale when asked about it a week later. But when the children were read two stories, they remembered less of either story.

Some memories, or at least the ability to access those memories, seem to fade with time. Workers trained in CPR techniques and tested three years later (with no practice in between) remembered only 15 percent of what they originally had learned. Memories never used are lost.

There are some ways to prod memories gone hazy, however, including environmental cues. One study showed that elderly people who returned to their high school campus were able to recall many more names of long-lost classmates than when they were asked to list classmates’ names from memory.

Notably, the authors cast doubts on the idea that people repress truly traumatic memories. Intense emotions like fear and anger tend to enhance the formation of memories. In fact, most haunted by memories of war, abuse and crimes remember their harrowing experiences all too well and may develop post-traumatic stress disorder.