"To initiate a memory is almost like creating a word processing file on a computer," explains the study's first author, Matthew Walker, Ph.D., instructor of psychiatry at Beth Israel Deaconess Medical Center and Harvard Medical School. "Once the file has been created, if you don't hit the 'save' button before shutting off the computer it will be lost. Our new research helps explain the process in our brains that enable us to first create the memories and then to stabilize and 'save' the memories we've created." The findings then go on to explain how memories can later be "edited" once they've been saved.
Walker, who conducted the research while at the Massachusetts Mental Health Center, and his colleagues focused on "procedural skill memory," the "how" type of memory that enables humans to learn coordination-based skills, such as driving, playing a sport, or learning to play a musical instrument or perform a surgical procedure. "This is the type of memory that we often take for granted," says Walker. "But for stroke patients or other individuals who have suffered neurological damage that has injured their motor skills functioning – including how they speak and how they move – it quickly becomes apparent how critically important this type of memory is to our daily existence." To identify these three stages of memory, the authors instructed a group of individuals (100 young healthy subjects, ages 18 to 27) in several different finger-tapping sequences (for example, 4,1,2,3,4) at various intervals and at various points of the sleep-wake cycle. Their resulting data disclosed several important findings, according to Walker.
"We first discovered that in order for a memory to be stabilized – and therefore become less vulnerable to competing information – it requires somewhere in the region of six waking hours," he explains. "So, this is when your brain is hitting the 'save' key and putting the file on the 'hard drive,' but instead of being saved in a matter of seconds like your computer file, a memory needs several hours to be saved."
From there, the researchers went on to discover that the second stage of memory processing occurs during sleep – and that it is "absolutely dependent on sleep in order to occur," according to Walker. Study subjects who were tested 24 hours after their finger-tapping lesson – and following a night's sleep – were found to have improved or enhanced memory from the previous day. "In keeping with the computer file analogy," says Walker, "this stage of memory would be comparable to an editor coming in and opening a stable but messy file, and reorganizing it, refining it and tightening it up." Furthermore, he explains, this discovery helps strengthen the argument that sleep is beneficial to the learning process. "If you don't get that full night's sleep, you may be shortchanging your brain of learning potential," he adds.
The final stage of memory identified by Walker and his colleagues is the "recall phase," which allows a previously stabilized memory to be modified. "What we found was that after the memory had been stabilized [after several waking hours] and enhanced [after a night's sleep] it once again became pliable so that it could be altered in the context of new ongoing experiences." In other words, although an individual may have learned to play a piano scale, then enhanced the skill after a night of sleep, by way of this third modifying stage of memory he could continue to tweak and refine this new skill.
This last stage may have important clinical implications in the treatment of patients with psychological disorders such as post-traumatic stress disorder (PTSD), says Walker. "In PTSD, individuals have specific memories with specific associations attached to them, which are negative, and thereby causing the disorder," he explains . "What we think behavioral and cognitive therapies do by having patients replay those memories and talk about them is that exact third memory stage. Over time, there may be the chance for these patients to redefine their memories and make them less traumatic."
This study was supported by grants from the National Science Foundation and the National Institute of Mental Health.
Study co-authors include Harvard Medical School researchers Robert Stickgold, PhD, of Beth Israel Deaconess Medical Center, formerly of the Massachusetts Mental Health Center; and J. Allan Hobson, MD, and Tiffany Brafkefield, BA, of the Massachusetts Mental Health Center. Beth Israel Deaconess Medical Center is a major patient care, teaching and research affiliate of Harvard Medical School, ranking third in National Institutes of Health funding among independent hospitals nationwide. The medical center is clinically affiliated with the Joslin Diabetes Center and is a founding member of the Dana-Farber/Harvard Cancer Center. BIDMC is the official hospital of the Boston Red Sox.
Journal
Nature