Prefrontal cortex atrophy limits deep sleep needed for memory retention.
A new study has, for the first time, linked structural brain changes that occur naturally with age to sleep-related memory problems. The finding offers an explanation as to why our ability to learn new information declines as we get older, a fact scientists have known for decades but have never fully understood.
A team of researchers at the University of California, Berkeley, designed an experiment which involved placing electrodes on the scalp of older subjects, to mimic the shape of slow-wave phase sleep. Humans spend an average of a quarter of each night engaged in deep low-wave phase sleep. This deep sleep has been found to play a major role in memory retention, as the brain moves temporary memories into long-term storage.
The use of electrodes to simulate deep sleep resulted in a demonstrable improvement in memory in several patients. The reason for this, scientists hypothesize, is that the brain’s prefrontal cortex loses volume with age, along with its ability to regulate patterns of deep sleep. This development compromises the brain’s ability to move short-term memories from their guest home in the hippocampus to the long-term vault of the prefrontal cortex.
While it may seem that diminishing brain volume would directly compromise memory, the study suggests that the indirect relationship between the prefrontal cortex and sleep is in fact responsible. “The analysis showed that the differences were due not to changes in capacity for memories, but to differences in sleep quality,” said Bryce A. Mander, a postdoctoral fellow at Berkeley and lead author of the study.
The researchers arrived at their conclusions by comparing memory retention in subjects of different ages after a full night of rest. First, the team analyzed brain images from 18 people in their early 20s, and compared these to images of 19 people of retirement age. On average, the medial prefrontal cortex was about one-third smaller in the older group as a result of natural atrophy.
The team then tested the subjects’ ability to memorize new, previously unknown information by having each group study a long list of 120 words coupled with random syllables, such as “arm-reconver” and “action-siblis”. After half an hour of study, participants were immediately tested on the list. Researchers found that the young group outscored their older counterparts by 25 percent in this initial test, suggesting an initial age-related difference in memory capacity unrelated to sleep.
Differences between the groups became much more pronounced after a night’s sleep, however. Using an electroencephalogram machine, or EEG, the team measured the shape and consistency of their subjects’ electrical brain waves, finding that the younger group spent about four times as many hours in deep sleep compared with the older group. As a result, the younger subjects outscored the older subjects by about 55 percent, as even the seniors who performed strongly just after studying the word pairs showed marked declines in memory retention.
The significant difference between older participants’ performance relative to that of younger participants after a night’s rest suggests sleep is an increasingly important factor affecting memory as people age.
“Essentially, with age, you lose tissue in this prefrontal area,” said Matthew P. Walker, a professor of psychology and neuroscience at Berkeley and a co-author of the study. “You get less quality deep sleep, and have less opportunity to consolidate new memories.”
Dr. Walker went on to clarify that medial prefrontal atrophy is not the only factor influencing age-related memory problems. A variety of changes occur throughout the brain as people age, and differences in our sleep patterns are likely to offer only a piece of the puzzle concerning memory degradation.
Getting enough sleep has implications beyond memory retention. Studies have shown that sleep affects a range of capabilities, including physical reflexes, fine motor skills, and even judgment. Some experiments have even shown that participants who were sleep deprived were more likely to believe that they were right when they were actually wrong.
While the exact connection between sleep and various mental processes remains largely unknown, the recent U.C. Berkeley study helps to demonstrate the importance of slow-wave sleep in connection to memory. Scientists also believe that certain types of memories crystallize in our subconscious during rapid eye movement (REM) sleep, as we dream.
The study also holds an important lesson for people of all ages– to get plenty of sleep in order to ensure productivity while awake. Measures like regular exercise and limiting stress can help to encourage deep, healthy sleep, along with a balanced and healthy lifestyle.
The U.C. Berkeley study was published on Sunday, January 27 in the journal Nature Neuroscience.