To understand the big picture, give it time -- and sleep

Memorizing a series of facts is one thing, understanding the big picture is quite another. Now a new study demonstrates that relational memory - the ability to make logical "big picture" inferences from disparate pieces of information – is dependent on taking a break from studies and learning, and even more important, getting a good night's sleep.

Led by researchers at Beth Israel Deaconess Medical Center (BIDMC) and Brigham and Women's Hospital (BWH), the findings appear on-line in today's Early Edition of the Proceedings of the National Academy of Sciences (PNAS).

"Relational memory is a bit like solving a jigsaw puzzle," explains senior author Matthew Walker, PhD, Director of the Sleep and Neuroimaging Laboratory at BIDMC and Assistant Professor of Psychology at Harvard Medical School (HMS). "It's not enough to have all the puzzle pieces – you also have to understand how they fit together."

Adds lead author Jeffrey Ellenbogen, MD, a postdoctoral fellow at HMS and sleep neurologist at BWH, "People often assume that we know all of what we know because we learned it directly. In fact, that's only partly true. We actually learn individual bits of information and then apply them in novel, flexible ways."

For instance, if a person learns that A is greater than B and B is greater than C, then he or she knows those two facts. But embedded within those is a third fact – A is greater than C – which can be deduced by a process called transitive inference, the type of relational memory that the researchers examined in this study.

Earlier research by Walker and colleagues had shown that sleep actively improves task-oriented "procedural memory" – for example, learning to talk, to coordinate limbs, musicianship, or to play sports. Because relational memory is fundamental to knowledge and learning, Walker and Ellenbogen decided to explore how and when this "inferential" knowledge emerges, hypothesizing that it develops during "off-line" periods and that, like procedural memory, would be enhanced following a period of sleep.

So, the researchers tested 56 healthy college students, each of whom was shown five pairs of unfamiliar abstract patterns – colorful oval shapes resembling Faberge' eggs. The students were then told that some of the patterns were "correct" while others were "incorrect," for example, Shape A wins over Shape B, Shape B wins over Shape C, and so on. All of the students learned the individual pairs but were not told that there was a hidden "hierarchy" linking all five of the pairs together.

After a 30-minute study period, the students were separated into three groups to test their understanding of the larger "big picture" relationship between the individual patterns: Group One was tested after a period of 20 minutes; Group Two was tested after a 12-hour period; and Group Three was tested after a 24-hour time span. In addition, approximately half of the students in Group Two slept during the 12-hour period, while the other half remained awake. All of the students in Group Three had a full night's sleep.