The Sleeping Brain Decides What to Remember and What to Forget

Durk Pearson & Sandy Shaw’s Life Extension News Volume 16 No. 4 – April 2013

A new paper¹ describes the sleep-dependent memory processing factory that decides what to do with all that information you encountered during the day. As the paper’s authors point out, only some of that information is consolidated so as to become a part of a long-term knowledge base. The paper sifts through evidence from studies of naps, sleep deprivation, correlations of sleep stages and memory processing, sleep physiology, regional brain activity measured during and after sleep with PET and fMRI studies, cellular firing patterns, and synaptic and intracellular measures of plasticity to conclude that there is convincing evidence of sleep processing of memory with improvement of the overall knowledge base.

The authors discuss the new understanding that not all information is uniformly preserved, but that there is an exquisite selection process of memories underway during sleep. For example, they report that emotional memories can be selectively consolidated, especially during rapid eye-movement (REM) sleep. It has also been found that memories can be selectively maintained when they contain information on potential monetary rewards. Interestingly, when subjects of sleep memory studies have been told that they would be tested on certain areas of information and not on others provided before sleep, they were found after sleep to have retained more of the information they were told they would be tested on. Hence, the brain “knew” what to do to recall the relevant information.

Moreover, the authors explain, it is possible for the brain to generate new information during the processing of the memory-derived information. “Whether consolidation necessarily precedes these integrative processes (serial processing) is not yet known, but no clear cases of integration without consolidation have been observed. We use the term ‘memory evolution’ to reflect both the qualitative changes that can occur during such integrative processing and the extended time course over which they occur.”¹ In gist extraction, the authors refer to the identification of commonalities between items in a collection of memories even when individual item memories are forgotten. Some studies have been found to show that during sleep subjects can extract overarching rules that govern recently studied sets of information, such that understanding of relationships between the sets is improved following sleep.

One study reported by the authors dealt with subjects taught a rote method for solving a class of mathematical problems for which there was a shortcut solution (about which subjects were not told). After a night of post-training sleep, however, subjects were found to be 2.6 times more likely to discover this shortcut than after an equal period of time awake (59.1 versus 22.7% of subjects).¹ But, interestingly, even those who did not discover the shortcut benefited from sleep by improving the speed with which they were able to perform the rote method of solving the problems. Those who became faster without discovering the shortcut improved their speed (using the rote method) three times more than either those who discovered the shortcut or those who remained awake.

This study examines important sleep processes at a systems level rather than at a neurotransmitter level. Understanding sleep involves comprehending its mechanisms from the micro-level details (neuro­transmitters and synapses) to the overarching system architecture.

Reference

1. Stickgold and Walker. Sleep-dependent memory triage: evolving generalization through selective processing. Nat Neurosci. 16(2):139-45 (2013).