Sleep deprivation, memory & brain imaging (JC)
Michael W.L. Chee and Lisa Y.M Chuah
Current Opinion in Neurology 2008, 21:417-423
Link to full online article (free)
This review paper provides an overview of studies using fMRI to gain insight into how sleep and sleep deprivation affect memory and cognition (as the title clearly states). The paper covers two topics. On the one hand studies on the effects of sleep deprivation on short-term memory. On the other hand studies on the role of sleep on long-term memory, more specifically memory consolidation. The described studies link these effects of sleep and sleep deprivation on short-term memory, and long-term memory respectively, to changes in brain activity or brain connectivity.
When people (or animals) perform a task, brain imaging shows activity in specific regions related to these tasks. E.g. for working memory this activity is most prevalent in the lateral prefrontal cortex and the parietal cortex. Sleep deprivation can affect this task-related activity, but studies show inconsistent findings with regard to the mechanism by which sleep deprivation alters this activity. To understand these inconsistent findings, one has to take into account task content, task difficulty, duration of sleep deprivation, methodological differences, and interindividual differences.
For short-term memory nice examples are given of brain regions affected by sleep deprivation with regard to different types of tasks (visual short-term memory, decision making, emotional responses…). Sleep deprivation often leads to compensatory brain activity (i.e. an increase in brain activity in the task-related brain region), which can sometimes result in less impaired performance. This means that performance isn’t altered much (or not up until a certain level of difficulty), but the brain mechanisms underlying this performance can be altered between sleep deprived people versus non-sleep deprived people (or between a night of sleep deprivation versus a night of no sleep deprivation, if you study the effects within an individual).
These studies however look at brain activity during tasks, and don’t look how brain activity is affected during sleep and how memories are consolidated during this stage of decreased awareness. The latter has been studied by Peigneux et al. (2004), who reported an increase in hippocampal blood flow during slow wave sleep (these are the deeper stages of sleep, with higher amplitude and lower frequency waves) following a hippocampus-dependent learning task (spatial memory task). In addition studies have shown improvements in performance when learning is followed by sleep.
It must be noted that aside from sleep affecting learning and providing a convenient time for memory consolidation and neuronal changes to take place, learning in itself also affects sleep. When certain brain areas are recruited during a learning task, these brain areas can show increased slow wave activity during sleep, as has been shown by Huber et al. (2004).