Local sleep

For our sleep module, I read an interesting paper from 2011 by Vladyslav Vyazovskiy of the University of Wisconsin-Madison, et al., entitled “Local sleep in awake rats.” You can find it in Nature 472 (pp. 443-447), or at the links at the bottom of the post.

The researchers set out to investigate the effects of sleep deprivation on neuronal activity by implanting rats with 16-channel microwire arrays in layers throughout their motor and parietal cortices and observing them while they were awake and asleep. Each array channel measured the local field potential (by EEG) and neural activity at that specific location in the rat’s brain. The readings for spontaneous awake and NREM sleep periods were consistent with previous studies, but the researchers found a few interesting results when they deprived the rats of sleep for four hours beginning at first daylight (rats are nocturnal, so the pressure to sleep increases steadily during this period).

The first finding is that, as sleep pressure increases during the deprivation period, small populations of neurons in different areas of the brain can, for short durations, go offline — into the OFF state for neuronal activity that characterizes NREM sleep. This occurs even though the rat appears to be fully awake, and its EEG readings are more or less normal. And the occurrence of these OFF states increases with increases in sleep pressure.

Secondly, the researchers recorded data that may suggest that sleep is not the global process that it was once thought to be. Subsets of neurons were found to enter OFF periods in one cortical area but not in another, and even in the same area, some neurons were ON while others were OFF. This is striking, but it seems to fit a “growing understanding,” according to Christopher Colwell at UCLA, “of sleep as a local and history-dependent phenomenon.” It perhaps suggests that the basic unit of sleep is the individual neuron, not the entire brain or regions of the brain.

Finally, the researchers confirmed that the performance of the sleep-deprived rats suffered in a task involving reaching for a food reward. This finding is again consistent with others that have shown decreased performance in humans and animals associated with sleep deprivation. However, the authors of this study were able to go one step further, showing a correlation between increases in the incidence of the OFF state with decreased performance.

So, what does this mean for the class? This paper may mark a step forward in understanding what sleep deprivation does to the brain and how it leads to declining cognitive performance. The class should take note of the mechanisms proposed here whenever they find themselves, or a friend or family member, attempting to make due with a severe lack of sleep (like I am doing in this blog post!). Further research is certainly needed to understand whether this increased occurrence of OFF states is a phenomenon that occurs in humans, too, and if so, to understand the costs and benefits of that behavior and the deeper mechanisms that underlie it (such as those that determine which neurons shut down).

That said, if human behavior is consistent with that observed in the rats in this study, there is potential harm in the fact that one can appear to be fully awake from the outside, yet be experiencing impaired cognitive performance due to sleep deprivation. Maybe there is an opportunity to use these findings to develop better tests for alertness in situations that require it such as driving, as even an EEG is not sufficient to identify these drops in neuronal activity. Or maybe we can use this compelling explanation as the centerpiece for a campaign against getting behind the wheel when one is exceedingly tired.

 

Vladyslav V. Vyazovskiy, Umberto Olcese, Erin C. Hanlon, Yuval Nir, Chiara Cirelli, and Giulio Tononi. (2011). Local sleep in awake rats. Nature. doi:10.1038/nature10009

Nature.com: http://www.nature.com/nature/journal/v472/n7344/full/nature10009.html

MIT Libraries: http://libproxy.mit.edu/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=mdc&AN=21525926&site=eds-live

 

Christopher S. Colwell. (2011). Neuroscience: Sleepy neurons? Nature. doi:10.1038/472427a

Nature.com: http://www.nature.com/nature/journal/v472/n7344/full/472427a.html

MIT Libraries: http://libproxy.mit.edu/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=mdc&AN=21525924&site=eds-live